A systematic TMRT observational study of Galactic ${}^{12}$C/${}^{13}$C ratios from Formaldehyde

Abstract

We present observations of the C-band $1_{10}-1_{11}$ (4.8 GHz) and Ku-band $2_{11}-2_{12}$ (14.5 GHz) K-doublet lines of H${}_2$CO and the C-band $1_{10}-1_{11}$ (4.6 GHz) line of H${}_2$${}^{13}$CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65-m radio telescope (TMRT). Our sample with 112 sources includes strong H${}_2$CO sources from the TMRT molecular line survey at C-band and other known H${}_2$CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34\%. Complementary observations of their continuum emission at both C- and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H${}_2$${}^{12}$CO/H${}_2$${}^{13}$CO 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 ${}^{12}$C/${}^{13}$C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from \citet{Wiesenfeld2013}. Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of ${}^{12}$C/${}^{13}$C = (5.08$\pm$1.10)D${}_{GC}$ + (11.86$\pm$6.60), with a correlation coefficient of 0.58. D${}_{GC}$ refers to Galactocentric distances. Our ${}^{12}$C/${}^{13}$C ratios agree very well with the ones deduced from CN and C${}^{18}$O but are lower than those previously reported on the basis of H${}_2$CO, tending to suggest that the bulk of the H${}_2$CO in our sources was formed on dust grain mantles and not in the gas phase.

27 pages, 8 figures, 7 tables. Accepted for publication in The Astrophysical Journal

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