Extended warm and dense gas towards W49A: starburst conditions in our Galaxy? (original) (raw)
Related papers
Dense molecular gas towards W49A: a template for extragalactic starbursts?
Astronomy & Astrophysics, 2010
Context. The HCN, HCO + , and HNC molecules are commonly used as tracers of dense star-forming gas in external galaxies, but such observations are spatially unresolved. Reliably inferring the properties of galactic nuclei and disks requires detailed studies of sources whose structure is spatially resolved. Aims. To understand the origin of extragalactic molecular line emission, we compare the spatial distributions and abundance ratios of HCN, HCO + , and HNC in W49A, the most massive and luminous star-forming region in the Galactic disk. Methods. Maps of a 2 (6.6 pc) field at 14 (0.83 pc) resolution of the J = 4−3 transitions of HCN, H 13 CN, HC 15 N, HCO + , H 13 CO + , HC 18 O + , and HNC are combined with supplementary observations of the J = 5−4 transition of DCN and the J = 3−2 transitions of HCN and H 13 CO +. Most of the data are from HARP/ACSIS, with supplementary data from JCMT Receiver A and the SCUBA archive. We use maps of the integrated intensity and line-profiles to pick out regions of the source to study in more detail. We compare column densities and abundance ratios towards these regions with each other and with predictions from gas-phase chemical models. Results. The kinematics of the molecular gas in W49A appears complex, with a mixture of infall and outflow motions. Both the line profiles and comparison of the main and rarer species show that the main species are optically thick. Two "clumps" of infalling gas that we look at in more detail appear to be at ∼40 K, compared to ≥100 K at the source centre, and may be ∼10× denser than the rest of the outer cloud. The chemical modelling suggests that the HCN/HNC ratio probes the current gas temperature, while the HCN/HCO + ratio and the deuterium fractionation were set during an earlier, colder phase of evolution. Conclusions. The similarity in the derived physical conditions in W49A and those inferred for the molecular gas in external galaxies suggest that W49A is an appropriate analogue of an extragalactic star forming region. Our data show that the use of HCN/HNC/HCO + line ratios as proxies for the abundance ratios is incorrect for W49A, suggesting that using these line ratios as abundance ratios in galactic nuclei is invalid too. On the other hand, our observed isotopic line ratios such as H 13 CN/H 13 CO + approach our modeled abundance ratios quite well in W49A. Second, the 4−3 lines of HCN and HCO + are much better tracers of the dense star-forming gas in W49A than the 1−0 lines, confirming similar indications for galactic nuclei. Finally, our observed HCN/HNC and HCN/HCO + ratios in W49A are inconsistent with homogeneous PDR or XDR models, indicating that irradiation does not strongly affect the gas chemistry in W49A. Overall, the W49A region appears to be a useful template for starburst galaxies.
Physical and chemical differentiation of the luminous star-forming region W49A
Astronomy & Astrophysics, 2015
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.
A comparison of dense molecular gas tracers towards the Galactic starburst analogue W49A
2010
Context. The HCN, HCO + , and HNC molecules are commonly used as tracers of dense star-forming gas in external galaxies, but such observations are spatially unresolved. Reliably inferring the properties of galactic nuclei and disks requires detailed studies of sources whose structure is spatially resolved. Aims. To understand the origin of extragalactic molecular line emission, we compare the spatial distributions and abundance ratios of HCN, HCO + , and HNC in W49A, the most massive and luminous star-forming region in the Galactic disk. Methods. Maps of a 2 (6.6 pc) field at 14 (0.83 pc) resolution of the J=4-3 transitions of HCN, H 13 CN, HC 15 N, HCO + , H 13 CO + , HC 18 O + , and HNC are combined with supplementary observations of the J=5-4 transition of DCN and the J=3-2 transitions of HCN and H 13 CO +. Most of the data are from HARP/ACSIS, with supplementary data from JCMT Receiver A and the SCUBA archive. We use maps of the integrated intensity and line-profiles to pick out regions of the source to study in more detail. We compare column densities and abundance ratios towards these regions with each other and with predictions from gas-phase chemical models. Results. We find evidence for large-scale infall, mostly from the East and West direction, across W49A. Two "clumps" of infalling gas that we look at in more detail appear to be at ∼40 K, compared to ≥100 K at the source centre, and may be ∼10× denser than the rest of the outer cloud. The chemical modelling suggests that the HCN/HNC ratio probes the current gas temperature, while the HCN/HCO + ratio and the deuterium fractionation were set during an earlier, colder phase of evolution. Conclusions. The derived physical conditions for W49A are very similar to those in active galactic nuclei so that a comparison is valid. Our data show that the use of HCN/HNC/HCO + line ratios as proxies for the abundance ratios is incorrect for W49A because the lines are optically thick, suggesting that using these line ratios as abundance ratios in AGN is invalid too. On the other hand, our observed isotopic line ratios such as H 13 CN/H 13 CO + approach our modeled abundance ratios quite well in W49A. Second, the 4-3 lines of HCN and HCO + are much better tracers of the dense star-forming gas in W49A than the 1-0 lines, confirming similar indications for galactic nuclei. Finally, our observed HCN/HNC and HCN/HCO + ratios in W49A are inconsistent with homogeneous PDR or XDR models, indicating the need for complex muti-component models.
