Quiescent H2 Emission From Pre-Main Sequence Stars in Chamaeleon I

TL;DR

This study reports the detection of quiescent molecular hydrogen emission from circumstellar disks of pre-main sequence stars in Chamaeleon I, linking it to accretion activity, inner disk gas, and stellar high-energy radiation.

Contribution

It provides new observational evidence of H$_2$ emission in pre-main sequence stars and explores its correlation with disk evolution, accretion, and high-energy photon excitation.

Findings

H$_2$ emission detected in six pre-main sequence stars.

Detected H$_2$ emission correlates with transitional disks and accretion signatures.

H$_2$ emission likely originates from gas at a few tens of AU from the stars.

Abstract

We report the discovery of quiescent emission from molecular hydrogen gas located in the circumstellar disks of six pre-main sequence stars, including two weak-line T Tauri stars (TTS), and one Herbig AeBe star, in the Chamaeleon I star forming region. For two of these stars, we also place upper limits on the 2->1 S(1)/1->0 S(1) line ratios of 0.4 and 0.5. Of the 11 pre-main sequence sources now known to be sources of quiescent near-infrared hydrogen emission, four possess transitional disks, which suggests that detectable levels of H$_2$ emission and the presence of inner disk holes are correlated. These H$_2$ detections demonstrate that these inner holes are not completely devoid of gas, in agreement with the presence of observable accretion signatures for all four of these stars and the recent detections of [Ne II] emission from three of them. The overlap in [Ne II] and H$_2$ detections hints at a possible correlation between these two features and suggests a shared excitation mechanism of high energy photons. Our models, combined with the kinematic information from the H$_2$ lines, locate the bulk of the emitting gas at a few tens of AU from the stars. We also find a correlation between H$_2$ detections and those targets which possess the largest H$\alpha$ equivalent widths, suggesting a link between accretion activity and quiescent H$_2$ emission. We conclude that quiescent H$_2$ emission from relatively hot gas within the disks of TTS is most likely related to on-going accretion activity, the production of UV photons and/or X-rays, and the evolutionary status of the dust grain populations in the inner disks.