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

Thi, W.F. and van Dishoeck, E.F. and Dartois, E. and Pontoppidan, K.M. and Schutte, W.A. and Ehrenfreund, P. and d'Hendecourt, L. and Fraser, H.J. (2006) VLT-ISAAC 3-5 μm spectroscopy of embedded young low-mass stars. III. Intermediate-mass sources in vela. Astronomy and Astrophysics, 449 (1). pp. 251-265. ISSN 0004-6361

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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.