ALMA Observations of Ethyl Formate toward Orion KL

TL;DR

This study used high-resolution ALMA observations to detect and analyze ethyl formate in Orion KL, revealing its spatial distribution, temperature, and possible formation pathways, thus advancing understanding of complex organic molecules in space.

Contribution

First detection of multiple ethyl formate lines in Orion KL with detailed spatial and physical characterization, suggesting grain-surface formation mechanisms for this complex molecule.

Findings

Detected 82 lines of ethyl formate, confirming its presence.

Derived temperature and column density for different regions.

Indicated grain-surface formation pathway involving methyl formate radicals.

Abstract

Orion KL is one of the prime templates of astrochemical and prebiotic chemical studies. We wish to explore more organic molecules with increasing complexity in this region. In particular, we have searched for one of the most complex organic molecules detected in space so far, ethyl formate (C$_{2}$H$_{5}$OCHO). This species is the next step in chemical complexity after the simplest member of esters (methyl formate, CH$_{3}$OCHO). The mechanisms leading to its formation are still poorly known. We have used high angular resolution ($\sim$ 1.$^{\prime\prime}$5) ALMA observations covering a large bandwidth from 214 to 247 GHz. We have detected 82 unblended lines of C$_{2}$H$_{5}$OCHO (49 and 33 of the trans and gauche conformers, respectively). The line images showed that C$_{2}$H$_{5}$OCHO arises mainly from the compact ridge and the hot core-southwest regions. The derived rotational temperatures and column densities are 122 $\pm$ 34 K, (0.9 $\pm$ 0.3) $\times$ 10$^{16}$ cm$^{-2}$ for the hot core-SW, and 103 $\pm$ 13 K, (0.6 $\pm$ 0.3) $\times$ 10$^{16}$ cm$^{-2}$ for the compact ridge. The comparison of spatial distribution and abundance ratios with chemically related molecules (methyl formate, ethanol and formic acid) indicates that C$_{2}$H$_{5}$OCHO is likely formed on the surface of dust grains by addition of CH$_{3}$ to functional-group radicals (CH$_{2}$OCHO) derived from methyl formate (CH$_{3}$OCHO).

Tables

Table 1. Table 1: Model fitting results of EF, MF, ET and t-HCOOH.

Molecular name	hot core-SW					compact ridge
	$v_{\mathrm{LSR}}$ aaThe $v_{\mathrm{LSR}}$ and $\Delta \upsilon$ toward two positions are the average values from the Gaussian fitting of several completely unblended lines.	$\Delta \upsilon$ aaThe $v_{\mathrm{LSR}}$ and $\Delta \upsilon$ toward two positions are the average values from the Gaussian fitting of several completely unblended lines.	Trot	N	$\chi$		$v_{\mathrm{LSR}}$ aaThe $v_{\mathrm{LSR}}$ and $\Delta \upsilon$ toward two positions are the average values from the Gaussian fitting of several completely unblended lines.	$\Delta \upsilon$ aaThe $v_{\mathrm{LSR}}$ and $\Delta \upsilon$ toward two positions are the average values from the Gaussian fitting of several completely unblended lines.	Trot	N	$\chi$
	$\left(km{s}^{-1}\right)$	$\left(km{s}^{-1}\right)$	($K$)	(${10}^{16}c{m}^{-2}$)	(${10}^{-8}$)		$\left(km{s}^{-1}\right)$	$\left(km{s}^{-1}\right)$	($K$)	(${10}^{16}c{m}^{-2}$)	(${10}^{-8}$)
C2H5OCHO	6.8$\pm$0.1	3.0$\pm$0.1	122$\pm$34	0.9$\pm$0.3	0.19$\pm$0.06		7.2$\pm$0.2	2.2$\pm$0.1	103$\pm$13	0.6$\pm$0.3	0.3$\pm$0.1
CH3OCHO	7.3$\pm$0.1	3.2$\pm$0.1	135$\pm$5	70.0$\pm$26.0	15$\pm$5		7.1$\pm$0.1	2.5$\pm$0.2	93$\pm$5	85.0$\pm$40.0	42$\pm$20
C2H5OH	7.1$\pm$0.1	3.5$\pm$0.1	144$\pm$13	12.9$\pm$4.8	2.7$\pm$1.0		7.7$\pm$0.1	2.5$\pm$0.1	111$\pm$15	2.0$\pm$0.5	1.0$\pm$0.3
t-HCOOH	6.7$\pm$0.1	4.0$\pm$0.2	120$\pm$16	0.6$\pm 0.2$	0.13$\pm$0.04		$\cdots$	$\cdots$	$\cdots$	$\cdots$

Table 2. Table 2: Parameters of EF, MF and ET from best-fit LTE analysis in three sources (Orion KL, W51 and Sgr B2(N)).

Species	Source	Num.1	Eu range	Obs.2	${\theta }_s$3	Trot	N	$\chi$	reference
			($K$)		(′′)	($K$)	(${10}^{16}c{m}^{-2}$)	(${10}^{-8}$)
	Orion KL (hc)	34	152$-$387	(ALMA)1.′′8$\times$1.′′3	1.6	122$\pm$34	0.9$\pm$0.3	0.19$\pm$0.06	this work
	Orion KL (cr)	51	170$-$411	(ALMA)1.′′8$\times$1.′′3	1.6	103$\pm$13	0.6$\pm$0.3	0.3$\pm$0.1	this work
EF	Orion KL (cr)	1	276$-$276	(ALMA)2.′′0$\times$1.′′5	3.0	100$\pm$20	0.2$\pm$0.04	$\cdots$	Tercero et al. (2015)
	Orion KL (cr)	90	$\cdots$	(IRAM 30m)11′′$-$29′′	15.0	150$\pm$20	0.045$\pm$0.01	$\cdots$	Tercero et al. (2013)
	W51	46	42$-$213	(IRAM 30m)10′′$-$27′′	2.4	78$\pm$10	2.0$\pm$0.3	1.0	Rivilla et al. (2017b)
	Sgr B2(N)	46	34$-$253	(IRAM 30m)22′′$-$29′′	3.0	100	5.4	0.36	Belloche et al. (2009)
	Orion KL (hc)	350	36$-$797	(ALMA)1.′′8$\times$1.′′3	1.6	135$\pm$5	70.0$\pm$26.0	15$\pm$5	this work
	Orion KL (hc)	20	57$-$312	(SMA+IRAM 30m)5.′′0$\times$4.′′0	$\cdots$	126$\pm$13	2.7$\pm$1.1	2$\pm$1	Feng et al. (2015)
	Orion KL (hc)	14	36$-$618	(PdBI)1.′′79$\times$0.′′79	$\cdots$	128$\pm$9	16.0$\pm$2.0	5.2	Favre et al. (2011)
	Orion KL (cr)	458	23$-$797	(ALMA)1.′′8$\times$1.′′3	1.6	93$\pm$5	85.1$\pm$40.0	42$\pm$20	this work
MF	Orion KL (cr)	1	122$-$122	(ALMA)2.′′0$\times$1.′′5	3.0	100$\pm$20	24.0$\pm$5.0	$\cdots$	Tercero et al. (2015)
	Orion KL (cr)	27	57$-$312	(SMA+IRAM 30m)5.′′0$\times$4.′′0	$\cdots$	102$\pm$5	9.7$\pm$2.1	15$\pm$3	Feng et al. (2015)
	Orion KL (cr)	12	2$-$143	(Effelsberg-100m)35′′$-$50′′	10	92$\pm$14	18$\pm$2	47$\pm$6	Gong et al. (2015)
	Orion KL (cr)	$\cdots$	$\cdots$	(HIFI, IRAM 30m)	10	110	13	33	Crockett et al. (2014)
	Orion KL (cr)aatoward the 7.6 km s-1 component for the compact ridge	20	36$-$366	(PdBI)1.′′79$\times$0.′′79	$\cdots$	79$\pm$2	16.0$\pm$1.0	3.1	Favre et al. (2011)
	W51	38	28$-$365	(IRAM 30m)10′′$-$27′′	2.4	112$\pm$3	50.1$\pm$0.6	25	Rivilla et al. (2017b)
	Sgr B2(N)	$\cdots$	$\cdots$	(IRAM 30m)22′′$-$29′′	4.0	80	45	3.5	Belloche et al. (2009)
	Orion KL (hc)	186	24$-$672	(ALMA)1.′′8$\times$1.′′3	1.6	144$\pm$13	12.9$\pm$4.8	2.7$\pm$1.0	this work
	Orion KL (hc)	5	86$-$263	(SMA+IRAM 30m)5.′′0$\times$4.′′0	$\cdots$	137$\pm$33	0.8$\pm$0.5	0.6$\pm$0.5	Feng et al. (2015)
	Orion KL (hc)	1	316$-$316	(ALMA)2.′′0$\times$1.′′5	3.0	100$\pm$20	6.0	$\cdots$	Tercero et al. (2015)
ET	Orion KL (cr)	162	24$-$548	(ALMA)1.′′8$\times$1.′′3	1.6	127$\pm$17	2.0$\pm$0.5	1.0$\pm$0.3	this work
	Orion KL (cr)	5	86$-$263	(SMA+IRAM 30m)5.′′0$\times$4.′′0	$\cdots$	68$\pm$11	0.6	0.9$\pm$0.5	Feng et al. (2015)
	Orion KL (cr)	$\cdots$	$\cdots$	(HIFI, IRAM 30m)	10	110	0.7	1.7	Crockett et al. (2014)
	W51	14	35$-$327	(IRAM 30m)10′′$-$27′′	2.4	95$\pm$3	15.8$\pm$0.4	7.9	Rivilla et al. (2017b)
	Sgr B2(N)	$\cdots$	$\cdots$	(IRAM 30m)22′′$-$29′′	3.0	100	84	5.7	Belloche et al. (2009)

Table 3. Table 3: Molecular column density ratios toward Orion KL in different works.

	[MF/EF]			[MF/ET]
	this work	Tercero et al. (2015)		this work	Feng et al. (2015)	Crockett et al. (2014)
hot core-SW	79$\pm$36	$-$		6$\pm$3	4$\pm$3	$-$
compact ridge	140$\pm$81	$\simeq 120$		42$\pm$24	17$\pm$12	20

Table 4. Table A.1: Selected unblended transitions of the T ​ r ​ a ​ n ​ s 𝑇 𝑟 𝑎 𝑛 𝑠 Trans and G ​ a ​ u ​ c ​ h ​ e 𝐺 𝑎 𝑢 𝑐 ℎ 𝑒 Gauche conformers of EF observed with the ALMA.