S R ] 2 A ug 2 01 5 Extremely Energetic Outflow and Decelerated Expansion in W 49 N
2018
W49N is a mini-starburst in the Milky Way and thus an ideal lab oratory for highmass star formation studies. Due to its large distance (11.1 +0.9 −0.7 kpc), the kinematics inside and between the dense molecular clumps in W49N are far from well understood. The SMA observations resolved the continuum emissio n int two clumps. The molecular line observation of SO 2 (284,24-283,25) suggests that the two clumps have a velocity difference of∼7 km s−1. The eastern clump is very close to two radio sources “G1” and “G2”, and the western clump coincides with a radio so urce “B”. The HCN (3-2) line reveals an extremely energetic outflow, which is a mong the most energetic molecular outflows in the Milky Way. This is the first report of high-velocity molecular outflow detection in W49N. The outflow jet might be in precessi on, which could account for the distribution, velocity and rotation of water m aser spots. Three absorption systems are identified in HCO + (3-2) spectra. The absorption feat...
A Near-Infrared and X-Ray Study of W49 B: A Wind Cavity Explosion
Astrophysical Journal, 2007
We present near-infrared narrowband images of the supernova remnant W49 B, taken with the WIRC instrument on the Hale 200 inch (5 m) telescope on Mount Palomar. The 1.64 μm [Fe II] image reveals a barrel-shaped structure with coaxial rings, which is suggestive of bipolar wind structures surrounding massive stars. The 2.12 μm shocked molecular hydrogen image extends 1.9 pc outside of the [Fe II] emission to the southeast. We also present archival Chandra data, which show an X-ray jetlike structure along the axis of the [Fe II] barrel, flaring at each end. Fitting single-temperature X-ray emission models reveals an enhancement of heavy elements, with particularly high abundances of hot Fe and Ni, and relatively metal-rich core and jet regions. We interpret these findings as evidence that W49 B originated inside a wind-blown bubble (R~5 pc) inside a dense molecular cloud. This suggests that W49 B's progenitor was a supermassive star that could significantly shape its surrounding environment. We also suggest two interpretations for the jet morphology, abundance variations, and molecular hydrogen emission: (1) the explosion may have been jet driven, interacting with the molecular cavity (i.e., a gamma-ray burst); or (2) the explosion could have been a traditional supernova, with the jet structure being the result of interactions between the shock and an enriched interstellar cloud.
Monthly Notices of the Royal Astronomical Society, 2009
We present high spatial resolution mid-infrared (IR) images of the ring of ultracompact H ii regions in W49A obtained at Gemini North, allowing us to identify the driving source of its powerful H 2 O maser outflow. These data also confirm our previous report that several radio sources in the ring are undetected in the mid-IR because they are embedded deep inside the cloud core. We locate the source of the water maser outflow at the position of the compact mid-IR peak of source G (source G:IRS1) to within 0. ′′ 07. This IR source is not coincident with any identified compact radio continuum source, but is coincident with a hot molecular core, so we propose that G:IRS1 is a hot core driving an outflow analogous to the wide-angle bipolar outflow in OMC-1. G:IRS1 is at the origin of a larger bipolar cavity and CO outflow. The water maser outflow is orthogonal to the bipolar CO cavity, so the masers probably reside near its waist in the thin cavity walls. Models of the IR emission require a massive protostar with M * ≃45 M ⊙ , L * ≃3×10 5 L ⊙ , and an effective envelope accretion rate of ∼10 −3 M ⊙ yr −1. Feedback from the central star could potentially drive the small-scale H 2 O maser outflow, but it has insufficient radiative momentum to have driven the largescale bipolar CO outflow, requiring that this massive star had an active accretion disk over the past 10 4 yr. Combined with the spatialy resolved morphology in IR images, G:IRS1 in W49 provides compelling evidence for a massive protostar that formed by accreting from a disk, accompanied by a bipolar outflow.
