Spectral classification of unidentified IRAS sources with ^*

¹ Indian Institute of Astrophysics, Bangalore 560 034, India
² Computer Sciences Corporation, System Sciences Division, Space Telescope Science Institute, Baltimore, Maryland, U.S.A.

Send offprint request to: K.V.K. Iyengar

Received: 4 December 1998
Accepted: 5 June 1998

Abstract

Spectral types of a large number of unidentified IRAS Point Sources with were determined; the majority are faint, oxygen-rich (M-type) or carbon-rich giant stars. The Guide Star Catalog has been used to find the photographic magnitudes of the newly classified IRAS sources with quality-3 flux densities at 12 m in order to determine their B_j -[12] colour index. The dependence of this and of the IRAS indices on spectral type is determined and discussed. The mean [12]-[25] colour of the M-type stars is found to increase monotonically from M3 to M6 and then levels off. Comparison of the [12]-[25] colours of these faint IRAS M stars with those of Bright Star Catalog M stars indicates that, at all types, the mean [12]-[25] index of the former group is higher than that of the latter by at least 0.2 magnitude, and this is found to be significant at the 95% confidence level. Comparison of the quality-3, mean [25]-[60] colours of the newly-classified, faint M stars with those of BSC stars over the same spectral type also shows the same trend. Possible reasons for this difference are discussed. The percentage of variable sources as a function of spectral type is seen to sharply increase from a nearly constant value of about 25% for sources of spectral type M3 to M7 to a value of about 50% at M10. The mean [12]-[25] colours of the IRAS unidentified sources (within the limits of the errors on their mean values) appear to be rather insensitive to the degree of variability.