Discs and the 10-μm silicate spectra of young stellar objects with non-photospheric continua

Dust emission in the non-photospheric 10-mum continua of HL Tau and Taurus-Elias 7 (Haro6-10, GV Tau) is distinguished from foreground silicate absorption using a simple disc model with radial power-law temperature and mass-density distributions based on the IR-submm model of T Tauri stars by Adams, Lada & Shu with foreground extinction. The resulting 10-mum absorption profiles are remarkably similar to those of the field star Taurus-Elias 16 obtained by Bowey, Adamson & Whittet. The fitted temperature indices are 0.44 (HL Tau) and 0.33 (Elias 7) in agreement with Boss's theoretical models of the 200-300 K region, but lower than those of IR-submm discs (0.5-0.61; Mannings & Emerson); a significant fraction of the modelled 10-mum emission of HL Tau is optically thin, whilst that of Elias 7 is optically thick. We suggest that HL Tau's optically thin component arises from silicate dust within low-density layers above an optically thick disc.