A Search for Spatially Resolved Infrared Ro-Vibrational Molecular Hydrogen Emission from the Disks of Young Stars

TL;DR

This study spatially resolves near-infrared H$_2$ emission in nine young star systems, revealing diverse structures including disks, shocks, and outflows, and providing new insights into the excitation mechanisms of circumstellar gas.

Contribution

First spatially resolved survey of near-infrared H$_2$ emission in multiple young star disks, identifying structures and excitation processes.

Findings

H$_2$ emission is spatially resolved in eight stars.

Anti-correlation between H$_2$ and X-ray luminosities suggests non-X-ray excitation.

Detection of spiral structures in AB Aur's disk matching ALMA CO maps.

Abstract

We present results from a survey searching for spatially resolved near-infrared line emission from molecular hydrogen gas in the circumstellar environments of nine young stars: AA Tau, AB Aur, DoAr 21, GG Tau, GM Aur, LkCa 15, LkH$\alpha$ 264, UY Aur, and V773 Tau. Prior high-resolution spectra of these stars showed the presence of ro-vibrational H$_2$ line emission at 2.12$\mu$m with characteristics more typical of gas located in proto-planetary disks rather than outflows. In this study, we spatially resolve the H$_2$ emission in the eight stars where it is detected. LkCa 15 is the only target that exhibits no appreciable H$_2$ despite a prior detection. We find an anti-correlation between H$_2$ and X-ray luminosities, likely indicating that the X-ray ionization process is not the dominant H$_2$ excitation mechanism in these systems. AA Tau, UY Aur, and V773 Tau show discrete knots of H$_2$, as typically associated with shocks in outflowing gas. UY Aur and V773 Tau exhibit spatially resolved velocity structures, while the other systems have spectrally unresolved emission consistent with systemic velocities. V773 Tau exhibits a complex line morphology indicating the presence of multiple excitation mechanisms, including red and blue-shifted bipolar knots of shock-excited outflowing gas. AB Aur, GM Aur, and LkH$\alpha$ 264 have centralized, yet spatially resolved H$_2$ emission consistent with a disk origin. The H$_2$ images of AB Aur reveal spiral structures within the disk, matching those observed in ALMA CO maps. This survey reveals new insights into the structure and excitation of warm gas in the circumstellar environments of these young stars.