12CO and 13CO J=3-2 observations toward N11 in the Large Magellanic Cloud

TL;DR

This study maps the molecular gas in the N11 nebula of the Large Magellanic Cloud using CO J=3-2 observations, revealing effects of stellar feedback, nebula expansion, and chemical processes affecting molecular cloud morphology and mass estimates.

Contribution

First comprehensive CO J=3-2 mapping of the entire N11 nebula, analyzing molecular cloud properties and feedback effects in different regions of the nebula.

Findings

Evidence of nebula expansion influencing cloud morphology

Selective photodissociation affecting CO isotopologue ratios

Clumps are likely pressure-supported and not gravitationally bound

Abstract

After 30 Doradus, N11 is the second largest and brightest nebula in the LMC. This large nebula has several OB associations with bright nebulae at its surroundings. N11 was previously mapped at the lowest rotational transitions of $^{12}$CO (J=1--0 and 2--1), and in some particular regions pointings of the $^{13}$CO J=1--0 and 2--1 lines were also performed. Using ASTE we mapped the whole extension of the N11 nebula in the $^{12}$CO J=3--2 line, and three sub-regions in the $^{13}$CO J=3--2 line. The regions mapped in the $^{13}$CO J=3--2 were selected based on that they may be exposed to the radiation at different ways: a region lying over the nebula related to the OB association LH10 (N11B), another one that it is associated with the southern part of the nebula related to the OB association LH13 (N11D), and finally a farther area at the southwest without any embedded OB association (N11I). We found that the morphology of the molecular clouds lying in each region shows some signatures that could be explained by the expansion of the nebulae and the action of the radiation. Fragmentation generated in a molecular shell due to the expansion of the N11 nebula is suggested. The integrated line ratios $^{12}$CO/$^{13}$CO show evidences of selective photodissociation of the $^{13}$CO, and probably other mechanisms such as chemical fractionation. The CO contribution to the continuum at 870 $\mu$m was directly derived. The distribution of the integrated line ratios $^{12}$CO J=3--2/2--1 show hints of stellar feedback in N11B and N11D. The ratio between the virial and LTE mass (M$_{\rm vir}$/M$_{\rm LTE}$) is higher than unity in all analyzed molecular clumps, which suggests that the clumps are not gravitationally bounded and may be supported by external pressure. A non-LTE analysis suggests that we are mapping gas with densities about a few 10$^{3}$ cm$^{-3}$.

Tables

Table 1. Table 1: Observed regions in 13 CO J=3–2 toward N11.

Region	RA (J2000)	Dec.(J2000)	Size	OB association	MCs evolutionary sequence𝐚𝐚\bf a𝐚𝐚\bf a$𝐚$probable evolutionary sequence (from Kawamura et al. 2009) of the molecular clouds in the regions.
N11B	04${}^{\text{h}}$ 56${}^{\text{m}}$ 48${}^{\text{s}}$	-66∘ 24′ 25′′	140${}_{}{}^{\prime \prime }\times$110′′	LH10 (embedded)	type III
N11D	04${}^{\text{h}}$ 55${}^{\text{m}}$ 41${}^{\text{s}}$	-66∘ 34′ 25′′	130${}_{}{}^{\prime \prime }\times$100′′	LH13 (to the north)	type II
N11I	04${}^{\text{h}}$ 57${}^{\text{m}}$ 50${}^{\text{s}}$	-66∘ 28′ 57′′	80${}_{}{}^{\prime \prime }\times$230′′	none	type I

Table 2. Table 2: Observed and derived parameters (assuming LTE) of each molecular clump.

Clump	1B	2B	1D	2D	1I	2I
Velocity [km s-1]	278.5	284.0	278.5	279.5	278.5	283.5
R [pc]	6.4	6.5	7.7	5.3	8.4	6.3
${\tau }_{12}$	8.8	9.5	10.6	15	8.8	8.2
${\tau }_{13}$	0.1	0.2	0.2	0.3	0.2	0.1
Tex [K]	8	13	11	7	10	8
N(H2) [$\times {10}^{22}$ cm-2]	1.6	2.3	2.5	2.3	3.9	1.0
MLTE [$\times {10}^4$ M⊙]	0.8	1.1	1.3	1.1	2.0	0.5

Table 3. Table 3: Used Δ Δ \Delta v 13 (in km s -1 ) and obtained virial mass (in × 10 4 absent superscript 10 4 \times 10^{4} M ⊙ ) of the molecular clumps from the 13 CO J=3–2 line.

	1B	2B	1D	2D	1I	2I
$\Delta$v13	4.0$\pm 0.5$	5.4$\pm 0.7$	3.7$\pm 0.4$	4.5$\pm 0.5$	6.0$\pm 1.2$	2.6$\pm 0.5$
Mvir	1.9$\pm 0.7$	3.7$\pm 1.4$	1.9$\pm 0.7$	2.0$\pm 0.8$	5.7$\pm 2.2$	0.8$\pm 0.3$

Table 4. Table 4: Average integrated ratios toward the peaks of the molecular clumps. The errors in all cases are about 10%.

