2008ApJ...672..398I

To study how the structure of the envelopes of young stellar objects (YSOs) evolve, we carried out a deep JHK'-band imaging survey of the M17 star-forming region using a near-infrared camera and the Infrared Camera and Spectrograph with adaptive optics mounted on the Subaru Telescope. In this survey, we found 51 dark silhouettes against bright near-infrared nebula emissions as background lights. They are regarded as envelopes associated with YSOs due to their size and association with the YSOs. We derived size, morphology, extinction, and mass for each silhouette envelope. The average mass of the envelopes was ∼0.02 M_☉, and the radius ranged from 1.5" to 2.8" (2250-4200 AU). We compared the properties of envelopes with the properties of the central YSO derived from color-color and color-magnitude diagrams. The radius and the mass of the envelope were found to decrease with the decrease in infrared excess, consistent with the widely accepted view in which the envelope accretes onto the central star/disk system during its evolution from Class I to Class II. However, the envelope still exists around the Class II object for which accretion is barely discernible, and it may have escaped detection due to its dimness. The envelope associated with Class II objects may not accrete onto the star or the disk, but it may dissipate in the future.