Dense molecular gas towards W49A: a template for extragalactic starbursts?

Fuller

et al. 2012

A&A

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Context. The star formation rates in starburst galaxies are orders of magnitude higher than in local star-forming regions, and the origin of this difference is not well understood. Aims. We use sub-mm spectral line maps to characterize the physical conditions of the molecular gas in the luminous Galactic starforming region W49A and compare them with the conditions in starburst galaxies. Methods. We probe the temperature and density structure of W49A using H 2 CO and HCN line ratios over a 2 × 2 (6.6 × 6.6 pc) field with an angular resolution of 15 (∼0.8 pc) provided by the JCMT Spectral Legacy Survey. We analyze the rotation diagrams of lines with multiple transitions with corrections for optical depth and beam dilution, and estimate excitation temperatures and column densities.Results. Comparing the observed line intensity ratios with non-LTE radiative transfer models, our results reveal an extended region (about 1 × 1 , equivalent to ∼3 × 3 pc at the distance of W49A) of warm (>100 K) and dense (>10 5 cm −3 ) molecular gas, with a mass of 2 × 10 4 −2 × 10 5 M (by applying abundances derived for other regions of massive star-formation). These temperatures and densities in W49A are comparable to those found in clouds near the center of the Milky Way and in starburst galaxies. Conclusions. The highly excited gas is likely to be heated via shocks from the stellar winds of embedded, O-type stars or alternatively due to UV irradiation, or possibly a combination of these two processes. Cosmic rays, X-ray irradiation and gas-grain collisional heating are less likely to be the source of the heating in the case of W49A.

Section: Introductionsupporting

confidence: 79%

Extended warm and dense gas towards W49A: starburst conditions in our Galaxy?

Nagy

Fuller

et al. 2012

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“…We selected these positions based on different kinematical signatures traced by dense gas tracers such as HCO + , HCN, and HNC (Roberts et al 2011), and are shown for example in Fig. 3.…”

Section: Line Identificationmentioning

confidence: 99%

“…This suggests the possible importance of PDR chemistry in W49A and may be tested using the ratio of CN/HCN, which is a well-known tracer of PDRs (e.g., Fuente et al 1996). The spatial distribution of HCN and its isotopomer HNC based on SLS data were analyzed in Roberts et al (2011). -C 2 H (ethynyl) is a commonly observed molecule in the interstellar medium, including in PDRs.…”

Section: Tracers Of Uv Irradiationmentioning

confidence: 99%

“…A recent study based on data from the JCMT Spectral Legacy Survey (SLS, Plume et al 2007) focused on an extended warm and dense gas component toward W49A seen in H 2 CO (Nagy et al 2012), and characterized the physical properties and excitation of the region using CH 3 OH, SO 2 , H 2 CO, and HCN transitions. Another study used the SLS line survey data to focus on dense gas tracers (Roberts et al 2011), such as HCN, HNC, DCN, HCO + , and their isotopologues, and compared the line ratios of these gas tracers to line ratios measured toward starburst galaxies and active galactic nuclei (AGN), as well as to chemical models, and found that W49A is a template for starburst galaxies rather than for AGNs.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Physical and chemical differentiation of the luminous star-forming region W49A

