Aims. Determining molecular abundance ratios is important not only for the study of the Galactic chemistry but also because they are useful to estimate physical parameters in a large variety of interstellar medium environments. The CO is one of the most important molecules to trace the molecular gas in the interstellar medium, 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 this kind of isotopes 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 isotopes observations towards a large sample of Galactic sources of different nature at different locations. Methods. To study the 13 CO/C 18 O abundance ratio it was used 12 CO J=3-2 data obtained form 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. It was analyzed a sample of 198 sources composed by young stellar objects (YSOs), Hii and diffuse Hii regions as catalogued in the Red MSX Source Survey in 27 • .5 ≤ l ≤ 46 • .5 and |b| ≤ 0 • .5. Results. Most of the analyzed sources are located in the galactocentric distance range 4.0-6.5 kpc. We found that YSOs have, in average, smaller 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 the predicted from the known elemental abundance relations. These results would be useful in future studies of molecular gas related to YSOs and Hii regions based on the observation of these isotopes.
Aims. Estimating molecular abundances ratios from the direct measurement of the emission of the molecules towards a variety of interstellar environments is indeed very useful to advance in our understanding of the chemical evolution of the Galaxy, and hence of the physical processes related to the chemistry. It is necessary to increase the sample of molecular clouds, located at different distances, in which the behaviour of molecular abundance ratios, such as the 13 CO/C 18 O ratio, is studied in detail. Methods. We selected the well-studied high-mass star-forming region G29.96−0.02, located at a distance of about 6.2 kpc, which is an ideal laboratory to perform this kind of studies. To study the 13 CO/C 18 O abundance ratio (X 13/18 ) towards this region it was 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 13 CO and C 18 O J=2-1 data retrieved from the CDS database which were observed with the IRAM 30 m telescope. The distribution of column densities and X 13/18 throughout the extension of the analyzed molecular cloud was studied based on LTE and non-LTE methods.Results. Values of X 13/18 between 1.5 to 10.5, with an average of about 5, were found across the studied region, showing that, besides the dependency between X 13/18 and the galactocentric distance, the local physical conditions may strongly affect this abundance ratio. We found that correlating the X 13/18 map with the location of the ionized gas and dark clouds allows us to suggest in which regions the far-UV radiation stalls in dense gaseous components, and in which ones it escapes and selectively photodissociates the C 18 O isotope.The non-LTE analysis shows that the molecular gas has very different physical conditions, not only spatially across the cloud, but also along the line of sight. This kind of studies may represent a tool to indirectly estimate (from molecular lines observations) the degree of photodissociation in molecular clouds, which is indeed useful to study the chemistry in the interstellar medium.
Context. The fragmentation of a molecular cloud that leads to the formation of high-mass stars occurs on a hierarchy of different spatial scales. The large molecular clouds harbor massive molecular clumps with massive cores embedded in them. The fragmentation of these cores may determine the initial mass function and the masses of the final stars. Therefore, studying the fragmentation processes in the cores is crucial to understanding how massive stars form. Aims. Detailed studies toward particular objects are needed to collect observational evidence that shed light on star formation processes on the smallest spatial scales. The hot molecular core G34–MM1, embedded in the filamentary infrared dark cloud (IRDC) G34.34+00.24 located at a distance of 3.6 kpc, is a promising object for studying fragmentation and outflow processes. Methods. Using data at 93 and 334 GHz obtained from the Atacama Large Millimeter Array (ALMA) database we studied in great detail the hot molecular core G34–MM1. The angular resolution of the data at 334 GHz is about 0.′′8, which allows us to resolve structures of about 0.014 pc (~2900 au). Results. We found evidence of fragmentation toward the molecular hot core G34–MM1 on two different spatial scales. The dust condensation MM1–A (about 0.06 pc in size) harbors three molecular subcore candidates (SC1 through SC3) detected in 12CO J = 3–2 emission, with typical sizes of about 0.02 pc and an average spatial separation among them of about 0.03 pc. From the HCO+ J = 1–0 emission, we identify, with better angular resolution than previous observations, two perpendicular molecular outflows arising from MM1–A. We suggest that subcores SC1 and SC2, embedded in MM1–A, respectively harbor the sources responsible for the main and the secondary molecular outflow. Finally, from the radio continuum emission at 334 GHz, we marginally detected another dust condensation, named MM1–E, from which a young (tdyn ~ 1.6 × 103 yr), massive (M ~ 5 M⊙), and energetic (E ~ 6 × 1046 ergs) molecular outflow arises. Conclusions. The fragmentation of the hot molecular core G34–MM1 at two different spatial scales, together with the presence of multiple molecular outflows associated with it, would support a competitive accretion scenario. Studies like this shed light on the relation between fragmentation and star formation processes occurring within hot molecular cores, only accessible through high angular resolution interferometric observations.
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