Abundant Refractory Sulfur in Protoplanetary Disks

Sulfur is one of the most abundant elements in the universe, with important roles in astro-, geo-, and biochemistry. Its main reservoirs in planet-forming disks have previously eluded detection: gaseous molecules only account for <1% of total elemental sulfur, with the rest likely in either ices or refractory minerals. We use a new method to measure the refractory component. Mechanisms such as giant planets can filter out dust from gas accreting onto disk-hosting stars. For stars above 1.4 solar masses, this leaves a chemical signature on the stellar photosphere that can be used to determine the fraction of each element that is locked in dust. Here, we present an application of this method to sulfur, zinc, and sodium. We analyze the accretion-contaminated photospheres of a sample of young stars and find (89 ± 8)% of elemental sulfur is in refractory form in their disks. The main carrier is much more refractory than water ice, consistent with sulfide minerals such as FeS.