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Session 81 - Variable Cool and Late Type Stars.
Display session, Wednesday, January 15
Model atmospheres and near-infrared (1--3 \mum) synthetic spectra have been computed for dwarf carbon (dC) stars in order to interpret their JHK colors. Since the known dC stars are believed to be main-sequence stars which have received carbon-rich material from a former AGB carbon star companion, we used the \sc marcs code to compute main-sequence models with the carbon increased to C/O = 1.07. Despite the increased opacity due to CN and C_2, the near-infrared spectra of dC stars are similar in character to those of K dwarfs and very different from those of carbon giants: in particular, the hump at the H^- opacity minimum is not very pronouned, and spectral features such as the CO bands are generally much weaker in carbon dwarfs than in carbon giants of the same temperature.
Using the response functions given by Bessell amp; Brett (PASP 100, 1134, 1988) for the JHK filters, we computed J--H and H--K colors from the model spectra. The principal absorbers (CO, CN, and C_2) were individually turned on and off when the synthetic spectra were computed in order to examine their separate effects on the colors. Polyatomic molecules including HCN, C_3 and C_2H_2 were also studied but have little effect at the relatively warm temperatures of the known dC stars. The effects of CO and C_2 on the J--H and H--K colors were found to be small and oppositely-directed in dC stars, so that their colors are nearly those of the continuum. In giant C stars the effects of molecular absorption are much greater. At a given T_eff, dwarf and giant C stars have very different J--H and H--K colors; however, the shift in the two-color diagram is roughly parallel to the sequence for stars of different T_eff. These results indicate that low-luminosity C stars would be hard to identify on the basis of JHK photometry alone, i.e. without a separate indicator of T_eff, but should be distinguishable by the simple addition of, say, a V--K color.
Program listing for Wednesday