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J/A+A/612/A117 CO isotopes towards Galactic YSOs and HII regions (Areal+, 2018)
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Beginning of ReadMe :J/A+A/612/A117 CO isotopes towards Galactic YSOs and HII regions (Areal+, 2018)
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Exploring the ^13^CO/C^18^O abundance ratio towards Galactic young stellar
objects and HII regions.
Areal M.B., Paron S., Celis Pena M., Ortega M.E.
<Astron. Astrophys., 612, A117 (2018)>
=2018A&A...612A.117A (SIMBAD/NED BibCode)
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ADC_Keywords: YSOs ; H II regions ; Carbon monoxide ; Abundances
Keywords: ISM: abundances - ISM: molecules - Galaxy: abundances -
H II regions - stars: formation
Abstract:
Aims. Determining molecular abundance ratios is important not only for
the study of Galactic chemistry, but also because they are useful to
estimate physical parameters in a large variety of interstellar medium
environments. One of the most important molecules for tracing the
molecular gas in the interstellar medium is CO, and the ^13^CO/C^18^O
abundance ratio is usually used to estimate molecular masses and
densities of regions with moderate to high densities. Nowadays isotope
ratios are in general indirectly derived from elemental abundances
ratios. We present the first ^13^CO/C^18^O abundance ratio study
performed from CO isotope observations towards a large sample of
Galactic sources of different natures at different locations. Methods.
To study the ^13^CO/C^18^O abundance ratio, we used ^12^CO J=3-2 data
obtained from the CO High-Resolution Survey, ^13^CO and C^18^O J=3-2
data from the ^13^CO/C^18^O (J=3-2) Heterodyne Inner Milky Way Plane
Survey, and some complementary data extracted from the James Clerk
Maxwell Telescope database. We analyzed a sample of 198 sources
composed of young stellar objects (YSOs), and HII and diffuse HII
regions as catalogued in the Red MSX Source Survey in
27.5deg<=l<=46.5deg and |b|<0.5deg. Results. Most of the
analyzed sources are located in the galactocentric distance range
4.0-6.5kpc. We found that YSOs have, on average, lower ^13^CO/C^18^O
abundance ratios than HII and diffuse HII regions. Taking into account
that the gas associated with YSOs should be less affected by the
radiation than in the case of the others sources, selective far-UV
photodissociation of C^18^O is confirmed. The ^13^CO/C^18^O abundance
ratios obtained in this work are systematically lower than those
predicted from the known elemental abundance relations. These results
will be useful in future studies of molecular gas related to YSOs and
HII regions based on the observation of these isotopes.
Description:
The data of the CO isotopes were extracted from two public databases
performed with the 15m James Clerk Maxwell Telescope (JCMT) in Hawaii.
The ^12^CO J=3-2 data were obtained from the CO High-Resolution Survey
(COHRS) with an angular and spectral resolution of 14" and 1km/s (see
Dempsey et al., 2013ApJS..209....8D). The data of the other CO
isotopes were obtained from the ^13^CO/C^18^O (J=3-2) Heterodyne Inner
Milky Way Plane Survey (CHIMPS), which have an angular and spectral
resolution of 15" and 0.5km/s (Rigby et al., 2016MNRAS.456.2885R).
Taking into account that the CHIMPS survey covers the Galactic region
in 27.5deg<l<46.5deg and |b|<=0.5deg, we selected all sources
catalogued as YSOs, and selected HII and diffuse HII regions lying in
this area from the Red MSX Source Survey (Lumsden et al., 2013, Cat.
J/ApJS/208/11), which is the largest statistically selected catalogue
of young massive protostars and HII regions to date.