A Hubble Space Telescope Survey of H2 Emission in the Circumstellar Environments of Young Stars

TL;DR

This study presents the largest spectrally resolved survey of H2 emission in young star systems, revealing that most molecular gas resides within 3 AU of the star, crucial for understanding planet formation.

Contribution

It provides new spectrally resolved H2 emission data for 34 young stars, including transitional disks, using Hubble Space Telescope observations, highlighting the spatial distribution of molecular gas.

Findings

H2 emission detected in all accreting stars and transitional disks

Most H2 emission originates within 3 AU of the star

Emission line widths are consistent with mid-IR CO spectra

Abstract

The formation timescale and final architecture of exoplanetary systems are closely related to the properties of the molecular disks from which they form. Observations of the spatial distribution and lifetime of the molecular gas at planet-forming radii (r < 10 AU) are important for understanding the formation and evolution of exoplanetary systems. Towards this end, we present the largest spectrally resolved survey of H2 emission around low-mass pre-main sequence stars compiled to date. We use a combination of new and archival far-ultraviolet spectra from the COS and STIS instruments on the Hubble Space Telescope to sample 34 T Tauri stars (27 actively accreting CTTSs and 7 non-accreting WTTSs) with ages ranging from roughly 1-10 Myr. We observe fluorescent H2 emission, excited by LyA photons, in 100 of the accreting sources, including all of the transitional disks in our sample (CS Cha, DM Tau, GM Aur, UX Tau A, LkCa15, HD 135344B and TW Hya). The spatial distribution of the emitting gas is inferred from spectrally resolved H2 line profiles. Some of the emitting gas is produced in outflowing material, but the majority of H2 emission appears to originate in a rotating disk. For the disk-dominated targets, the H2 emission originates predominately at r < 3 AU. The emission line-widths and inner molecular radii are found to be roughly consistent with those measured from mid-IR CO spectra.