Volume 585, January 2016
|Number of page(s)||12|
|Section||Interstellar and circumstellar matter|
|Published online||22 December 2015|
Molecular gas in absorption and emission along the line of sight to W31C G10.62-0.38
1 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
2 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Université Paris 6, École Normale Supérieure, 75005 Paris, France
Received: 29 August 2015
Accepted: 5 October 2015
Context. The sightline to W31C G10.62-0.38 was extensively observed in absorption under the PRISMAS program on Herschel.
Aims. We relate absorbing material to the older view of Galactic molecules gained from CO emission.
Methods. We used the Arizona Radio Observatory Kitt Peak 12m antenna to observe emission from the J = 1−0 lines of carbon monoxide, HCO+, and HNC, and the J = 2−1 line of CS toward and around the continuum peak used for absorption studies and we compare them with CH, HNC, C+, and other absorption spectra from PRISMAS. We develop a kinematic analysis that allows a continuous description of the spectral properties and relates them to viewing geometry in the Galaxy.
Results. As it is for CH, HF, C+, HCO+, and other species observed in absorption, mm-wave emission in CO, HCO+, HNC, and CS is continuous over the full velocity range expected for material between the Sun and W31 4.95 kpc away. CO emission is much stronger than average in the Galactic molecular ring and the mean H2 density derived from CH, 4 cm-3 ≲ 2⟨ n(H2) ⟩ ≲ 10 cm-3 at 4 ≲ R ≲ 6.4 kpc, is similarly elevated. The CO-H2 conversion factor falls in a narrow range XCO= 1−2 × 1020 H2 cm-2 (K−km s-1)-1 if the emitting gas is mostly on the near side of the subcentral point, as we suggest. The brightnesses of HCO+, HNC, and CS are comparable (0.83%, 0.51%, and 1.1%, respectively, relative to CO) and have no variation in galactocentric radius with respect to CO. Comparison of the profile-averaged HCO+ emission brightness and optical depth implies local densities n(H) ≈ 135 ± 25 cm-3 with most of the excitation of HCO+ from electrons. At this level of density, a consistent picture of the H2-bearing gas, which also accounts for the CO emission, has a volume filling factor of 3% and a 5 pc clump or cloud size.
Key words: ISM: molecules / ISM: clouds / Galaxy: structure
© ESO, 2015
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.