2008A&A...488..235G

Molecular hydrogen is the main constituent of circumstellar disks and could be an important tracer for the evolution and structure of such disks. So far, H₂ has only been detected in a few disks and only through spectroscopic observations, resulting in a limited knowledge of the spatial distribution of the H₂ emitting gas. We report the detection of quiescent H₂ emission in a spatially resolved ring-like structure within 100AU of T Tau N. We present evidence to show that the emission most likely arises from shocks in the atmosphere of a nearly face-on disk around T Tau N. Using high spatial resolution 3D spectroscopic K-band data, we trace the spatial distribution of several H₂ NIR rovibrational lines in the vicinity of T Tau N. We examine the structure of the circumstellar material around the star through SED modeling. Then, we use models of shocks and UV+X-ray irradiation to reproduce the H₂ line flux and line ratios in order to test how the H₂ is excited. We detect weak H₂ emission from the v=1-0 S(0), S(1), Q(1) lines and the v=2-1 S(1) line in a ring-like structure around T Tau N between 0.1" (∼15AU) and 0.7" (∼100AU) from the star. The v=1-0 S(0) and v=2-1 S(1) lines are detected only in the outer parts of the ring structure. Closer to the star, the strong continuum limits our sensitivity to these lines. The total flux of the v=1-0 S(1) line is 1.8x10^–14erg/s/cm2, similar to previous measurements of H₂ in circumstellar disks. The velocity of the H₂ emitting gas around T Tau N is consistent with the rest velocity of the star, and the H₂ does not seem to be part of a collimated outflow. Both shocks impinging on the surface of a disk and irradiation of a disk by UV-photons and X-rays from the central star are plausible candidates for the H₂ excitation mechanism. However, irradiation should not create a large degree of excitation at radii larger than 20AU. Most likely the H₂ emission arises in the atmosphere of a flared disk with radius 85-100 AU and mass 0.005-0.5M_☉, where the gas is excited by shocks created when a wide-angle wind impinges on the disk. The H₂ emission could also originate from shock excitation in the cavity walls of an envelope, but this requires an unusually high velocity of the wide-angle wind from T Tau N.