Clump	$\frac{{}_{}{}^{12}\text{CO}\left(3-2\right)}{{}_{}{}^{13}\text{CO}\left(3-2\right)}$	$\frac{{}_{}{}^{12}\text{CO}\left(3-2\right)}{{}_{}{}^{12}\text{CO}\left(1-0\right)}$	$\frac{{}_{}{}^{12}\text{CO}\left(3-2\right)}{{}_{}{}^{12}\text{CO}\left(2-1\right)}$
1B	8.3	0.7	0.7
2B	7.2	1.2	0.9
1D	6.5	0.5	0.8
2D	6.8	0.7	0.7
1I	7.0	0.4	0.6
2I	13.0	0.4	0.8

Table 5. Table 5: Line parameters from the spectra shown in Figs.  10 . The 12 CO J=2–1 and 3–2, and 13 CO J=3–2 spectra were convolved with the 45″ resolution of the J=1–0 line.

Clump	Emission	$T_{\text{mb}}$	$V_{\text{LSR}}$	$\Delta$ v
		(K)	(km s-1)	(km s-1)
2B	12CO (1–0)	2.0 $\pm$ 0.2	285.8 $\pm$ 0.3	5.9 $\pm$ 0.1
	12CO (2–1)	1.3 $\pm$ 0.1	285.8 $\pm$ 0.1	6.2 $\pm$ 0.7
	12CO (3–2)	1.2 $\pm$ 0.3	285.4 $\pm$ 0.7	5.7 $\pm$ 1.7
	13CO (3–2)	0.30 $\pm$ 0.03	285.4 $\pm$ 1.0	4.8 $\pm$ 0.6
1D	12CO (1–0)	2.3 $\pm$ 0.2	280.1 $\pm$ 0.2	4.5 $\pm$ 0.5
	12CO (2–1)	1.3 $\pm$ 0.1	280.2 $\pm$ 0.3	4.3 $\pm$ 0.6
	12CO (3–2)	1.1 $\pm$ 0.4	279.9 $\pm$ 0.7	4.1 $\pm$ 0.8
	13CO (3–2)	0.40 $\pm$ 0.03	279.9 $\pm$ 1.0	3.1 $\pm$ 0.4
1I	12CO (1–0)	2.9 $\pm$ 0.9	279.7 $\pm$ 0.2	4.9 $\pm$ 0.4
	12CO (2–1)	1.6 $\pm$ 0.1	279.9 $\pm$ 0.2	5.1 $\pm$ 0.5
	12CO (3–2)	0.9 $\pm$ 0.3	279.8 $\pm$ 0.7	4.8 $\pm$ 1.5
	13CO (3–2)	0.20 $\pm$ 0.03	279.1 $\pm$ 0.6	4.4 $\pm$ 0.9

Table 6. Table 6: Radex results from the 12 CO 1–0 , 2–1 and 3–2(50%) lines with T = K 20 {}_{\rm K}=20 K. Errors are about 20% and 30% in the limits of the ranges of N(CO) and n H 2 subscript H 2 {}_{\rm H_{2}} , respectively.

Clump	N(CO) (${10}^{15}$cm-2)	n${}_{H_2}$ (${10}^3$cm-3)
2B	4.4 – 10.1	1.5 – 8.5
1D	3.3 – 9.1	1.1 – 6.9
1I	7.9 – 14.4	1.0 – 2.4

Equations

$$T_{ex}(3\to 2)=\frac{16.59\text{K}}{\text{ln}[1+16.59\text{K}/(T_{max}{(}^{12}\text{CO})+0.036\text{K})]}$$

$$\frac{{}^{12}{\text{T}}_{\text{mb}}}{{}^{13}{\text{T}}_{\text{mb}}}=\frac{1-\text{exp}(-{\tau }_{12})}{1-\text{exp}(-{\tau }_{12}/X)}$$

$$N{(}^{13}\text{CO})=8.28\times 10^{13}{e}^{\frac{15.85}{T_{ex}}}\frac{T_{ex}+0.88}{1-e^{\frac{-15.85}{T_{ex}}}}\frac{1}{J(T_{ex})-J(T_{BG})}\int T_{mb}dv$$

$$J(T)=\frac{h\nu /k}{exp\left(\frac{h\nu }{kT}\right)-1}$$

$$M=\mu m_H{D}^2\Omega \sum _i{\mathrm{N}}_i({\mathrm{H}}_2)$$

$${\mathrm{M}}_{\mathrm{vir}}/{\mathrm{M}}_{\odot }=\mathrm{k}\text{leavevmode}R/pc\text{leavevmode}(\mathrm{\Delta }{\mathrm{v}}_{13}/km\text{leavevmode}{\mathrm{s}}^{-1}{)}^2$$

Full text

*Methods. *Using the Atacama Submillimeter Telescope Experiment we mapped the whole extension of the N11 nebula in the 12CO J=3–2 line, and three sub-regions in the 13CO J=3–2 line. The regions mapped in the 13CO J=3–2 were selected based on that they may be exposed to the radiation at different ways: a region lying over the nebula related to the OB association LH10 (N11B), another one that it is associated with the southern part of the nebula related to the OB association LH13 (N11D), and finally a farther area at the southwest without any embedded OB association (N11I).

