Sulfur Chemistry in Protoplanetary Disks: CS and H₂CS

Abstract

The nature and abundance of sulfur chemistry in protoplanetary disks (PPDs) may impact the sulfur inventory on young planets and therefore their habitability. PPDs also present an interesting test bed for sulfur chemistry models, since each disk present a diverse set of environments. In this context, we present new sulfur molecule observations in PPDs, and new S-disk chemistry models. With ALMA we observed the CS 5 − 4 rotational transition toward five PPDs (DM Tau, DO Tau, CI Tau, LkCa 15, MWC 480), and the CS 6 − 5 transition toward three PPDs (LkCa 15, MWC 480 and V4046 Sgr). Across this sample, CS displays a range of radial distributions, from centrally peaked, to gaps and rings. We also present the first detection in PPDs of 13 CS 6 − 5 (LkCa 15 and MWC 480), C 34 S 6 − 5 (LkCa 15), and H 2 CS 8 17 − 7 16 , 9 19 − 8 18 and 9 18 − 8 17 (MWC 480) transitions. Using LTE models to constrain column densities and excitation temperatures, we find that either 13 C and 34 S are enhanced in CS, or CS is optically thick despite its relatively low brightness temperature. Additional lines and higher spatial resolution observations are needed to distinguish between these scenarios. Assuming CS is optically thin, CS column density model predictions reproduce the observations within a factor of a few for both MWC 480 and LkCa 15. However, the model underpredicts H 2 CS by 1-2 orders of magnitude. Finally, comparing the H 2 CS/CS ratio observed toward the MWC 480 disk and toward different ISM sources, we find the closest match with prestellar cores. two of the Taurus disks: the Herbig Ae disk MWC 480, and the T Tauri disk LkCa 15. The spectral resolution for these lines was ∼ 975 kHz, i.e. corresponding to a velocity resolution of ∼ 1 km/s. The 12 CS 6 − 5 transition was observed in three execution blocks on January 17, 2016, April 23, 2016 and December 12, 2016. For the first execution block 31 antennas were included and covered baselines from 15 to 331 m. 36 antennas were included for the two other execution blocks, covering baselines from 15 to 463 m and 15 to 650 m, respectively. The first and third blocks used the source J0510+1800 as band-pass and flux calibrators, and the source J0438+3004 as phase calibrator. The second block of execution used the source J0238+1636 as band-pass calibrator, the source J0433+2905 as phase calibrator, and the source J0510+1800 as flux calibrator. The total on-source integration time were 17.6, 12.6 and 20.7 min., respectively.The 13 CS and C 34 S 6−5, and H 2 CS 8 17 −7 16 rotational transitions were observed in a single execution block, on January 17, 2016, with the same calibrator sources as those used for the first and second execution blocks used for 12 CS 6 − 5 (see above) but with 36 antennas and a total on-source integration time of ∼ 19.2 min.

High resolution observations of molecular emission lines toward the CI Tau proto-planetary disc: planet-carved gaps or shadowing?

“…A ring morphology is in agreement with observations and modelling of CS in other discs (e.g., Le Gal et al 2019), indicating that the ring is likely produced by chemical effects rather than a variation of the gas surface density or gas temperature. Our analysis shows that the ring is slightly offset from the central star, consistent with the CS emission originating from a relative height of / ∼ 0.1 in the disc.…”

“…They find the emission originates from / ∼ 0.15-0.2 (above one scale height), broadly in agreement with their model predictions of a CS reservoir between / ∼ 0.2-0.4. More recently, Le Gal et al (2019) analysed CS emission from a broader sample of discs (including DM Tau). While relative heights were not directly measured, their models show agreement with the above, placing CS reservoirs at / > 0.1.…”

“…In this context, CI Tau is a source which shows three gaps and rings (Clarke et al 2018;Long et al 2018). It has a bright continuum disc (Guilloteau et al 2011) and several molecular species have been detected at sub-mm wavelengths (Guilloteau et al 2014(Guilloteau et al , 2016Bergner et al 2019;Le Gal et al 2019;Pegues et al 2020), making it a natural target for sensitive line studies. In this paper we report the results of high-resolution (100-150 marcsec) observations of this source in the 12 CO & 13 CO =3-2 and CS = 7-6 emission lines.…”

Organosulfur Compounds Formed by Sulfur Ion Bombardment of Astrophysical Ice Analogs: Implications for Moons, Comets, and Kuiper Belt Objects

“…On Europa, the formation of H 2 SO 4 has been supposed via radiolysis from energetic charged particle bombardment and is supported both by experiments (Strazzulla et al 2007) and observations (Dalton et al 2013). Next to the presented numerous observations of sulfur molecules in space, their chemistry, especially those of organosulfur, is not well understood yet (e.g., problem of sulfur depletion in dense clouds and star-forming regions but no sulfur depletion in diffuse clouds in t h eI S M ;C a l m o n t ee ta l .2016; Le Gal et al 2019).…”

“…A hypothesis suggested that the sulfur is locked in the form of OCS in icy grain mantles (Palumbo et al 1997). A recent study has modeled that the "missing" sulfur is indeed trapped in the form of organosulfur molecules on grains (Laas & Caselli 2019).I n protoplanetary disks, the planets' places of birth around young stars, CS or H 2 CS chemistry has been discussed (Semenov et al 2018;Le Gal et al 2019). Nevertheless, the number of sulfur molecules detected in disks is significantly lower than in the diffuse ISM, indicating a high reactivity of sulfur in the gas phase (Semenov et al 2018).…”