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Issue A&A
Volume 401, Number 2, April II 2003
Page(s) 613 - 624
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20030158



A&A 401, 613-624 (2003)
DOI: 10.1051/0004-6361:20030158

ISOCAM-CVF spectroscopy of the circumstellar environment of young stellar objects

R. D. Alexander1, M. M. Casali1, P. André2, P. Persi3 and C. Eiroa4

1  UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
2  Service d'Astrophysique, CEA/DSM/DAPNIA, C.E. Saclay, 91191 Gif-sur-Yvette Cedex, France
3  IASF, CNR Roma, Via Fosso del Cavaliere, 00133 Roma, Italy
4  Dpto. Física Teórica, C-XI, Facultad de Ciencias, UAM, Cantoblanco, 28049 Madrid, Spain

(Received 28 August 2002 / Accepted 4 February 2003)

Abstract
We present the results of a mid-infrared (5-16.5  $\mu$m) imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, $\rho$ Ophiuchi, Serpens and Chamaeleon I. This survey was performed using ISOCAM and its Circular Variable Filters (CVF) and observed 42 YSO candidates: we were able to obtain complete 5-16.5  $\mu$m spectra for 40 of these with a spectral resolving power of $\lambda/\Delta\lambda\simeq40$. A number of spectral features were measured, most notably the 9.7  $\mu$m silicate feature, the bending modes of both water and CO 2 ices at 6.0 and 15.2  $\mu$m respectively and the well-known unidentified feature at 6.8  $\mu$m. The strength of the unidentified feature was observed to correlate very well with that of the water ice bending mode and far less strongly with the CO 2 ice bending mode. This suggests, in a manner consistent with previous observations, that the carrier of the unidentified feature is a strongly polar ice. Absorption profiles of the bending mode of CO 2 ice are observed to show a significant long wavelength wing, which suggests that a significant fraction of the CO 2 ice observed exists in a polar (H 2O-rich) phase. The sources observed in RCrA, $\rho$ Oph and Serpens show similar spectral characteristics, whilst the sources observed in Cha I are somewhat anomalous, predominantly showing silicate emission and little or no absorption due to volatile ices. However this is consistent with previous studies of this region of the Cha I cloud, which contains an unusual cluster of YSOs. From comparisons of the strengths of the water ice and silicate bands we detect an apparent under-abundance of water ice towards the sources in $\rho$ Oph, relative to both RCrA and Serpens. This may be indicative of differences in chemical composition between the different clouds, or may be due to evaporation. Finally the CO 2:H 2O ice ratios observed towards the sources in $\rho$ Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in $\rho$ Oph.


Key words: ISM: dust, extinction -- stars: circumstellar matter -- stars: pre-main-sequence -- ISM: molecules -- ISM: lines and bands -- infrared: ISM

Offprint request: R. D. Alexander, rda@ast.cam.ac.uk

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© ESO 2003


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