Gas and Dust Shadows in the TW Hydrae Disk

TL;DR

This study uses new CO emission observations and reanalyzed scattered light data to investigate the gas and dust shadow dynamics in the TW Hya disk, providing empirical constraints on dust-gas collisional timescales.

Contribution

It introduces a novel analysis linking gas and dust shadow offsets to collisional timescales, offering the first empirical constraints on $t_{coll}$ in a protoplanetary disk.

Findings

Azimuthal variations in CO emission up to 20% beyond 180 au.

An oscillatory model better explains the dust shadow evolution.

Derived collisional timescales range from near instantaneous to ~10 years.

Abstract

We present new observations of CO J=2-1 emission from the protoplanetary disk around TW Hya. Emission is detected out to 240 au (4") and found to exhibit azimuthal variations up to 20% beyond 180 au (3"), with the west side of the disk brighter than the east. This asymmetry is interpreted as tracing the shadow previously seen in scattered light. A reanalysis of the multi-epoch observations of the dust shadow in scattered light from Debes et al. (2017) suggests that an oscillatory motion would provide a better model of the temporal evolution of the dust shadow rather than orbital motion. Both models predict an angular offset between the dust shadow and the gas shadow of up to ~100 deg. We attribute this offset to the finite rate at which dust grains and gas molecules can exchange heat, dominated by the collisional rate between gas molecules and dust grains, $t_{\rm coll}$. The angular offsets derived are equivalent to collisional timescales that range from the near instantaneous up to $t_{\rm coll}$ ~ 10 years, depending on whether a straight or a curved dust shadow, as suggested by HST observations reported by Debes et al. (2017), is adopted. The inferred range of $t_{\rm coll}$ are consistent with those predictions based on representative gas densities, temperatures, gas-to-dust ratios and grain sizes. These results represent the first time empirical constraints can be placed on $t_{\rm coll}$.