INTERSTELLAR DUST ABSORPTION FEATURES IN THE INFRARED SPECTRUM OF HH 100-IR: SEARCHING FOR THE NITROGEN COMPONENT OF THE ICES

摘要

We present observations of solid-state absorption features due to water ice, CO ice, and silicate dust in the spectrum of the bright infrared source (IRS 1) associated with the Herbig-Haro nebula HH 100 in the R Coronae Australis dark cloud. These absorptions are shown to arise predominantly in the molecular cloud rather than in circumstellar matter associated with the infrared source itself, which we deduce to be a pre-main-sequence star obscured by ～25 mag of visual extinction. In common with other lines of sight, the spectra indicate the presence of distinct hydrogen-rich (polar) and hydrogen-poor (nonpolar) phases in the ice toward HH 100-IR. The nonpolar phase is dominated by CO. The strength of the solid CO feature at 4.67 μm suggests that as much as ～40% of all CO in the line of sight may be in the solid phase. Our data show a lack of significant absorption at 4.62 μm that might arise in CN-bearing molecules formed by energetic processing of the mantles. A previous report of structure in the profile of the 3 μm water-ice feature in HH 100-IR that might be attributed to the N-H resonance in condensed ammonia at ～2.96 μm is not confirmed. The abundance ratio NH_3:H_2O in the ices is found to be no more than ～8%, which is probably insufficient to account for the 3.3-3.6 μm "ice-band wing" in terms of ammonium hydrate. However, we propose that the wing might be explained by hydrates containing other bases in addition to ammonia/Further progress is likely to require new laboratory data for appropriate ice mixtures.