The Photoevaporative Wind from the Disk of TW Hya

TL;DR

This study uses high-resolution spectroscopy to investigate the photoevaporative wind of TW Hya's disk, confirming the [Ne II] line traces unbound gas within 10AU, and analyzing optical lines to understand the disk's structure and dispersal mechanisms.

Contribution

It provides new observational constraints on the photoevaporative wind in TW Hya, especially confirming the origin of [Ne II] emission and analyzing optical lines to distinguish between different disk gap formation scenarios.

Findings

[Ne II] traces unbound wind within 10AU

Optical forbidden lines are centered at stellar velocity

Most optical emission arises from bound gas, not the wind

Abstract

Photoevaporation driven by the central star is expected to be a ubiquitous and important mechanism to disperse the circumstellar dust and gas from which planets form. Here, we present a detailed study of the circumstellar disk surrounding the nearby star TW Hya and provide observational constraints to its photoevaporative wind. Our new high-resolution (R ~ 30,000) mid-infrared spectroscopy in the [Ne II] 12.81 {\mu}m line confirms that this gas diagnostic traces the unbound wind component within 10AU from the star. From the blueshift and asymmetry in the line profile, we estimate that most (>80%) of the [Ne II] emission arises from disk radii where the midplane is optically thick to the redshifted outflowing gas, meaning beyond the 1 or 4AU dust rim inferred from other observations. We re-analyze high-resolution (R ~ 48, 000) archival optical spectra searching for additional transitions that may trace the photoevaporative flow. Unlike the [Ne II] line, optical forbidden lines from OI, SII, and MgI are centered at the stellar velocity and have symmetric profiles. The only way these lines could trace the photoevaporative flow is if they arise from a disk region physically distinct from that traced by the [Ne II] line, specifically from within the optically thin dust gap. However, the small (~10 km/s) FWHM of these lines suggest that most of the emitting gas traced at optical wavelengths is bound to the system rather than unbound. We discuss the implications of our results for a planet-induced versus a photoevaporation-induced gap.