Millimeter and far-IR study of the IRCD SDC 341.232-0.268

TL;DR

This study investigates the molecular gas and dust in the IRDC SDC 341.232-0.268 using millimeter and far-IR data to understand its physical properties, evolutionary stage, and gas dynamics such as infall and outflow motions.

Contribution

It combines multi-wavelength observations and molecular line analysis to characterize the physical conditions and gas motions in an infrared dark cloud for the first time.

Findings

Identified two dense clumps with >10^4 cm^-3 density.

Detected infall and outflow motions in the molecular gas.

Found cold dust associated with the molecular clumps and EGOs.

Abstract

We analyze the molecular gas and dust associated with the infrared dark cloud SDC 341.232-0.268 in order to investigate the characteristics and parameters of the gas, determine the evolutionary status of four embedded EGO candidates and establish possible infall or outflow gas motions. We based our study on $^{12}$CO(2-1), $^{13}$CO(2-1), and C$^{18}$O(2-1) data obtained with the APEX telescope, molecular data of high density tracers from the MALT90 survey and IR images from Spitzer, Herschel and ATLASGAL. The study reveals two clumps at -44 km/s towards the IRDC, with densities of > 10$^4$ cm$^{-3}$ , typical of IRDCs, while high density tracers show H$_2$ densities > 10$^5$ . FIR images reveals the presence of cold dust linked to the molecular clumps and EGOs. A comparison of spectra of the optically thin and optically thick molecular lines towards the EGOs suggests the existence of infall and outflow motions.

Tables

Table 1. Table 1: Observational parameters of the molecular transitions.

Transition	${\nu }_0$	$\theta$	$\Delta v_{res}$	rms
	GHz	\arcsec	km s-1	K
12CO(2-1)	230.538	30	0.1	0.35
13CO(2-1)	220.398	28.5	0.1	0.35
C18O(2-1)	219.560	28.3	0.1	0.35
HNC(1-0)	90.664	38.0	0.11	0.35
HCO+(1-0)	89.189	38.0	0.11	0.35
N2H+(1–0)	93.173	38.0	0.11	0.35

Table 2. Table 2: Parameters of the gaussian fits for C ​ l ​ u ​ m ​ p ​ A 𝐶 𝑙 𝑢 𝑚 𝑝 𝐴 Clump\,A  and C ​ l ​ u ​ m ​ p ​ B 𝐶 𝑙 𝑢 𝑚 𝑝 𝐵 Clump\,B .

	Line	$v$	$\Delta$v	Tpeak	Area
		km s-1	km s-1	K	K km s-1
$ClumpA$	13CO(2-1)	–44.56(0.03)	5.56(0.07)	6.82	40.33(0.41)
	C18O(2-1)	–44.23(0.02)	4.46(0.05)	1.37	6.49(0.06)
$ClumpB$	13CO(2-1)	–43.85(0.01)	2.56(0.08)	9.52	25.96(0.18)
	C18O(2-1)	–43.70(0.01)	2.61(0.03)	2.46	6.84(0.07)

Equations

$$\frac{T_{\mathrm{mb}}^{13}}{T_{\mathrm{mb}}^{18}}=\frac{1-e^{-{\tau }^{13}}}{1-e^{-{\tau }^{13}/7.6}}$$

$${\tau }_{18}=\frac{1}{7.6}{\tau }_{13}\left(\frac{\Delta {\mathrm{v}}_{13}}{\Delta {\mathrm{v}}_{18}}\right){\left(\frac{{\nu }^{13}}{{\nu }^{18}}\right)}^2$$

$$T_{\mathrm{exc}}=\frac{T^{\ast }}{ln\left[{\left(\frac{T_{\mathrm{mb}}^{13}}{T^{\ast }}+\frac{1}{\exp (T^{\ast }/T_{\mathrm{bg}})-1)}\right)}^{-1}+1\right]},$$

$$N{(}^{13}\mathrm{CO})=2.4\times 10^{14}\exp \left(\frac{T^{\ast }}{T_{\mathrm{exc}}}\right)\times \frac{T_{exc}}{1-e^{-T^{\ast }/T_{\mathrm{exc}}}}\int {\tau }_{13}d\mathrm{v}$$

$$T_{exc}\int {\tau }_{13}d\mathrm{v}\simeq \times \frac{{\tau }^{13}}{1-exp(-{\tau }^{13})}\int T_{\mathrm{mb}}^{13}d\mathrm{v}$$

$$M(H_2)=(m_{sun}{)}^{-1}\mu m_{\mathrm{H}}AN(H_2)d^2$$

$$n_{{\mathrm{H}}_2}=\frac{M(H_2)}{\frac{4}{3}\pi r^3\mu m_H}$$

$$\frac{M_{\mathrm{vir}}}{M_{\odot }}=k_2\left[\frac{r}{\mathrm{pc}}\right]\left[\frac{\Delta {\mathrm{v}}^2}{\mathrm{km}{\mathrm{s}}^{-1}}\right]$$

$$\frac{\int T_{\mathrm{mb},1}d\mathrm{v}}{\int T_{\mathrm{mb},2}d\mathrm{v}}=\frac{1-exp(-{\tau }_1)}{1-exp(-{\tau }_2)}=\frac{1-exp(-{\tau }_1)}{1-exp(-a{\tau }_1)}$$

$$T_{\mathrm{exc}}=4.47/ln\left[1+{\left(\frac{T_{\mathrm{mb}}}{4.47(1-exp(-\tau ))}+0.236\right)}^{-1}\right]$$

$$N({\mathrm{N}}_2{\mathrm{H}}^{+})=\frac{3kW}{8{\pi }^3\nu S{\mu }^2}\left(\frac{T_{\mathrm{exc}}}{T_{\mathrm{exc}}-T_{\mathrm{bg}}}\right)\left(\frac{\tau }{1-exp(-\tau )}\right)$$

$$\times Q(T_{\mathrm{exc}})exp(E_u/k{T}_{\mathrm{exc}})$$

$$M_{dust+gas}=\frac{{\tau }_{{\nu }_{fit}}}{{\kappa }_{870}}{\left(\frac{345GHz}{{\nu }_{fit}}\right)}^{{\beta }_d}{d}^2{R}_{\mathrm{d}}$$

$${\dot{M}}_{\inf }=\frac{4}{3}\pi n_{H2}\mu m_H{r}_{\mathrm{clump}}^2{\mathrm{v}}_{\inf }$$

