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

Abstract

We present the results of a mid-infrared (5-16.5m) imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, 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 5-16.5m spectra for 40 of these with a spectral resolving power of=' 40. A number of spectral features were measured, most notably the 9.7m silicate feature, the bending modes of both water and CO2 ices at 6.0 and 15.2m respectively and the well-known unidentified feature at 6.8 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 CO2 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 (H2O-rich) phase. The sources observed in RCrA, 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 Oph, relative to both RCrA and Serpens. This may be indicative of dierences in chemical composition between the dierent clouds, or may be due to evaporation. Finally the CO2:H2O ice ratios observed towards the sources in Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in Oph.