VLT-ISAAC 3-5 m spectroscopy of embedded young low-mass stars
III. Intermediate-mass sources in VelaW.-F. Thi1, 2, 3, E. F. van Dishoeck2, E. Dartois4, K. M. Pontoppidan2, 5, W. A. Schutte2, P. Ehrenfreund2, L. d'Hendecourt4 and H. J. Fraser2, 6
1 Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
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)
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.
Key words: stars: circumstellar matter -- astrochemistry -- ISM: molecules
© ESO 2006