Rest Frequency	Transition	$S_{ij}{\mu }^2$	Eu	hot core-SW					compact ridge				$\sigma$
				$v_{\mathrm{LSR}}$	$\Delta V$	$I_p$	Detection		$v_{\mathrm{LSR}}$	$\Delta V$	$I_p$	Detection
(GHz)		(Debye${}_{}{}^2)$	(K)	(km s-1)	(km s-1)	(K)	Level		(km s-1)	(km s-1)	(K)	Level	(K)
$Trans$ conformer
214.11074	$39(13,26)-38(13,25)$	118.64	327	B					7.2$\pm$0.1	1.7$\pm$0.3	0.42$\pm$0.07	7$\sigma$	0.06
214.43282	$39(9,31)-38(9,30)$	126.36	264	B					7.0$\pm$0.1	2.0$\pm$0.1	1.09$\pm$0.05	23$\sigma$	0.05
214.43284	$39(9,30)-38(9,29)$	126.36	264
214.60560	$39(4,36)-38(4,35)$	131.98	218	B					7.0$\pm$0.1	2.0$\pm$0.1	0.65$\pm$0.05	11$\sigma$	0.06
214.60858	$39(8,32)-38(8,31)$	127.85	251	6.5$\pm$0.1	4.2$\pm$0.1	1.40$\pm$0.04	25$\sigma$		6.9$\pm$0.1	2.5$\pm$0.1	1.15$\pm$0.05	20$\sigma$	0.06
214.60911	$39(8,31)-38(8,30)$	127.85	251
214.87062	$39(7,33)-38(7,32)$	129.18	241	7.3$\pm$0.2	4.0$\pm$0.4	0.78$\pm$0.05	14$\sigma$		7.4$\pm$0.1	1.7$\pm$0.3	0.59$\pm$0.06	10$\sigma$	0.06
214.88185	$39(7,32)-38(7,31)$	129.17	241	6.8$\pm$0.1	3.7$\pm$0.3	0.80$\pm$0.04	14$\sigma$		7.2$\pm$0.1	2.1$\pm$0.2	0.61$\pm$0.06	11$\sigma$	0.06
215.46693	$39(5,35)-38(5,34)$	131.27	224	B					7.1$\pm$0.1	1.7$\pm$0.1	0.84$\pm$0.06	15$\sigma$	0.06
217.05267	$40(3,38)-39(3,37)$	135.84	222	7.1$\pm$0.1	2.5$\pm$0.3	0.86$\pm$0.6	13$\sigma$		7.0$\pm$0.1	1.8$\pm$0.2	0.78$\pm$0.07	12$\sigma$	0.07
218.03650	$41(2,40)-40(2,39)$	139.64	225	B					7.2$\pm$0.1	1.7$\pm$0.2	0.81$\pm$0.09	11$\sigma$	0.08
218.23915	$40(2,38)-39(2,37)$	135.92	222	6.9$\pm$0.1	3.0$\pm$0.2	1.18$\pm$0.07	14$\sigma$		7.1$\pm$0.1	1.8$\pm$0.3	0.66$\pm$0.09	8$\sigma$	0.09
218.53650	$39(3,36)-38(3,35)$	132.54	216	PB					6.9$\pm$0.1	1.7$\pm$0.1	1.13$\pm$0.07	17$\sigma$	0.07
220.14861	$40(8,33)-39(8,32)$	131.42	262	B					7.0$\pm$0.1	2.0$\pm$0.1	0.86$\pm$0.05	18$\sigma$	0.05
220.14940	$40(8,32)-39(8,31)$	131.42	262
220.44705	$40(7,33)-39(7,32)$	132.70	252	PB					7.1$\pm$0.1	2.3$\pm$0.2	0.84$\pm$0.06	15$\sigma$	0.06
221.00793	$40(5,36)-39(5,35)$	134.73	235	B					7.0$\pm$0.1	1.8$\pm$0.2	0.82$\pm$0.07	12$\sigma$	0.07
223.26160	$42(1,41)-41(1,40)$	143.07	236	7.3$\pm$0.1	2.7$\pm$0.3	0.72$\pm$0.04	15$\sigma$		PB				0.05
225.36427	$41(4,38)-40(4,37)$	138.86	239	7.0$\pm$0.2	2.1$\pm$0.5	0.40$\pm$0.06	6$\sigma$		7.0$\pm$0.1	1.7$\pm$0.2	0.87$\pm$0.07	13$\sigma$	0.07
225.48591	$41(9,33)-40(9,32)$	133.57	285	B					7.1$\pm$0.1	2.0$\pm$0.1	1.13$\pm$0.05	24$\sigma$	0.05
225.48595	$41(9,32)-40(9,31)$	133.57	285
228.34140	$43(2,42)-42(2,41)$	146.49	247	B					7.2$\pm$0.1	1.9$\pm$0.1	0.78$\pm$0.05	17$\sigma$	0.05
228.34705	$42(2,40)-41(2,39)$	142.72	243	7.2$\pm$0.1	2.7$\pm$0.2	0.67$\pm$0.04	14$\sigma$		7.0$\pm$0.1	1.5$\pm$0.1	0.66$\pm$0.05	14$\sigma$	0.05
228.39919	$43(1,42)-42(1,41)$	146.48	247	6.0$\pm$0.1	4.2$\pm$0.3	0.56$\pm$0.09	12$\sigma$		7.2$\pm$0.1	1.4$\pm$0.1	0.47$\pm$0.05	10$\sigma$	0.05
230.71849	$42(4,39)-41(4,38)$	142.31	250	6.3$\pm$0.1	3.3$\pm$0.1	1.06$\pm$0.04	22$\sigma$		PB				0.05
230.75105	$42(11,32)-41(11,31)$	133.90	325	B					7.2$\pm$0.2	2.8$\pm$0.6	0.59$\pm$0.05	10$\sigma$	0.06
230.861687	$42(10,33)-41(10,32)$	135.61	310	6.4$\pm$0.1	3.8$\pm$0.3	0.79$\pm$0.03	21$\sigma$		B				0.04
230.861689	$42(10,32)-41(10,31)$	135.61	310
230.91185	$41(4,37)-40(4,36)$	139.12	241	7.1$\pm$0.1	2.3$\pm$0.2	0.60$\pm$0.05	13$\sigma$		B				0.05
231.01571	42(9, 24)-41(9, 33)	137.16	296	7.9$\pm$0.1	2.3$\pm$0.1	1.01$\pm$0.05	21$\sigma$		B				0.05
231.59355	$42(7,35)-41(7,34)$	139.74	274	B					7.5$\pm$0.1	1.9$\pm$0.3	0.58$\pm$0.06	10$\sigma$	0.06
232.32584	$42(6,36)-41(6,35)$	140.82	265	7.9$\pm$0.2	1.8$\pm$0.3	0.28$\pm$0.05	6$\sigma$		7.7$\pm$0.1	1.9$\pm$0.2	0.63$\pm$0.05	13$\sigma$	0.05
233.53793	$44(1,43)-43(1,42)$	149.90	258	B					7.1$\pm$0.1	1.8$\pm$0.2	0.70$\pm$0.08	9$\sigma$	0.08
236.381882	$43(10,34)-42(10,33)$	139.22	321	8.0$\pm$0.1	2.7$\pm$0.2	1.19$\pm$0.07	16$\sigma$		7.6$\pm$0.1	1.6$\pm$0.3	0.68$\pm$0.11	9$\sigma$	0.08
236.381885	$43(10,33)-42(10,32)$	139.22	321
237.56673	$43(6,38)-42(6,37)$	144.31	276	7.8$\pm$0.1	1.5$\pm$0.2	0.70$\pm$0.07	12$\sigma$		PB				0.06
237.57528	$43(5,39)-42(5,38)$	145.14	268	7.0$\pm$0.1	2.9$\pm$0.3	0.53$\pm$0.04	11$\sigma$		7.3$\pm$0.1	1.9$\pm$0.1	0.80$\pm$0.05	17$\sigma$	0.05
239.33504	$46(1,46)-45(1,45)$	157.20	272	PB					7.1$\pm$0.1	2.3$\pm$0.1	1.32$\pm$0.06	20$\sigma$	0.07
239.33580	$46(0,46)-45(0,45)$	157.20	272
239.39187	$43(3,40)-42(3,39)$	146.09	260	7.2$\pm$0.1	3.0$\pm$0.2	0.99$\pm$0.05	15$\sigma$		6.9$\pm$0.1	1.8$\pm$0.1	0.81$\pm$0.07	12$\sigma$	0.07
241.30348	$32(4,28)-32(1,31)$	0.30	152	3.8$\pm$0.1	1.7$\pm$0.2	0.42$\pm$0.05	9$\sigma$		PB				0.05
241.68105	$44(12,32)-43(12,31)$	139.38	364	B					7.8$\pm$0.2	1.5$\pm$0.3	0.35$\pm$0.06	6$\sigma$	0.06
242.76463	$44(7,37)-43(7,36)$	146.78	297	7.8$\pm$0.1	2.2$\pm$0.1	0.82$\pm$0.07	14$\sigma$		PB				0.06
243.56050	$45(2,43)-44(2,42)$	152.99	278	B					7.0$\pm$0.1	2.2$\pm$0.1	0.128$\pm$0.006	19$\sigma$	0.007
243.67449	$46(1,45)-45(2,44)$	16.76	281	6.1$\pm$0.1	2.9$\pm$0.3	0.70$\pm$0.04	15$\sigma$		6.4$\pm$0.1	2.1$\pm$0.3	0.32$\pm$0.05	7$\sigma$	0.05
243.78836	$46(2,45)-45(2,44)$	156.77	281	B					7.3$\pm$0.1	1.6$\pm$0.2	0.72$\pm$0.05	15$\sigma$	0.05
246.12899	$44(5,39)-43(5,38)$	148.70	281	7.5$\pm$0.1	3.5$\pm$0.2	0.70$\pm$0.05	15$\sigma$		7.7$\pm$1.3	1.3$\pm$0.9	0.35$\pm$0.09	7$\sigma$	0.05
$Gauche$ conformer
213.807356	$33(1,33)-32(1,32)$	68.12	272	6.0$\pm$0.1	3.7$\pm$0.1	1.76$\pm$0.05	31$\sigma$		6.9$\pm$0.1	2.4$\pm$0.1	1.32$\pm$0.04	23$\sigma$	0.06
213.807357	$33(0,33)-32(0,32)$	68.12	272
213.86579	$30(9,22)-29(9,21)$	56.64	279	5.8$\pm$0.3	3.4$\pm$0.8	0.28$\pm$0.06	5$\sigma$		7.0$\pm$0.2	1.6$\pm$0.4	0.30$\pm$0.05	6$\sigma$	0.06
220.2051952	$34(1,34)-33(1,33)$	70.19	283	B					7.0$\pm$0.1	1.9$\pm$0.1	1.21$\pm$0.05	21$\sigma$	0.06
220.2051959	$34(0,34)-33(0,33)$	70.19	283
222.39509	$31(8,23)-30(8,22)$	60.04	285	7.2$\pm$0.2	3.0$\pm$0.5	0.29$\pm$0.04	6$\sigma$		7.1$\pm$0.1	1.7$\pm$0.2	0.46$\pm$0.05	10$\sigma$	0.05
226.16523	$33(4,30)-32(4,29)$	66.58	289	B					7.2$\pm$0.1	2.1$\pm$0.2	0.41$\pm$0.04	11$\sigma$	0.04
226.56953	$32(21,11)-31(21,10)$	37.80	411	7.0$\pm$0.3	2.1$\pm$0.6	0.21$\pm$0.04	5$\sigma$		7.1$\pm$0.2	2.1$\pm$0.5	0.21$\pm$0.04	5$\sigma$	0.04
226.6013827	$35(1,35)-34(1,34)$	72.27	294	6.5$\pm$0.1	3.4$\pm$0.1	1.75$\pm$0.04	37$\sigma$		PB				0.05
226.6013832	$35(0,35)-34(0,34)$	72.27	294
227.12119	$32(12,21)-31(12,20)$	57.07	320	6.7$\pm$0.1	2.5$\pm$0.2	1.10$\pm$0.04	23$\sigma$		PB				0.05
227.12122	$32(12,20)-31(12,19)$	57.07	320
229.98245	$32(8,24)-31(8,23)$	62.25	296	7.9$\pm$0.1	2.6$\pm$0.3	0.51$\pm$0.05	11$\sigma$		B				0.05
230.77940	$32(5,27)-31(5,26)$	64.13	287	$\cdots$			$\le$3$\sigma$		7.1$\pm$0.2	2.6$\pm$0.4	0.30$\pm$0.04	8$\sigma$	0.04
230.86702	$33(4,29)-32(4,28)$	66.21	294	8.0$\pm$0.4	2.1$\pm$0.8	0.16$\pm$0.05	3$\sigma$		8.1$\pm$0.1	2.5$\pm$0.3	0.45$\pm$0.05	10$\sigma$	0.05
232.02663	$33(5,29)-32(4,28)$	18.18	294	B					6.8$\pm$0.4	1.8$\pm$0.7	0.20$\pm$0.07	4$\sigma$	0.06
232.89572	$12(11,2)-11(11,1)$	25.44	170	$\cdots$			3$\sigma$		7.4$\pm$0.1	1.7$\pm$0.2	0.57$\pm$0.04	15$\sigma$	0.04
233.17815	$32(7,25)-31(7,24)$	63.36	292	8.1$\pm$0.3	3.6$\pm$0.3	0.60$\pm$0.05	11$\sigma$		8.2$\pm$0.2	2.0$\pm$0.4	0.33$\pm$0.06	6$\sigma$	0.06
233.61188	$33(18,15)-32(18,14)$	48.10	387	6.5$\pm$0.2	2.2$\pm$0.4	0.42$\pm$0.06	6$\sigma$		6.9$\pm$0.1	1.8$\pm$0.2	0.57$\pm$0.07	9$\sigma$	0.07
233.61188	$33(18,16)-32(18,15)$	48.10	387
233.88192	$33(14,19)-32(14,18)$	56.15	347	B					6.7$\pm$0.2	2.7$\pm$0.4	0.37$\pm$0.04	8$\sigma$	0.05
234.23279	$14(10,5)-13(10,4)$	23.33	171	PB					7.3$\pm$0.1	2.0$\pm$0.3	0.31$\pm$0.03	8$\sigma$	0.04
234.59985	$33(11,23)-32(11,22)$	60.86	324	B					6.2$\pm$0.2	2.0$\pm$0.5	0.18$\pm$0.04	5$\sigma$	0.04
234.60109	$33(11,22)-32(11,21)$	60.86	324
234.77083	$35(3,33)-34(3,32)$	71.17	306	6.2$\pm$0.1	2.8$\pm$0.3	0.85$\pm$0.06	13$\sigma$		B				0.07
234.77186	$35(2,33)-34(2,32)$	71.17	306
235.68320	$33(9,25)-32(9,24)$	63.38	312	B					7.2$\pm$0.2	2.1$\pm$0.4	0.36$\pm$0.06	13$\sigma$	0.06
239.38860	$37(1,37)-36(1,36)$	76.43	316	7.0$\pm$0.1	3.5$\pm$0.1	1.57$\pm$0.04	28$\sigma$		7.3$\pm$0.1	2.1$\pm$0.1	1.27$\pm$0.06	22$\sigma$	0.06
239.38861	$37(0,37)-36(0,36)$	76.43	316
242.84401	$34(5,29)-33(5,28)$	68.08	310	PB					7.0$\pm$0.2	1.7$\pm$0.4	0.38$\pm$0.07	6$\sigma$	0.07
243.38631	$35(4,31)-34(4,30)$	70.32	317	B					7.3$\pm$0.1	1.6$\pm$0.3	0.44$\pm$0.06	9$\sigma$	0.05
245.7795365	$38(1,38)-37(1,37)$	78.49	328	8.0$\pm$0.1	2.8$\pm$0.2	1.29$\pm$0.09	17$\sigma$		7.5$\pm$0.1	1.6$\pm$0.3	1.73$\pm$0.08	23$\sigma$	0.08
245.7795366	$38(0,38)-37(0,37)$	78.49	328

Table 5. Table A.2: Selected unblended transitions of MF observed with the ALMA.

