Spitzer spectroscopic survey of ices around low-mass young stellar objects

With the goal to study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low luminosity YSOs (L ∼ 0.1− 10 L⊙) using 5–38 μm Spitzer Space Telescope and 3–4 μm ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well studied massive YSOs (L ∼ 10 L⊙). This paper focuses on the origin of the prominent absorption features in the 5-8 μm spectral region. The long-known 6.0 and 6.85 μm bands are detected toward all sources, with the Class 0-type low mass YSOs showing the deepest bands ever observed. In almost all sources the 6.0 μm band is deeper, by up to a factor of 3, than expected from the bending mode of pure solid H2O, based on the optical depths of the 3.0 μm stretching and 13 μm libration modes. The depth and shape variations of the remaining 5–7 μm absorption indicate that it consists of 5 independent components, which, by comparison to laboratory studies, must be from at least 8 different carriers. Together with information from the 3-4 μm spectra and the additionally detected weak 7.25, 7.40, 9.0, and 9.7 μm features it is argued that overlapping bands of simple species are responsible for much of the absorption in the 5-7 μm region, at abundances of 1-30% for CH3OH, 3-8% for NH3, 1-5% for HCOOH, ∼6% for H2CO, and ∼0.3% for HCOO − with respect to solid H2O. The 6.85 μm band likely consists of one or two carriers, of which one is less volatile than H2O because its abundance relative to H2O is enhanced at lower H2O/τ9.7 ratios. It does not survive in the diffuse interstellar medium (ISM), however. The similarity of the 6.85 μm bands for YSOs and background stars indicates that its carrier(s) must be formed early in the molecular cloud evolution. If an NH4 salt is the carrier its abundance with respect to solid H2O is typically 7%, and low temperature acid-base chemistry or cosmic ray induced reactions must have been involved in its formation. Possible origins are discussed for the carrier of an enigmatic, very broad absorption between 5 and 8 μm. It shows large depth variations toward both lowand high-mass YSOs. Weak evidence is found that it correlates with temperature tracers. Finally, all the phenomena observed for ices toward massive YSOs are also observed toward low mass YSOs, indicating that processing of the ices by internal ultraviolet radiation fields is a minor factor in the early chemical evolution of the ices. Subject headings: infrared: ISM — ISM: molecules — ISM: abundances — stars: formation — infrared: stars— astrochemistry 1 Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. 2 The VLT/ISAAC spectra were obtained at the European Southern Observatory, Paranal, Chile, within the observing programs 164.I-0605, 69.C-0441, and 272.C-5008 3 Division of PMA, Mail Code 105-24, California Institute of Technology, Pasadena, CA 91125, USA 4 AURA/NOAO-South, Gemini Science Center, Casilla 603, La Serena, Chile 5 current address: IPAC, NASA Herschel Science Center, Mail Code 100-22, California Institute of Technology, Pasadena, CA 91125, USA (email: aboogert@ipac.caltech.edu) 6 Division of GPS, Mail Code 150-21, California Institute of Technology, Pasadena, CA 91125, USA 7 Hubble Fellow 8 Department of Astronomy, University of Maryland, College Park, MD 20742, USA 9 Leiden Observatory, PO Box 9513, 2300 RA Leiden, the Netherlands 10 SRON, PO Box 800, 9700 AV Groningen, the Netherlands 11 Department of Astronomy, University of Texas at Austin, 1