A Multiline Analysis of the Distribution and Excitation of CS and H₂CS in the HD 163296 Disk

Abstract The abundance and distribution of sulfur-bearing molecules in protoplanetary disks directly influence the composition and potential habitability of nascent planets in addition to providing powerful probes of the physical gas conditions in the disks themselves. Here, we present new and archival Atacama Large Millimeter/submillimeter Array and Submillimeter Array observations of CS and H 2 CS, and their C 34 S and H 2 C 34 S isotopologues, at high angular resolution (≈0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> </mml:math> 2–0 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>″</mml:mi> </mml:mrow> </mml:mover> </mml:math> 4; 20–40 au) in the HD 163296 disk, which reveal a central cavity and multiringed emission structure. These observations comprise the most comprehensive, multiline CS data in a planet-forming disk to date, spanning a wide range of excitation conditions from E u = 7.1 to 129.3 K, and include new detections of C 34 S, H 2 CS, and H 2 C 34 S in this system. Using these data, we derive spatially resolved rotational temperature and column density profiles for all species. We find a column density ratio N (H 2 CS)/ N (CS) ≈ 0.5, which is comparable to that of the similar MWC 480 disk and suggests that organic sulfur compounds may constitute a large fraction of the volatile sulfur reservoir in disks around Herbig stars generally. We derive 32 S/ 34 S ratios of ≈5 (CS/C 34 S) and ≈2 (H 2 CS/H 2 C 34 S) based on disk-averaged and spatially resolved analyses. Both values are consistent across these two pairs of optically thin molecules and are well below the expected ratio in the interstellar medium of ≈22, suggesting significant sulfur fractionation. We also constrain the CS emitting layer ( z / r ≲ 0.1) using the vertical separations of the disk surfaces in the channel maps and based on the known 2D gas structure of the HD 163296 disk combined with our excitation analysis.