Results. We found that the morphology of the molecular clouds lying in each region shows some signatures that could be explained by the expansion of the nebulae and the action of the radiation. Fragmentation generated in a molecular shell due to the expansion of the N11 nebula is suggested. The integrated line ratios 12CO/13CO show evidences of selective photodissociation of the 13CO, and probably other mechanisms such as chemical fractionation. The values found for the integrated line ratios 12CO J=3–2/1–0 are in agreement with values that were assumed in previous works, and the CO contribution to the continuum at 870 $\mu$m was directly derived. The distribution of the integrated line ratios 12CO J=3–2/2–1 show hints of stellar feedback in N11B and N11D. The ratio between the virial and LTE mass (Mvir/MLTE) is higher than unity in all analyzed molecular clumps, which suggests that the clumps are not gravitationally bounded and may be supported by external pressure. A non-LTE analysis suggests that we are mapping gas with densities about a few 103 cm*-3*. The molecular clump at N11B, the unique molecular feature with direct evidence of ongoing star formation, is the densest one among the analyzed clumps.

1 Introduction

Studying low-metallicity interstellar media toward other galaxies, whose physical conditions may resemble those that existed in the early Milky Way, is a very important issue because it can shed light on the primeval processes of star formation that occurred in our Galaxy.

Due to its proximity (about 50 kpc) the Large Magellanic Cloud (LMC) is an ideal laboratory to study the physical properties of molecular clouds under conditions different from those found in our Galaxy. Indeed, Magellanic Clouds are a unique laboratory to study the effects of metallicity and galaxy mass on molecular gas and star formation at high spatial resolution (Jameson et al., 2016). The metallicity in the LMC is Z $\sim$ 0.5 Z*⊙* (Keller & Wood, 2006) and the gas-to-dust ratio is a factor of 4 higher than in our Galaxy. As the LMC is seen nearly face-on with an inclination angle of 35*∘*, many active star-forming and Hii regions were found and studied (e.g. Ochsendorf et al. 2017, Paron et al. 2015, 2014).

After 30 Doradus, N11 (Henize, 1956) is the second largest and brightest nebula of the LMC. It is located at the north-western corner and it is one of the most important star-forming region in the galaxy. N11 has a ring morphology (see Fig. 1) with a cavity of 170 pc in diameter enclosing the OB association LH9 (Lucke & Hodge, 1970) also known as NGC 1760. This region also presents several OB associations with bright nebulae on its surroundings (Rosado et al., 1996); for instance, LH10 (NGC 1763) lying at the northern rim and exciting the N11B nebula, and LH13 (NGC 1769) at the east and associated with the N11D nebula. Parker et al. (1992) and Walborn & Parker (1992) proposed sequential star formation among the OB associations; LH9, an older association, may have generated the formation of LH10, which in turn is likely triggering new star formation in the N11B nebula surroundings (Barbá et al., 2003). Moreover, Hatano et al. (2006) have proposed that LH9 is indeed triggering star formation in the surroundings molecular clouds. Concerning the molecular gas related to N11, Israel et al. (2003) and Herrera et al. (2013) provided catalogues of physical properties of individual molecular clouds distributed through the N11 complex based on 12CO J=1–0 and J=2–1 data. They pointed out that the molecular gas component related to N11 exhibits a shell morphology with massive clumps embedded, which is the usual configuration for a triggered star formation scenario around Hii regions (e.g. Elmegreen & Lada 1977; Pomarès et al. 2009). Additionally, Israel & Maloney (2011) observed that the distribution of CO and [CII] emissions are quite similar, suggesting that a large-scale dissociation of CO and subsequent ionization of the resulting neutral carbon is indeed ongoing in the region. More recently, using Spitzer, Herschel and APEX/LABOCA data, Galametz et al. (2016) performed a complete study about the dust properties toward N11 and investigated variations of the gas-to-dust ratio across the region.

Observations of other CO isotopes and higher rotational transitions of 12CO are needed to map gas with lower optical depths and higher critical densities, useful to study, in a more accurate way, the physical conditions of the gas component. In this paper, we present for the first time 12CO and 13CO J=3–2 observations toward N11. The 12CO J=3–2 line, tracer of warmer and denser gas than 12CO J=1–0, was observed in the entire N11 region (see the region enclosed by dashed lines in Fig. 1). 13CO J=3–2 observations were done toward three sub-regions (see boxes in Fig. 1), which according to Herrera et al. (2013), have the highest CO peak temperatures and contain molecular clouds at different evolutionary stages.

2 Presentation of the analyzed sub-regions in N11

In this section we briefly describe the sub-regions mapped in the 13CO J=3–2 line (see Fig. 1).