Rest Frequency	Transition	$S_{ij}{\mu }^2$	Eu	Detection
(GHz)		(Debye${}_{}{}^2)$	(K)	hot core-SW	compact ridge
214.23362	$13(4,9)-13(1,12)E$	0.13	65	B	$\surd$
214.31522	$13(4,9)-13(1,12)A$	0.13	65	B	$\surd$
214.62285	$10(4,7)-9(3,6)E$	1.71	43	$\surd$	$\surd$
214.63169	$17(5,12)-16(5,11)E$	41.28	108	$\surd$	$\surd$
214.65259	$17(5,12)-16(5,11)A$	41.28	108	$\surd$	$\surd$
214.67036	$10(4,7)-9(3,6)A$	1.75	43	$\surd$	$\surd$
214.78236	$18(3,16)-17(3,15)E$	46.05	106	$\surd$	$\surd$
214.79249	$18(3,16)-17(3,15)A$	45.98	106	$\surd$	$\surd$
214.96936	$34(7,28)-34(6,29)E$	8.23	388	$\surd$	$\surd$
214.99967	$34(7,28)-34(6,29)A$	8.34	388	B	$\surd$
215.96593	$19(3,17)-18(2,17)E$	6.59	109	$\surd$	$\surd$
215.97199	$19(1,18)-18(2,17)A$	6.59	109	$\surd$	$\surd$
216.10973	$19(2,18)-18(2,17)E$	49.29	109	$\surd$	$\surd$
216.11549	$19(2,18)-18(2,17)A$	49.28	109	$\surd$	$\surd$
216.13438	$29(9,20)-29(6,23)E$	7.68	312	$\surd$	$\surd$
216.17531	$39(7,32)-39(6,33)E$	9.84	507	$\surd$	$\surd$
216.21086	$19(3,17)-18(3,16)E$	49.29	109	$\surd$	$\surd$
216.21646	$19(1,18)-18(1,17)A$	49.29	109	$\surd$	$\surd$
216.23612	$39(7,32)-39(6,33)A$	9.84	507	$\surd$	$\surd$
216.35465	$19(2,18)-18(3,16)E$	6.59	109	$\surd$	$\surd$
216.35995	$19(2,18)-18(1,17)A$	6.58	109	$\surd$	$\surd$
216.51828	$34(9,26)-34(8,27)A$	9.51	408	$\surd$	$\surd$
216.54009	$34(7,27)-34(10,25)E$	9.51	408	B	$\surd$
216.58863	$33(9,25)-33(8,26)A$	9.04	388	$\surd$	$\surd$
216.61044	$33(9,25)-33(8,26)E$	9.04	388	$\surd$	$\surd$
216.75896	$32(6,27)-32(5,28)E$	6.88	338	B	$\surd$
216.80896	$32(6,27)-32(5,28)A$	6.96	338	B	$\surd$
216.83020	$18(2,16)-17(2,15)E$	46.14	106	$\surd$	$\surd$
216.83885	$18(2,16)-17(2,15)A$	46.07	106	$\surd$	$\surd$
216.96479	$20(1,20)-19(1,19)E$	52.71	112	B	B
216.96590	$20(1,20)-19(1,19)A$	52.82	111	B	B
216.96625	$20(0,20)-19(0,19)E$	52.81	112	B	B
216.96742	$20(0,20)-19(0,19)A$	52.83	111	B	B
217.00429	$30(4,26)-30(4,27)E$	2.62	291	$\surd$	$\surd$
217.07727	$30(4,26)-30(4,27)A$	2.63	291	$\surd$	$\surd$
217.14275	$35(9,27)-35(8,28)A$	9.70	429	$\surd$	$\surd$
217.16536	$35(9,27)-35(8,28)E$	9.70	429	B	$\surd$
217.21584	$32(9,24)-32(8,25)A$	8.67	368	$\surd$	$\surd$
217.23592	$32(9,24)-32(8,25)E$	8.67	368	B	$\surd$
217.26668	$30(4,26)-30(3,27)A$	5.56	291	$\surd$	$\surd$
217.33797	$37(10,27)-37(9,28)E$	10.41	485	$\surd$	$\surd$
217.51710	$8(5,4)-8(3,5)A$	0.02	38	B	$\surd$
217.79416	$32(6,27)-32(4,28)A$	2.98	338	$\surd$	$\surd$
218.05680	$19(6,14)-19(4,15)E$	0.41	137	$\surd$	$\surd$
218.06057	$19(6,14)-19(4,15)A$	0.41	137	$\surd$	$\surd$
218.25959	$31(9,23)-31(8,24)A$	8.31	349	$\surd$	$\surd$
218.28084	$17(1,16)-16(1,15)E$	43.54	100	$\surd$	$\surd$
218.29789	$17(3,14)-16(3,13)A$	43.60	100	$\surd$	$\surd$
218.58532	$36(9,28)-36(8,29)A$	10.14	451	B	$\surd$
218.59334	$27(7,21)-27(5,22)A$	1.23	258	B	$\surd$
218.60704	$36(9,28)-36(8,29)E$	9.99	451	$\surd$	$\surd$
219.08994	$34(7,28)-34(5,29)E$	3.09	388	B	$\surd$
219.10866	$34(7,28)-34(5,29)A$	3.10	388	B	$\surd$
219.26318	$36(6,30)-36(6,31)A$	3.43	429	B	$\surd$
219.41727	$30(5,26)-30(4,27)E$	5.49	292	$\surd$	$\surd$
219.48370	$30(5,26)-30(4,27)A$	5.55	292	$\surd$	$\surd$
219.57904	$28(9,19)-28(8,20)A$	7.29	295	$\surd$	$\surd$
219.59210	$28(9,19)-28(6,22)E$	7.26	295	$\surd$	$\surd$
219.60016	$30(9,22)-30(8,23)E$	7.94	330	$\surd$	$\surd$
219.60736	$30(5,26)-30(3,27)E$	2.61	292	B	$\surd$
219.67311	$30(5,26)-30(3,27)A$	2.62	292	B	$\surd$
220.16689	$17(4,13)-16(4,12)E$	42.92	103	$\surd$	$\surd$
220.19029	$17(4,13)-16(4,12)A$	42.93	103	$\surd$	$\surd$
220.43282	$33(5,28)-33(5,29)A$	3.06	357	B	$\surd$
220.78615	$28(3,25)-28(3,26)E$	2.13	248	$\surd$	$\surd$
220.81185	$18(3,16)-17(2,15)E$	4.43	106	$\surd$	$\surd$
220.81519	$18(3,16)-17(2,15)A$	4.48	106	$\surd$	$\surd$
220.81955	$24(2,23)-24(1,24)A$	1.37	169	B	$\surd$
220.86870	$28(3,25)-28(3,26)A$	2.13	248	$\surd$	$\surd$
220.88894	$18(17,2)-17(17,1)A$	5.17	293	B	$\surd$
220.90106	$18(17,1)-17(17,0)E$	5.17	293	$\surd$	$\surd$
220.91035	$18(17,2)-17(17,1)E$	5.20	293	$\surd$	$\surd$
220.92617	$18(16,2)-17(16,1)A$	10.05	271	$\surd$	$\surd$
220.93518	$18(16,2)-17(16,1)E$	10.05	271	$\surd$	$\surd$
220.96661	$37(9,29)-37(8,30)E$	10.24	473	B	$\surd$
220.97798	$18(15,3)-17(15,2)A$	14.69	250	$\surd$	$\surd$
220.99833	$18(15,4)-17(15,3)E$	14.69	250	$\surd$	$\surd$
221.02507	$36(6,30)-36(5,31)A$	8.45	429	$\surd$	$\surd$
221.04999	$18(14,4)-17(14,3)E$	18.98	231	$\surd$	$\surd$
221.06693	$18(14,5)-17(14,4)E$	18.98	231	$\surd$	$\surd$
221.07598	$29(9,21)-29(8,22)A$	7.66	312	$\surd$	$\surd$
221.08618	$29(9,21)-29(8,22)E$	7.57	312	$\surd$	$\surd$
221.13972	$18(13,5)-17(13,4)E$	22.97	213	B	B
221.14113	$18(13,5)-17(13,4)A$	22.98	213	B	B
221.15854	$18(13,6)-17(13,5)E$	22.98	213	$\surd$	$\surd$
221.26068	$18(12,6)-17(12,5)E$	26.59	196	$\surd$	$\surd$
221.26560	$18(12,7)-17(12,6)A$	26.59	196	$\surd$	$\surd$
221.26569	$18(12,6)-17(12,5)A$	26.68	196	$\surd$	$\surd$
221.28090	$18(12,7)-17(12,6)E$	26.59	196	$\surd$	$\surd$
221.29389	$28(4,25)-28(3,26)E$	4.12	248	$\surd$	$\surd$
221.32184	$28(4,25)-28(2,26)E$	2.12	248	$\surd$	$\surd$
221.37482	$28(4,25)-28(3,26)A$	4.11	248	$\surd$	$\surd$
221.42457	$18(11,7)-17(11,6)E$	29.99	181	B	$\surd$
221.43302	$18(11,7)-17(11,6)A$	30.08	181	$\surd$	$\surd$
221.44563	$18(11,8)-17(11,7)E$	29.99	181	$\surd$	$\surd$
221.64934	$18(10,8)-17(10,7)E$	33.09	167	$\surd$	$\surd$
221.66048	$18(4,15)-17(4,14)E$	45.31	112	$\surd$	$\surd$
221.67075	$18(10,9)-17(10,8)E$	33.09	167	$\surd$	$\surd$
221.67467	$18(4,15)-17(4,14)A$	45.32	112	$\surd$	$\surd$
221.71716	$10(4,6)-9(3,7)A$	1.68	43	$\surd$	$\surd$
221.74611	$26(5,22)-26(2,24)E$	2.77	207	$\surd$	$\surd$
221.74862	$26(5,22)-26(3,24)E$	1.53	207	$\surd$	$\surd$
221.76298	$26(2,24)-26(1,25)A$	2.75	207	$\surd$	$\surd$
221.84111	$26(3,24)-26(2,25)A$	2.75	207	$\surd$	$\surd$
221.84391	$26(3,24)-26(1,25)A$	1.53	207	$\surd$	$\surd$
221.96442	$18(9,9)-17(9,8)E$	35.98	155	B	B
221.97932	$18(9,10)-17(9,9)A$	35.90	155	$\surd$	$\surd$
221.98570	$18(9,10)-17(9,9)E$	35.89	155	$\surd$	$\surd$
222.00501	$11(5,6)-11(3,9)E$	0.05	56	B	$\surd$
222.28943	$37(9,29)-36(10,26)E$	1.97	473	B	$\surd$
222.35551	$27(9,18)-27(8,19)A$	6.92	278	$\surd$	$\surd$
222.36695	$27(9,18)-27(8,19)E$	6.69	278	$\surd$	$\surd$
222.42140	$18(8,10)-17(8,9)E$	38.41	144	$\surd$	$\surd$
222.43825	$18(8,11)-17(8,10)A$	38.41	144	B	B
222.44036	$18(8,10)-17(8,9)A$	38.41	144	B	B
222.44206	$18(8,11)-17(8,10)E$	38.41	144	B	B
222.61360	$8(5,4)-7(4,4)E$	2.10	38	$\surd$	$\surd$
222.63075	$28(9,20)-28(8,21)A$	7.28	295	$\surd$	$\surd$
222.63508	$28(9,20)-28(8,20)E$	7.25	295	$\surd$	$\surd$
222.64933	$8(5,4)-7(4,3)A$	2.12	38	$\surd$	$\surd$
222.65732	$8(5,3)-7(4,3)E$	2.10	38	$\surd$	$\surd$
222.70916	$8(5,3)-7(4,4)A$	2.12	38	$\surd$	$\surd$
222.86138	$18(7,12)-17(7,10)E$	0.31	134	$\surd$	B
223.03786	$19(4,16)-18(3,15)E$	4.94	117	$\surd$	$\surd$
223.05172	$19(2,17)-18(3,16)A$	4.95	117	$\surd$	$\surd$
223.11923	$18(7,12)-17(7,11)A$	40.62	134	$\surd$	$\surd$
223.12509	$18(7,11)-17(7,10)E$	40.39	134	$\surd$	$\surd$
223.13501	$18(7,12)-17(7,11)E$	40.32	134	$\surd$	$\surd$
223.16274	$18(7,11)-17(7,10)A$	40.70	134	$\surd$	$\surd$
223.34801	$18(6,13)-17(6,11)E$	0.35	125	B	$\surd$
223.39866	$18(7,11)-17(7,11)E$	0.30	134	B	$\surd$
223.46534	$11(4,8)-10(3,7)E$	1.76	50	$\surd$	$\surd$
223.50046	$11(4,8)-10(3,7)A$	1.77	50	$\surd$	$\surd$
223.53851	$43(8,35)-43(7,36)E$	11.36	618	B	$\surd$
223.59223	$43(8,35)-43(7,36)A$	11.36	618	$\surd$	$\surd$
223.62449	$37(8,30)-37(7,31)E$	9.54	463	B	$\surd$
223.63491	$37(8,30)-37(7,31)A$	9.54	463	$\surd$	$\surd$
223.64534	$12(5,7)-12(3,10)A$	0.10	63	B	$\surd$
223.82145	$35(11,25)-35(2,33)E$	8.37	409	$\surd$	$\surd$
223.85393	$35(7,29)-35(6,30)A$	8.36	409	$\surd$	B
224.02187	$18(6,13)-17(6,12)E$	42.26	125	B	B
224.02410	$18(6,13)-17(6,12)A$	42.62	125	B	B
224.16796	$27(9,19)-27(8,20)E$	6.68	278	$\surd$	$\surd$
224.17335	$27(9,19)-27(8,20)A$	6.91	278	B	$\surd$
224.29599	$38(9,30)-38(8,31)A$	10.64	496	B	$\surd$
224.31315	$18(5,14)-17(5,13)E$	44.18	118	$\surd$	$\surd$
224.32831	$18(5,14)-17(5,13)A$	44.18	118	$\surd$	$\surd$
224.48622	$26(9,18)-26(8,18)E$	0.89	262	B	$\surd$
224.58221	$18(6,12)-17(6,11)E$	42.19	125	$\surd$	$\surd$
224.60001	$26(9,17)-26(8,18)A$	6.54	262	B	$\surd$
224.60934	$18(6,12)-17(6,11)A$	42.53	125	$\surd$	$\surd$
224.61879	$26(9,17)-26(8,18)E$	5.65	262	$\surd$	$\surd$
224.96406	$36(10,26)-36(9,27)A$	10.09	463	$\surd$	$\surd$
225.02937	$36(10,26)-36(9,27)E$	9.92	463	$\surd$	$\surd$
225.12058	$27(6,21)-26(7,20)A$	1.78	252	B	$\surd$
225.25615	$18(6,12)-17(6,12)E$	0.35	125	B	$\surd$
225.60877	$19(3,16)-18(3,15)E$	48.63	117	$\surd$	$\surd$
225.61867	$19(3,17)-18(3,16)A$	48.63	117	$\surd$	$\surd$
225.85536	$6(6,1)-5(5,1)E$	2.53	36	$\surd$	$\surd$
225.90065	$6(6,0)-5(5,0)E$	2.53	36	B	$\surd$
225.92860	$6(6,1)-5(5,0)A$	2.53	36	$\surd$	$\surd$
225.92861	$6(6,0)-5(5,1)A$	2.53	36	$\surd$	$\surd$
225.99938	$30(9,21)-29(8,21)E$	1.83	312	$\surd$	$\surd$
226.08117	$30(7,23)-29(8,22)A$	1.78	312	B	$\surd$
226.33511	$25(9,17)-25(8,17)E$	1.54	246	B	$\surd$
226.46764	$25(9,16)-25(8,17)E$	4.62	246	$\surd$	$\surd$
226.53018	$33(2,31)-33(3,30)E$	6.96	358	$\surd$	$\surd$
226.56142	$35(7,29)-35(5,30)A$	3.23	409	B	$\surd$
226.57502	$33(8,25)-32(9,24)E$	1.87	379	B	$\surd$
226.58135	$33(6,28)-33(5,29)A$	6.96	358	B	$\surd$
226.62935	$20(1,19)-19(2,18)E$	6.98	120	$\surd$	$\surd$
226.63521	$20(1,19)-19(2,18)A$	7.06	120	$\surd$	$\surd$
226.71306	$20(2,19)-19(2,18)E$	52.04	120	$\surd$	$\surd$
226.71869	$20(2,19)-19(2,18)A$	52.05	120	$\surd$	$\surd$
226.77313	$20(1,19)-19(1,18)E$	52.04	120	$\surd$	$\surd$
226.77871	$20(1,19)-19(1,18)A$	51.94	120	$\surd$	$\surd$
226.85682	$20(2,18)-19(3,17)E$	7.06	120	$\surd$	$\surd$
226.86218	$20(2,19)-19(1,18)A$	7.06	120	$\surd$	$\surd$
226.97409	$25(9,17)-25(8,18)E$	4.62	246	$\surd$	$\surd$
227.01949	$19(4,16)-18(4,15)E$	48.68	117	$\surd$	$\surd$
227.02807	$19(2,17)-18(2,16)A$	48.69	117	$\surd$	$\surd$
227.10666	$25(9,16)-25(8,18)E$	1.54	246	B	$\surd$
227.15250	$33(6,28)-33(4,29)E$	3.04	358	$\surd$	$\surd$
227.20175	$33(6,28)-33(4,29)A$	3.05	358	$\surd$	$\surd$
227.56094	$21(1,21)-20(1,20)E$	55.48	122	B	B
227.56199	$21(1,21)-20(1,20)A$	55.49	122	B	B
227.56274	$21(0,21)-20(0,20)A$	55.37	122	B	B
227.67322	$26(7,20)-26(5,21)A$	0.97	242	B	$\surd$
227.71420	$26(7,20)-26(5,21)E$	0.97	242	B	$\surd$
227.87135	$31(4,27)-31(4,28)E$	2.66	309	$\surd$	$\surd$
227.94332	$31(4,27)-31(4,28)A$	2.66	309	B	$\surd$
227.95445	$24(9,15)-24(8,16)A$	5.77	231	B	$\surd$
227.98625	$31(4,27)-31(3,28)E$	5.49	309	B	$\surd$
227.99455	$24(9,15)-24(8,16)E$	4.34	231	$\surd$	B
228.05789	$31(4,27)-31(3,28)A$	5.49	309	B	$\surd$
228.16491	$18(6,13)-18(4,14)A$	0.31	125	B	$\surd$
228.20584	$24(9,16)-24(8,17)E$	4.38	231	$\surd$	$\surd$
228.27042	$24(9,16)-24(8,17)A$	5.82	231	$\surd$	$\surd$
228.35844	$24(9,15)-24(8,17)E$	1.43	231	$\surd$	$\surd$
228.49487	$14(4,10)-14(1,13)E$	0.12	73	B	$\surd$
228.59002	$14(4,10)-14(1,13)A$	0.12	73	$\surd$	$\surd$
228.62888	$18(5,13)-17(5,12)E$	44.24	119	$\surd$	$\surd$
228.65140	$18(5,13)-17(5,12)A$	44.25	119	$\surd$	$\surd$
228.66505	$39(9,31)-39(8,32)A$	10.83	520	B	$\surd$
228.67921	$39(9,31)-39(8,32)E$	10.64	520	$\surd$	$\surd$
228.71258	$36(9,27)-35(10,26)E$	1.99	452	$\surd$	$\surd$
229.08216	$23(9,15)-23(8,15)E$	0.76	217	B	$\surd$
229.22423	$23(9,14)-23(8,15)A$	5.43	217	$\surd$	$\surd$
229.25931	$23(9,14)-23(8,15)E$	4.67	217	B	$\surd$
229.32015	$23(9,15)-23(8,16)E$	4.67	217	$\surd$	$\surd$
229.38892	$23(9,15)-23(8,16)A$	5.42	217	$\surd$	$\surd$
229.40502	$18(3,15)-17(3,14)E$	46.12	111	$\surd$	$\surd$
229.42034	$18(3,15)-17(3,14)A$	46.12	111	$\surd$	$\surd$
229.47410	$20(1,19)-19(4,15)E$	3.49	134	$\surd$	$\surd$
229.49206	$31(5,27)-31(4,28)A$	5.49	309	B	$\surd$
229.49722	$23(9,14)-23(8,16)E$	0.76	217	B	$\surd$
229.50463	$20(3,17)-19(4,16)A$	3.49	134	$\surd$	$\surd$
229.59502	$19(3,17)-18(2,16)A$	4.97	117	$\surd$	$\surd$
229.60676	$31(5,27)-31(3,28)A$	2.67	310	B	$\surd$
230.11030	$22(9,14)-22(8,14)E$	0.21	203	B	$\surd$
230.12282	$35(8,28)-35(6,29)A$	2.57	419	$\surd$	$\surd$
230.16097	$35(8,28)-35(6,29)E$	2.55	419	B	$\surd$
230.29395	$22(9,13)-22(8,14)A$	5.08	203	B	$\surd$
230.31231	$22(9,13)-22(8,14)E$	4.87	203	$\surd$	$\surd$
230.31580	$22(9,14)-22(8,15)E$	4.87	203	B	$\surd$
230.37659	$22(9,14)-22(8,15)A$	5.08	203	$\surd$	$\surd$
230.43575	$25(1,24)-25(0,25)E$	1.38	182	$\surd$	$\surd$
230.44077	$25(2,24)-25(1,25)E$	1.38	182	$\surd$	$\surd$
230.92011	$29(3,26)-29(3,27)E$	2.15	264	$\surd$	$\surd$
230.93636	$29(3,26)-29(2,27)E$	4.12	264	B	$\surd$
231.00438	$29(3,26)-29(3,27)A$	2.16	264	$\surd$	$\surd$
231.01900	$12(4,9)-11(3,8)E$	1.80	57	$\surd$	$\surd$
231.04600	$12(4,9)-11(3,8)A$	1.81	57	B	$\surd$
231.18769	$21(9,13)-21(8,14)E$	4.71	190	$\surd$	$\surd$
