¹²C/¹³C in Metal-poor Field Halo Giants

We have estimated ¹²C/¹³C in 15 metal-poor (-2.4 ≤ [Fe/H] ≤ -1.0) field halo giant stars from spectra of the ¹³CO v = 3–1 and v = 2–0 band heads and surrounding ¹²CO and ¹³CO R-branch lines. Our isotope ratios are consistent with previous measurements for stars in our sample with ¹²C/¹³C determined either from the infrared first-overtone bands of CO or from optical G-band spectra of CH and red system bands of CN. We have also compiled carbon isotope ratios from the literature for a much larger sample of field and cluster red giant branch (RGB) stars spanning a wide range of metallicities (-2.4 ≤ [Fe/H] ≤ solar). Combining these data, we confirm the decline of the isotope ratio as stars evolve up the RGB and we have identified a trend toward higher levels of mixing in more metal-poor stars. Standard RGB first dredge-up models do not predict the carbon isotope ratios that we observe in the more evolved (higher luminosity) metal-poor stars, but more recent models that account for other mixing mechanisms can explain these data; even for very metal-poor stars such as those that we have observed in the Galactic halo.