Galactic interstellar sulfur isotopes: A radial ${}^{32}$S$/$${}^{34}$S gradient?

Abstract

We present observations of ${}^{12}$C${}^{32}$S, ${}^{12}$C${}^{34}$S, ${}^{13}$C${}^{32}$S and ${}^{12}$C${}^{33}$S 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 ${}^{12}$C/${}^{13}$C ratio, the ${}^{32}$S$/$${}^{34}$S isotope ratio was determined from the integrated ${}^{13}$C${}^{32}$S/${}^{12}$C${}^{34}$S line intensity ratios for our sample. Our analysis shows a ${}^{32}$S$/$${}^{34}$S gradient from the inner Galaxy out to a galactocentric distance of 12\,kpc. An unweighted least-squares fit to our data yields ${}^{32}$S$/$${}^{34}$S = (1.56 $\pm$ 0.17)${\mathrm{D}}_{\mathrm{GC}}$ + (6.75 $\pm$ 1.22) with a correlation coefficient of 0.77. Errors represent 1$\sigma$ standard deviations. Testing this result by (a) excluding the Galactic center region, (b) excluding all sources with C${}^{34}$S opacities $>$ 0.25, (c) combining our data and old data from previous study, and (d) using different sets of carbon isotope ratios leads to the conclusion that the observed ${}^{32}$S$/$${}^{34}$S gradient is not an artefact but persists irrespective of the choice of the sample and carbon isotope data. A gradient with rising ${}^{32}$S$/$${}^{34}$S values as a function of galactocentric radius implies that the solar system ratio should be larger than that of the local interstellar medium. With the new carbon isotope ratios we obtain indeed a local ${}^{32}$S$/$${}^{34}$S isotope ratio about 10$\%$ below the solar system one, as expected in case of decreasing ${}^{32}$S$/$${}^{34}$S ratios with time and increased amounts of stellar processing. However, taking older carbon isotope ratios based on a lesser amount of data, such a decrease is not seen. No systematic variation of ${}^{34}$S$/$${}^{33}$S ratios along galactocentric distance was found.

arXiv comment28 pages, 11 figures, 5 tables. Accepted for publication in ApJ

References in corpus (21)

• A computer program for fast non-LTE analysis of interstellar line spectra
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole
• Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way
• Abundance Gradients and the Formation of the Milky Way
• The Distance to the Perseus Spiral Arm in the Milky Way
• A Parallax-based Distance Estimator for Spiral Arm Sources
• Kinematic Distances to Molecular Clouds identified in the Galactic Ring Survey
• Stellar populations dominated by massive stars in dusty starburst galaxies across cosmic time
• Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey
• The RMS Survey: Ammonia and water maser analysis of massive star forming regions
• The Bolocam Galactic Plane Survey. VII. Characterizing the Properties of Massive Star-Forming Regions
• The Bolocam Galactic Plane Survey. XIV. Physical Properties of Massive Starless and Star Forming Clumps
• A systematic TMRT observational study of Galactic ${}^{12}$C/${}^{13}$C ratios from Formaldehyde
• The Bolocam Galactic Plane Survey -- III. Characterizing Physical Properties of Massive Star-Forming Regions in the Gemini OB1 Molecular Cloud
• ATCA Survey of Ammonia in the Galactic Center: The Temperatures of Dense Gas Clumps between SgrA* and SgrB2
• The abundance of 36S in IRC+10216 and its production in the Galaxy
• Physical characterisation of southern massive star-forming regions using Parkes NH${}_3$ observations
• A spectral line survey of IRC +10216 between 13.3 and 18.5 GHz
• Searching for Inflow Towards Massive Starless Clump Candidates Identified in the Bolocam Galactic Plane Survey
• The Evolution Of Carbon, Sulphur, and Titanium Isotopes from High-Redshift to the Local Universe

Cited by in corpus (8)

• ALCHEMI: an ALMA Comprehensive High-resolution Extragalactic Molecular Inventory. Survey presentation and first results from the ACA array
• Direct measurements of carbon and sulfur isotope ratios in the Milky Way
• Sulfur isotope ratios in the Large Magellanic Cloud
• Interstellar Nitrogen Isotope Ratios: New NH3 Data from the Galactic Center out to the Perseus Arm
• A Systematic Observational Study on Galactic Interstellar Ratio 18O/17O. II. C18O and C17O J=2-1 Data Analysis
• A Possible Chemical Clock in High-mass Star-forming Regions: N(HC3N)/N(N2H+)?
• The GUAPOS project. VI: the chemical inventory of shocked gas
• Looking for evidence of high-mass star formation at core scale in a massive molecular clump