CO isotopes towards Galactic YSOs and HII regions

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 27.5{deg}<=l<=46.5{deg} and |b|<0.5{deg}.

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.