CARBON-ISOTOPE RATIOS FROM THE FIRST OVERTONE CO BANDS IN METAL-POOR GIANTS

We report observations of the first overtone bands of (CO)-C-12 and (C O)-C-13 using the high-resolution, infrared spectrometer Phoenix to de termine carbon isotope ratios in held metal-poor giant stars. The isot ope ratios we have determined are in good agreement with previous work using blue CH and red CN features. Carbon isotope ratios in metal-poo r stars appear to decline abruptly to very low values at log g similar or equal to 2 on the first ascent of the giant branch. The decline is consistent with the onset of mixing of the convective envelope into t he hydrogen burning shell. Evidence for a continuing decline in the is otope ratio with increasing luminosity in metal poor giants from log g =2 to log g=0 is weak, but such a decline cannot be ruled out. (C) 199 7 American Astronomical society.