Nagy

Fuller

et al. 2015

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Context. The massive and luminous star-forming region W49A is a well-known Galactic candidate to probe the physical conditions and chemistry similar to those expected in external starburst galaxies. Aims. We aim to probe the physical and chemical structure of W49A on a spatial scale of ∼0.8 pc based on the JCMT Spectral Legacy Survey, which covers the frequency range between 330 and 373 GHz. Methods. The wide 2 × 2 arcmin field and the high spectral resolution of the HARP instrument on JCMT provides information on the spatial structure and kinematics of the cloud traced by the observed molecular lines. For species where multiple transitions are available, we estimate excitation temperatures and column densities using a population diagram method that takes beam dilution and optical depth corrections into account. Results. We detected 255 transitions corresponding to 63 species in the 330-373 GHz range at the center position of W49A. Excitation conditions can be probed for 14 molecules, including the complex organic molecules CH 3 CCH, CH 3 CN, and CH 3 OH. The chemical composition suggests the importance of shock, photon-dominated region (PDR), and hot core chemistry. Many molecular lines show a significant spatial extent across the maps including CO and its isotopologues, high density tracers (e.g., HCN, HNC, CS, HCO + ), and tracers of UV irradiation (e.g., CN and C 2 H). The spatially extended species reveal a complex velocity-structure of W49A with possible infall and outflow motions. Large variations are seen between the subregions with mostly blue-shifted emission toward the eastern tail, mostly red-shifted emission toward the northern clump, and emission peaking around the expected source velocity toward the southwest clump. Conclusions. A comparison of column density ratios of characteristic species observed toward W49A to Galactic PDRs suggests that while the chemistry toward the W49A center is driven by a combination of UV irradiation and shocks, UV irradiation dominates for the northern clump, eastern tail, and southwest clump regions. A comparison to a starburst galaxy and an active galactic nucleus suggests similar C 2 H, CN, and H 2 CO abundances (with respect to the dense gas tracer 34 CS) between the ∼0.8 pc scale probed for W49A and the >1 kpc regions in external galaxies with global star formation.

“…The targets are the Orion Bar, a prototypical photodissociation region ( Van der Wiel et al 2009); NGC1333 IRAS 4, a low-mass star-forming region; W49A, an active star formation complex (Roberts et al 2011); and the isolated high-mass star-forming region AFGL2591, which is the topic of this study. The SLS is complemented at higher frequencies (360)(361)(362)(363)(364)(365)(366)(367)(368)(369)(370)(371)(372)(373) The 16 receptors of the HARP receiver are distributed in a 4 × 4 pattern, regularly spaced at 30 intervals.…”

Section: Harp-b and The Jcmt Spectral Legacy Surveymentioning

confidence: 99%

The JCMT Spectral Legacy Survey: physical structure of the molecular envelope of the high-mass protostar AFGL2591

Wiel

Spaans

et al. 2011

A&A

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Context. The understanding of the formation process of massive stars ( > ∼ 8 M ) is limited by a combination of theoretical complications and observational challenges. The high UV luminosities of massive stars give rise to chemical complexity in their natal molecular clouds and affect the dynamical properties of their circumstellar envelopes. Aims. We investigate the physical structure of the large-scale (∼10 4 -10 5 AU) molecular envelope of the high-mass protostar AFGL2591. Methods. Observational constraints are provided by spectral imaging in the 330-373 GHz regime from the JCMT Spectral Legacy Survey and its high-frequency extension. While the majority of the ∼160 spectral features from the survey cube are spatially unresolved, this paper uses the 35 that are significantly extended in the spatial directions. For these features we present integrated intensity maps and velocity maps. The observed spatial distributions of a selection of six species are compared with radiative transfer models based on (i) a static spherically symmetric structure; (ii) a dynamic spherical structure; and (iii) a static flattened structure. Results. The maps of CO and its isotopic variations exhibit elongated geometries on scales of ∼100 , and smaller scale substructure is found in maps of N 2 H + , o-H 2 CO, CS, SO 2 , C 2 H, and various CH 3 OH lines. In addition, a line-of-sight velocity gradient is apparent in maps of all molecular lines presented here, except SO, SO 2 , and H 2 CO. We find two emission peaks in warm (E up ∼ 200 K) CH 3 OH separated by 12 (12 000 AU), indicative of a secondary heating source in the envelope. The spherical models are able to explain the distribution of emission for the optically thin H 13 CO + and C 34 S, but not for the optically thick HCN, HCO + , and CS or for the optically thin C 17 O. The introduction of velocity structure mitigates the optical depth effects, but does not fully explain the observations, especially in the spectral dimension. A static flattened envelope viewed at a small inclination angle does slightly better. Conclusions. Based on radiative transfer modeling, we conclude that a geometry of the envelope other than an isotropic static sphere is needed to circumvent line optical depth effects. We propose that this could be achieved in circumstellar envelope models with an outflow cavity and/or an inhomogeneous structure on scales 10 4 AU. The picture of inhomogeneity is supported by substructure observed in at least six different species.