Spatially resolved H2 emission from the disk around T Tau N

Context: 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.
Aims: We report the detection of quiescent H2 emission in a spatially resolved ring-like structure within 100 AU 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.
Methods: 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.
Results: 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 arcsec ( 15 AU) and 0.7 arcsec ( 100 AU) 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 1.8 × 10^-14 erg s^-1 cm^-2, 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 20 AU. Most likely the H2 emission arises in the atmosphere of a flared disk with radius 85-100 AU and mass 0.005-0.5 M_⊙, 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.

Based on observations collected at the European Southern

Observatory, Paranal, Chile under the programme 60.A-9041(A).