Full text

Analizamos el gas molecular y el polvo asociado a la nube oscura infrarroja SDC 341.232-0.268 con el fin de investigar las características y parámetros físicos del gas, determinar el estado evolutivo de los cuatro EGOs embebidos y establecer posibles movimientos de acreción o flujo molecular. Nos basamos en datos de 12CO(2-1), 13CO(2-1) y C18O(2-1) obtenidos con el telescopio APEX, trazadores de alta densidad extraídos de MALT90, e imágenes infrarrojas de Spitzer, Herschel y ATLASGAL. El estudio revela dos grumos moleculares a $-$44 km s*-1* coincidentes con la IRDC con una densidad $>$ 104cm*-3*, típica de IRDCs. Los trazadores de alta densidad arrojan densidades de H2 $>$ 105. Las imágenes en el lejano IR muestran polvo frío asociado a los grumos moleculares y a los EGOs. La comparación de espectros moleculares ópticamente gruesos y finos sugiere la existencia de acreción y flujos moleculares.

\addkeyword

ISM: kinematics and dynamics \addkeywordISM: molecules \addkeywordStars: formation \addkeyword(ISM:) dust, extinction \addkeywordISM: clouds \addkeywordISM: individual objects: IRDC SDC 341.232-0.268

0.1 Introduction

Infrared Dark Clouds (IRDCs) are sites of recently star formation within molecular clouds. These are detected as dark silhouettes against a bright background in the mid-infrared. These regions, which are detected in the FIR, are believed to be places that may contain compact cores which probably host the early stages of high-mass star formation (e.g., Rathborne et al. 2007; Chambers et al. 2009). In fact, their cold (T $<$ 25 K) and dense (102-104 cm*-3*) ambient conditions, and their sizes of $\sim$1-3 pc and masses of 102-104 M⊙ (Rathborne et al., 2006, 2007) favor star formation mechanisms. Evidences of active high-mass star formation in IRDCs are inferred from the presence of ultracompact Hii regions (Battersby et al., 2010), hot cosres (Rathborne et al., 2008), embedded 24 $\mu$m sources (Chambers et al., 2009), maser emission (Wang, 2006; Chambers et al., 2009), and/or outflow and infall processes (e.g. Sanhueza et al. 2010; Chen et al. 2011; Ren et al. 2011; Beuther et al. 2002; Zhang et al. 2007).

Many IRDCs are also associated with extended enhanced 4.5 $\mu$m emission (named Extended Green Objects, EGOs, for the common coding of the [4.5] band in green in the 3-color composite IRAC images), the so-called “green fuzzies”, indicating the presence of shocks. Following photometric criteria, Cyganowski et al. (2008) suggested that most of EGOs fall in the region of the color-color space occupied by the youngest MYSOs, surrounded by accreting envelopes (see Figure 13 in their work). This hypothesis is supported by Chen et al. (2010), whose observations are consistent with the speculation that EGOs trace a population with ongoing outflow activity and active rapid accretion stage of massive protostellar evolution. The emission at 4.5 $\mu$m includes both H2 (v = 0-0, S(9, 10, 11)) lines and CO (v = 1-0) band heads (Cyganowski et al., 2008), which are excited by shocks, such as those expected when protostellar outflows impinge on the ambient interstellar medium (ISM) (Cyganowski et al., 2008).

Using IRAC images, Peretto & Fuller (2009) catalogued and characterized many IRDCs using 8 and 24 $\mu$m Spitzer data. In this catalogue, the IRDCs were defined as connected structures with H2 column density peaks $N_{\rm H_{2}}>$ 2$\times$1022 cm*-2* and boundaries defined by $N_{\rm H_{2}}$ = 1$\times$1022 cm*-2*. Single-peaked structures found in the H2 column density maps were identified by the authors as fragments. These dark and dense patches are associated with molecular gas and dust emission (Sanhueza et al., 2012).

As a part of a project aimed at studying in detail the physical properties of the IR bubble complex S21-S24, we present here an analysis of the molecular gas and dust associated with SDC 341.232-0.268, a poorly studied IRDC of the southern hemisphere, which is member of this complex. Our aims are to determine the physical parameters and kinematics of the gas and to chracterize the interstellar dust associated with the IRDC, to investigate the presence of embedded young stellar objects and to establish possible infall or outflow gas motions. To accomplish this project, we analyze molecular data of the 12CO(2-1), 13CO(2-1), and C18O(2-1) lines obtained with the APEX telescope, and HCO*+(1-0), HNC(1-0), and N2H+*(1-0) lines from the MALT90 survey (Jackson et al., 2013; Foster et al., 2013, 2011)111http://malt90.bu.edu/data.html, mid- and far-IR continuum data from Spitzer-IRAC, Herschel-PACS and -SPIRE, and ATLASGAL (APEX) images (Schuller et al., 2009). We introduce the IRDC SDC 341.232-0.268 in § 0.2, the used database in § 0.3, the results of the molecular gas in § 0.4, and those of the infrared emission in § 0.5, signs of star formation are discussed in § 0.6, and a summary in § 0.7.

