Direct measurements of carbon and sulfur isotope ratios in the Milky Way

Abstract

With the IRAM 30 meter telescope, we performed observations of the $J$ = 2-1 transitions of CS, C${}^{33}$S, C${}^{34}$S, C${}^{36}$S, ${}^{13}$CS, ${}^{13}$C${}^{33}$S, and ${}^{13}$C${}^{34}$S as well as the $J$ = 3-2 transitions of C${}^{33}$S, C${}^{34}$S, C${}^{36}$S, and ${}^{13}$CS toward a large sample of 110 HMSFRs. We measured the ${}^{12}$C/${}^{13}$C, ${}^{32}$S/${}^{34}$S, ${}^{32}$S/${}^{33}$S, ${}^{32}$S/${}^{36}$S, ${}^{34}$S/${}^{33}$S, ${}^{34}$S/${}^{36}$S, and ${}^{33}$S/${}^{36}$S abundance ratios with rare isotopologs of CS, thus avoiding significant saturation effects. With accurate distances obtained from parallax data, we confirm previously identified ${}^{12}$C/${}^{13}$C and ${}^{32}$S/${}^{34}$S gradients as a function of galactocentric distance (RGC). In the CMZ, ${}^{12}$C/${}^{13}$C ratios are higher than suggested by a linear fit to the disk values as a function of RGC. While ${}^{32}$S/${}^{34}$S ratios near the Galactic center and in the inner disk are similar, this is not the case for ${}^{12}$C/${}^{13}$C, when comparing central values with those near RGC of 5 kpc. As was already known, there is no ${}^{34}$S/${}^{33}$S gradient but the average ratio of 4.35~$\pm$~0.44 derived from the $J$ = 2-1 transition lines of C${}^{34}$S and C${}^{33}$S is well below previously reported values. A comparison between solar and local interstellar ${}^{32}$S/${}^{34}$S and ${}^{34}$S/${}^{33}$S ratios suggests that the Solar System may have been formed from gas with a particularly high ${}^{34}$S abundance. For the first time, we report positive gradients of ${}^{32}$S/${}^{33}$S, ${}^{34}$S/${}^{36}$S, ${}^{33}$S/${}^{36}$S, and ${}^{32}\mathrm{S}{/}^{36}\mathrm{S}$ in our Galaxy. The predicted ${}^{12}$C/${}^{13}$C ratios from the latest GCE models are in good agreement with our results. While ${}^{32}$S/${}^{34}$S and ${}^{32}$S/${}^{36}$S ratios show larger differences at larger RGC, ${}^{32}$S/${}^{33}$S ratios show an offset across the entire inner 12 kpc of the Milky Way.

HMSFRs: high-mass star-forming regions. CMZ: central molecular zone. GCE: Galactic chemical evolution. 68 pages, 12 tables, 18 figures. Accepted for publication in A&A