Measuring the $\mathrm{^{34}S}$ and $\mathrm{^{33}S}$ isotopic ratios of volatile sulfur during planet formation

TL;DR

This study measures sulfur isotope ratios in a planet-forming disk using ALMA, revealing variations from solar values and providing insights into sulfur chemistry and isotopic fractionation during planet formation.

Contribution

First constraints on 32S/34S and 32S/33S isotope ratios in a planet-forming disk using ALMA observations of SO and SO2 isotopologues.

Findings

SO depleted in heavy sulfur compared to solar values.

SO2 enriched in heavy sulfur relative to solar values.

Sulfur isotope ratios suggest non-solar isotopic compositions in disk reservoirs.

Abstract

Stable isotopic ratios constitute powerful tools for unraveling the thermal and irradiation history of volatiles. In particular, we can use our knowledge of the isotopic fractionation processes active during the various stages of star, disk and planet formation to infer the origins of different volatiles with measured isotopic patterns in our own solar system. Observations of planet-forming disks with the Atacama Large Millimeter/submillimeter Array (ALMA) now readily detect the heavier isotopologues of C, O and N, while the isotopologue abundances and isotopic fractionation mechanisms of sulfur species are less well understood. Using ALMA observations of the SO and SO2 isotopologues in the nearby, molecule-rich disk around the young star Oph-IRS 48 we present the first constraints on the combined 32S/34S and 32S/33S isotope ratios in a planet-forming disk. Given that these isotopologues likely originate in relatively warm gas (>50 K), like most other Oph-IRS 48 volatiles, SO is depleted in heavy sulfur while SO2 is enriched compared to solar system values. However, we cannot completely rule out a cooler gas reservoir, which would put the SO sulfur ratios more in line with comets and other solar system bodies. We also constrain the S18O/SO ratio and find the limit to be consistent with solar system values given a temperature of 60 K. Together these observations show that we should not assume solar isotopic values for disk sulfur reservoirs, but additional observations are needed to determine the chemical origin of the abundant SO in this disk, inform on what isotopic fractionation mechanism(s) are at play, and aid in unravelling the history of the sulfur budget during the different stages of planet formation.