Ice and Dust in the Quiescent Medium of Isolated Dense Cores

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 µm wavelength range are combined. The baseline for the broad and overlapping ice features is modeled, using calculated spectra of giants, H 2 O 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 µm is ∼50% relative to that at 2.2 µm. The strengths of the 6.0 and 6.85 µm absorption bands are in line with those of YSOs. Thus, their carriers, which, besides H 2 O and CH 3 OH, may include NH + 4 , HCOOH, H 2 CO and NH 3 , are readily formed in the dense core phase, before stars form. The 3.53 µm C-H stretching mode of solid CH 3 OH was discovered. The CH 3 OH/H 2 O 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 µm band of silicates relative to the continuum extinction at 2.2 µm 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.

Spectroscopic constraints on CH3OH formation: CO mixed with CH3OH ices towards young stellar objects

“…Observational studies show that H 2 O freezes out at much lower extinctions (Whittet et al 2001;Chiar et al 2011;Boogert et al 2013;Whittet et al 2013) than CO, which needs a higher density to freeze out (Chiar et al 1994). Methanol forms at even higher extinction as shown by ice observations (Boogert et al 2011;Chiar et al 2011;Whittet et al 2011), which is consistent with the picture that a relatively high density is needed to efficiently convert CO into CH 3 OH Boogert, Gerakines & Whittet 2015).…”

Vacuum-UV spectroscopy of interstellar ice analogs

“…Our VUV-absorption spectra can be directly applied to astrophysical icy environments practically made of a single compound, for example, either CO 2 or H 2 O largely dominate the composition of different regions at the south pole of Mars (Bibring et al 2004). Nevertheless, interstellar ice mantles are either thought to be a mixture of different species or to present a layered structure (e.g., Gerakines et al 1995;Dartois et al 1999;Pontoppidan et al 2008;Boogert et al 2011;Öberg et al 2011;Kim et al 2012). Our study on pure ices can be used to estimate the absorption of multilayered ice mantles, but a priori, it cannot be extrapolated to ice mixtures.…”

Water in low-mass star-forming regions withHerschel

“…Surveys of large samples of low-and high-mass protostars as well as background sources reveal typical water ice column density ratios N s-H 2 O /N H ∼ 5 × 10 −5 (e.g., Smith et al 1993;Gibb et al 2004;Pontoppidan et al 2004;Boogert et al 2004Boogert et al , 2008Boogert et al , 2011Whittet et al 2001Whittet et al , 2007Whittet et al , 2013Öberg et al 2011). Here N H is the column density of hydrogen nuclei, inferred either from the silicate optical depth or the colour excess toward the star.…”