Abstract:
Spectral types of a large number of unidentified IRAS Point Sources with F{_nu }(12 μm) >= F{_nu }(25 μm) 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 μm 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. Table 6 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130 79 128 5) or via http://cdsweb.u-strasbg.fr/Abstract.html
