Observations and analysis of carbon monoxide in cool stars at five microns.

Portions of the 5- m fundamental vibration-rotation band of CO have been observed and analyzed for a number of late-type giant and supergiant stars. In at least one of these, 0 Cet, lines of H2 0 also appear to be present in the spectrum near 5 m. Analysis using synthetic spectra computed from model atmospheres yields values for 12C/13C of approximately 30 in a Her, 15 in a Sco, and 10 in a Ori. 160/170 is 500 in a Her; similar values are indicated for a Ori and a Sco. No evidence for values of 160/180 significantly different from the terrestrial value is found for these three stars. The elemental abundance ratio [C/H] is found to be lower than the cosmic value for all three stars; however, the ratios are dependent on the assumed continuum temperatures at 5 Lm, which are somewhat uncertain. The present results imply the mixing of CN-cycle products of main-sequence burning into the envelopes of these stars. Post-main-sequence processing of the envelopes of a Sco and a Ori is implied by their low `2C/'3() values. If the envelope of a Her has not undergone post-main-sequence CN processing (as is suggested by its `2C/13C ratio), then mixing down to the region of main-sequence NO processing is suggested by its low value of 160/170. Key words: stellar spectra-infrared-late-type stars