Ice and Dust in the Quiescent Medium of Isolated Dense Cores

TL;DR

This study investigates the composition and properties of ices in isolated dense cores, revealing that complex ices form before star formation and differ from those in YSOs, with implications for initial conditions in star-forming regions.

Contribution

It provides the first detailed comparison of ice features in quiescent dense cores and YSOs, showing that complex ices form prior to star formation and vary with environment.

Findings

Solid CH3OH detected with 5-12% abundance relative to H2O.

Ices in dense cores show no signs of thermal processing seen in YSOs.

Silicate absorption features are shallower than in diffuse ISM.

Abstract

The relation between ices in the envelopes and disks surrounding YSOs and those in the quiescent interstellar medium is investigated. For a sample of 31 stars behind isolated dense cores, ground-based and Spitzer spectra and photometry in the 1-25 um wavelength range are combined. The baseline for the broad and overlapping ice features is modeled, using calculated spectra of giants, H2O ice and silicates. The adopted extinction curve is derived empirically. Its high resolution allows for the separation of continuum and feature extinction. The extinction between 13-25 um is ~50% relative to that at 2.2 um. The strengths of the 6.0 and 6.85 um absorption bands are in line with those of YSOs. Thus, their carriers, which, besides H2O and CH3OH, may include NH4+, HCOOH, H2CO and NH3, are readily formed in the dense core phase, before stars form. The 3.53 um C-H stretching mode of solid CH3OH was discovered. The CH3OH/H2O abundance ratios of 5-12% are larger than upper limits in the Taurus molecular cloud. The initial ice composition, before star formation occurs, therefore depends on the environment. Signs of thermal and energetic processing that were found toward some YSOs are absent in the ices toward background stars. Finally, the peak optical depth of the 9.7 um band of silicates relative to the continuum extinction at 2.2 um is significantly shallower than in the diffuse interstellar medium. This extends the results of Chiar et al. (2007) to a larger sample and higher extinctions.