5 Summary

Spectral-type assignments have been made for more than 14000 IRAS sources which are primarily unidentified in the PSC and which have $F{_\nu}(12~{\mu}{\rm m}) \geq F{_\nu}(25~{\mu}{\rm m})$.A large number of them are either O-rich (M-type) or C-rich (C-type) stars. The mean $B_{\rm j}$-[12], [12]-[25], and [25]-[60] colours of a sample of the M stars (from the 14000 IRAS sources referred to above) have been obtained and compared with those of corresponding BSC M giants. The classification of these sources would not have been possible on the basis of their IRAS colours alone as most of them are detected with best-quality flux densities at only 12 ${\mu}$m. The mean colours of the unidentified sources are higher than those of BSC giants of the same spectral type implying that the more distant IRAS sources have thicker CSE. A plot of the [25]-[60] colours vs. [12]-[25] colours (as in VH) indicates that 1/3 of the sources which are likely to be classified as carbon stars on the basis of their location in the VH diagram are found to be of M spectral type as judged from their spectra. The spectral classification of the faint, unidentified sources discussed in this study therefore constitutes an important contribution to the available information on these galactic IRAS sources.

Acknowledgements

KVKI thanks the Director of the Indian Institute of Astrophysics (IIA) for kindly providing him the facilities of IIA while this work was being carried out. He also thanks the Smithsonian Institution, Washington DC, U.S.A. for the award of a Short-Term Visiting Appointment (and also for a short extension of this appointment) to work at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass, U.S.A. where a major part of this work was carried out. He thanks Dr. G.G. Fazio for his kind hospitality and fruitful discussions. He also thanks the Research Programs Office of the Space Telescope Science Institute (STScI), Baltimore, MD, U.S.A. for providing financial assistance to enable him to visit DJM for discussions related to this work. DJM thanks the STScI for salary support during the visit of KVKI and during analysis of this material. We thank Profs. H. Bhatt and M. Parthasarathy (both of IIA) for several useful

discussions and Dr. C. Barnbaum for a careful reading of the manuscript. We also thank Dr. van der Veen, (the referee of this paper) for several useful comments and suggestions which helped to improve the manuscript. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. DJM's classification work was supported by the NASA Astrophysics Data Program under JPL Contracts 766490 and 957272.