A Search for Near - Infrared Molecular Hydrogen Emission in the CTTS LkH α 264 and the debris disk 49 Cet ⋆

We report on the first results of a search for molecular hydrogen emission from protoplanetary disks using CRIRES, ESO's new VLT Adaptive Optics high resolution near-infrared spectrograph. We observed the classical T Tauri star LkHα 264 and the debris disk 49 Cet, and searched for υ = 1 − 0 S(1) H 2 emission at 2.1218 µm, υ = 1 − 0 S(0) H 2 emission at 2.2233 µm and υ = 2 − 1 S(1) H 2 emission at 2.2477 µm. The H 2 line at 2.1218 µm is detected in LkHα 264 confirming the previous observations by Itoh et al. (2003). In addition, our CRIRES spectra reveal the previously observed but not detected H 2 line at 2.2233 µm in LkHα 264. An upper limit of 5.3 ×10 −16 ergs s −1 cm −2 on the υ = 2 − 1 S(1) H 2 line flux in LkHα 264 is derived. The detected lines coincide with the rest velocity of LkHα 264. They have a FWHM of ∼20 km s −1. This is strongly suggestive of a disk origin for the lines. These observations are the first simultaneous detection of υ = 1 − 0 S(1) and υ = 1 − 0 S(0) H 2 emission from a protoplanetary disk. 49 Cet does not exhibit H 2 emission in any of the three observed lines. We derive the mass of optically thin H 2 at T ∼ 1500 K in the inner disk of LkHα 264 and derive stringent limits in the case of 49 Cet at the same temperature. There are a few lunar masses of optically thin hot H 2 in the inner disk (∼ 0.1 AU) of LkHα 264, and less than a tenth of a lunar mass of hot H 2 in the inner disk of 49 Cet. The measured 1-0 S(0)/1-0 S(1) and 2-1 S(1)/1-0 S(1) line ratios in LkHα 264 indicate that the H 2 emitting gas is at a temperature lower than 1500 K and that the H 2 is most likely thermally excited by UV photons. The υ = 1 − 0 S(1) H 2 line in LkHα 264 is single peaked and spatially unresolved. Modeling of the shape of the line suggests that the disk should be seen close to face-on (i < 35 o) and that the line is emitted within a few AU of the LkHα 264 …