Evaporation of grain-surface species by shock waves in proto-planetary disk

TL;DR

This study investigates how shock waves in proto-planetary disks can cause evaporation of grain-surface molecules like SO and H2O, explaining observed warm gas emissions around forming protostars.

Contribution

It demonstrates that thermal desorption in shock waves efficiently releases molecules from grains, providing a physical mechanism for observed warm gas in protostellar disks.

Findings

Thermal desorption dominates over sputtering in post-shock regions.

SO can be desorbed at lower velocities than H2O under certain conditions.

Warm post-shock gas column density is approximately 10^21 cm^-2.

Abstract

Recent ALMA (Atacama Large Millimeter/submillimeter Array) observations of young protostellar objects detected warm SO emission, which could be associated with a forming protostellar disk. In order to investigate if such warm gas can be produced by accretion shock onto the forming disk, we calculate the sputtering and thermal desorption of various grain surface species in one dimensional shock waves. We find that thermal desorption is much more efficient than the sputtering in the post-shock region. While H$_{2}$O can be thermally desorbed, if the accretion velocity is larger than 8 km s$^{-1}$ with the pre-shock gas number density of 10$^{9}$ cm$^{-3}$, SO is desorbed, if the accretion velocity $\gtrsim$ 2 km s$^{-1}$ and $\gtrsim$ 4km s$^{-1}$, with the pre-shock density of 10$^{9}$ cm$^{-3}$ and 10$^{8}$ cm$^{-3}$, respectively. We also find that the column density of hydrogen nuclei in warm post-shock gas is $N_{{\rm warm}} \sim 10^{21}$ cm$^{-2}$.