Galactic environment and the 10- m silicate feature of young stellar objects

Abstract

Disc and sphere dust models are used to fit 8‐13 µm flux spectra of 19 low-mass young stellar objects (YSOs) and five Herbig AeBe stars. The 13 non-photospheric low-mass YSOs in quiescent environments and the five Herbig AeBe stars have mean disc temperature indices of 0.4, indicating that the emission arises from optically thin layers above a flared optically thick disc; 10 out of 14 of the low-mass YSO and four out of five of the Herbig AeBe features contain an optically thin silicate emission component. The radius of the peak 10-µm emission for nine out of the 13 low-mass YSOs is 10‐130 au, and three out of the five Herbig AeBe stars are 10‐30 au in size. In contrast, the five YSOs from disrupted molecular clouds that have been shaped by expanding supernova remnants have temperature indices of between 0.3 and 0.8; four out of the five are optically thick and three out of the five have radii 2 au. The photosphere-like continuum of Taurus-Elias 18 could be fitted only with truncated optically thick models, implying the presence of a void between the >500 K and cold (100 K) foreground dust. Silicates surrounding low-mass YSOs in quiescent molecular clouds are similar to those in the Trapezium region of the Orion Nebula except when AV 2 mag. In the low-AV case and in low-mass YSOs in disrupted molecular clouds the silicates are similar to circumstellar dust around the evolved star µ Cephei.