Spectroscopy of Molecular Hydrogen Emission from KH 15D

Abstract

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 (1000) reveal the presence of prominent molecular hydrogen emission in 5 lines near 2 microns. The relative line strengths are consistent with thermal excitation at T = 2800 +/-300K. Observations out of eclipse, at both low and high spectral resolution (20,000), 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 more than 4 arc-sec both east and west of the stellar point spread function (more than 3000 AU). Observed at high spectral resolution, the velocity and intensity structure of the 1-0 S(1) profile are both asymmetric. East of the stellar PSF (by 1.1 - 2.3 arc-sec) the emission is blueshifted (-63 km/sec), and has significantly greater intensity than the marginally redshifted component (+2 km/sec, consistent with zero) which 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 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.