0.2 IRDC SDC 341.232-0.268

The IRDC SDC 341.232-0.268 (RA, Dec.(J2000)= 16:52:35.67, –44:28:21.8) has a mid-IR radiation field intensity $I_{\rm MIR}\sim$ 88.3 MJy ster*-1* at 8.0 $\mu$m and a peak opacity of 1.09. From the Multiband Infrared Photometer for Spitzer (MIPS) images Peretto & Fuller (2009) detected a 24 $\mu$m point source in the field of the IRDC and two clumps. The clumps were classified by Bergin & Tafalla (2007) as structures with sizes $\sim$10*-1*-100 pc, masses $\sim$10-103 M*⊙, and volume densities $\sim$103-104 cm-3*.

Figure 1 shows the Spitzer-IRAC images at 4.5 (green) and 8.0 $\mu$m (red), and the Spitzer-MIPS image at 24 $\mu$m (blue) in the region of SDC 341.232-0.268. Green contours correspond to low emission levels at 8 $\mu$m, revealing a region of high extinction at 8.0 $\mu$m at the location of the IRDC, in contrast with the environment, a typical feature of these objects. The 90 MJy/ster contour delineates the IRDC. Four candidate EGOs catalogued by Cyganowski et al. (2008) appear projected toward this region: the “likely” MYSO outflow candidates G 341.22-0.26(b)(R.A., Decl.(J2000) = 16:52:30.3, –44:28:40.0), G 341.22-0.26(a) (16:52:32.2, –44:28:38.0), the “possible” MYSO outflow candidate G 341.23-0.27 (16:52:34.2, –44:28:36.0), and the “likely” MYSO outflow candidate G 341.24-0.27 (16:52:37.3, –44:28:09.0). These authors categorized the EGOs as a “likely” or “possible” MYSO outflow candidate based primarily on the angular extent of the extended excess 4.5 $\mu$m emission. Any source in which it was possible that multiple nearby point sources and/or image artifacts from a bright IRAC source could be confused with truly extended 4.5 $\mu$m emission was considered a “possible” candidate, which are likely still good YSO candidates, but not necessarily MYSOs with outflows and so likely to be actively accreting.

From here on, they will be named EGO 1, EGO 2, EGO 3, and EGO 4, in increasing order of R.A. EGO 1 is associated with 8 and 24 $\mu$m emission and is located in the border of the IRDC, while EGOs 2, 3, and 4 are detected at 4.5 and 8 $\mu$m. EGO 1 and EGO 4 coincide with ATLASGAL Compact Sources AGAL 341.219-00.259 and AGAL 341.236-00.271. Their integrated flux densities are 12.85$\pm$2.20 Jy and 16.81$\pm$2.80 Jy, and their effective radii are $\sim$ 37\arcsec and 47\arcsec, respectively (Contreras et al., 2013).

Methanol maser emission was detected towards EGO 2 and EGO 4 at 6 and 95 GHz, within the velocity range $-$43 to $-$52 km s*-1*(Caswell et al., 2010; Chen et al., 2011; Hou & Han, 2014; Yang et al., 2017). Methanol masers provide signpost to the very earliest stages of the massive star formation process, prior the onset of the UCHII region phase. They are associated with embedded sources whose bolometric luminosities suggest the will soon become OB stars (Burton et al., 2002; Sobolev et al., 2005). The masers are independent tracers and they give proven signatures of ongoing star formation as the methanol masers (see Ellingsen 2006; Breen et al. 2013). In other hand, masers of water or hydroxyl have also been detected in in star-forming regions, as well as in evolved stars or supernova remnants. Therefore, the presence of these maser types in the vicinity of these EGOs could be an excellent signspot of ongoing star formation.

0.3 Database

0.3.1 Molecular line observations

The 12CO(2-1), 13CO(2-1), and C18O(2-1) data were acquired with the APEX-1 receiver of the Swedish Heterodyne Facility Instrument (SHeFI; Vassilev et al. 2008) in the Atacama Pathfinder EXperiment (APEX) telescope, located in the Puna de Atacama (Chile). The backend for the observations was the eXtended bandwidth Fast Fourier Transform Spectrometer2 (XFFTS2) with a 2.5 GHz bandwidth divided into 32768 channels. The main parameters of the molecular transitions (rest frequency $\nu_{0}$, half-power beam-width $\theta$, velocity resolution $\Delta{\rm v}_{\rm res}$, and rms noise of the individual spectra obtained using the OTF mode) are listed in Table 1. The selected off-source position free of molecular emission was RA, Dec.(J2000)= (16:36:40.56, $-$42:03:40.6).

Calibration was done using the chopper-wheel technique. The antenna temperature scale was converted to the main-beam brightness-temperature scale by $T_{\rm mb}$ = $T_{\rm A}$/$\eta_{\rm mb}$, where $\eta_{\rm mb}$ is the main beam efficiency. For the SHeFI/APEX-1 receiver we adopted $\eta_{\rm mb}$ = 0.75. Ambient conditions were good, with a precipitable water vapuor (PWV) between 1.5 $-$ 2.0 mm.

The molecular spectra were reduced using the CLASS90 software of the IRAM’s GILDAS software package.

In addition, we used molecular data from the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90) taken with the Mopra spectrometer (MOPS). We used beam efficiencies between 0.49 at 86 GHz and 0.42 at 230 GHz (Ladd et al., 2005). The data analysis was conducted with CLASS90 software. Emission was detected in the HCO+(1-0), HNC(1-0), and N2H+(1-0) lines, which were used to detect high density regions within SDC 341.232-0.268. Their main parameters are included in Table 1.

