Timescale for disk survival: A search for the molecular hydrogen component of protoplanetary disks orbiting T Tauri stars.

摘要

We have surveyed several X-ray bright, classical and weak-lined T Tauri stars (TTS)—young, Sun-like stars with ages of 1–20 Myr—located in three nearby star forming regions (D 200 pc) for near-infrared line emission at 2.1218 μm from quiescent, molecular hydrogen gas. Using high-resolution near-infrared spectrometers and 3- to 4-m class telescopes, we have detected H₂ emission from 4 of the 27 TTS surveyed: TW Hya, LkCa 15, GG TauA, and DoAr 21. Based upon the velocity information obtained from the emission features associated with these detections as well as the lack of extended emission in the images, I argue that for each star the emission is closely associated with the young source and most likely resides in a gaseous disk component orbiting the young source.; Gas temperatures found in the circumstellar disks (T ≃ 10 2−3 K) observed to surround TTS are, on average, much less than the temperatures (T ≃ 2000 K) thought to be required to produce observable levels of H₂ emission from rotation-vibrational transitions within the ground state of the H₂ molecule. In past observations of near-infrared H₂ emission, stimulation mechanisms have been shown to be capable of producing the H₂ emission when thermally excited emission is negligible. We review several possible stimulation mechanisms including shocks, Lyα photons, UV fluorescence, and X-ray ionization, all previously suggested as ways to stimulate H₂ emission. Based upon the double-peaked H₂ emission feature associated with LkCa 15, the penetration depths we estimate for UV and X-ray photons from models of the disks of classical TTS, and the temperature independent H₂ gas masses that range between 10−10 and 10−12 $M\odot$, we determined that the emission must reside in the upper atmospheres of these disks at radii of 10 to 30 AU from the sources. Using the existing models associated with UV fluorescence and X-ray ionization, we find that one or both of these mechanisms may be capable of producing the observed level of H₂ emission. Therefore, we suggest that the X-ray/UV fluxes associated with TTS are sufficient to stimulate the observed emission and imply the existence of a significant amount of H₂ in the disks of these sources.; The detection of H₂ emission from a disk surrounding the weak-lined TTS DoAr 21 not only indicates the presence of a gaseous component of a disk orbiting this young star, but implies the presence of an undetected dust component. Previously, sources such as DoAr 21 have been thought to be ‘naked’ or without circumstellar disk material and, therefore, incapable of forming planets. Stimulated H₂ emission from sources such as DoAr 21 suggests that disks may survive beyond the disk lifetimes inferred from detections of disk tracers (i.e., dust and CO).; High-resolution near-infrared surveys capable of detecting X-ray and/or UV stimulated emission from quiescent H₂ gas residing in the circumstellar disks of young stars should provide insight into the timescale for planet formation by allowing astronomers to search for the most abundant component of such disks.