The first one is a molecular cloud in the northern part of N11, N11B, associated with LH10, a young (3 Myr) OB association. LH10 is younger than LH9 (Walborn et al., 1999; Mokiem et al., 2007), and Parker et al. (1992) discover that its stellar content presents a higher ratio of higher-mass to lower-mass stars than in LH9. This cloud has bright free-free emission, [CII] 158 $\mu$m and 8 $\mu$m emission. According to Galametz et al. (2016), it shows the highest radiation field across N11. Israel & Maloney (2011) obtain a $G_{0}$ value, which is the flux over the range 6–13.6 eV normalized to $1.6\times 10^{-3}$ erg cm*-2* s*-1*, of about 180, the highest in the region. There is evidence of ongoing star formation in N11B. About 20 HAeBe stars, which are intermediate-mass pre-main sequence stars, have been detected in this region (Barbá et al., 2003; Hatano et al., 2006).

Second, N11D, a molecular cloud with bright free-free emission but not massive star formation (no direct evidence of star formation activity is observed in this region) in the eastern border of N11. This cloud is in the southern part of the nebula generated by the OB association LH13 (Rosado et al., 1996). Galametz et al. (2016) show that the region also has a significant radiation field intensity but not so extended nor intense than in the former described region. Israel & Maloney (2011) obtain $G_{0}$ about 26 toward this region.

Third, N11I, a molecular cloud in the south-western border of N11, that does not have bright free-free emission, indicating more quiescent molecular gas.

These clouds can be placed in the evolutionary sequence of molecular clouds proposed by Kawamura et al. (2009): N11B would be a type III cloud (clouds associated with young stars), N11D, a type II (clouds associated with Hii regions), and N11I, a type I (no Hii region associated).

The center and sizes of the observed regions in 13CO J=3–2, and some of the above mentioned characteristics, are summarized in Table 1.

3 ASTE observations and data reduction

The observations of the 12CO $J$=3–2 emission line, mapping the entire N11 region (see the region enclosed by dashed lines in Fig. 1), were performed during November and December 2014 with the 10 m Atacama Submillimeter Telescope Experiment (ASTE). The CATS345 GHz band receiver used is a two single-band SIS receiver remotely tunable in the LO frequency range of 324-372 GHz. The spectrometer MAC was used with a bandwidth of 128 MHz and a natural spectral resolution of 0.125 MHz. The spectral velocity resolution was 0.11 km s*-1* and the half-power beamwidth (HPBW) was 22″at 345 GHz. We mapped the N11 region in the on-the-fly mapping mode covering the region with two different maps in both X and Y scan directions in order to remove any residual on the maps due to the scan pattern. The system temperatures values changed from 300 to 700 K during the observations. Pointing was performed toward R Dor, every 1 or 1.5 hours, depending on the weather conditions. We also observe Orion KL to check the absolute value of the calibrations, which agrees within 10%.

During August 2015, three regions toward N11 (boxes in Fig. 1, and see Table 1) were mapped in the 13CO J=3–2 line using the same telescope with the CATS345 receiver and the MAC spectrometer with the same configuration as explained above. The spectral velocity resolution was 0.11 km s*-1* and the HPBW was 23″at 330 GHz. The observations were done also in the on-the-fly mapping mode. The system temperature was between 200 and 400 K. In both cases the main beam efficiency was about $\eta_{mb}=$ 0.6.

The observations were reduced with NOSTAR (Sawada et al., 2008) and some spectra processed using the XSpec software package111XSpec is a spectral line reduction package for astronomy which has been developed by Per Bergman at Onsala Space Observatory.. The spectra were Hanning-smoothed to improve the signal-to-noise ratio and low-degree polynomials were used for baseline fitting. The typical rms noise level is 200 and 40 mK for the 12CO and 13CO, respectively.

4 Results

Figure 2 presents in contours the 12CO J=3–2 emission integrated between 270 and 290 km s*-1* toward the entire N11 region (the surveyed region is enclosed with the dashed lines).

Concerning the sub-regions mapped in the 13CO (boxes in Fig. 1), named N11B, N11D, and N11I, Fig. 3 (left) presents the integrated 13CO J=3–2 line between 270 and 290 km s*-1*. For comparison we also show in Fig. 3 (right) the same sub-regions as seen in the integrated 12CO J=3–2 line emission.

In order to study the morphology and velocity distribution of the molecular gas mapped in each isotope toward N11B, N11D and N11I, we present in Figs. 4, 5,and 6 the 12CO and 13CO J=3–2 emission displayed in channel maps integrated in bins of 2 km s*-1*. The velocity ranges displayed in these figures are the same as those presented in the 12CO J=2–1 channel map in Herrera et al. (2013). We based our work on the 13CO emission, the optically thinner tracer, to identify molecular clumps. Thus, in N11B we identified two clumps, 1B and 2B (they are marked in the channel maps at 280.8 and 282.9 km s*-1* in Fig. 4), in N11D we identified clumps 1D and 2D (marked in the channel maps at 276.6 km s*-1* in Fig. 5), and finally in N11I, clumps 1I and 2I (marked in the channel maps at 278.6 and 282.9 km s*-1* in Fig. 6). In the last case, another molecular clump can be identified toward the east in the channel map at 274.4 km s*-1*, but as the clump is not completely observed we do not catalogued in order to calculate its physical parameters.