0.3.2 Images in the infrared

We used near- and mid-infrared (NIR, MIR) images from the Spitzer-IRAC archive at 4.5 and 8.0 $\mu$m of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 222http://irsa.ipac.caltech.edu/data/SPITZER/GLIMPSE/ (Benjamin et al., 2003), and the Multiband Imaging Photometer for Spitzer (MIPS) image at 24 $\mu$m from the MIPS Inner Galactic Plane Survey (MIPSGAL)333http://irsa.ipac.caltech.edu/data/SPITZER/MIPSGAL/ (Carey et al., 2005) to delineate the IRDC and investigate their correlation with the EGO candidates.

To trace the cold dust emission we utilized far-infrared (FIR) images from the Herschel Space Observatory belonging to the Infrared GALactic (Hi-GAL) plane survey key program (Molinari et al., 2010). The data were carried out in parallel mode with the instruments PACS (Poglitsch et al., 2010) at 70 and 160 $\mu$m, and SPIRE (Griffin et al., 2010) at 250, 350, and 500 $\mu$m. The angular resolutions for the five photometric bands spans from 8\arcsec to 35\farcs2 for 70 $\mu$m to 500 $\mu$m. Herschel Interactive Processing Environment (HIPE v12444HIPE is a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIRE consortia members, see http://herschel.esac.esa.int/HerschelPeople.shtml., Ott 2010) was used to reduce the data, with reduction scripts from standard processing. The data reduction and calibration (including zero-level and color correction) is described in details in section 2.2 of Cappa et al. (2016).

We also used images at 870 $\mu$m from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) (Schuller et al., 2009) with a beam size of 19\farcs2. This survey covers the inner Galactic plane, with an rms noise in the range 0.05–0.07 Jy beam*-1*. The calibration uncertainty in the final maps is about 15%.

0.4 Molecular characterization of the IRDC

0.4.1 CO data: morphological and kinematical description

To visualize the spatial distribution of the molecular emission the upper panel of Fig. 2 exhibits the 13CO(2-1) brightness temperature distribution from $\sim$ –47.0 to –42.5 km s*-1* in steps of 0.3 km s*-1*. The most intense 13CO(2-1) emission appears in the range $-$45.2 to $-$44 km s*-1*. The bottom central panel of Fig. 2 shows the average 13CO(2-1) emission in the range $-$47.0 to $-$42.8 km s*-1*, revealing two molecular clumps centered at RA, Dec.(J2000)= (16:52:37.06, $-$44:28:13.9) (Clump A) and RA, Dec.(J2000)= (16:52:29.72, $-$44:28:40.1) (Clump B). The coincidence of the clumps with EGO 1, 2, and 4 (indicated with crosses in this figure) is clear. The effective radii of the clumps are 37\farcs6 and 45\farcs6 for $Clump\,A$ and $Clump\,B$, respectively, as obtained taking into account the average 13CO(2-1) emission higher that 6.2 K. Finally, the bottom left and right panels of the same figure display the C18O(2-1) and 12CO(2-1) average emission in velocity intervals similar to that in the image of 13CO(2-1). Both clumps A and B are detected at C18O(2-1) revealing the existence of dense molecular gas in the clumps. The 12CO(2-1) emission delineates the IRDC but the clumps are not well defined.

Figure 3 shows the averaged 12CO(2-1), 13CO(2-1), and C18O(2-1) spectra within the area corresponding to $Clump\,A$ and $Clump\,B$. The rms noise of the averaged spectra is 0.019 K for $Clump\,A$, and 0.017 for $Clump\,B$. For $Clump\,A$, the 12CO(2-1) line exhibits a multi-peak structure with components in the interval from –70 to –20 km s*-1*, with the most intense features between –48 and –35 km s*-1*. Three velocity componentes are detected within the last velocity interval, peaking at –46.6 km s*-1*, –42.2 km s*-1*, and –36.8 km s*-1*. The 13CO(2-1) and C18O(2-1) spectra peak at around –44.0 km s*-1*, coincident with a minimum in the 12CO(2-1) spectrum. Components outside this range will be analyzed in § 0.6.2. For $Clump\,B$, the 12CO(2-1) line also shows a multi-peak structure between –55 and –25 km s*-1*, with the maximum at –45.4 km s*-1*. The 13CO(2-1) and C18O(2-1) profiles peak at –44.0 km s*-1*.

Taking into account that the C18O(2-1) emission is generally optically thin, we adopted systemic velocities (${\rm v}_{\rm sys}$) of –44.0 km s*-1* for $Clump\,A$ and $Clump\,B$. The adopted systemic velocity coincide with the velocity for EGO 2 and 4 reported by Yang et al. (2017) (–44.6 and –44.9, respectively) from methanol maser emision, and from masers by Chen et al. (2011). It is also compatible with the velocity of NH3 clouds identified by Purcell et al. (2012) at –43.7 km s*-1* at a position distant 15\arcsec from EGO 3, and from the CS(2-1) line emission obtained by Bronfman et al. (1996), who observed toward [RA, Dec.(J2000)= (16:52:34.2, –44:28:36.0)], revealing the presence of high density regions ($n_{crit}$ = 3.0$\times$105 cm*-3*). Circular galactic rotation models predict that gas with velocities of –44 km s*-1* lies at the near kinematical distance of 3.6 kpc (see, for example, Brand & Blitz 1993). Adopting a velocity dispersion of 6 km s*-1*, the uncertainty in distance is 0.5 kpc (15%). The distance coincides also with that of S 24 (Cappa et al. 2016) indicating that the IRDC belongs to the same complex.

