Galactic Interstellar Sulfur Isotopes: A Radial ³²S/³⁴S Gradient?

Abstract

We present observations of 12C32S, 12C34S, 13C32S, and 12C33S J = 2−1 lines toward a large sample of massive star-forming regions by using the Arizona Radio Observatory 12 m telescope and the IRAM 30 m. Taking new measurements of the carbon 12C/13C ratio, the 32S/34S isotope ratio was determined from the integrated 13C32S/12C34S line intensity ratios for our sample. Our analysis shows a 32S/34S gradient from the inner Galaxy out to a galactocentric distance of 12 kpc. An unweighted least-squares fit to our data yields 32S/34S = (1.56 ± 0.17)D GC + (6.75 ± 1.22) with a correlation coefficient of 0.77. Errors represent 1σ standard deviations. Testing this result by (a) excluding the Galactic center region, (b) excluding all sources with C34S opacities >0.25, (c) combining our data and old data from previous study, and (d) using different sets of carbon isotope ratios leads to the conclusion that the observed 32S/34S gradient is not an artifact but persists irrespective of the choice of sample and carbon isotope data. A gradient with rising 32S/34S values as a function of galactocentric radius implies that the solar system ratio should be larger than that of the local interstellar medium. With the new carbon isotope ratios, we indeed obtain a local 32S/34S isotope ratio about 10% below the solar system one, as expected in the case of decreasing 32S/34S ratios with time and increased amounts of stellar processing. However, taking older carbon isotope ratios based on a lesser amount of data, such a decrease is not seen. No systematic variation of 34S/33S ratios along galactocentric distance was found. The average value is 5.9 ± 1.5, the error denoting the standard deviation of an individual measurement.

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

“…These are only slightly lower than the value of 22 in the Solar System. There is an obvious and confirmed 32 S/ 34 S gradient (Chin et al 1996;Yu et al 2020) from the inner Galaxy out to a galactocentric distance of 12.0 kpc. Nevertheless, there is a lack of data at small and large galactocentric distances.…”

“…To enlarge this sample, we chose 92 sources from the Bar and Spiral Structure Legacy (BeSSeL) Survey 1 in 2020. These 92 targets were recently released by the BeSSeL project (Reid et al 2019) and not observed by Yu et al (2020). In total, 110 objects in the Galaxy are part of our survey.…”

“…CS is unique in that it is a simple diatomic molecule exhibiting strong line emission and possessing eight stable isotopologs, which allows us to determine the above-mentioned carbon and sulfur isotope ratios. Six isotopologs have been detected so far in the ISM (e.g., Chin et al 1996;Mauersberger et al 1996;Humire et al 2020;Yu et al 2020).…”

“…We are performing systematic observational studies on isotope ratios in the Milky Way, including 12 C/ 13 C (Yan et al 2019), 14 N/ 15 N (Chen et al 2021), 18 O/ 17 O (Zhang et al 2015(Zhang et al , 2020a(Zhang et al , 2020bLi et al 2016), and 32 S/ 34 S (Yu et al 2020). We have thus performed a more systematic study on CS and its isotopologs toward 110 high-mass star-forming regions (HMSFRs).…”

A Possible Chemical Clock in High-mass Star-forming Regions: N(HC3N)/N(N2H+)?

“…where η BD is the beam-filling factor and θ s and θ beam are the source size and the HPBW, respectively (Zhang et al 2017;Yu et al 2020). According to T mb values measured by IRAM 30 m and ARO 12 m, the source size (HC 3 N J = 10−9) is estimated for those 33 sources except G010.32−00.15 and G109.87 +02.11 (unreasonable results of size, larger than the HPBW of ARO 12 m, may be nonuniform with a clumpy structure).…”