Taking into account that we have for the first time data of the 13CO J=3–2 line toward N11, we use it to estimate the mass of the molecular clumps identified in Figs. 4,  5, and 6 by assuming local thermodynamic equilibrium (LTE). The excitation temperature was obtained from:

[TABLE]

The 12CO and 13CO optical depths, $\tau_{12}$ and $\tau_{13}$, were obtained from:

[TABLE]

where 12T${}_{\textrm{mb}}$ and 13T${}_{\textrm{mb}}$ are the peak temperatures of the 12CO and 13CO J=3–2 lines and $X=50$ is the assumed isotope abundance ratio (Wang et al., 2009). As shown in Table 2, the 13CO J=3–2 line is optically thin, thus we estimate its column density from:

[TABLE]

with

[TABLE]

We assume an abundance ratio of [H2/13CO] $=1.8\times 10^{6}$ (Garay et al., 2002) to obtain the molecular hydrogen column density N(H2). Finally, the mass was derived from

[TABLE]

where $\mu$ is the mean molecular weight, assumed to be 2.8 by taking into account a relative helium abundance of 25$\%$, $m_{H}$ is the hydrogen mass, $D$ is the distance (50 kpc) and $\Omega$ is the solid angle subtended by the beam size. We summed over all beam positions of the 13CO molecular structures. The central velocity, and the radii are presented in the first two lines in Table 2. In the case of clumps 1B and 2D, as they have an elliptical shape, we provide the effective radius obtained from R$=(R_{a}\times R_{b})^{1/2}$, where $R_{a}$ and $R_{b}$ are the semi-axes of the elliptical molecular feature. The other files of the Table present all the parameters obtained under the LTE assumption.

We estimate the virial mass of each clump from the 13CO J=3–2 emission from the following equation:

[TABLE]

where k=190 by assuming clouds with density distributions \propto$$r^{-1} (MacLaren et al., 1988), $\Delta$v13 the line velocity width (FWHM) of the 13CO J=3–2 emission obtained at the peak position of each molecular structure, and R is the radius presented in Table 2. The $\Delta$v13 and the obtained virial mass are presented in Table 3.

4.1 Line ratio maps

In order to study evidences of molecular photodissociation we calculate the isotopic integrated line ratios $\int{\rm T^{{}^{12}CO}dv}/\int{\rm T^{{}^{13}CO}dv}$ (12CO/13CO J=3–2). A similar analysis was done for the $\int{\rm T^{3-2}dv}/\int{\rm T^{2-1}dv}$ (12CO 3–2/2–1) and $\int{\rm T^{3-2}dv}/\int{\rm T^{1-0}dv}$ (12CO 3–2/1–0) ratios in order to study possible evidences of shocks in the gas, how the gas is affected by the radiation, and to better constrain the relation between the J=3–2 and J=1–0 lines in order to evaluate CO contribution to the continuum at 870 $\mu$m.

We construct maps of 12CO/13CO J=3–2 for each region, which are presented in Fig. 7 in the context of the N11 complex. To construct the maps of the 12CO/13CO ratio it was considered pixels above the 3$\sigma$ noise level in both, the 13CO and 12CO integrated maps.

Using the 12CO J=2–1 and J=1–0 from Herrera et al. (2013) together with our observations of 12CO J=3–2, we construct maps of the integrated line ratios 12CO 3–2/2–1 and 12CO 3–2/1–0 (see Figs. 8 and 9). To compare these lines, we need to have the data at the same angular resolution. Thus, we convolve the 12CO J=3–2 data to the 23″ and 45″ resolutions of the J=2–1 and J=1–0 data, respectively.

Table 4 lists the obtained average values for these ratios toward the peak position of each molecular clump.

4.2 Non-LTE analysis

Using the 12CO J=1–0, 2–1, and 3–2 and 13CO J=3–2 lines we perform a non-LTE analysis to estimate the physical conditions toward the peaks of the most intense molecular features (i.e. clouds 2B, 1D, and 1I). To estimate the column density and the H2 volume density, we use the RADEX code333RADEX is a statistical equilibrium radiative transfer code, available as part of the Leiden Atomic and Molecular Database (http://www.strw.leidenuniv.nl/moldata/). (Van der Tak et al., 2007).

The inputs of RADEX are: kinetic temperature (TK), line velocity width at FWHM ($\Delta$v), and line peak temperature (Tpeak), and the code yields column and volume molecular densities (N and n${}_{\rm H_{2}}$). Assuming that the dust and gas are coupled, we use T${}_{\rm K}=20$ K based on the dust temperature obtained in previous works (Galametz et al., 2016; Herrera et al., 2013). In the case of the 12CO J=2–1 and 3–2 and 13CO J=3–2 lines, the data were convolved with the 45″ beam of the J=1–0 line. The Tpeak and $\Delta$v were obtained from Gaussian fits to the spectra (see Fig. 10), whose results are presented in Table 5. Results from the 13CO J=3–2 line are included for comparison.