0.4.2 Parameters derived from CO data

Bearing in mind that the spatial distribution of the 13CO(2-1) and C18O(2-1) emission coincide with the IRDC, we used these lines to estimate the main parameters of the molecular gas assuming local thermodynamic equilibrium (LTE). All the calculations were carried out from the gaussian fits to the lines averaged in the area of $Clump\,A$ and $Clump\,B$  shown in Table 2. The columns list the central velocity of each component $v$, the full-width at half-maximum, the peak temperature, and the integrated emission. We apply the following expression to estimate optical depths (e.g. Scoville et al., 1986):

[TABLE]

where $\tau^{13}$ is the optical depth of the 13CO(2-1) gas and 7.6 = [13CO(2-1)]/[C18O(2-1)] (Sanhueza et al., 2010) is the isotope abundance ratio. The 13CO(2-1) optical depths are indicated in column 2 of Table 3. To estimate the C18O(2-1) optical depth we use

[TABLE]

where $\Delta{\rm v}$ is the full-width at half-maximum of the C18O(2-1) and 13CO(2-1) profiles. The results are listed in column 3 of Table 3. Bearing in mind that 13CO(2-1) is moderately optically thick we calculate the excitation temperature from the 13CO(2-1) line using

[TABLE]

where $T^{*}$ = $\frac{h\nu}{k}$, $\nu$ is the frequency of the 13CO(2-1) line and $T_{\rm bg}$ = 2.7 K. Values estimated for $T_{\rm exc}$ are listed in Table 3.

The column density for 13CO(2-1) was derived using Rohlfs & Wilson (2004)

[TABLE]

In this case we used the approximation for $\tau_{13}$ $>$ 1,

[TABLE]

This approximation helps to elliminate to some extent optical depths effects. The integral was evaluated as $T_{mean-mb}\times\Delta{\rm v}$, where $T_{mean-mb}$ is equal to the average $T_{mb}$ within the area of each clump.

To estimate $N(\rm H_{2})$ (column 6 in Table 3) we adopted an abundance [H2][13CO]= 5$\times$105 (Dickman, 1978). Then, the molecular mass (column 9 in Table 3) is calculated from the equation

[TABLE]

where $m_{sun}$ = 2$\times$1033 g is the solar mass, the mean molecular weight $\mu$ = 2.76 (which includes a relative helium abundance of 25% by mass, Allen 1973), and $m_{\rm H}$ is the hydrogen atomic mass. In this expression $d$ is the distance, $N(H_{2})$ is the H2 column density, and $A$ is the area of the source in cm*-2*. The effective radius of each clump as seen in the 13CO(2-1) line and the volume densities are listed in colums 7 and 9 in Table 3.

The ambient volume densities, $n_{\rm H_{2}}$, was calculated assuming a spherical geometry for the clumps, using the formula

[TABLE]

.

Ambient densities are listed in column 9 of Table 3. The parameters calculated in this work agree with Bergin & Tafalla (2007).

The virial mass can be determined following MacLaren et al. (1988):

[TABLE]

where $r$ and $\Delta$v are the radius of the region and the velocity width measured from the gaussian fit of the C18O(2-1) emission, and $k_{2}$ depends on the geometry of the ambient gas in the region, being 190 or 126 according to $\rho\propto r^{-1}$ or $\rho\propto r^{-2}$, respectively. $M_{\rm vir}$ values are listed in column 10 in Table 3. The ratios $M_{vir}/M(H_{2})$ suggest that both clumps may collapse to form new stars, since if the virial mass value is lower than the LTE mass value then this clump does not have enough kinetic energy to stop the gravitational collapse.

Uncertainties in both the molecular mass derived using LTE conditions, $M(\rm H_{2})$, and the virial mass $M_{\rm vir}$ are affected by distance indetermination (15%) yielding 30% error in $M(\rm H_{2})$ and 15% in $M_{\rm vir}$. Inaccuracies in the borders of the clumps originate errors in their sizes and thus additional uncertainties in the masses, suggesting errors of 40% in $M(\rm H_{2})$. Virial masses are not free of uncertainties, because of the existence of magnetic field support that may overestimate the derived values (see MacLaren et al., 1988) and the unknown density profile of the clump.

Analysis of the MALT90 data

The MALT90 data cubes of this region show emission of HCO+(1–0), HNC(1–0), and N2H+(1–0) molecules, which are the most often detected molecules towards IRDCs (Sanhueza et al., 2012; Rathborne et al., 2016). These molecules have critical densities of 2$\times$105, 3$\times$105 and 3$\times$105 cm*-3*, respectively (Sanhueza et al., 2012), allowing us to infer a lower limit for the density of the clumps. These values are higher than the H2 ambient densities derived from CO lines (see Table 3).

Spectra of these lines, averaged over the area of the clumps, are shown in Fig. 5. By applying a gaussian fitting to the HCO+ and HNC lines, a mean velocity of –46 km s*-1* was derived. For the case of the N2H+ line, a multi gaussian function was applied to take into account hyperfine structure. This molecule has seven hyperfine components, in two blended groups of three lines (groups 2 and 3) and one isolated component (group 1). The rest frequencies of groups 1, 2 and 3 are 93.176, 93.173, and 93.171 GHz, respectively. Its mean velocity is –44 km s*-1*. The velocities of these lines coincide with the systemic velocities derived from CO data.

These tracers of dense gas provide slightly different information: HCO+ often shows infall signatures and outflow wings (e.g. Rawlings et al., 2004; Fuller et al., 2005). However, in our case no signs of infall can be identified. HNC is specially preponderant in cold gas and is a commonly used as tracer of dense gas in molecular clouds. Finally, N2H+ is more resistant to freeze out on grains than the carbon-bearing species like CO.

Figure 5 shows the HCO+(1–0), HNC(1–0) and N2H+(1–0) maps, integrated from –47.6 to –44.0 km s*-1*, –46.7 to -43.1 km s*-1*, and –47 to –41.4 km s*-1*, respectively. Both clumps are detected in the three molecular lines indicating mean densities of 105 cm*-3*. Carbon-bearing species, like CO, tend to disappear from the gas phase in the high density centers of the cores, while nitrogen-bearing species like N2H+ survive almost unaffected up to much higher densities. That is evidenced in the fact that the spacial distribution of the N2H+ emission is similar to dust continuum emission (see Figure 6).