231.19926	$21(9,12)-21(8,13)A$	4.78	190	$\surd$	$\surd$
231.23920	$21(9,13)-21(8,14)A$	4.75	190	$\surd$	$\surd$
231.33127	$29(4,26)-29(2,27)A$	2.15	264	B	$\surd$
231.40224	$40(7,33)-40(6,34)E$	9.76	532	$\surd$	$\surd$
231.41445	$35(10,25)-35(9,26)A$	9.48	441	$\surd$	$\surd$
231.42446	$38(8,31)-38(7,32)E$	9.60	485	$\surd$	$\surd$
231.43718	$38(8,31)-38(7,32)A$	9.75	485	$\surd$	B
231.46966	$35(10,25)-35(9,26)E$	9.47	441	$\surd$	B
231.51343	$27(2,25)-27(1,26)E$	2.75	222	B	$\surd$
231.61200	$27(2,25)-27(1,26)A$	2.75	222	$\surd$	$\surd$
231.65806	$27(3,25)-27(2,26)A$	2.75	222	$\surd$	$\surd$
231.93866	$20(9,12)-20(8,13)E$	4.41	178	$\surd$	$\surd$
231.95522	$20(9,11)-20(8,12)E$	4.41	178	$\surd$	$\surd$
231.96702	$20(9,11)-20(8,12)A$	4.41	178	B	$\surd$
232.30577	$34(8,27)-34(6,28)A$	2.22	398	$\surd$	$\surd$
232.35615	$34(8,27)-34(6,28)E$	2.22	398	$\surd$	$\surd$
232.37214	$34(9,26)-34(3,31)E$	3.12	377	B	$\surd$
232.43549	$34(5,29)-34(5,30)A$	3.12	378	$\surd$	$\surd$
232.57943	$19(9,11)-19(8,12)E$	4.08	166	$\surd$	$\surd$
232.59727	$19(9,10)-19(8,11)E$	4.07	166	B	$\surd$
232.61714	$19(9,10)-19(8,11)A$	4.10	166	$\surd$	$\surd$
232.62520	$19(9,11)-19(8,12)A$	4.10	166	$\surd$	$\surd$
232.76069	$34(9,26)-34(4,30)E$	6.98	377	$\surd$	$\surd$
232.82276	$34(5,29)-34(4,30)A$	6.88	377	B	$\surd$
233.03244	$22(0,22)-21(5,17)E$	2.53	165	$\surd$	$\surd$
233.08217	$22(4,18)-21(5,17)A$	2.53	165	B	$\surd$
233.12216	$18(9,10)-18(8,11)E$	3.74	155	B	$\surd$
233.14065	$18(9,9)-18(8,10)E$	3.76	155	$\surd$	$\surd$
233.16610	$18(9,9)-18(8,10)A$	3.76	155	$\surd$	$\surd$
233.16945	$18(9,10)-18(8,11)A$	3.77	155	$\surd$	$\surd$
233.18150	$37(6,31)-37(6,32)A$	3.50	452	$\surd$	$\surd$
233.20020	$19(17,2)-18(17,1)A$	10.12	304	B	$\surd$
233.21277	$19(4,16)-18(4,15)E$	48.03	123	$\surd$	$\surd$
233.22221	$19(17,3)-18(17,2)E$	10.12	304	$\surd$	$\surd$
233.22675	$19(4,16)-18(4,15)A$	47.96	123	$\surd$	$\surd$
233.24679	$19(16,3)-18(16,2)A$	14.76	282	$\surd$	$\surd$
233.25601	$19(16,3)-18(16,2)E$	14.76	282	$\surd$	$\surd$
233.26859	$19(16,4)-18(16,3)E$	14.76	282	$\surd$	$\surd$
233.31012	$19(15,4)-18(15,3)A$	19.11	261	$\surd$	$\surd$
233.31578	$19(15,4)-18(15,3)E$	19.11	261	$\surd$	$\surd$
233.33121	$19(15,5)-18(15,4)E$	19.11	261	$\surd$	$\surd$
233.39466	$19(14,5)-18(14,4)A$	23.18	242	B	B
233.39668	$19(14,5)-18(14,4)E$	23.18	242	B	B
233.41443	$19(14,6)-18(14,5)E$	23.18	242	$\surd$	$\surd$
233.50498	$19(13,6)-18(13,5)E$	26.87	224	B	B
233.50660	$19(13,7)-18(13,6)A$	26.87	224	B	B
233.52463	$19(13,7)-18(13,6)E$	26.96	224	$\surd$	$\surd$
233.57852	$17(9,9)-17(8,10)E$	3.41	144	$\surd$	$\surd$
233.64988	$19(12,7)-18(12,6)E$	30.47	208	$\surd$	$\surd$
233.65527	$19(12,8)-18(12,7)A$	30.37	208	$\surd$	$\surd$
233.67098	$19(12,8)-18(12,7)E$	30.46	208	$\surd$	$\surd$
233.75392	$18(2,16)-17(2,15)E$	45.69	114	$\surd$	$\surd$
233.77752	$18(4,14)-17(4,13)A$	45.76	114	$\surd$	$\surd$
233.84523	$19(11,8)-18(11,7)E$	33.69	192	$\surd$	$\surd$
233.85423	$19(11,9)-18(11,8)A$	33.59	192	$\surd$	$\surd$
233.86711	$19(11,9)-18(11,8)E$	33.59	192	$\surd$	$\surd$
233.92728	$42(11,31)-42(10,32)A$	11.96	618	B	$\surd$
233.95901	$16(9,8)-16(8,9)E$	3.07	134	$\surd$	$\surd$
233.97864	$16(9,7)-16(8,8)E$	3.09	134	$\surd$	$\surd$
234.02371	$42(11,31)-42(10,32)E$	11.96	618	$\surd$	$\surd$
234.04769	$40(9,32)-40(8,33)A$	10.78	543	B	$\surd$
234.05863	$40(9,32)-40(8,33)E$	10.78	544	$\surd$	$\surd$
234.11233	$19(10,9)-18(10,8)E$	36.62	179	$\surd$	$\surd$
234.12486	$19(10,10)-18(10,9)A$	36.52	179	$\surd$	$\surd$
234.13460	$19(10,10)-18(10,9)E$	36.62	179	$\surd$	$\surd$
234.25981	$37(6,31)-37(5,32)E$	8.30	451	$\surd$	$\surd$
234.27312	$15(9,7)-15(8,8)E$	2.73	125	$\surd$	$\surd$
234.29326	$15(9,6)-15(8,7)E$	2.74	125	$\surd$	$\surd$
234.31623	$37(6,31)-37(5,32)A$	8.29	451	B	$\surd$
234.32876	$15(9,6)-15(8,7)A$	2.74	125	$\surd$	$\surd$
234.47929	$9(5,5)-8(4,4)E$	0.20	43	$\surd$	$\surd$
234.48639	$19(9,10)-18(9,9)E$	39.28	166	$\surd$	$\surd$
234.50223	$19(9,11)-18(9,10)A$	39.19	166	$\surd$	$\surd$
234.50864	$19(9,11)-18(9,10)E$	39.18	166	$\surd$	$\surd$
234.52939	$14(9,6)-14(8,7)E$	2.39	116	$\surd$	$\surd$
234.55012	$14(9,5)-14(8,6)E$	2.38	116	B	$\surd$
234.58806	$14(9,5)-14(8,6)A$	2.40	116	$\surd$	$\surd$
234.72653	$20(2,18)-19(3,17)E$	5.39	128	$\surd$	$\surd$
234.73565	$13(9,5)-13(8,6)E$	2.03	107	B	B
234.73749	$9(5,5)-8(4,4)A$	2.16	43	B	B
234.73903	$20(2,18)-19(3,17)A$	5.45	128	B	B
234.75688	$13(9,4)-13(8,5)E$	2.03	107	B	$\surd$
234.78336	$9(5,4)-8(4,4)E$	1.95	43	$\surd$	$\surd$
234.79710	$13(9,4)-13(8,5)A$	2.04	107	$\surd$	$\surd$
234.89899	$12(9,4)-12(8,5)E$	1.67	100	$\surd$	$\surd$
234.91680	$9(5,4)-8(4,5)A$	2.15	43	$\surd$	$\surd$
234.92057	$12(9,3)-12(8,4)E$	1.68	100	$\surd$	$\surd$
234.94342	$36(7,30)-36(5,31)A$	3.35	430	$\surd$	$\surd$
234.96309	$12(9,3)-12(8,4)A$	1.67	100	$\surd$	$\surd$
235.02583	$11(9,3)-11(8,4)E$	1.29	93	$\surd$	$\surd$
235.02994	$19(8,11)-18(8,10)E$	41.56	155	$\surd$	$\surd$
235.04648	$19(8,12)-18(8,11)A$	41.57	155	$\surd$	$\surd$
235.05138	$19(8,12)-18(8,11)E$	41.65	155	$\surd$	$\surd$
235.09216	$11(9,2)-11(8,3)A$	1.30	93	$\surd$	$\surd$
235.12201	$10(9,2)-10(8,3)E$	0.89	86	$\surd$	$\surd$
235.14449	$10(9,1)-10(8,2)E$	0.89	86	B	$\surd$
235.19049	$10(9,1)-10(8,2)A$	0.90	86	$\surd$	$\surd$
235.19274	$9(9,1)-9(8,2)E$	0.47	80	$\surd$	$\surd$
235.26327	$9(9,1)-9(8,2)A$	0.47	80	$\surd$	$\surd$
235.31520	$11(3,8)-10(2,9)E$	0.68	46	$\surd$	$\surd$
235.37523	$11(3,8)-10(2,9)A$	0.68	46	B	$\surd$
235.60227	$19(7,13)-18(7,11)E$	1.11	145	B	$\surd$
235.84454	$19(7,13)-18(7,12)A$	43.75	145	$\surd$	$\surd$
235.86597	$19(7,13)-18(7,12)E$	42.63	145	$\surd$	$\surd$
235.88711	$19(7,12)-18(7,11)E$	42.63	145	$\surd$	$\surd$
235.93235	$19(7,12)-18(7,11)A$	43.66	145	$\surd$	$\surd$
236.15070	$19(7,12)-18(7,12)E$	1.12	145	$\surd$	$\surd$
236.36553	$20(3,18)-19(3,17)A$	51.27	128	$\surd$	$\surd$
236.44334	$15(5,10)-15(3,13)E$	0.16	88	B	$\surd$
236.49247	$34(6,29)-34(5,30)E$	6.87	378	$\surd$	$\surd$
236.58655	$6(4,2)-5(2,3)A$	0.02	23	B	$\surd$
236.74370	$19(5,15)-18(5,14)E$	47.01	130	$\surd$	$\surd$
236.74879	$34(10,24)-34(9,25)E$	9.04	420	B	$\surd$
236.75969	$19(5,15)-18(5,14)A$	47.02	130	$\surd$	$\surd$
236.80059	$19(6,14)-18(6,13)E$	45.46	137	$\surd$	$\surd$
236.81031	$19(6,14)-18(6,13)A$	45.56	137	$\surd$	$\surd$
236.88101	$34(6,29)-34(4,30)E$	3.10	378	B	$\surd$
237.23860	$33(8,26)-33(6,27)A$	1.87	377	$\surd$	$\surd$
237.26120	$21(1,20)-20(2,19)E$	7.44	132	$\surd$	$\surd$
237.26679	$21(1,20)-20(2,19)A$	7.52	132	$\surd$	$\surd$
237.29747	$20(4,17)-19(4,16)E$	51.30	128	$\surd$	$\surd$
237.30597	$20(2,18)-19(2,17)A$	51.40	128	$\surd$	$\surd$
237.30954	$21(2,20)-20(2,19)E$	54.70	132	$\surd$	$\surd$
237.31508	$21(2,20)-20(2,19)A$	54.71	132	$\surd$	$\surd$
237.34487	$21(1,20)-20(1,19)E$	54.71	132	$\surd$	$\surd$
237.35039	$21(1,20)-20(1,19)A$	54.71	132	$\surd$	$\surd$
237.36505	$8(3,6)-7(1,7)E$	0.04	27	B	$\surd$
237.39319	$21(2,19)-20(3,18)E$	7.53	132	$\surd$	$\surd$
237.39858	$21(2,20)-20(1,19)A$	7.44	132	B	$\surd$
237.80763	$19(6,13)-18(6,12)E$	45.46	137	$\surd$	$\surd$
237.82983	$19(6,13)-18(6,12)A$	45.56	137	$\surd$	$\surd$
237.87993	$17(6,12)-17(4,13)E$	0.23	115	$\surd$	$\surd$
237.91834	$17(6,12)-17(4,13)A$	0.24	115	B	$\surd$
238.11798	$7(6,2)-6(5,2)E$	2.53	40	$\surd$	$\surd$
238.15588	$22(1,22)-21(1,21)E$	58.16	134	B	B
238.15631	$22(0,22)-21(0,21)E$	58.16	134	B	B
238.15730	$22(0,22)-21(0,21)A$	58.17	134	B	B
238.19009	$7(6,2)-6(5,1)A$	2.53	40	$\surd$	$\surd$
238.35264	$25(7,19)-25(5,20)A$	0.76	226	B	$\surd$
238.39195	$25(7,19)-25(5,20)E$	0.76	226	$\surd$	$\surd$
238.48438	$32(4,28)-32(4,29)E$	2.69	328	$\surd$	$\surd$
238.53427	$39(10,30)-39(9,31)A$	11.05	531	B	$\surd$
238.55621	$32(4,28)-32(4,29)A$	2.70	328	B	$\surd$
238.57345	$39(10,30)-39(9,31)E$	10.85	531	$\surd$	$\surd$
238.57772	$38(10,29)-38(9,30)A$	10.51	508	$\surd$	$\surd$
238.62473	$32(4,28)-32(3,29)A$	5.49	328	$\surd$	$\surd$
238.92680	$20(3,17)-19(4,16)E$	5.47	128	$\surd$	$\surd$
238.93248	$20(3,18)-19(2,17)A$	5.47	128	$\surd$	$\surd$
239.04127	$48(12,36)-48(11,37)A$	14.53	797	B	$\surd$
239.15577	$48(12,36)-48(11,37)E$	14.15	797	$\surd$	$\surd$
239.24086	$37(10,28)-37(9,29)A$	10.32	485	B	$\surd$
239.24469	$40(10,31)-40(9,32)A$	11.38	555	B	$\surd$
239.27843	$37(10,28)-37(9,29)E$	10.15	485	$\surd$	$\surd$
239.28305	$40(10,31)-40(9,32)E$	11.17	555	B	$\surd$
239.41674	$10(2,8)-9(1,9)E$	0.15	36	$\surd$	$\surd$
239.47271	$32(5,28)-32(4,29)E$	5.49	328	B	$\surd$
239.49359	$10(2,8)-9(1,9)A$	0.15	36	B	$\surd$
239.54125	$32(5,28)-32(4,29)A$	5.48	328	$\surd$	$\surd$
239.89169	$8(3,6)-7(0,7)A$	0.05	27	B	$\surd$
239.92044	$39(8,32)-39(7,33)E$	9.62	509	$\surd$	B
239.92752	$44(8,36)-44(7,37)E$	11.23	645	B	$\surd$
239.93517	$39(8,32)-39(7,33)A$	9.79	509	$\surd$	$\surd$
240.02108	$19(1,18)-18(1,17)E$	48.55	122	$\surd$	$\surd$
240.03461	$19(3,16)-18(3,15)A$	48.55	122	$\surd$	$\surd$
240.16280	$26(1,25)-26(1,26)E$	0.82	196	$\surd$	$\surd$
240.16561	$26(2,25)-26(1,26)E$	1.38	196	$\surd$	$\surd$
240.28104	$26(1,25)-26(1,26)A$	0.82	196	B	$\surd$
240.28383	$26(2,25)-26(1,26)A$	1.38	196	B	$\surd$
240.36208	$10(2,8)-9(0,9)E$	0.09	36	B	$\surd$
240.40880	$41(9,33)-41(8,34)E$	10.88	569	$\surd$	$\surd$
240.43837	$10(2,8)-9(0,9)A$	0.09	36	B	$\surd$
241.06847	$30(3,27)-30(2,28)A$	4.16	281	$\surd$	B
241.25789	$30(4,27)-30(2,28)A$	2.17	281	$\surd$	B
241.33567	$28(2,26)-28(1,27)E$	2.75	237	$\surd$	B
241.43753	$28(2,26)-28(1,27)A$	2.77	237	$\surd$	B
242.43959	$14(4,11)-13(3,10)E$	1.96	73	$\surd$	$\surd$
242.45343	$14(4,11)-13(3,10)A$	1.94	73	B	$\surd$
242.83539	$37(7,31)-37(6,32)E$	8.26	452	B	$\surd$
242.87157	$19(5,14)-18(5,13)E$	47.14	130	$\surd$	$\surd$
242.89602	$19(5,14)-18(5,13)A$	47.15	130	$\surd$	$\surd$
243.56460	$34(10,25)-34(9,26)A$	9.03	420	$\surd$	$\surd$
243.59166	$34(8,27)-34(7,27)E$	9.14	420	B	$\surd$
243.86206	$35(5,30)-35(5,31)E$	3.16	398	$\surd$	$\surd$
243.97365	$37(7,31)-37(5,32)E$	3.45	452	B	$\surd$
244.00438	$37(7,31)-37(5,32)A$	3.46	452	B	$\surd$
244.10297	$35(5,30)-35(4,31)E$	6.97	398	$\surd$	$\surd$
244.16231	$35(5,30)-35(4,31)A$	6.97	398	$\surd$	$\surd$
244.48967	$32(10,22)-32(9,23)A$	8.40	380	B	$\surd$
244.51900	$32(10,22)-32(9,23)E$	8.23	380	B	$\surd$
244.58030	$20(4,16)-19(4,15)E$	50.64	135	$\surd$	$\surd$
244.59401	$20(4,17)-19(4,16)A$	50.64	135	$\surd$	$\surd$
244.65312	$16(5,11)-16(3,14)A$	0.16	98	B	$\surd$
244.84960	$32(8,25)-32(6,26)A$	1.54	357	B	$\surd$
244.87111	$11(3,8)-10(1,9)E$	0.30	46	$\surd$	$\surd$
245.13823	$21(1,20)-20(4,16)E$	4.05	147	$\surd$	$\surd$
245.16576	$21(3,18)-20(4,17)A$	4.05	147	$\surd$	$\surd$
245.31530	$15(4,11)-15(2,14)E$	0.07	83	B	$\surd$
245.37027	$33(10,24)-33(9,25)A$	8.77	399	$\surd$	$\surd$
245.39342	$33(8,26)-33(7,27)E$	8.74	399	B	$\surd$
245.42720	$15(4,11)-15(2,14)A$	0.07	83	$\surd$	$\surd$
245.44689	$20(19,2)-19(19,1)A$	5.18	363	$\surd$	$\surd$
245.46557	$20(19,1)-19(19,0)E$	5.18	363	B	$\surd$
245.46846	$20(19,2)-19(19,1)E$	5.21	363	B	$\surd$
245.47575	$20(18,2)-19(18,1)A$	10.11	339	$\surd$	$\surd$
245.49181	$20(18,2)-19(18,1)E$	10.16	339	$\surd$	$\surd$
245.50908	$25(5,20)-24(6,19)E$	2.18	214	$\surd$	$\surd$
245.51755	$20(17,3)-19(17,2)A$	14.83	316	$\surd$	$\surd$
245.53030	$20(17,3)-19(17,2)E$	14.83	315	$\surd$	$\surd$
245.58397	$20(16,4)-19(16,3)E$	19.23	294	$\surd$	$\surd$
245.59730	$20(16,5)-19(16,4)E$	19.23	294	B	$\surd$
245.65108	$20(15,6)-19(15,5)A$	23.27	273	$\surd$	$\surd$
245.65678	$20(15,5)-19(15,4)E$	23.36	273	$\surd$	$\surd$
245.67298	$20(15,6)-19(15,5)E$	23.37	273	$\surd$	$\surd$
245.75215	$20(14,7)-19(14,6)A$	27.13	254	B	B
245.75227	$20(14,6)-19(14,5)A$	27.23	254	B	B
245.77264	$20(14,7)-19(14,6)E$	27.12	254	$\surd$	$\surd$
245.81939	$41(7,34)-41(6,35)E$	9.71	556	B	$\surd$
245.88310	$20(13,7)-19(13,6)E$	30.71	236	B	B
245.88524	$20(13,7)-19(13,6)A$	30.83	236	B	B
245.90368	$20(13,8)-19(13,7)E$	30.82	236	$\surd$	$\surd$
246.02750	$21(4,18)-20(3,17)E$	5.95	140	$\surd$	$\surd$
246.03889	$21(2,19)-20(3,18)A$	5.87	140	$\surd$	$\surd$
246.05482	$20(12,8)-19(12,7)E$	34.15	219	$\surd$	$\surd$
246.06074	$20(12,9)-19(12,8)A$	34.03	219	B	B
246.06083	$20(12,8)-19(12,7)A$	34.15	219	B	B
246.12596	$7(4,3)-6(2,4)A$	0.04	27	ND	$\surd$
246.19601	$38(6,32)-38(6,33)A$	3.57	474	B	$\surd$
246.28534	$20(11,9)-19(11,8)E$	37.09	204	$\surd$	$\surd$
246.29507	$20(11,10)-19(11,9)A$	37.09	204	$\surd$	$\surd$
246.30823	$20(11,10)-19(11,9)E$	37.09	204	$\surd$	$\surd$
246.44490	$15(4,11)-15(3,13)E$	0.10	83	B	$\surd$
246.45615	$10(5,6)-9(4,5)E$	0.67	49	B	$\surd$
246.57737	$35(6,30)-35(5,31)E$	6.87	398	$\surd$	$\surd$
246.60000	$20(10,10)-19(10,9)E$	39.88	190	$\surd$	$\surd$
246.61336	$20(10,11)-19(10,10)A$	39.89	190	$\surd$	$\surd$
246.62319	$20(10,11)-19(10,10)E$	39.99	190	$\surd$	$\surd$

