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Astron. Astrophys. Suppl. Ser. 133, 201-209

Spectral classification of unidentified IRAS sources with $ F{_\nu}(12~{\mu}{\rm m}) \geq F{_\nu}(25~{\mu}{\rm m})$[*]

K.V.K. Iyengar1 - D.J. MacConnell2

Send offprint request: K.V.K. Iyengar

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.

Received December 4; accepted June 5, 1998


Spectral types of a large number of unidentified IRAS Point Sources with $ F{_\nu}(12~{\mu}{\rm m}) \geq F{_\nu}(25~{\mu}{\rm m})$ 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 ${\mu}$m in order to determine their $B_{\rm j}$-[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.

Key words: stars: AGB -- stars: carbon -- stars: late-type -- stars: mass-loss -- infrared: stars

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