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

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 (TTSs) 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 H₂ emission and the presence of inner disk holes are correlated. These H₂ 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 H₂ 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 H₂ lines, locate the bulk of the emitting gas at a few tens of AU from the stars. We also find a correlation between H₂ detections and those targets which possess the largest Hα equivalent widths, suggesting a link between accretion activity and quiescent H₂ emission. We conclude that quiescent H₂ emission from relatively hot gas within the disks of TTSs is most likely related to ongoing accretion activity, the production of UV photons and/or X-rays, and the evolutionary status of the dust grain populations in the inner disks.