Table 6. Table A.3: Transitions of the T ​ r ​ a ​ n ​ s 𝑇 𝑟 𝑎 𝑛 𝑠 Trans and G ​ a ​ u ​ c ​ h ​ e 𝐺 𝑎 𝑢 𝑐 ℎ 𝑒 Gauche conformers of ET observed with the ALMA.

Rest Frequency	Transition	$S_{ij}{\mu }^2$	Eu	Detection
(GHz)		(Debye${}_{}{}^2)$	(K)	hot core-SW	compact ridge
$Trans$ conformer
213.85626	$13(0,13)-12(1,12)$	12.83	74	$\surd$	$\surd$
214.18012	$22(5,17)-22(4,18)$	23.10	245	$\surd$	$\surd$
214.26774	$9(2,8)-8(1,7)$	6.53	43	$\surd$	$\surd$
214.95629	$18(11,7)-19(10,10)$	2.02	295	$\surd$	ND
215.37564	$21(2,20)-21(1,21)$	22.88	196	$\surd$	$\surd$
215.45224	$26(4,23)-25(5,20)$	9.50	316	$\surd$	$\surd$
215.61072	$20(3,17)-19(4,16)$	7.27	191	B	$\surd$
216.62542	$9(8,1)-10(7,4)$	0.32	118	$\surd$	ND
217.80369	$5(3,3)-4(2,2)$	5.99	24	$\surd$	$\surd$
218.46123	$5(3,2)-4(2,3)$	5.98	24	B	$\surd$
218.55450	$21(5,16)-21(4,17)$	21.98	226	$\surd$	$\surd$
218.65408	$7(2,5)-6(1,6)$	5.50	29	$\surd$	$\surd$
218.94327	$35(4,31)-35(3,32)$	37.58	560	$\surd$	B
219.17369	$30(3,27)-30(2,28)$	32.39	410	B	$\surd$
220.15478	$24(3,22)-24(2,23)$	25.92	263	$\surd$	$\surd$
221.61296	$34(6,28)-34(5,29)$	35.94	548	$\surd$	$\surd$
222.21729	$20(5,15)-20(4,16)$	20.86	208	$\surd$	$\surd$
222.51950	$26(4,23)-26(3,24)$	27.84	316	$\surd$	$\surd$
224.11927	$24(4,20)-23(5,19)$	8.46	275	B	$\surd$
225.21036	$19(5,14)-19(4,15)$	19.74	191	$\surd$	$\surd$
226.39549	$22(2,21)-22(1,22)$	23.95	214	$\surd$	$\surd$
226.58134	$26(2,24)-26(1,25)$	28.24	305	$\surd$	B
226.66170	$10(2,9)-9(1,8)$	7.05	51	$\surd$	$\surd$
227.60608	$18(5,13)-18(4,14)$	18.62	175	B	$\surd$
229.37325	$33(6,27)-33(5,28)$	34.84	519	$\surd$	$\surd$
229.49113	$17(5,12)-17(4,13)$	17.48	160	$\surd$	$\surd$
229.77930	$27(4,24)-27(3,25)$	28.92	339	$\surd$	$\surd$
230.14482	$25(3,23)-25(2,24)$	27.00	284	B	B
230.95378	$16(5,11)-16(4,12)$	16.34	146	$\surd$	B
230.98460	$29(5,25)-28(6,22)$	10.17	399	$\surd$	ND
230.99137	$14(0,14)-13(1,13)$	13.89	86	$\surd$	$\surd$
231.56090	$20(5,16)-20(4,17)$	20.87	208	$\surd$	$\surd$
231.73761	$19(5,15)-19(4,16)$	19.74	191	$\surd$	B
232.03463	$18(5,14)-18(4,15)$	18.62	175	$\surd$	$\surd$
232.31850	$23(5,19)-23(4,20)$	24.21	264	$\surd$	$\surd$
232.40481	$17(5,13)-17(4,14)$	17.48	160	$\surd$	$\surd$
232.54796	$17(11,7)-18(10,8)$	1.66	280	$\surd$	$\surd$
232.80883	$16(5,12)-16(4,13)$	16.34	146	$\surd$	$\surd$
232.85088	$28(5,23)-27(6,22)$	9.66	375	$\surd$	ND
232.92850	$14(5,9)-14(4,10)$	14.02	120	$\surd$	$\surd$
233.57102	$13(5,8)-13(4,9)$	12.84	108	$\surd$	$\surd$
233.95112	$13(5,9)-13(4,10)$	12.84	108	$\surd$	$\surd$
234.05112	$12(5,7)-12(4,8)$	11.65	97	$\surd$	$\surd$
234.25516	$12(5,8)-12(4,9)$	11.65	97	$\surd$	$\surd$
234.40645	$11(5,6)-11(4,7)$	10.43	87	$\surd$	$\surd$
234.48134	$31(3,28)-31(2,29)$	33.46	436	$\surd$	$\surd$
234.66627	$10(5,5)-10(4,6)$	9.18	78	$\surd$	$\surd$
234.71457	$10(5,6)-10(4,7)$	9.18	78	$\surd$	$\surd$
234.72560	$23(1,22)-23(0,23)$	50.23	233	B	B
234.75882	$6(3,4)-5(2,3)$	6.41	29	$\surd$	$\surd$
234.85303	$9(5,4)-9(4,5)$	7.90	69	$\surd$	B
234.87361	$9(5,5)-9(4,6)$	7.90	69	$\surd$	$\surd$
234.98424	$8(5,3)-8(4,4)$	6.56	62	$\surd$	$\surd$
234.99191	$8(5,4)-8(4,5)$	6.56	62	$\surd$	B
235.07349	$7(5,2)-7(4,3)$	5.15	55	$\surd$	$\surd$
235.13151	$6(5,1)-6(4,2)$	3.64	49	$\surd$	$\surd$
235.98333	$14(1,14)-13(0,13)$	16.03	86	$\surd$	$\surd$
236.29907	$6(3,3)-5(2,4)$	6.41	29	$\surd$	$\surd$
236.32517	$26(5,22)-26(4,23)$	27.52	327	$\surd$	$\surd$
237.52412	$23(2,22)-23(1,23)$	25.02	233	$\surd$	$\surd$
237.63879	$28(4,25)-28(3,26)$	30.01	362	$\surd$	$\surd$
238.70176	$11(2,10)-10(1,9)$	7.58	60	$\surd$	$\surd$
240.11024	$27(2,25)-27(1,26)$	29.31	327	$\surd$	$\surd$
240.45095	$26(3,24)-26(2,25)$	28.08	305	$\surd$	B
243.55694	$8(2,6)-7(1,7)$	6.01	36	$\surd$	$\surd$
244.90813	$32(6,26)-31(7,25)$	10.85	491	$\surd$	ND
245.15728	$29(5,25)-29(4,26)$	30.80	399	$\surd$	$\surd$
$Gauche$ conformer
213.76008	$13(1,13)-12(1,12)$,vt=1-1	20.65	135	$\surd$	$\surd$
213.87270	$32(7,26)-32(6,26)$, vt=0-1	18.92	556	$\surd$	ND
214.05616	$21(10,11)-22(9,13)$, vt=1-0	2.15	376	$\surd$	ND
214.43462	$17(3,15)-16(4,13)$, vt=1-0	3.24	200	$\surd$	ND
215.36727	$13(1,13)-12(1,12)$, vt=0-0	20.65	131	$\surd$	$\surd$
215.93156	$11(0,11)-10(1,9)$, vt=1-0	6.06	115	$\surd$	$\surd$
216.02627	$10(3,8)-10(2,8)$, vt=1-0	6.02	118	$\surd$	$\surd$
216.41562	$13(0,13)-12(0,12)$, vt=0-0	20.77	131	$\surd$	$\surd$
216.52169	$8(4,4)-7(3,4)$, vt=0-1	5.00	106	$\surd$	$\surd$
216.65968	$8(4,5)-7(3,5)$, vt=0-1	5.00	106	$\surd$	$\surd$
217.54815	$5(1,4)-4(0,4)$, vt=1-0	3.64	76	$\surd$	$\surd$
217.84716	$30(7,24)-30(6,24)$, vt=0-1	17.65	504	$\surd$	ND
218.79621	$35(7,28)-35(6,30)$, vt=0-1	20.80	642	$\surd$	ND
218.94832	$36(7,29)-36(6,31)$, vt=0-1	21.43	672	$\surd$	ND
219.06702	$22(4,19)-22(3,19)$, vt=1-0	13.31	292	$\surd$	B
219.27029	$9(3,7)-9(2,7)$, vt=1-0	5.37	110	$\surd$	$\surd$
219.39206	$14(2,12)-13(1,12)$, vt=0-1	5.19	151	B	$\surd$
220.12701	$10(2,8)-10(1,10)$, vt=1-0	6.25	113	$\surd$	ND
220.44347	$30(7,23)-30(6,25)$, vt=0-1	17.65	504	$\surd$	$\surd$
221.81168	$8(3,6)-8(2,6)$, vt=1-0	4.72	103	$\surd$	$\surd$
222.14170	$26(7,20)-26(6,20)$, vt=0-1	15.10	409	$\surd$	$\surd$
222.16062	$27(7,20)-27(6,22)$, vt=0-1	15.74	431	$\surd$	$\surd$
222.62600	$26(2,24)-25(3,22)$, vt=0-1	6.43	358	$\surd$	ND
222.64283	$26(7,19)-26(6,21)$, vt=0-1	15.10	409	$\surd$	$\surd$
222.67711	$13(2,12)-12(2,11)$, vt=0-0	20.28	137	$\surd$	$\surd$
222.74214	$13(2,12)-12(2,11)$, vt=1-1	20.28	142	$\surd$	B
222.75240	$25(7,19)-25(6,19)$, vt=0-1	14.46	387	$\surd$	B
223.23324	$12(2,11)-11(1,11)$, vt=0-1	4.60	126	B	$\surd$
223.24146	$24(7,18)-24(6,18)$, vt=0-1	13.81	366	$\surd$	$\surd$
223.62958	$23(7,17)-23(6,17)$, vt=0-1	13.16	346	$\surd$	$\surd$
223.69502	$7(3,5)-7(2,5)$, vt=1-0	4.06	96	$\surd$	$\surd$
223.74813	$23(7,16)-23(6,18)$, vt=0-1	13.16	346	$\surd$	B
224.07085	$7(7,1)-7(6,1)$, vt=0-1	1.14	140	B	$\surd$
224.14513	$8(7,2)-8(6,2)$, vt=0-1	2.15	147	B	$\surd$
224.16627	$21(7,15)-21(6,15)$, vt=0-1	11.85	309	$\surd$	B
224.22200	$9(7,3)-9(6,3)$, vt=0-1	3.07	154	$\surd$	$\surd$
224.29910	$10(7,4)-10(6,4)$, vt=0-1	3.93	162	$\surd$	$\surd$
224.30383	$17(1,17)-16(2,15)$, vt=1-0	4.28	185	$\surd$	B
224.33928	$20(7,14)-20(6,14)$, vt=0-1	11.18	291	$\surd$	$\surd$
224.36174	$20(7,13)-20(6,15)$, vt=0-1	11.18	291	$\surd$	B
224.37376	$11(7,5)-11(6,5)$, vt=0-1	4.75	171	$\surd$	$\surd$
224.44306	$12(7,6)-12(6,6)$, vt=0-1	5.54	181	$\surd$	$\surd$
224.47371	$19(7,12)-19(6,14)$, vt=0-1	10.51	275	$\surd$	B
224.50376	$13(7,7)-13(6,7)$, vt=0-1	6.29	192	$\surd$	$\surd$
224.54091	$18(7,12)-18(6,12)$, vt=0-1	9.84	259	$\surd$	$\surd$
224.54724	$18(7,11)-18(6,13)$, vt=0-1	9.84	259	$\surd$	$\surd$
224.55244	$14(7,8)-14(6,8)$, vt=0-1	7.03	204	$\surd$	$\surd$
224.59698	$16(7,10)-16(6,10)$, vt=0-1	8.46	230	$\surd$	B
224.66995	$3(2,1)-2(1,1)$, vt=1-0	2.02	71	$\surd$	$\surd$
224.73929	$11(3,9)-10(2,9)$, vt=0-1	5.01	123	$\surd$	$\surd$
224.82313	$13(1,12)-12(1,11)$, vt=0-0	20.65	135	$\surd$	$\surd$
224.87264	$13(11,2)-12(11,1)$, vt=1-1	5.90	286	$\surd$	$\surd$
224.88357	$13(10,3)-12(10,2)$, vt=1-1	8.48	260	$\surd$	$\surd$
224.90658	$13(9,5)-12(9,4)$, vt=1-1	10.81	237	$\surd$	$\surd$
224.92383	$16(2,15)-15(3,13)$, vt=1-0	3.45	179	$\surd$	$\surd$
224.94604	$13(8,6)-12(8,5)$, vt=1-1	12.90	216	$\surd$	$\surd$
225.00120	$6(3,4)-6(2,4)$, vt=1-0	3.38	90	$\surd$	$\surd$
225.00899	$13(7,6)-12(7,5)$, vt=1-1	14.75	197	$\surd$	$\surd$
225.10519	$13(10,3)-12(10,2)$, vt=0-0	8.48	255	$\surd$	$\surd$
225.10988	$13(6,7)-12(6,6)$, vt=1-1	16.35	181	$\surd$	$\surd$
225.13050	$13(8,6)-12(8,5)$, vt=0-0	12.90	211	$\surd$	$\surd$
225.17061	$13(7,6)-12(7,5)$, vt=0-0	14.75	192	$\surd$	$\surd$
225.24881	$13(6,7)-12(6,6)$, vt=0-0	16.35	176	$\surd$	$\surd$
225.27885	$13(5,9)-12(5,8)$, vt=1-1	17.70	168	$\surd$	$\surd$
225.28285	$13(5,8)-12(5,7)$, vt=1-1	17.70	168	$\surd$	$\surd$
225.39973	$13(5,9)-12(5,8)$, vt=0-0	17.70	163	$\surd$	$\surd$
225.45691	$13(3,11)-12(3,10)$, vt=1-1	19.66	148	$\surd$	$\surd$
225.54569	$13(4,10)-12(4,9)$, vt=1-1	18.81	157	$\surd$	$\surd$
225.56011	$13(3,11)-12(3,10)$, vt=0-0	19.67	144	$\surd$	B
225.66028	$13(4,10)-12(4,9)$, vt=0-0	18.80	152	$\surd$	B
225.67110	$13(4,9)-12(4,8)$, vt=1-1	18.80	157	$\surd$	$\surd$
225.79689	$13(4,9)-12(4,8)$, vt=0-0	18.80	152	$\surd$	$\surd$
225.83609	$5(3,3)-5(2,3)$, vt=1-0	2.67	85	$\surd$	$\surd$
226.05693	$14(0,14)-13(0,13)$, vt=1-1	22.37	146	$\surd$	$\surd$
226.73025	$3(3,0)-3(2,2)$, vt=1-0	1.06	78	B	$\surd$
227.10890	$5(3,2)-5(2,4)$, vt=1-0	2.67	85	$\surd$	B
227.29475	$13(3,10)-12(3,9)$, vt=1-1	19.67	149	$\surd$	$\surd$
227.58083	$12(3,9)-11(2,9)$, vt=0-1	5.30	133	$\surd$	$\surd$
227.59077	$6(3,3)-6(2,5)$, vt=1-0	3.38	90	$\surd$	$\surd$
227.76082	$3(2,2)-2(1,2)$, vt=1-0	2.02	71	B	$\surd$
227.89191	$13(1,12)-12(1,11)$, vt=1-1	20.65	140	$\surd$	$\surd$
228.02905	$13(3,10)-12(3,9)$, vt=0-0	19.67	144	$\surd$	$\surd$
228.28855	$11(1,10)-10(2,8)$, vt=1-0	2.48	119	$\surd$	$\surd$
228.52283	$7(3,4)-7(2,6)$, vt=1-0	4.06	96	$\surd$	$\surd$
228.88665	$9(3,6)-9(2,8)$, vt=1-0	5.37	110	$\surd$	$\surd$
229.24255	$20(4,16)-19(5,14)$, vt=1-0	3.64	257	$\surd$	$\surd$
229.24944	$25(3,22)-25(2,24)$, vt=0-1	15.31	343	$\surd$	ND
230.23074	$13(2,11)-12(2,10)$, vt=1-1	20.28	143	$\surd$	B
230.67255	$13(2,11)-12(2,10)$, vt=0-0	20.28	139	$\surd$	$\surd$
230.69655	$8(3,5)-8(2,7)$, vt=1-0	4.72	103	$\surd$	$\surd$
230.79376	$6(5,1)-5(4,1)$, vt= 0-1	5.56	105	$\surd$	$\surd$
230.90000	$23(11,12)-24(10,14)$, vt=1-0	2.30	439	B	$\surd$
231.56869	$18(1,18)-17(2,16)$, vt=1-0	4.58	199	$\surd$	B
231.66873	$14(1,14)-13(1,13)$, vt=0-0	22.26	142	$\surd$	$\surd$
231.78985	$31(5,27)-31(4,27)$, vt=1-0	18.71	506	$\surd$	$\surd$
232.00849	$11(2,9)-11(1,11)$, vt=1-0	6.86	122	$\surd$	ND
232.49137	$14(0,14)-13(0,13)$, vt=0-0	22.37	142	$\surd$	$\surd$
232.59655	$14(1,14)-13(1,13)$, vt=1-1	22.26	147	B	B
233.71428	$9(4,5)-8(3,5)$, vt=0-1	5.25	114	$\surd$	$\surd$
233.92471	$13(0,13)-12(1,11)$, vt=1-0	7.27	136	$\surd$	$\surd$
234.36056	$15(0,15)-14(1,13)$, vt=1-0	8.49	158	B	$\surd$
235.75642	$35(5,30)-35(4,32)$, vt=0-1	21.18	616	$\surd$	ND
237.59811	$12(3,9)-12(2,11)$, vt=1-0	7.29	138	$\surd$	$\surd$
238.49875	$19(1,19)-18(2,17)$, vt=1-0	4.88	214	$\surd$	ND
238.56819	$6(1,5)-5(0,5)$, vt=1-0	4.24	81	$\surd$	$\surd$
239.18595	$16(0,16)-15(1,14)$, vt=1-0	9.09	171	$\surd$	$\surd$
239.47808	$14(2,13)-13(2,12)$, vt=0-0	21.91	148	$\surd$	$\surd$
239.55137	$14(2,13)-13(2,12)$, vt=1-1	21.91	153	$\surd$	$\surd$
239.91214	$21(4,18)-20(5,16)$, vt=1-0	3.93	274	$\surd$	ND
240.50454	$33(2,31)-32(3,29)$, vt=0-1	8.54	531	$\surd$	ND
240.65398	$4(2,2)-3(1,2)$, vt=1-0	2.27	75	$\surd$	$\surd$
240.78219	$15(2,13)-14(1,13)$, vt=0-1	5.50	163	$\surd$	$\surd$
242.17545	$14(11,4)-13(11,3)$, vt=1-1	8.56	297	$\surd$	$\surd$
242.19130	$14(10,4)-13(10,3)$, vt=1-1	10.96	272	$\surd$	$\surd$
242.22128	$14(9,6)-13(9,5)$, vt=1-1	13.12	248	$\surd$	$\surd$
242.27115	$14(8,6)-13(8,5)$, vt=1-1	15.06	228	$\surd$	$\surd$
242.34984	$14(7,7)-13(7,6)$, vt=1-1	16.78	209	$\surd$	$\surd$
242.42989	$14(10,4)-13(10,3)$, vt=0-0	10.96	266	$\surd$	$\surd$
242.47558	$14(6,8)-13(6,7)$, vt=1-1	18.26	193	$\surd$	$\surd$
242.52421	$14(7,8)-13(7,7)$, vt=0-0	16.78	204	$\surd$	$\surd$
242.62569	$14(6,8)-13(6,7)$, vt=0-0	18.26	188	$\surd$	$\surd$
242.68501	$14(5,10)-13(5,9)$, vt=1-1	19.52	180	$\surd$	$\surd$
242.69303	$14(5,9)-13(5,8)$, vt=1-1	19.52	180	$\surd$	$\surd$
242.77010	$14(3,12)-13(3,11)$, vt=1-1	21.34	160	$\surd$	$\surd$
242.81645	$14(5,10)-13(5,9)$, vt=0-0	19.51	175	$\surd$	$\surd$
242.82510	$14(5,9)-13(5,8)$, vt=0-0	19.51	175	$\surd$	$\surd$
242.99596	$14(4,11)-13(4,10)$, vt=1-1	20.54	169	$\surd$	$\surd$
243.12032	$14(4,11)-13(4,10)$, vt=0-0	20.54	164	$\surd$	$\surd$
243.13859	$13(3,10)-12(2,10)$, vt=0-1	5.59	144	B	$\surd$
243.20652	$14(4,10)-13(4,9)$, vt=1-1	20.54	169	B	$\surd$
243.30550	$17(0,17)-16(1,15)$, vt=1-0	9.70	185	$\surd$	$\surd$
243.34972	$14(4,10)-13(4,9)$, vt=0-0	20.54	164	$\surd$	$\surd$
243.48628	$35(2,33)-34(3,31)$, vt=0-1	9.14	587	$\surd$	ND
244.00985	$12(3,10)-11(2,10)$, vt=0-1	5.30	133	$\surd$	$\surd$
244.63395	$14(1,13)-13(1,12)$, vt=1-1	22.25	152	$\surd$	$\surd$
245.32714	$14(3,11)-13(3,10)$, vt=1-1	21.34	160	$\surd$	$\surd$
245.44511	$12(2,10)-12(1,12)$, vt=1-0	7.48	132	B	B
246.41476	$14(3,11)-13(3,10)$, vt=0-0	21.34	156	$\surd$	$\surd$