Morphological and spectral properties of the W51 region measured with the MAGIC telescopes
Astronomy & Astrophysics, 2012
Context. The W51 complex hosts the supernova remnant W51C which is known to interact with the molecular clouds in the star forming region W51B. In addition, a possible pulsar wind nebula CXO J192318.5+140305 was found likely associated with the supernova remnant. Gamma-ray emission from this region was discovered by Fermi/LAT (between 0.2 and 50 GeV) and H.E.S.S. (>1 TeV). The spatial distribution of the events could not be used to pinpoint the location of the emission among the pulsar wind nebula, the supernova remnant shell and/or the molecular cloud. However, the modeling of the spectral energy distribution presented by the Fermi/LAT collaboration suggests a hadronic emission mechanism. The possibility that the gamma-ray emission from such an object is of hadronic origin can contribute to solvingthe long-standing problem of the contribution to galactic cosmic rays by supernova remnants. Aims. Our aim is to determine the morphology of the very-high-energy gamma-ray emission of W51 and measure its spectral properties. Methods. We performed observations of the W51 complex with the MAGIC telescopes for more than 50 hours. The energy range accessible with MAGIC extends from 50 GeV to several TeV, allowing for the first spectral measurement at these energies. In addition, the good angular resolution in the medium (few hundred GeV) to high (above 1 TeV) energies allow us to perform morphological studies. We look for underlying structures by means of detailed morphological studies. Multi-wavelength data from this source have been sampled to model the emission with both leptonic and hadronic processes. Results. We detect an extended emission of very-high-energy gamma rays, with a significance of 11 standard deviations. We extend the spectrum from the highest Fermi/LAT energies to ∼ 5 TeV and find that it follows a single power law with an index of 2.58 ± 0.07 stat ± 0.22 syst . The main part of the emission coincides with the shocked cloud region, while we find a feature extending towards the pulsar wind nebula. The possible contribution of the pulsar wind nebula, assuming a point-like source, shows no dependence on energy and it is about 20% of the overall emission. The broad band spectral energy distribution can be explained with a hadronic model that implies proton acceleration above 100 TeV. This result, together with the morphology of the source, tentatively suggests that we observe ongoing acceleration of ions in the interaction zone between supernova remnant and cloud.
Extremely Energetic Outflow and Decelerated Expansion in W49N
The Astrophysical Journal, 2015
W49N is a mini-starburst in the Milky Way and thus an ideal laboratory for highmass star formation studies. Due to its large distance (11.1 +0.9 −0.7 kpc), the kinematics inside and between the dense molecular clumps in W49N are far from well understood.
The supernova remnant W49B as seen with H.E.S.S. and Fermi-LAT
Astronomy & Astrophysics, 2018
The supernova remnant (SNR) W49B originated from a core-collapse supernova that occurred between one and four thousand years ago, and subsequently evolved into a mixed-morphology remnant, which is interacting with molecular clouds (MC). Gamma-ray observations of SNR-MC associations are a powerful tool to constrain the origin of Galactic cosmic rays, as they can probe the acceleration of hadrons through their interaction with the surrounding medium and subsequent emission of non-thermal photons. We report the detection of a γ-ray source coincident with W49B at very high energies (VHE; E > 100 GeV) with the H.E.S.S. Cherenkov telescopes together with a study of the source with five years of Fermi-LAT high-energy γ-ray (0.06–300 GeV) data. The smoothly connected, combined source spectrum, measured from 60 MeV to multi-TeV energies, shows two significant spectral breaks at 304 ± 20 MeV and 8.4−2.5+2.2 GeV; the latter is constrained by the joint fit from the two instruments. The detec...
The dense gas mass fraction in the W51 cloud and its protoclusters
Astronomy & Astrophysics, 2015
Context. The density structure of molecular clouds determines how they will evolve. Aims. To map the velocity-resolved density structure of the most vigorously star-forming molecular cloud in the Galactic disk, the W51 Giant Molecular Cloud. Methods. We present new 2 cm and 6 cm maps of H2CO, radio recombination lines, and the radio continuum in the W51 star forming complex acquired with Arecibo and the Green Bank Telescope at ∼ 50 ′′ resolution. We use H2CO absorption to determine the relative line-of-sight positions of molecular and ionized gas. We measure gas densities using the H2CO densitometer, including continuous measurements of the dense gas mass fraction (DGMF) over the range 10 4 cm −3 < n(H2) < 10 6 cm −3 -this is the first time a dense gas mass fraction has been measured over a range of densities with a single data set. Results. The DGMF in W51 A is high, f 70% above n > 10 4 cm −3 , while it is low, f < 20%, in W51 B. We did not detect any H2CO emission throughout the W51 GMC; all gas dense enough to emit under normal conditions is in front of bright continuum sources and therefore is seen in absorption instead. The data set has been made public at http://dx.doi.org/10.7910/DVN/26818.