Spectroscopy of Molecular Hydrogen Emission from KH 15D

We report infrared spectroscopy of the unusual eclipsing pre-main-sequence object KH 15D, obtained using NIRSPEC on Keck II. During eclipse, observations using low spectral resolution (λ/δλ~1000) reveal the presence of prominent molecular hydrogen emission in five lines near 2 μm. The relative line strengths are consistent with thermal excitation at T~2800+/-300 K. Observations out of eclipse, at both low and high spectral resolution (λ/δλ~2×10⁴), show reduced contrast with the stellar continuum. The change in contrast for the strongest line, 1-0 S(1), is consistent with an approximately constant emission line superposed on a variable stellar continuum. Emission in the 1-0 S(1) line is observed to extend by >~4" both east and west of the stellar point-spread function (PSF; >~3000 AU). Observed at high spectral resolution, the velocity and the intensity structure of the 1-0 S(1) profile are both asymmetric. East of the stellar PSF (by 1.1"-2.3") the emission is blueshifted (-63 km s^-1) and has significantly greater intensity than the marginally redshifted component (+2 km s^-1, approximately consistent with zero) that dominates west of the stellar PSF. The spatial extent of the emission and the excitation temperature suggest shock excitation of ambient gas by a bipolar outflow from the star and/or the disk. However, it is difficult to account for the observed radial velocity unless the outflow axis is inclined significantly to the plane of the sky.

Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.