Table 7. Table A.4: Unblended lines of the formic acid (t-HCOOH) toward the hot core-SW and hot core-north.

Rest Frequency	Transition	$S_{ij}{\mu }^2$	Eu	Detection
(GHz)		(Debye${}_{}{}^2)$	(K)	hot core-SW	hot core-north
220.03797	$10(0,10)-9(0,9)$	20.14	59	$\surd$	$\surd$
223.91557	$10(2,9)-9(2,8)$	19.39	72	$\surd$	$\surd$
224.91183	$10(7,3)-9(7,2)$	10.30	215	$\surd$	$\surd$
224.92908	$10(6,5)-9(6,4)$	12.93	174	B	$\surd$
224.97676	$10(5,6)-9(5,5)$	15.15	139	$\surd$	$\surd$
225.08544	$10(4,7)-9(4,6)$	16.97	110	$\surd$	$\surd$
225.09121	$10(4,6)-9(4,5)$	16.97	110	$\surd$	$\surd$
225.51255	$10(3,7)-9(3,6)$	18.38	88	B	$\surd$
228.54408	$10(2,8)-9(2,7)$	19.40	73	$\surd$	B
231.50561	$10(1,9)-9(1,8)$	19.99	64	$\surd$	B
236.71720	$11(1,11)-10(1,10)$	22.03	72	$\surd$	$\surd$
241.14625	$11(0,11)-10(0,10)$	22.15	70	$\surd$	$\surd$

Equations

$$N_{H_2}=8.1\times 10^{17}\frac{e^{h\nu /\kappa T_d}-1}{Q_{\nu }\Omega }\left(\frac{S_{\nu }}{Jy}\right){\left(\frac{\nu }{GHz}\right)}^{-3}(c{m}^{-2}),$$

Full text

ISM:abundances — ISM:individual (Orion KL) — ISM:molecules — radio lines: ISM — star:formation

1 INTRODUCTION

Almost 200 molecules have been detected in the interstellar medium (ISM) and circumstellar environments, of which more than 65 are considered complex organic molecules (COMs), molecules with $\geq$ 6 atoms containing carbon. The formation of these COMs in the ISM is still poorly understood, and is subject of intense debate. Two general theories have been proposed: gas-phase chemistry (e.g., Millar et al. 1991; Balucani et al. 2015; Taquet et al. 2016) and dust grain surface chemistry (e.g., Garrod & Herbst 2006; Fedoseev et al. 2015; Chuang et al. 2016).

Orion KL is a high mass star-forming region that provides a superb template for constraining the chemical models (e.g., Brouillet et al. 2013; Crockett et al. 2015; Feng et al. 2015; Favre et al. 2017; Carroll et al. 2017; Friedel & Looney 2017). Orion KL consists of different spatial and chemical components, namely: the hot core, the compact ridge, the extended ridge and the plateau (Blake et al., 1987). In particular two regions, the hot core and the compact ridge, have received special attention because they are very rich in organic species (e.g., Blake et al. 1987; Wang et al. 2011; Favre et al. 2011). While the hot core is rich in nitrogen-bearing molecules (Peng et al., 2013, 2017; Pagani et al., 2017), the compact ridge is richer in oxygen-bearing species (Blake et al., 1987; Beuther et al., 2005; Friedel & Snyder, 2008; Gong et al., 2015). The hot core region may be heated by shocks from source I outflow (e.g., Goddi et al. 2011) or by explosive event occurred around 500 yr ago (e.g., Zapata et al. 2011; Rivilla et al. 2013; Bally et al. 2011, 2017; Wright & Plambeck 2017). Whether this explosive event impacted the release of complex molecules to the gas phase is under debate (e.g., Friedel & Looney 2017; Favre et al. 2017).

The ester family of molecules is essential for many prebiotic processes. The simplest representative is the 8 atom molecule methyl formate (CH3OCHO, hereafter MF), which is relatively abundant in many high-mass (e.g., Favre et al. 2011; Rivilla et al. 2017b) and low-mass star-forming regions (e.g., Cazaux et al. 2003). Ethyl formate (C2H5OCHO, hereafter EF) is the next step in chemical complexity of esters, with eleven atoms. EF has two conformers: the $trans$-conformer (also called $anti$-conformer) where the heavy atoms C-C-O-C=O form a planar zigzag chain, and the $gauche$-conformer produced by an out-of-plane rotation of the ethyl group (C2H5) around the O-C bond (Medvedev et al., 2009). The formation routes of EF are still poorly known. Several chemical pathways have been proposed, including reactions on the surface of dust grains and reactions in the gas-phase. On one hand, Charnley et al. (1995) used gas-phase reactions starting with the ethanol (C2H5OH, hereafter ET) to produce the EF. Recently, the formation of EF has been included in a new gas-phase chemical network (Taquet et al., 2016), in which formic acid (HCOOH) and ammonia (NH3) favor the formation of EF. On the other hand, Garrod et al. (2008) proposed an alternative scenario based on grain-surface chemistry, in which EF can be formed from ET and MF via cosmic ray-induced photodissociation.

