Tianma Telescope's Systematic Observation And Study Of Formaldehyde And Its Isotope Molecules In The Milky Way

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.In particular,the¹²C/¹³C ratio is one of the most useful tracers of the relative degree of primary to secondary processing.We present observations of the C-band 1₁₀-1₁₁（4.8 GHz）and Ku-band 2₁₁-2₁₂（14.5GHz）K-doublet lines of H₂CO and the C-band 1₁₀-1₁₁（4.6 GHz）line of H₂¹³CO to-ward 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 iso-tope ratio H₂¹²CO/H₂¹³CO 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 obtain¹²C/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（2013）.Also implementing distance values from trigonometric parallax measurements for our sources,we obtain a linear fit of¹²C/¹³C=（5.08±1.10）D_GC+（11.86±6.60）,with a correlation coeffi-cient of 0.58.D_GCrefers to Galactocentric distances.Our¹²C/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,it’s helpful for us to better understand the chemical evolution of the Milky Way.