Infrared spectra of late-type stars

Abstract

Spectra have been obtained in the 2μ atmospheric window with a resolution of 32 and 65A of late-type dwarfs, giants, supergiants, and long period variables. In the non-variable stars, the first overtone band of C0 at 2.29μ shows a systematic increase with both increasing luminosity and decreasing temperature. This increasing strength is interpreted as arising from an increase in turbulence and a decrease in atmospheric opacity. The 1.87μ band of H 2 O appears only in stars of spectral type M6 and later. The observed strength of this band is considerably weaker than that predicted by model atmospheres. Strong bands of C 13 O 16 are visible in most of the stars, and they are interpreted as indicating a number ratio [C 12 /C 13 ] Variations in the strengths of the H 2 O and C0 bands were measured in 18 long period variables as well as variations in the continuum flux at 2.25μ, f(225). There is a phase lag between the visual and f(225) light curves in the sense that the maximum of the latter occurs after that of the former. The size of the lag is directly proportional to the range at 2.25μ. The molecular absorption bands are stronger in the variables than in the non-variables of the same spectral type. The value of [C 12 /C 13 ] for the variables is comparable to that for the non-variables. There is evidence for differences in the value of [0/C] among the variable stars. NML Tau seems to have a value similar to that of U Ori, a typical M-type variable. R Aur, T Cas, X Cyg, and RS Lib appear to have abnormally low values of [0/C]. A model involving the motion of a shock wave through the star's atmosphere is discussed as a possible means of explaining variations in both visual and infrared spectra. The evolutionary history of and the effect of convective mixing in late-type stars is discussed and it is concluded that: 1.) the value of [C 12 /C 13 ] indicates that mixing has occurred; 2.) the degree of mixing may be greater in the non-variables than in the variables; 3.) the observations are not inconsistent with a high value of [0/C] in most of the stars observed, i.e. [0/C] ~2.