The different chemical routes proposed should be checked with observations of EF and its chemically related species. However, since EF is a very complex and hence low abundant molecule, it has been detected so far only towards 4 sources: three massive star forming regions Sgr B2(N) (Belloche et al., 2009), W51 e2 (Rivilla et al., 2017b), Orion KL (Tercero et al., 2013, 2015), and one low-mass star forming region Barnard 1b (Marcelino et al., 2018). The works by Belloche et al. (2009), Rivilla et al. (2017b) and Tercero et al. (2013) all used single-dish IRAM 30m data. Single-dish observations give the line intensity integrated over a large beam area and do not provide the spatial distribution of EF nor an accurate value of the source-averaged column density. To better constrain the formation of this species, interferometric observations at high angular resolution and good sensitivity are needed. Tercero et al. (2015) provided an ALMA map of a single transition of EF (at 237.567 GHz, with Eu = 275.9 K), which peaks mainly towards the compact ridge. However, a more complete multi-transition study involving many EF transitions at different energy levels is needed to: i) determine the excitation conditions of the molecule and thus the column density in the different components of Orion KL, and ii) compare the spatial distribution and abundances of EF with that of the molecules proposed to play a role in its formation.

With this purpose, we study in this paper multiple transitions of EF, covering the spectral range 214-247 GHz, and energy levels from 56 to 788 K, using interferometric ALMA observations with high angular resolution ($\sim$1.*′′*5). We identified EF, and also the chemically related molecules ET, MF and HCOOH. We studied their relative abundances and relative spatial distributions, and we compared with the predictions of the theoretical gas-phase and grain-surface models, so that we can further understand the chemical routes of EF. This paper is organized as follows. In Section 2, we introduce the observations. The results and derivation of physical parameters are presented in Section 3 and Section 4, respectively. A general discussion is presented in Section 5. We summarize the results in Section 6.

2 OBSERVATIONS

We used data from the ALMA science verification (SV) program111The data are available to the public and can be accessed from the ALMA-SV website at https://almascience.nrao.edu/alma-data/science-verification.. The observations were carried with 16 antennas of 12 m in January 2012. The phase center of the observations is R.A.(J2000)=$05^{\mathrm{h}}35^{\mathrm{m}}14^{\mathrm{s}}.35$ and Decl.(J2000)=$-05^{\circ}22^{{}^{\prime}}35^{\prime\prime}.00$. The baseline lengths range from 17 to 265 m. A total of 20 spectral windows of 1.875 GHz completely cover a frequency range from 214 GHz to 247 GHz. Each spectral window consists of 3840 channels with a uniform spectral resolution of 0.448 MHz ($\sim 0.7$ km s*-1*). Callisto and the quasar J0607-085 were used for the flux and phase calibration, respectively. The 1 Jy beam*-1* correspond to a main beam brightness temperature of $\sim$ 9 K. The primary beam size of each 12 m antenna is about 30*′′*. The calibrated UV data were exported to Miriad format, and images were using the Miriad software package (Sault et al., 1995). The synthesized beam size with natural weighting for spectral images is approximately $1.^{\prime\prime}8\times 1.^{\prime\prime}3$ ($\sim$ 745 AU$\times$ 538 AU) with a PA of $-2.6^{\circ}$.

3 RESULTS

3.1 Line Identification

We identified spectral line transitions using the eXtended CASA Line Analysis Software Suite (XCLASS; Möller et al. 2017), which accesses the Cologne Database for Molecular Spectroscopy (CDMS222http://cdms.de; Müller et al. 2001, 2005) and Jet Propulsory Laboratory (JPL333http://spec.jpl.nasa.gov; Pickett et al. 1998) molecular databases. We used the CDMS entries for EF, ET and t-HCOOH. We used the JPL entry for MF. To identify lines, we set the following constraints (Snyder & Lovas., 2005; Brouillet et al., 2015): the rest frequency must be consistent with the value from laboratory measurement; all detected transitions for a given species must have similar Local standard of rest (LSR) velocity of the source; the transitions at different frequency have different beam dilution effect; for a specific species, transitions having similar excitations condition should have similar spatial distribution. Over the full spectral coverage, we identified 82 (above 5$\sigma$) unblended lines of EF (49 of the trans conformer and 33 of the gauche conformer) with upper level energies (Eu) in the range 152–411 K, 469 lines of MF with Eu 36–797 K, 204 lines of ET (67 trans and 137 gauche) with Eu ranging from 24 to 672 K, and 12 lines of trans-HCOOH with Eu of 59–262 K. In this work, all lines below the $5\sigma$ detection level are ignored, where $\sigma$ is the rms noise. In order to compute rms of a certain transition, we measured the rms value considering the line-free channels among the nearby 50 channels. We show in Figure 1 selected transitions of EF that appear unblended or only slightly blended towards the two selected positions (see below): the compact ridge and/or the hot core southwest region. We also present the full observed spectra (black curve) in Figure A.1, and overplot all the detectable transitions of EF (unblended and blended). The observed spectra of MF and ET are showed in Figure 2 and t-HCOOH is showed in Figure 3. The spectra of EF, MF and ET are extracted from the positions of the intensity peaks of EF (see Figure 4a), which are R.A.=$05^{\mathrm{h}}35^{\mathrm{m}}14^{\mathrm{s}}.430$, Decl.=$-05^{\circ}22^{{}^{\prime}}34^{"}.26$ for the hot core-SW and R.A.=$05^{\mathrm{h}}35^{\mathrm{m}}14^{\mathrm{s}}.107$, Decl.=$-05^{\circ}22^{{}^{\prime}}36^{"}.84$ for the compact ridge. The spectra of t-HCOOH are extracted from the intensity peaks of t-HCOOH (see Figure 4d, R.A.=$05^{\mathrm{h}}35^{\mathrm{m}}14^{\mathrm{s}}.485$, Decl.=$-05^{\circ}22^{{}^{\prime}}32^{"}.56$ for the hot core-SW, and R.A.=$05^{\mathrm{h}}35^{\mathrm{m}}14^{\mathrm{s}}.584$, Decl.=$-05^{\circ}22^{{}^{\prime}}27^{"}.46$ for the hot core-N). The rest frequencies, quantum numbers, the line strength (S$\mu^{2}$), and the upper level energies (Eu) of selected lines of EF are listed in Table A.1. We performed Gaussian fits to every transition. The LSR velocity, full-width half maximun and peak intensities of each transitions are given in columns 5 to 7 (for the hot core-southwest) and 9 to 11 (for the compact ridge). The detection level of each transition is given in column 8 and 12 in Table A.1. The last column of Table A.1 lists the 1$\sigma$ channel rms noise level of each transition. Table A.2, A.3, A.4 list the unblended lines of MF, ET and formic acid, respectively. From the Figure 1 and A.1, one can see that EF lines are less blended toward the compact ridge than in the hot core-SW, and that the molecular lines have emission intensities slightly weaker toward the compact ridge than toward the hot core-SW.

3.2 Spatial Distributions

We show in Figure 4 the line integrated maps of EF, MF, ET and t-HCOOH at different (lower, intermediate and higher) upper level energies, with the aim of evaluating how the molecular emission varies with the energy of the transitions. It should be noted that it is difficult to present the EF integrated maps at low (e.g., Eu = 222 K) and high (e.g., Eu = 596 K) energies due to contamination from other species, especially toward the hot core-SW. We show three lines at intermediate energy levels (306–320 K) that are completely free from contamination in Figure 4a. In Figure 4, the peak positions of continuum hot core (R.A.=$05^{\mathrm{h}}35^{\mathrm{m}}14^{\mathrm{s}}.543$, Decl.=$-05^{\circ}22^{{}^{\prime}}31^{"}.21$) and compact ridge (R.A.=$05^{\mathrm{h}}35^{\mathrm{m}}14^{\mathrm{s}}.064$, Decl.=$-05^{\circ}22^{{}^{\prime}}36^{"}.97$) are marked as red crosses. The coordinates of two peak positions are from Peng et al. (2017) that used the same dataset.

The emission of EF distributes over the compact ridge and the hot core-SW. The gas map of MF with low-energy level (Eu = 236 K) indicates widespread emission and has two main concentrations: the compact ridge and the hot core, in agreement with previous studies (e.g., Favre et al. 2011; Widicus Weaver & Friedel 2012; Favre et al. 2014; Sakai et al. 2015). However, at intermediate (Eu = 462 K) and higher (Eu = 617 K) energy levels, the emissions of MF becomes more compact and are mainly from the compact ridge and the hot core-SW. We note that the emission of MF at the compact ridge is stronger than at the hot core for low energies, and emission at the hot core becomes stronger at higher upper level energies (e.g., Eu = 617 K; see Figure 2 and Figure 4(b)), consistent with the results presented by Favre et al. (2011). This indicates that lines having higher upper level energies are easily excited in hotter environment. A salient result is that the emission of EF and MF mainly come from the hot core-SW and compact ridge, and peak at the same position toward these two regions. Unlike EF and MF, ET has no strong emission in the compact ridge, and it mainly arises from the hot core-SW, which agrees with the previous works (e.g., Guélin et al. 2008; Feng et al. 2015).

The velocity integrated intensity maps of t-HCOOH with three different energies (70–215 K) are shown in Figure 4d. One emission peak of t-HCOOH is mainly at the hot core-SW, while another one is at the north of hot core (hereafter hot core-N). No compact emission of these relatively high energy transitions of HCOOH is detected towards the compact ridge molecular peak traced by EF and MF. We note that previous observations of lower excitation lines (14–70 K) of HCOOH (Liu et al. 2002; Widicus Weaver & Friedel 2012; Pagani et al. 2017) showed widespread distribution towards the S-SW of the compact ridge. However, this gas component, seen more clearly with poorer spatial resolution (3*′′-6′′*; Liu et al. 2002; Widicus Weaver & Friedel 2012), do not seem related with the compact gas traced by EF and MF.

4 DERIVATION OF PHYSICAL PARAMETERS

4.1 Rotational temperatures and column Densities

The combined usage of XCLASS and MAGIX (Modeling and Analysis Generic Interface for eXternal numerical codes; Möller et al. 2013, 2017) provides the best fit to the data. We have fitted the spectra extracted from the intensity peaks of EF in the hot core-SW and the compact ridge region, and spectra from the intensity peaks of HCOOH toward the hot core-SW and the hot core-N (see Section 3.1). Under the assumption of local thermodynamical equilibrium (LTE), XCLASS takes into account the beam dilution, the line opacity and line blending (Möller et al., 2017), and generates the synthetic spectrum of the different molecular species. The main modeling parameters of XCLASS for each molecule are the source size, the line full width at half maximum $\Delta\upsilon$, the velocity offset with respect to the systemic velocity of the object voff, the rotational temperature Trot and the column density N. We have derived the velocity offsets and line widths from Gaussian fits. We have derived the source size by performing two-dimension Gaussian fits to the line images, by using Miriad task IMFIT. We obtain the source sizes of 1.6*′′* for the hot core-SW and compact ridge based on the integrated maps of EF. We have fixed the $\Delta\upsilon$ and voff. The rotational temperature and the column density are left as free parameters. These two parameters produced by XCLASS are used as initial guesses, and then MAGIX is used to optimize the fit and find the best solutions of Trot and N, and provide corresponding error estimates. The algorithms we adopted are the Genetic, Levenberg-Marquardt and Error estimation using Interval-Nested-Sampling.

We stress that in the case of the conformers of EF and ET, we are able to fit both conformers using the same physical parameters. So both conformers of EF and ET are fitted together. We show the LTE model spectra overlaid on the observed spectra in Figure 1 (EF) and Figure 2 (MF and ET) toward the hot core-SW and the compact ridge. Figure 3 shows the LTE spectra of t-HCOOH toward the hot core-SW and the hot core-N. The derived parameters from two positions, including rotational temperature $T_{\rm rot}$, total column density N and abundance $\chi$ are listed in Table 1. See more details about abundances in section 4.2. In the following we comment individually the rotational temperature and column density obtained from each species:

EF (C2H5OCHO): The detected unblended transitions cover a large Eu range (152–411 K), which allow to constrain well the rotational temperature, and then the column density. We have obtained Trot = 122 $\pm$ 34 K, and a column density of N = (9.0 $\pm$ 3.0) $\times$ 1015 cm*-2* for the hot core-SW, and Trot = 103 $\pm$ 13 K, N = (6.0 $\pm$ 3.0) $\times$ 1015 cm*-2* for the compact ridge. Meanwhile, for the purpose of comparison with other works, we list the number of detected lines, corresponding Eu range, observations information and source size they used in Table 2. The temperature Trot we obtained is similar to that derived in W51 (78 $\pm$ 10 K) by Rivilla et al. (2017b), and to that assumed in Orion KL compact ridge (100 $\pm$ 20 K) by Tercero et al. (2015) and Sgr B2(N) (100 K) by Belloche et al. (2009). The column density we obtained in the compact ridge is higher to that of Tercero et al. (2015). They assumed Trot of 100 K and $\theta_{\rm s}$ of 3*′′* for one single transition to obtain column density N of (0.2 $\pm$ 0.04) $\times$ 1016 cm*-2*. This value would be 0.7$\times$ 1016 cm*-2*, similar to our value, if we consider the same source size of 1.6*′′*.

As mentioned before, multiple unblended or slightly blended lines of the both conformers of EF are clearly identified, as shown in Figure 1. We note that a few transitions (e.g., 218.74259 GHz, 218.74470 GHz, 220.00368 GHz, 223.89863 GHz, 224.28239 GHz) are weaker than the LTE fit. A possible explanation is a poor baseline subtraction at these spectral ranges, since Orion is a line-rich source, and it is difficult to find free-line channels, and then the continuum is not perfectly substracted. On the other hand, some of the observed lines might be weaker than the LTE fit due to the presence of nearby absorption lines from other species (e.g. 218.752 GHz, 219.658 GHz and 225.240 GHz). There are some lines, at 219.40243 GHz, 219.65815 GHz, 219.78133 GHz, 219.78771 GHz and 220.17206 GHz, for which the LTE fit predicts higher line intensities than observed. We found that the spectroscopic parameters (in particular S$\mu^{2}$) of these five transitions show discrepancies between the CDMS and the JPL entries. Other lines that have similar spectroscopic parameters in two database have good fitting results. We suspect that there are some issues affecting the spectroscopic information of some of the transitions with discrepancies.

MF (CH3OCHO): We note that the stronger lines are overestimated toward the hot core-SW, this could be due to line opacity effects. These stronger lines have high S$\mu^{2}$. If we leave the source size as a free parameter, all lines of MF can be fitted better, including high S$\mu^{2}$ lines, which appears saturated with optical depth of $\sim$4. The strong lines are expected to have more extended gas distribution, and interferometric observations of ALMA will partially filter out diffuse gas component. This is likely to cause decreasing intensities of strong lines. The best fit parameters of MF are Trot = 135 $\pm$ 5 K and N = (7 $\pm$ 3) $\times$ 1017 cm*-2* toward the hot core-SW, and 93 $\pm$ 5 K and (9 $\pm$ 4) $\times$ 1017 cm*-2* for the compact ridge. Our derived rotational temperatures in the hot core-SW and compact ridge are mostly consistent with the other works, but column densities in the two regions are larger than the values provided by others (see Table 2, e.g., Favre et al. 2011; Feng et al. 2015; Tercero et al. 2015). Our column density is higher than results from Favre et al. (2011) by a factor of $\sim$4–5 probably because they cover lower Eu transitions, especially in the compact ridge. The lower column densities found by Feng et al. (2015) is easily explained because of their large beam (5.′′0$\times$4.′′0), which also includes less dense gas. The difference of column density between our work and Tercero et al. (2015) can be easily explained due to the different source size used. If one consider $\theta_{\rm s}$ of 1.6′′ instead of 3*′′, the Tercero et al. (2015) estimate would be 84 $\times$ 1016* cm*-2*, in remarkably agreement with our results.