Following Purcell et al. (2009) we estimated the N2H+ optical depth and column density. Assuming the line widths of the individual hyperfine components are all equal, the integrated intensities of the three blended groups should be in the ratio of 1:5:3 under optically thin conditions. The optical depth can then be derived from the ratio of the integrated intensities of any group using the following equation:

[TABLE]

where $a$ is the expected ratio of $\tau_{1}/\tau_{2}$ under optically thin conditions.

To determine the optical depth we used only the intensity ratio of group 1/group 2, as Caselli et al. (1995) report anomalous excitation of the F1,F = 1,0 $\rightarrow$ 1,1 and 1,2 $\rightarrow$ 1,2 components (in group 3). Thus we obtain $\tau_{1}$ = 0.11 and 0.19 for $Clump\,A$ and $Clump\,B$, respectively.

Based on the expression for $T_{\rm mb}$ given by Rohlfs & Wilson (2004), the excitation temperature for N2H+ can be calculated with the following formula:

[TABLE]

and the column densities can be derived using (Chen et al., 2013):

[TABLE]

where $k$ is the Boltzmann constant, $W$ is the observed line integrated intensity (obtained from gaussian fit), $\nu$ is the frequency of the transition, and $S\mu^{2}$ is the product of the total torsion-rotational line strength and the square of the electric dipole moment. $T_{\rm exc}$ and $T_{\rm bg}$ are the excitation temperature and background brightness temperature, respectively. $E_{u}/k$ is the upper level energy in K, $Q(T_{\rm exc})$ is the partition function at temperature $T_{\rm exc}$ and $\tau$ is the optical depth. For group 1 the values of $\nu$, $S\mu^{2}$ and $E_{u}/k$ are 93176.2526 MHz, 12.42 D2 and 4.47 K, respectively. These values were taken from the SPLATALOGUE catalogue 555SPLATALOGUE at http://www.splatalogue.net/.

We derive $T_{\rm exc}$ = 17.4 and 12.5 K and $N({\rm N_{2}H^{+}})$ = 8.7$\times 10^{13}$ and 8.1$\times 10^{13}$ for $Clump\,A$ and $Clump\,B$, respectively. Considering an abundance [N2H+]/[H2] = 5$\times$10*-10* for dark molecular clouds (Ohishi et al., 1992) we obtain H2 column densities of 1.7 and 1.6 $\times$1023 for $Clump\,A$ and $Clump\,B$, respectively. These column densities are higher than those obtained from CO calculations. Ambient densities are $n_{H_{2}}\approx$ 1.7$\times$105 cm*-3* for each clump with an effective radii in N2H+ of 0.53 pc. This difference may be explained by considering that the nitrogen-bearing species survive almost unaffected up to much higher densities that carbon-bearing molecules.

0.5 Warm and cold dust distribution

Figure 6 shows Herschel and ATLASGAL maps, in original resolution, of the SDC 341.232-0.268 region. At $\lambda<$ 160 $\mu$m the beam resolution allows to identify the emission associated with EGOs 1, 2, and 4. Warm dust coincident with the EGOs is also revealed by the emission in 24 $\mu$m, which can be seen in Figure 1. The presence of this emission allows us to classify the clumps as “active”, according to the classification of Chambers et al. (2009). These authors proposed an evolutionary sequence in which “quiescent” clumps (contains neither IR indicator) evolve into “intermediate” (contain either a “green fuzzy” or a 24 $\mu$m point source, but not both), “active”(caracterized by the presence of a “green fuzzy” coincident with an embedded 24 $\mu$m source, such as those observed toward EGOs 1, 2, and 4), and “red” clumps (dominated by 8 $\mu$m emission, which contains PAH features). At $\lambda>$ 160 $\mu$m two dust clumps are detected superimposed onto more extended submillimeter emission, indicating dust related to $Clump\,A$ and $Clump\,B$.

The characterization of the dust properties of these clumps is limited by the resolution of FIR data. We study the integrated dust properties of each clump from the spectral energy distribution (SED) using the Herschel (160 up to 500 $\mu$m) and ATLASGAL maps convolved at common beam resolution of the 500 $\mu$m map (36\arcsec). The flux densities are obtained from a circular aperture photometry integration (radio of 37\farcs6 and 45\farcs6 for $Clump\,A$ and $Clump\,B$, respectively), and subtracting a background level computed from a rectangular region (width = 52\farcs5 and height 25\farcs8 in the North of the clumps). For flux uncertainty estimation, we considered the standard deviation of surface brightness in the background region and the flux calibration uncertainties. For each clumps, the final SED is depicted in Figure 7 and the fluxes and their error bars are listed in Table 4. We perform a thermal dust fit to the data considering a single-component modified blackbody (grey-boby), which depend on the optical depth, the emissivity spectral index ($\beta_{d}$) and the dust temperature (Td). In the optically thin regime, the best fitting provides similar dust temperature and emissivity for both clumps, with values of Td = 13.5$\pm$0.47 k and $\beta_{d}$ = 2.7. These dust temperatures are of the order on those found by Guzmán et al. (2015) (18.6$\pm$0.2) for proto-stellar clumps.

For $Clump\,A$ and $Clump\,B$, the total mass ($M_{\rm dust+gas}$) were calculated from the optical depth of the dust obtained from the fit, using the following expression

[TABLE]

where $\tau_{\nu_{fit}}$ is the dust optical depth (= 10*-26* $\times$ fit amplitude), $\nu_{fit}$=1 Hz is the frequency of the fit, $R_{\rm d}$ = 100 is the typical gas-to-dust ratio, $d$ is the distance and $\kappa_{870}$=1.00 cm*-2* gr*-1* is the dust opacity at 345 GHz (Ossenkopf & Henning, 1994). The mass uncertainties are computed propagating the error bars for dust optical depth and distance. Additional errors may result from taking different values for the gas-to-dust ratio (in our galaxy take typical values between 100 and 150).

