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.'
author:
- 'Y. T. Yan'
- 'J. S. Zhang'
- 'C. Henkel'
- 'T. Mufakharov'
- 'L. W. Jia'
- 'X. D. Tang'
- 'Y. J. Wu'
- 'J. Li'
- 'Z. A. Zeng'
- 'Y. X. Wang'
- 'Y. Q. Li'
- 'J. Huang'
- 'J. M. Jian'
title: 'A systematic TMRT observational study of Galactic $^{12}$C/$^{13}$C ratios from Formaldehyde'
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Introduction {#sec:intro}
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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 (@Wilson1994). In particular, the $^{12}$C/$^{13}$C ratio is one of the most useful tracers of the relative degree of primary to secondary processing. $^{12}$C is known to be produced primarily via Helium burning in massive stars, on rapid time scales, whereas $^{13}$C is thought to be formed primarily through CNO processing 