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

Disc and sphere dust models are used to fit 8-13 mum 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-mum 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 less than or similar to2 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 (less than or similar to100 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 A(V)less than or similar to2 mag. In the low-A(V) case and in low-mass YSOs in disrupted molecular clouds the silicates are similar to circumstellar dust around the evolved star mu Cephei.