VLT-ISAAC 3–5 μm spectroscopy of embedded young low-mass stars

III. Intermediate-mass sources in Vela

¹ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: ewine@strw.leidenuniv.nl
² Leiden Observatory, PO Box 9513, 2300, Leiden, The Netherlands
³ ESA Research Fellow, Research Support Science Department, ESTEC, Keplerlaan 1, PO 2201 AZ, Noordwijk, The Netherlands
⁴ Astrochimie Expérimentale, Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
⁵ Divisions of Geological and Planetary Sciences, California Institute of Technology, Passadena, CA 91125, USA
⁶ Department of Physics, University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow G4 ONG, UK

Received: 24 February 2005
Accepted: 28 November 2005

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 μm) and M (4.55–4.8 μm) 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 HO at 3 μm is detected in all sources, including the companion objects. CO ice at 4.67 μm 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 μm. The abundance of H₂O-rich CO likely correlates with that of water ice. A weak feature at 3.54 μm attributed to solid CHOH and a broad feature near 4.62 μm are observed toward LLN 17, but not toward the other sources. The derived abundances of solid CHOH and OCN^- are ~% and ~% of the HO ice abundance respectively. The HO 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.