INFRARED PROPERTIES OF IRAS SOURCES ASSOCIATED WITH NEARBY DARK MOLECULAR CLOUDS

We present an analysis of the infrared properties of a sample of IRAS sources associated with nearby dark molecular clouds. Examination of the luminosities and distribution of sources in colour-colour space suggests that they are T-Tauri stars and younger 'embedded cores', with mass typically approximately 1 M.. The broad range of colour temperatures as a function of wavelength implies the presence of extended circumstellar dust components with considerable temperature variations. Sources which are optically visible on the Palomar Observatory Sky Survey plates generally have colour temperatures higher than for invisible sources, although the difference is not large. In particular, we identify a subset of sources which have embedded core colours and yet are optically visible, illustrating that the IRAS data alone are not sufficient to distinguish between the youngest 'protostellar' sources and certain more evolved premain-sequence stars. We combine the IRAS data with existing broad-band photometry between approximately 1-mu-m and 20-mu-m to construct the spectral energy distributions (SED's) for a small number of sources. By comparing the SED's with theoretical models, the young stellar objects can be classified in terms of their evolutionary status. Statistical arguments are used to extend the classification to the rest of the sample. In the notation of Lada, the optically visible sources with embedded core colours are likely to be class 11-D sources, representing a transient phase between protostar and visible premain-sequence star. We estimate that the duration of the class I phase is approximately 2 x 10(5) yr and derive a similar result for the 11-D phase. This conflicts with analogous results for the embedded cluster in rho-Ophiuchus.