Abstract
Abstract We present observations of the C-band 110–111 (4.8 GHz) and Ku-band 211–212 (14.5 GHz) K-doublet lines of H2CO and the C-band 110–111 (4.6 GHz) line of H2 13CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65 m radio telescope (TMRT). Our sample with 112 sources includes strong H2CO sources from the TMRT molecular line survey at C-band and other known H2CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34%. Complementary observations of their continuum emission at both C- and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H2 12CO/H2 13CO for each source. To evaluate photon trapping caused by sometimes significant opacities in the main isotopologue’s rotational mm-wave lines connecting our measured K-doublets, and to obtain 12C/13C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from Wiesenfeld & Faure. Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of 12C/13C = (5.08 ± 1.10)D GC + (11.86 ± 6.60), with a correlation coefficient of 0.58. D GC refers to Galactocentric distances. Our 12C/13C ratios agree very well with the ones deduced from CN and C18O but are lower than those previously reported on the basis of H2CO, tending to suggest that the bulk of the H2CO in our sources was formed on dust grain mantles and not in the gas phase.
Highlights
Isotope abundance ratios provide a powerful tool to trace stellar nucleosynthesis, to evaluate the enrichment of the interstellar medium (ISM) by stellar ejecta and to constrain the chemical evolution of the Milky Way (Wilson & Rood 1994)
We present observations of the C-band 110 − 111 (4.8 GHz) and Ku-band 211 − 212 (14.5 GHz) K-doublet lines of H2CO and the C-band 110 − 111 (4.6 GHz) line of H213CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65-m radio telescope (TMRT)
Our 12C/13C ratios agree very well with the ones deduced from CN and C18O but are lower than those previously reported on the basis of H2CO, tending to suggest that the bulk of the H2CO in our sources was formed on dust grain mantles and not in the gas phase
Summary
Isotope abundance ratios provide a powerful tool to trace stellar nucleosynthesis, to evaluate the enrichment of the interstellar medium (ISM) by stellar ejecta and to constrain the chemical evolution of the Milky Way (Wilson & Rood 1994). 12C is known to be produced primarily via Helium burning in massive stars, on rapid time scales, whereas 13C is thought to be formed primarily through CNO processing of 12C seeds from earlier stellar generations. This occurs on a slower timescale during the red giant phase in low and intermediate-mass stars or novae (Henkel et al 1994, Meyer 1994; Wilson & Rood 1994). A 12C/13C ratio cannot be directly deduced from 12CO (hereafter CO) and 13CO, since CO (and sometimes 13CO) is optically thick Their isotopologues C18O and 13C18O, exhibiting optically thin lines, are sometimes used to determine 12C/13C (Langer & Penzias 1990). The goal of our study is to critically review their results using new distances and H2CO collision rates as well as a larger sample of sources and to examine in how far previous results have to be modified
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