As done in a previous work toward N159 (Paron et al., 2015), we assume that the lower 12CO transitions (J=1–0 and 2–1) arise mainly from the cold gas component, while it is likely that the J=3–2 transition arises from both, a cold and a warmer one. Then, we run the RADEX code for two cases: 100 and 50 per cent of the 12CO J=3–2 emission assigned to the cold component at 20 K. The results from RADEX are presented in Figure 11. Table 6 presents the results obtained by considering the 50 per cent of the 12CO J=3–2 emission, which as Fig. 11 shows (see the gray shaded area in each panel) are the more tight results between the analyzed possibilities. Taking into account the errors in the peak temperatures, the uncertainties in the results from RADEX are about 20% and 30% in the limits of the ranges of N(CO) and n${}_{\rm H_{2}}$, respectively.

5 Discussion

The 12CO and 13CO J=3–2 line were mapped for the first time toward N11. These data complement previous studies about the molecular gas and dust properties in the region (e.g. Israel et al. 2003; Herrera et al. 2013; Galametz et al. 2016). The 12CO J=3–2 emission shows that the molecular gas is distributed in a fragmented shell around N11 (see Fig. 2). As Deharveng et al. (2005) point out, the presence of either a dense molecular shell surrounding the ionized gas of an Hii region or massive fragments regularly spaced along the ionization front, suggests triggered star formation through the collect and collapse mechanism. This scenario agrees with sequential star formation among the OB associations proposed in Parker et al. (1992) and Walborn & Parker (1992), and suggests that it is likely that the triggered star formation processes observed in our Galaxy (e.g. Deharveng et al. 2009; Zavagno et al. 2010; Duronea et al. 2017) also occur in the LMC.

The morphology of the molecular features, as seen in the 12CO and 13CO J=3–2 lines (see Fig. 3 and also the channel maps in Figs. 4, 5,and 6), shows signatures that allow us to discuss some issues about the effects of the Hii regions/OB associations on the molecular gas. For instance, in the case of N11D, which is a region southwards the LH13 OB association, the observed molecular cloud has a filament-like structure with a maximum toward the north. This feature presents a slight curvature with the concavity pointing to the center of N11 nebula. Even though this cloud can be affected by the action of LH13 OB association, taking into account its morphology and that it is located at a border of the N11 complex, the ionizing bubble around LH9 (the N11 central OB cluster) should be important in shaping it.

In general, the analyzed molecular features in the regions that have been observed in 13CO show a clumpy morphology with at least two well defined molecular clumps in each one. In the case of N11D we identified clumps 1D and 2D belonging to the molecular structure described above. In N11B, there are two molecular structures: a more intense one with a circular shape (clump 2B), and a weaker one with a roughly elliptical shape (clump 1B). These molecular features positionally coincide with the nebula generated by LH10 and clump 2B is located slightly to the north of a maximum of [OIII] emission generated by the LH10 OB association, where also a methanol maser lies (Barbá et al., 2003). The molecular feature in N11I presents an even more clumpy morphology, with clump 1I being the more intense one in the region. The presence of these clumps in the 13CO J=3–2 line can be interpreted as an evidence of the fragmentation generated in a molecular shell due to the expansion of the N11 nebula as mentioned above. However, a general collapse of giant molecular clouds giving place to these molecular fragments can not be discarded.

From the mass characterization of these clumps we find that the ratio between the virial and LTE mass (Mvir/MLTE) is higher than unity in all cases, which suggests that they are not gravitationally bounded, and it is likely that we are mapping mainly turbulent molecular gas. This is consistent with the presence of OB clusters and Hii regions, known sources of kinetic energy and turbulence in molecular clouds (e.g. Garay et al. 2002). Herrera et al. (2013), using the 12CO J=2–1, obtained similar relations between Mvir and MLTE for most of their catalogued clumps in N11. Our results, obtained with the 13CO, an optically thinner tracer, show that even deeper parts of the molecular clumps are not gravitationally bounded and may be supported by external pressure, as found in molecular clouds around Galactic Hii regions (Rathborne et al., 2002; Massi et al., 1997), and in the LMC, in N113 (Paron et al., 2014) and Cloud B of Complex No.37 (Garay et al., 2002).

Clump 1I exhibits the largest virial mass among the studied clumps ($\sim 5.7\times 10^{4}$ M⊙) which is almost three times larger than its LTE mass. This suggests, as mentioned above, that the molecular clump would not be gravitationally supported. Taking a look to the corresponding 12CO J=1-0, J=2-1, and J=3-2 spectra (see Fig. 10), a slight red-skewed asymmetry in the profiles is observed. Discarding the presence of a second velocity component, and given that it is very likely that this clump lacks stellar activity in its interior, the observed red-skewed asymmetry in the spectra could be ascribed to the global expansion of N11 originated by the feedback of the OB association LH9. Taking into account that the VLSR associated with the 12CO emission is about 280 kms*-1* and the average systemic velocity of the association LH9 is about 295 km s*-1* (Evans et al., 2007), the blue wings exhibited in the spectra could be reflecting the presence of gas pushed toward us by the expansion of the nebula.