ET (C2H5OH): Regarding ET, we have found significant discrepancies in the spectroscopic parameters between the CDMS and JPL entries affecting some transitions, e.g., those at 213.760 and 213.856 GHz. Comparing the LTE spectra of both databases, we have seen that the CDMS entry is more consistent with the observed spectrum, so that we have used it to derive the physical properties. The LTE fitting gives Trot = 144 $\pm$ 13 K and N = (1.3 $\pm$ 0.5)$\times$ 1017 cm*-2* toward the hot core-SW, and 111 $\pm$ 15 K and (2.0 $\pm$ 0.5) $\times$ 1016 cm*-2* for the compact ridge. The column densities derived from our work are higher than those reported in Feng et al. (2015) (see Table 2), owing to our higher angular resolution. Again for result from Tercero et al. (2015), if one consider $\theta_{\rm s}$ =1.6*′′, the column density would be 21 $\times$ 1016* cm*-2*, which is of the same order than our value (less than a factor of 2).

Formic acid (t-HCOOH): We derive Trot = 120 $\pm$ 16 K and N = (6.2 $\pm$ 1.8) $\times$ 1015 cm*-2* toward the hot core-SW peak, and 106 $\pm$ 27 K and (2.8 $\pm$ 0.8) $\times$ 1015 cm*-2* toward the hot core-N.

We give in Table 3 the ratios of [MF/EF] and [MF/ET] from our work and a few other works toward the hot core-SW and the compact ridge. The [MF/EF] of 140$\pm$81 toward the compact ridge in our work is reasonable agreement with the ratio of 120 in Tercero et al. (2015). Toward the hot core-SW, the [MF/ET] of 6$\pm$3 is in agreement with that of 4$\pm$3 in Feng et al. (2015), and toward the compact ridge our result of 42$\pm$24 is similar to those from Feng et al. (2015) (17$\pm$12) and Crockett et al. (2014) (20) if considering the uncertainties.

4.2 Molecular abundances

The fractional abundance of a certain molecule relative to H2 depends on the column density of the certain molecule, N, and that of the H2 molecule ($\chi$ = N/N${}_{\rm H_{2}}$). The estimations of N${}_{\rm H_{2}}$ for each Orion KL component are presented in many works (e.g., Tercero et al. 2010; Crockett et al. 2014; Favre et al. 2011; Feng et al. 2015), and N${}_{\rm H_{2}}$ varies in every work because of different spatial resolutions. We used the dust continuum emission of the ALMA observations to estimate the column density of H2. In the assumption of optically thin emission from dust, and taking a grain radius of 0.1 $\mu$m, a grain density of 3 g cm*-3*, and a gas-to-dust ratio of 100, the column density of H2 can be derived by the formula (e.g., Lis et al. 1991; Qin et al. 2010):

[TABLE]

where Td is the dust temperature, Qν is the grain emissivity at frequency $\nu$, Sν is the peak intensity of the continuum and $\Omega$ is the beam solid angle. We assumed that the dust temperature equals to the rotation temperature of the core. We used dust temperature of 200 K for the hot core and 100 K for the compact ridge (e.g., Wu et al. 2014; Favre et al. 2011), and grain emissivity of 2 $\times$ 10*-5* at 230 GHz (Lis et al., 1991). We adopt Sν of 1.34 $\pm$ 0.06 Jy beam*-1* for the hot core and 0.28 $\pm$ 0.03 Jy beam*-1* for the compact ridge from Wu et al. (2014) that used same ALMA data. Therefore we obtain H2 column densities of (4.7 $\pm$ 0.2) $\times$ 1024 cm*-2* and (2.0 $\pm$ 0.2) $\times$ 1024 cm*-2* for the hot core and the compact ridge, respectively. We list the abundances relative to H2 of EF, MF, ET and t-HCOOH toward the two positions in Table 1. In comparison with abundances for each species from other works (see Table 2), one can see that:

1. The abundance of EF towards the hot core and the compact ridge are very similar, 0.19 $\times$ 10*-8* and 0.3 $\times$ 10*-8* respectively. This value is in good agreement with the ones derived in the other two massive star forming regions where EF has been detected: 1 $\times$ 10*-8* in W51 (Rivilla et al., 2017b) and 0.36 $\times$ 10*-8* on Sgr B2(N) (Belloche et al., 2009).

2. The abundances of ET and MF we have derived are in better agreement with those from Feng et al. (2015), compared with the column densities. This is because the effect of the different beams are compensated by dividing by N${}_{\rm H_{2}}$ (namely, their N is lower but also their N${}_{\rm H_{2}}$).

3. The abundances of MF from Favre et al. (2011) are lower, especially in the compact ridge (3.1 $\times$ 10*-8*). This could be because the range of energies of the lines used in the compact ridge by Favre et al. (2011) is much lower than the one we have used (see Table 2), and thus they are tracing less dense gas.

4. Our derived abundance of MF ((42$\pm$20) $\times$ 10*-8*) in the compact ridge agrees very well with those derived by Gong et al. (2015) ((47$\pm$6) $\times$ 10*-8*) and Crockett et al. (2014) (33 $\times$ 10*-8*).

We compare graphically the abundances (Figure 5a) and rotational temperatures (Figure 5b) of each molecule derived in different regions (Orion KL, W51 and Sgr B2(N)) with the theoretical values from the grain-surface model by Garrod et al. (2008) and gas-phase model by Taquet et al. (2016). In the next section we will discuss which of the two proposed chemistries is in better agreement with the observations in star-forming regions.

5 Discussion

We have shown that both conformers of EF can be well fitted simultaneously, and hence that they share the same abundance. The relative abundance of the two conformers is $N_{\rm g}$/$N_{\rm t}$ = 2e${}^{-\Delta E/T_{\rm kin}}$ (Rivilla et al., 2017b), where $T_{\rm kin}$ is kinetic temperature of the gas and $\Delta E$ is energy difference between the two ground states of two conformers. $\Delta E$ is in the range of 64–124 K (Riveros & Wilson., 1967), which implies a $T_{\rm kin}$ in the range 93–178 K when considering $N_{\rm g}$/$N_{\rm t}$ = 1. The $T_{\rm kin}$ would be equal to the rotational temperature Trot, if we assume that EF is thermalized. The Trot derived in our work (122 $\pm$ 34 K for hot core-SW and 103 $\pm$ 13 K for compact ridge, see Table 1) are compatible to the range of 93–178 K, which suggests that EF is indeed thermalized.

As we stated above, three molecules (MF, ET and HCOOH) may play a important role in forming EF, so it is necessary to take these related species into account to study the proposed chemical routes to EF. Table 1 and Figure 5b showed that $T_{\rm rot}$ in the hot core are in the range of 120–144 K, and 93–111 K in the compact ridge, indicating that these four molecules share similar excitation conditions, and hence they are expected to trace similar gas components. This means that if there is no emission in some region (for instance ET in the compact ridge), this is due to a chemical effect.

Two main pathways have been proposed to form EF, one based on gas-phase chemistry and other based on chemistry on the surface of dust grains. We compare in the following the result found in star-forming regions (Orion KL, W51 and Sgr B2(N)) with the predictions of the proposed models.

1. Gas-phase formation. Based on the model from Charnley et al. (1995), their gas-phase route is able to produce EF abundance of $\sim$10*-10* in 104-105 yr (C2H5OH + H*+* $\rightarrow$ C2H5OH${}^{+}_{2}$, C2H5OH${}^{+}_{2}$ + H2CO $\rightarrow$ HC(OH)OC2H${}^{+}_{5}$ + H2, HC(OH)OC2H${}^{+}_{5}$ + e*-* $\rightarrow$ C2H5OCHO + H). The observed EF abundances of 0.2 $\times$ 10*-8* (hot core) and 0.3 $\times$ 10*-8* (compact ridge) (see Table 1) are much larger than that predicted by gas-phase route. The estimated abundances in W51 (1.0 $\times$ 10*-8*) and Sgr B2(N) (3.6 $\times$ 10*-9*) are also larger than that predicted ones. So this gas-phase reaction can not produce enough abundance of EF. A new gas-phase network for the formation of COMs have been proposed by Taquet et al. (2016). Considering proton transfer (PT) reaction with HCOOH and NH3 (C2H5OH${}^{+}_{2}$ + HCOOH $\rightarrow$ HC(OH)OC2H${}^{+}_{5}$ + H2O, HC(OH)OC2H${}^{+}_{5}$ + NH3 $\rightarrow$ C2H5OCHO +NH${}^{+}_{4}$), their model is able to produce the observed EF abundances of 10*-8*–10*-7* in a reasonable timescale for hot molecular cores of 104 $-$ 105 yr. In the hot core-SW, our derived HCOOH abundance of (1.3$\pm$0.4) $\times$ 10*-9* is much lower than those required for gas-phase model (see panel a(4) of Figure 5). Moreover, the ALMA maps show that there is no similarity in spatial distributions and emission peaks between HCOOH and EF (see Figure 4).

Regarding the distribution of NH3 (transitions with Eu $>$ 408 K) detected by Goddi et al. (2011), it peaks towards the hot core and IRC7 regions, while no emission is detected towards the compact ridge peak traced by EF and MF (see their Fig. 2). Since both the hydrogen column densities and rotational temperatures found towards the two positions studied in our work, hot core-SW (N${}_{\rm H_{2}}$ = (4.7 $\pm$ 0.2) $\times$ 1024 cm*-2*, Trot = 120–144 K) and compact ridge (N${}_{\rm H_{2}}$ = (2.0 $\pm$ 0.2) $\times$ 1024 cm*-2*, Trot = 93–111 K), are relatively similar, the clear presence of NH3 in the hot core region and the lack in the compact ridge peak, point to a chemical differentiation and not to excitation effects. Therefore, similarly to HCOOH, there is no spatial coincidence between NH3 and EF. In the hot core region, Goddi et al. (2011) derived a column density of NH3 of (1-4) $\times$ 1017 cm*-2*, which implies an abundance of (2$-$9) $\times$ 10*-8*. This value is lower than the gas model value by more than an order of magnitude. In summary, the molecular abundances and spatial distributions of HCOOH and NH3 indicate that the proposed gas-phase routes are not able to produce the observed EF abundance in Orion KL, as also noted by Rivilla et al. (2017b) in W51.

2. Grain-surface formation. According to the coupled gas-phase and grain-surface chemical network by Garrod et al. (2008), EF can be formed on grain surfaces by the reactions between radicals CH3 and CH2OCHO or between HCO and C2H5O. MF is the primary source of precursor radical of EF (CH3OCHO + h$\nu$ $\rightarrow$ CH2OCHO, CH2OCHO + CH3 $\rightarrow$ C2H5OCHO). When MF evaporates, ET (C2H5OH) becomes dominant in forming EF ( C2H5OH + h$\nu$ $\rightarrow$ C2H5O; C2H5O + HCO $\rightarrow$ C2H5OCHO). The predicted abundance of EF is 2.3 $\times$ 10*-9* in this model (see Belloche et al. 2009), which is in reasonably agreement with the range of observed values in our work ((2$-$3)$\times$10*-9*) and in W51 e2 and Sgr B2(N) ((3.6$-$10)$\times$10*-9*). In addition, the predicted molecular ratio MF/EF is 72/1 (in Basic model, see Table 15 of Belloche et al. 2009), which is close to our observed ratio values of (79$\pm$36)/1 for the hot core and (140$\pm$81)/1 for the compact ridge. The excitation temperature of EF (122$\pm$34 K for the hot core and 103$\pm$13 K for the compact ridge) and MF (135$\pm$5 K for the hot core and 93$\pm$5 K for the compact ridge) derived in Orion KL are also close to the temperature associated with the peak gas-phase temperatures of Garrod et al. (2008) model (110 K for EF and 81 K for MF) (see Figure 5b). The abundances of ET derived from our fitting ((2.7$\pm$1.0) $\times$ 10*-8* for the hot core and (1.0$\pm$0.3) $\times$ 10*-8* for the compact ridge), 7.9$\times$10*-8* in W51 and 5.7$\times$10*-8* in Sgr B2(N) (see Table 2 and Figure 5a) are lower than abundance from Garrod et al. (2008) model (1.0$\times$10*-7*). This may indicate that the dominant route to the formation of EF is grain surface chemistry from MF and not from ET. This is also supported by the spatial distribution of the different molecules revealed by the ALMA observations (see Figure 4): EF and MF share the same intensity peaks toward the hot core-SW and the compact ridge, while there is no strong ET emission towards the compact ridge. Considering all this together, EF is likely formed on grains by addition of CH3 to functional-group radicals (CH2OCHO) derived from CH3OCHO and then evaporated to the gas phase.

6 Summary

As the the next step in chemical complexity of esters, ethyl formate (C2H5OCHO) is a low abundant molecule and it is detected so far only toward three sources. The formation routes of EF are still poorly known. Multiple transitions of EF covering a wide range of energy levels are needed to explore it’s excitation conditions and formation routes. The different chemical routes proposed should be checked with observations of EF and its chemically related species (methyl formate (CH3OCHO), ethanol (C2H5OH) and trans-HCOOH). We have identified and imaged multiple lines of EF, MF, ET and t-HCOOH observed with ALMA at band 6 (214–247 GHz). The identified transitions of three molecules span a wide range of upper-level energy so that we can perform a detailed LTE analysis to derive accurate physical parameters. We have compared our results with the two main chemical routes proposed to the formation of EF (gas-phase chemistry and grain surface chemistry). The main findings are as follows:

1. We have identified 82 unblended lines of EF (49 of the trans conformer and 33 of the gauche conformer), 469 lines of MF, 204 lines of ET (67 of the trans conformer and 137 of the gauche conformer) and 12 lines of t-HCOOH over the full ALMA band 6.

2. The spatial distribution of EF exhibits two main emission regions (compact ridge and hot core-SW). The gas emission of MF is very similar to that of EF, while ET mainly originates from the hot core-SW but not from the compact ridge. t-HCOOH mainly originate from the south-west of hot core and hot core-N.

3. The derived rotational temperatures and column densities of EF are 122 $\pm$ 34 K, (9.0 $\pm$ 3.0) $\times$ 1015 cm*-2* for the hot core-SW, and 103 $\pm$ 13 K, (6.0 $\pm$ 3.0) $\times$ 1015 cm*-2* for the compact ridge. We also derived the best fit parameters of chemically related molecules MF, ET and t-HCOOH.

4. We have compared our results with the two proposed routes to form EF, based on gas-phase chemistry (Taquet et al., 2016) and grain surface chemistry (Garrod et al., 2008; Belloche et al., 2009). The observed abundances of NH3 and HCOOH are much lower than those required for gas-phase model to produce the observed abundances of EF. Moreover, there is no similarity in spatial distributions and emission peaks among HCOOH, NH3 and EF. So these gas-phase routes may not produce the observed EF abundance in Orion KL and other star-forming regions. The derived EF abundance of (2-3)$\times$10*-9* is in good agreement with the one predicted by the grain surface model. The observed ratios of MF/EF ((79$\pm$36)/1 for the hot core and (140$\pm$81)/1 for the compact ridge are also in good agreement with the grain-surface model. In addition, the gas emissions of EF and MF peak at same position in the hot core and compact ridge, while ET spatial distribution is different, lacking in the compact ridge. Altogether, these results suggest that EF is likely formed on grains by addition of CH3 to functional-group radicals (CH2OCHO) derived from CH3OCHO and then evaporated to the gas phase.

This paper makes use of the following ALMA data: ADS/JAO.ALMA# 2011.0.00009.SV. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. We thank Sheng-Li Qin for carefully reading the paper and providing useful comments and suggestions. This work has been supported by the National Natural Science Foundation of China under grant No. 11433004. V.M. R. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 664931, and from the Italian Ministero dell’Istruzione, Università e Ricerca through the grant Progetti Premiali 2012 - iALMA (CUP C52I13000140001). JX acknowledges support from FONDECYT grant 3170768 and the ”Light of West China” program of Chinese Academy of Sciences (CAS).