Search for 17 μm H2Pure Rotational Emission from Circumstellar Disks

Abstract

We report the negative detection of the S(1) pure rotational line emission of molecular hydrogen at 17.035 μm for four young stars, HD 163296, MWC 863, CQ Tau, and LkCa 15, for which Infrared Space Observatory (ISO) observations detected the S(1) emission, with the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the 8.2 m Subaru Telescope. We did not detect the line emission toward the central star nor within 15'' of the central star along the slit for any of the present targets. Upper limits of the present observations are much smaller than previous line flux estimates based on ISO Short Wavelength Spectrometer ( SWS) observations, irrespective of the intrinsic line width. The present results indicate that the emission detected by the ISO SWS is not concentrated within the disk. Our detection limit of the H2 S(1) emission corresponds to upper limits of the disk masses of (0.72-3.8) × 10-4 M☉ within a radius of ~30-45 AU for the optically thin emission from the gas of 150 K. The upper limits of the disk masses are significantly lower than those of the warm molecular hydrogen mass predicted by the model (Chiang & Goldreich 1997), suggesting that the optically thick emission from dust dominates in the radiation from the disks in the mid-infrared wavelength. We point out that detectability of the H2 emission in young stars depends on the evolution of the disks, particularly in the sedimentation and growth of dust grains in the disk.