5.1 Integrated line ratios

The average values (between 7 and 8) in the 12CO/13CO integrated line ratio found toward the peaks of clumps 2B, 1D, 2D, and 1I are similar to the values found toward N159 and N113 (Paron et al., 2014, 2015), while the values at the peaks of clumps 2I and 1B (larger than 8) are quite similar to the values found toward N132, N166-A, and N166-B (Paron et al., 2015), regions not as active as N133 and N159. In general, the 12CO/13CO maps (see Fig. 7) show that the molecular peaks present a quite uniform ratio, with larger values at the borders. By assuming that the integrated line ratios resemble the 12CO/13CO abundance ratio, the selective photodissociation of the 13CO at the external layers of the clumps can explain what is observed (van Dishoeck & Black, 1988; Visser et al., 2009). This phenomenon has been observed in Galactic molecular clouds exposed to UV radiation (Langer & Penzias, 1993; Visser et al., 2009). In the case of N11D, a clear gradient is found extending from right to left, with lower values at the border exposed to the N11 central OB cluster (LH9) and larger values at the opposite one. This cannot be explained just by selective photodissociation generated by a FUV flux from right to left, suggesting either the influence of an external source of flux impinging from left to right in N11D such as a strong interstellar radiation field, or most likely, competing mechanisms among selective photodissociation, chemical fractionation and non-isotope-selective reactions (Szűcs et al., 2014; Federman et al., 2003).

The average value of the 12CO J=3–2/1–0 ratio obtained from the six analyzed molecular clumps is about 0.65, which is in agreement with the assumed value in Galametz et al. (2016) to estimate the contribution of the 12CO J=3–2 line to the 870 $\mu$m flux observed with LABOCA. The bandwidth of LABOCA camera is wide and includes the 12CO line, thus, it is contaminated and the contribution of the 12CO emission to the continuum flux has to be removed. Given that observations of 12CO J=3–2 were lacked, Galametz et al. (2016) used the observed 12CO J=1–0 line to estimate the emission of the J=3–2 line. Our results confirm the estimation performed by the authors. However, considering the variations in the 12CO J=3–2/1–0 ratio among the regions (see Fig. 8), the use of the 12CO J=3–2 emission presented here could improve the estimation of the actual contribution of the dust emission at 870 $\mu$m. In Sect. 5.2 we discuss this contribution from a comparison with the LABOCA data.

Additionally, from Fig. 8, we note that N11B presents 12CO J=3–2/1–0 ratios higher than unity across most of the mapped area, which is in agreement with Minamidani et al. (2008), who point out that this ratio is enhanced to 1.0 – 1.5 toward Hii regions or clouds with young clusters, as it is the case for N11B. On the other hand, the authors found ratios less than 1 toward clumps with neither Hii regions nor clusters, as it is the case for N11I. N11D also presents 12CO J=3–2/1–0 ratios less than 1, suggesting that N11B and N11D may be affected in different ways by the radiation.

The values of 12CO J=3–2/2–1 ratio are quite constant among the peaks of the analyzed molecular clumps, with an average of 0.75. This value is similar to what is found toward regions along molecular loops at the Central Molecular Zone in our Galaxy (Kudo et al., 2011); in particular toward some protrusions in these features, that have larger values than in the surrounding gas, but lower than in strong shocked regions (with values as high as 2.5). Indeed, our values in the 12CO J=3–2/2–1 ratios are close to the value (0.8) found in typical galactic disk clouds (Enokiya et al., 2018). The maps presented in Fig. 9 show that the values in the 12CO J=3–2/2–1 ratio increase toward the south of N11B and the north of N11D, in coincidence with the presence of the local OB associations in these regions.

5.2 12CO J=3–2 line contribution to the 870 $\mu$m continuum

Following the above discussion, we calculate the 12CO J=3–2 line contribution to the continuum emission at 870 $\mu$m by comparing with LABOCA data (kindly provided by Galametz M.). The continuum data were convolved to the angular resolution of the 12CO data. The integrated 12CO J=3–2 line (in K km s*-1*) was converted to pseudo-continuum flux (mJy beam*-1*) using the conversion factor $C$ (see Drabek et al. 2012). The maps presented in Fig. 12 show the percentage of the line contribution to the 870 $\mu$m continuum toward the analyzed regions. The map of N11B shows similar values as presented in Galametz et al. (2016). On the other hand, in the case of N11D, our map shows lower percentages in comparison with the range presented by the mentioned authors.

It is worth noting that region N11I presents the larger line contribution to the 870 $\mu$m continuum among the three studied regions. This could be due to the fact that this region is less irradiated by UV photons and hence the molecular gas, in comparison with the dust, should be more abundant than in regions more intensely irradiated.

5.3 Non-LTE considerations

Concerning to the non-LTE analysis, a T${}_{\rm K}=20$ K was considered based on the assumption that the gas is coupled to the dust, and hence they should have same temperatures. We find that the best convergence in the models (i.e. obtaining ranges of results more tight in a N(CO)–n${}_{\rm H_{2}}$ diagram) occurs when we consider that the 12CO J=3–2 emission likely arises from both, a cold and a warmer gas component. Thus, we roughly consider that 50% of its emission corresponds to gas with the assumed T${}_{\rm K}=20$ K while the other 50% to gas at higher temperatures.

