VLT-ISAAC 3-5 μm spectroscopy of embedded young low-mass stars. III. Intermediate-mass sources in vela

W.F. Thi, E.F. van Dishoeck, E. Dartois, K.M. Pontoppidan, W.A. Schutte, P. Ehrenfreund, L. d'Hendecourt, H.J. Fraser

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Abstract

Aims. We study in this paper the ice composition in the envelope around intermediate-mass class I Young Stellar Objects (YSOs).
Methods. We performed a spectroscopic survey toward five intermediate-mass class I YSOs located in the Southern Vela molecular cloud in the L (2.85-4.0 m) and M (4.55-4.8 m) bands at resolving powers -800 up to 10 000, using the Infrared Spectrometer and Array Camera mounted on the Very Large Telescope-ANTU. Lower mass companion objects were observed simultaneously in both bands.
Results. Solid H 2O at 3 m is detected in all sources, including the companion objects. CO ice at 4.67 m is detected in a few main targets and one companion object. One object ( LLN 19 ) shows little CO ice but strong gas-phase CO ro-vibrational lines in absorption. The CO ice profiles are different from source to source. The amount of water ice and CO ice trapped in a water-rich mantle may correlate with the flux ratio at 12 and 25 m. The abundance of H2O-rich CO likely correlates with that of water ice. A weak feature at 3.54 m attributed to solid CH 3OH and a broad feature near 4.62 m are observed toward , but not toward the other sources. The derived abundances of solid CH 3OH and OCN- are ~% and ~% of the H 2O ice abundance respectively. The H 2O optical depths do not show an increase with envelope mass, nor do they show lower values for the companion objects compared with the main protostar. The line-of-sight CO ice abundance does not correlate with the source bolometric luminosity.
Conclusions. Comparison of the solid CO profile toward LLN 17 , which shows an extremely broad CO ice feature, and that of its lower mass companion at a few thousand AU, which exhibits a narrow profile, together with the detection of OCN- toward LLN 17 provide direct evidences for local thermal processing of the ice.
Original languageEnglish
Pages (from-to)251-265
Number of pages14
JournalAstronomy and Astrophysics
Volume449
Issue number1
DOIs
Publication statusPublished - Apr 2006

Keywords

  • astrophysics
  • interstellar ice
  • stars
  • young stellar objects

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