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

Abstract

Abstract. We present the results of a mid-infrared (5-16.5 µm) imaging spectroscopy survey of Young Stellar Objects (YSOs) and their surrounding environment in four low-mass star formation regions: RCrA, ρ 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 µm spectra for 40 of these with a spectral resolving power of λ/∆λ 40. A number of spectral features were measured, most notably the 9.7 µm silicate feature, the bending modes of both water and CO 2 ices at 6.0 and 15.2 µm respectively and the well-known unidentified feature at 6.8 µm. 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 2 O-rich) phase. The sources observed in RCrA, ρ 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 ρ 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 2 O ice ratios observed towards the sources in ρ Oph show significantly greater scatter than in the other regions, possibly due to varying local conditions around the YSOs in ρ Oph.

Disk Masses for Embedded Class I Protostars in the Taurus Molecular Cloud

“…Furthermore, all of our targets have been observed with the Spitzer IRS spectrograph, and most have silicate and/or CO 2 ice absorption in their spectra, commonly associated with embedded sources (e.g., Alexander et al 2003;Boogert et al 2004;Watson et al 2004;Pontoppidan et al 2008). …”

POISSON project

“…This last feature is detected in absorption in all sources except #7 and #18, where it is observed in emission, usually indicating more evolved objects (see, e.g., Watson et al 2004). These broadband features are usually connected to dust and ice mantels on dust in the interstellar medium and in the YSO envelope, and their optical depths have been used in several works to compute extinction towards the protostellar photospheres (see, e.g., Alexander et al 2003;Chiar et al 2007). …”

“…Boogert et al 2008). Then, we applied to derive τ λ , the following relationship (see, e.g., Alexander et al 2003;Chiar et al 2007):…”

“…τ λ (CO 2 ) ice can be converted to a column density, using the following relationship (see, e.g., Alexander et al 2003):…”

“…where τ λ dλ is the equivalent width, λ peak is the wavelength of the feature peak optical depth, and A is the ice band strength (1.1 × 10 17 cm molecule −1 ) assuming pure CO 2 ice (Gerakines et al 1995;Alexander et al 2003).…”