The results are listed in Table 4, which includes the derived masses of the clumps and their volume densities. Uncertainties in masses and ambient densities are about 35% and 60%, respectively. Total masses derived from molecular gas and dust emission are in good agrement (within errors).

0.6 Star formation

0.6.1 Search for additional YSOs coincident with the IRDC

To investigate the coincidence of the IRDC with other candidate young stellar objects (YSOs), we analyze the characteristics of the point sources in the WISE and Spitzer catalogues (Wright et al., 2010; Benjamin et al., 2003) projected in the region. In Figure 8 we marked the positions of Class I and II YSO candidates identified in the area of the molecular clumps. An inspection of the figure reveals that EGOs 1, 3, and 4 coincide with identified YSOs.

To study in some detail the nature of the EGOs we plotted their spectral energy distributions (SEDs) using the Robitaille’s SED fitting tool (Robitaille et al., 2007). To perform the analysis we were able to used available data from 2MASS, IRAC-GLIMPSE, and MIPSGAL at 24 $\mu$m only. The obtained SEDs are shown in Figure 9. Clearly, EGO 1, 3 and 4 have characteristics of YSOs, and as seen in Fig. 8 coincide with additional star formation evidences. As regards EGO 2, the fit is not good, it does not coincide with other sources or with the molecular clumps. So, its status is doubtful. In Table 5 we display the main parameters obtained from the SEDs. Column 1 gives the name of the source; columns 2 and 3, the age and mass, $M_{\rm stellar}$, of the central source; column 4, the mass of the envelope, $M_{\rm env}$; column 5, the infall rate, $\dot{M}_{\rm acr}$; and column 6, the total luminosity. To perform the SEDs we adopted d = 3.6$\pm$0.5 kpc and a visual extinction of 3-4 mag.

Following Robitaille et al. (2007) an estimate of the evolutionary stage of the sources can be obtained based on the ratio of the infall rate and the mass of the central source. For the four candidate EGOs the ratio $\dot{M}_{\rm acr}/M_{\rm stellar}>$ 10*-6* indicates Stage 0/I. The age of some of the sources suggest that they are still immersed in their envelopes. According to the fitting, EGO 4 would be the most massive object in the sample and EGO 2 seems to be the most evolved one. These results should be taken with caution because of the limitations of the fitting tool that may come from: (1) YSOs are complex 3d objects with not very well axisymmetric density structures, so the models are incorrect compared to actual density distributions, (2) there are often mixtures of sources and many objects appear as a single YSO, but they are often two or more objects, and (3) even in the case of an isolated YSO variability is an additional complication, since the data with which SEDs are usually constructed belong to different elevations that have been performed in different years and can produce discontinuities in SEDs (Robitaille, 2008; Deharveng et al., 2012; Offner et al., 2012).

As pointed out before, the emission at 24 $\mu$m, detected toward EGO 1, 3, and 4, suggests the existence of warm dust and embedded protostars (Jackson et al., 2008).

0.6.2 Evidence for inflow/outflow motions

One way to visualize molecular outflows or infalls is to compare the optically thick 12CO(2-1) emission line with the optically thin C18O(2-1) molecular line toward the candidate EGOs, as shown in Fig. 10 for EGO 3 and EGO 4. The 12CO(2-1) line present a double peak structure with the blue shifted peak brighter than the red-shifted one, and a minimum at the systemic velocity, while the C18O(2-1) line presents a simple peak centered at the absorption dip of the optically thick line. According to Chen et al. (2011) these spectra displays characteristics typical of infall motions: a double-peak structure with the blue-shifted peak brighter than the red-shifted one, while an emission peak of an optically thin line appears centered at the absorption dip of the optically thick line. According to these authors, the infall motion is the only process that would consistently produce the blue profile asymmetry. The blue-shifted emission can be explained due the high-excitation approaching warm gas located on the far side of the center of contraction. The emission of this gas undergoes less extinction than the emission from the red-shifted receding nearside material, given that the excitation temperature of the molecules increases toward the center of the region (Zhou, 1992). Thus, the spectra of EGO 3 and EGO 4 suggest the presence of infall motions.

Following Mardones et al. (1997), we use the asymmetry parameter $\delta$v (the velocity difference between the peaks of the optically thick line and the optically thin lines, normalized by the FWHM of the thin line), to quantify the blue asymmetry. The parameter is defined as $\delta$v = (vthick – vthin) / $\Delta$vthin. An statistically significant excess of blue asymmetric line profiles with $\delta$v $<$ –0.25 indicate that the molecular gas is infalling onto the clump. We consider the velocity and width of the C18O(2-1), from Table 2, as vthin and $\Delta$vthin, and calculate vthick from the gaussian fits of 12CO(2-1) profiles (–46.9 and –45.3 from $Clump\,A$ and $Clump\,B$, respectively). We find, for both $Clump\,A$ and $Clump\,B$  $\delta$v $\sim$ –0.06, which supports the infall hypothesis.

The interpretation presented above is supported by the $M_{\rm vir}/M({\rm H_{2}})$ ratio, previously derived for the objects. As the classical virial equilibrium analysis establishes, a ratio $M_{\rm vir}/M({\rm H_{2}})\gtrsim$ 1 indicates that a clump has too much kinetic energy and is stable against gravitational collapse. Then, derived ratios suggest that $Clump\,A$ and $Clump\,B$ are not stable and could be collapsing.

