Compact Hydrogen Sulfide Emission Indicates Sulfur-bearing Ice Sublimation in the Inner Disk of HD 163296

TL;DR

Deep ALMA observations of hydrogen sulfide in the HD 163296 disk reveal compact emission from the inner 3-5 au, indicating sulfur-bearing molecule sublimation in warm, planet-forming regions.

Contribution

First high-resolution detection and modeling of H₂S, SO, and SO₂ emission in a protoplanetary disk, constraining their origin and sulfur chemistry in the inner disk region.

Findings

H₂S and SO emission originate from within 3-5 au of the star.

Gas temperatures of 90-120 K suggest sublimation of sulfur molecules.

H₂S may be a significant sulfur reservoir in the disk.

Abstract

The sulfur chemistry in protoplanetary disks directly affects the composition and potential habitability of nascent planets, but its volatile inventory remains highly uncertain. Here, we present deep Atacama Large Millimeter/submillimeter Array (ALMA) observations of hydrogen sulfide (H$_2$S) along with SO and SO$_2$ in the disk around HD 163296 at an angular resolution of $\approx0.\!\!^{\prime\prime}3$ (or $\approx$30 au). We detect unresolved, compact emission of H$_2$S and SO (and tentatively SO$_2$) at the disk center with a broad line width of $\sim$40 km s$^{-1}$, suggesting that the emission is originating from the innermost regions. By fitting line profiles with a geometrically-thin Keplerian-rotating disk model, we constrain the emitting radii and gas temperatures of these molecules to be $\approx$3-5 au and $\gtrsim$90-120 K, respectively, consistent with sublimation of sulfur-bearing molecules along with water ice in the inner warm region. While the higher or comparable column density of H$_2$S with respect to SO and SO$_2$ indicates that H$_2$S is an important volatile sulfur reservoir in the disk, the limited constraints mean that we cannot rule out significantly depleted volatile sulfur as also commonly inferred in other planet-forming disks. Further observations are needed to better constrain disk sulfur inventory, unravel how sulfur compounds are reprocessed in disks, and shed light on the nature of less-volatile species, such as salts and sulfide minerals, which may occupy a significant portion of sulfur budget.