RADEX yields n${}_{\rm H_{2}}$ ranges of 1.5 – 8.5, 1.1 – 6.9, 1.0 – 2.4 $\times$ 103 cm*-3* for clumps 2B, 1D, and 1I, respectively. The range with largest values in n${}_{\rm H_{2}}$ is found toward 2B, which is the unique region with direct evidence of ongoing star formation, while a more narrow range with lower values is found toward 1I, the region without any activity within it. The average value of n${}_{\rm H_{2}}$ obtained in clump 2B, and the largest values in clump 1D are in quite agreement with the density obtained toward the star-forming region N113 by Wang et al. (2009) from a LVG analysis.

6 Summary

The N11 nebula, one of the most important star-forming region in the Large Magellanic Cloud, was mapped for the first time in the 12CO and 13CO J=3–2 line using the ASTE Telescope. The 12CO J=3–2 line, mapped in the whole region, shows that the molecular gas is distributed in a fragmented shell around N11. Three sub-regions (N11B, N11D, and N11I) that may be affected by the radiation in different ways, were also mapped in the 13CO J=3–2 line. N11B and N11D are related to the OB associations LH10 and LH13, respectively, and N11I is a farther area at the southwest without any embedded OB association. The main results are summarized as follows:

(1) We found that the molecular features lying in each analyzed sub-region are in general clumpy (at least two well defined clumps were found in each one) and show some signatures that could be explained by the expansion of the nebula and the action of the radiation. In N11D, which is a region southwards the LH13 OB association, the molecular cloud has a curved filament-like morphology with the concavity pointing to the center of N11 nebula, showing that the ionizing bubble around LH9 (the N11 central OB cluster) seems to shape the N11D molecular cloud, influencing its star-formation and chemistry as suggested by the isotopic ratio. The molecular cloud in N11B has a peak slightly to the north of a maximum of [OIII] emission generated by the LH10 OB association. The molecular feature in N11I, which is even more clumpy and it is not related to any embedded OB association, reinforces the hypothesis of the fragmentation generated in a large molecular shell due to the expansion of the N11 nebula.

(2) The fragmented molecular clouds observed along the N11 bubble are in line with the sequential star formation scenario proposed by previous works but also observed in our Galaxy.

(3) The ratio between the virial and LTE mass (Mvir/MLTE) is higher than unity in all analyzed molecular clumps, which suggests that they are not gravitationally bounded and may be supported by external pressure.

(4) A non-LTE analysis suggests that at clumps 2B, 1D, and 1I, we are mainly observing cold gas (TK about 20 K if we assume that the gas is coupled to the dust) with n${}_{\rm H_{2}}$ about a few 103 cm*-3*. The analysis yields that clump 2B is the densest one among the three analyzed clumps. It is probable that the 12CO J=3–2 line arises from both, a cold component and a warmer one.

(5) The maps of the integrated line 12CO/13CO ratios show quite uniform values at the peaks of the clumps, with larger values at the borders, while a gradient is found extending from the border exposed to the N11 central cluster to the opposite one in N11D. Selective photodissociation of the 13CO can explain what is observed in N11B and N11I, while chemical fractionation probably should be taken into account in N11D.

(6) We obtained an average value of 0.65 in the 12CO J=3–2/1–0 ratio among the analyzed molecular clumps, confirming the assumption done by Galametz et al. (2016) to estimate the contribution of the 12CO J=3–2 line to the 870 $\mu$m flux. However, the variations among the clumps shows that the direct observation of this line could improve the estimation of the actual contribution of the dust emission at 870 $\mu$m. Maps of the 12CO J=3–2 line contribution to the continuum emission at 870 $\mu$m were done. It was found that the N11I presents the larger line contribution to the 870 $\mu$m continuum among the three studied regions.

(7) It was found that N11B and N11D present 12CO J=3–2/1–0 ratios higher and lesser than the unity, respectively. This is an evidence that the molecular gas in both regions is affected in differnt ways by the radiation. Values in the 12CO J=3–2/2–1 ratios across the whole region were found to be similar to the value (about 0.8) measured in typical galactic disk clouds. The high 12CO J=3–2/2–1 ratios in the northern part of N11D shows that there is stellar feedback from the LH13 cluster. The same is observed in N11B around the LH10 cluster

Acknowledgements.

We thank the anonymous referee for her/his very helpful comments and corrections. We wish to thank to M. Galametz for kindly provide us with the LABOCA data. The ASTE project is led by Nobeyama Radio Observatory (NRO), a branch of National Astronomical Observatory of Japan (NAOJ), in collaboration with University of Chile, and Japanese institutes including University of Tokyo, Nagoya University, Osaka Prefecture University, Ibaraki University, Hokkaido University, and the Joetsu University of Education. M.C.P. is a doctoral fellow of CONICET, Argentina. S.P. and M.E.O. are members of the Carrera del investigador científico of CONICET, Argentina. This work was partially supported by grants awarded by ANPCYT and UBA (UBACyT) from Argentina. M.R. wishes to acknowledge support from Universidad de Chile VID grant ENL22/18 and partial support from CONICYT project Basal AFB-170002. M.R. and M.C.P. acknowledge support from CONICYT (CHILE) through FONDECYT gran No1140839.