For the sake of completeness, we calculate the eventual infall rate estimated from Klaassen & Wilson (2007)

[TABLE]

where $n_{\rm H2}$ is the H2 volume density, $r_{\rm clump}$ is the linear radius of the infall region, and ${\rm v}_{\rm inf}$ is the infall velocity of the material. One way to estimate the infall velocity is considering the two layer radiative transfer model of Myers et al. (1996). Form the 12CO(2-1) and C18O(2-1) spectra corresponding to $Clump\,A$ from Fig. 3, we calculated v${}_{\rm inf}=$ 0.53 km s*-1using equation (9) in Myers et al. (1996). In order to get a better estimate, we also calculated the infall velocity using the Hill5 model (De Vries & Myers, 2005) in the PySpecKit package (Ginsburg & Mirocha, 2011) 666https://pyspeckit.readthedocs.io/en/latest/hill5infall_model.html. The Hill5 model employs an excitation temperature profile increasing linearly toward the center, rather than the two-slab model of Myers et al. (1996), so the Hill5 model is thought to provide a better fit of infall motions (De Vries & Myers, 2005). From this model we obtained v${}_{\rm inf}=$ 1.53$\pm$0.26 km s-1*. Considering this value of v${}_{\rm inf}=$ and the parameters listed in Table 3, we obtained $\dot{M}_{\rm inf}\sim$ 7.47$\times$10*-4* M⊙ yr*-1*. This result is consistent with those derived from surveys of massive YSOs such as Klaassen & Wilson (2007). $Clump\,B$ does not show clear observational evidences of collapse.

A typical outflow appears as spatially confined wings beyond the emission from the cloud core. For EGO 3 and EGO 4, we can distinguish the presence of 12CO(2-1) blue wings from $\sim$ –60 to –50 km s*-1*, shown in the spectra fit in Figure 10, while the velocity range of the optically thin C18O(2-1) emission is $\sim$ –47 to –40 km s*-1*. The dashed line marks the systemic velocity of the clumps, that coincides with the central velocity of the C18O(2-1) line. Red wings can not be identified, although multiple components in the 12CO(2-1) emission are present.

The typical signature of outflows can be seen in the position-velocity diagram. As shown in Fig. 11 for EGO 4, the extension of the emission toward velocities of about –55 km s*-1*would correspond to the blue wings shown in Figure 10. Note that the declination of this extension and the right ascension of the map coincides with the position of EGO 4.

These characteristics suggest that, although they are not spatially resolved, there may be outflows associated with EGO 3 and EGO 4.

0.7 Summary

Based on 12CO(2-1), 13CO(2-1), and C18O(2-1) images obtained using the APEX telescope and high density molecular tracers from the MALT90 survey such as HCO+(1-0), HNC(1-0) and N2H+(1-0) we investigate the molecular component of the IRDC SDC 341.232-0.268. The 13CO(2-1) and C18O(2-1) along with MALT90 data reveal two molecular clumps ($Clump\,A$ and $Clump\,B$) linked to SDC 341.232-0.268 with systemic velocities of –44.0 km s*-1*, indicating a kinematical distance of 3.6$\pm$0.5 kpc. Four EGOs (EGO 1 at RA,Dec = 16:52:30.3, –44:28:40.0; EGO 2 at RA,Dec = 16:52:32.2, –44:28:38.0; EGO 3 at RA,Dec = 16:52:34.2, –44:28:36.0 and EGO 4 at RA,Dec = 16:52:37.3, –44:28:09.9) coincide with the molecular clumps. We calculate masses of 2400$\pm$960 and 1200$\pm$480 M⊙, for $Clump\,A$ and $Clump\,B$, respectively, and H2 ambient densities $>$104 cm*-3*. Ambient densities estimated using the N2H+(1-0) line are higher ($\sim$ 105 cm*-3*) and agree with the critical density of this molecule, which would be tracing the densest part of the clumps.

Both $Clump\,A$ and $Clump\,B$ are detected in the FIR (Herschel images) at $\lambda>$ 160 $\mu$m. In the NIR and MIR, at $\lambda<$ 160 $\mu$m (24 and 70 $\mu$m), three of four EGOs seem to be resolved mainly at 24 $\mu$m. Molecular masses derived from the emission at 870 $\mu$m are roughly in agreement with those calculated from the molecular lines.

Spectral energy distributions (SEDs) for $Clump\,A$ and $Clump\,B$ built using fluxes in the FIR indicate dust temperatures of 13.5 K, typical for an IRDC.

Our search for additional signs of star formation indicates that some of the EGOs coincides with young stellar objects classified as Class I detected from point sources of the Spitzer and WISE catalogs.

Additionally, the 12CO(2-1) spectra toward EGO 3 and EGO 4 present a double-peak structure with the blue-shifted peak brighter than the red-shifted one, whith the maximum in the 13CO(2-1) and C18O(2-1) spectra coincident with the absorption dip, which disclose the existence of infall material. This fact together with the values obtained from $M_{\rm vir}/M({\rm H_{2})}$ ratio derived for the clumps reveal that this would be collapsing. Blue extended wings in the C18O(2-1) spectra are also present toward these EGOs suggesting the presence of outflows.

Acknowledgements

This project was partially financed by CONICET of Argentina under projects PIP 00356, and PIP 00107 and from UNLP, projects PPID092, PPID/G002, and 11/G139. M.R. wishes to acknowledge support from CONICYT (CHILE) through FONDECYT grant No1140839. VF acknowledges support from CONICYT Astronomy Program-2015 Research Fellow GEMINI-CONICYT (32RF0002). V.F. also acknowledges support from the Faculty of the European Space Astronomy Centre (ESAC), and would like to thank Ivan Valtchanov, Bruno Altieri, and Luca Conversi for their support and valuable assistance in Herschel data processing. We thank to the referee for the careful reading of the manuscript and constructive comments which largely improved this presentation. This work is based [in part] on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.