Carbon Isotope Ratios in Metal-Poor Halo Red Giant Stars: Chemical Mixing on the Red Giant Branch

Details Carbon Isotope Ratios in Metal-Poor Halo Red Giant Stars: Chemical Mixing on the Red Giant Branch Carbon Isotope Ratios in Metal-Poor Halo Red Giant Stars: Chemical Mixing on the Red Giant Branch

Nov 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...198.9401k&link_type=abstract

American Astronomical Society, 198th AAS Meeting, #94.01; Bulletin of the American Astronomical Society, Vol. 33, p.1181

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The ratio of carbon isotope abundances (12C/13C) in the atmospheres of low mass stars correlate with evolution up the red giant branch (RGB) since 12C is present in the photospheres of main sequence stars, but 13C is only present after products of the CN cycle of nuclear reactions reach the surface via convective mixing. We have estimated 12C/13C in a large sample of metal-poor (-2.4<[Fe/H]<-1.0) field halo giant stars from spectra of the 13CO v=3-1 and v=2-0 bandheads 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 the optical G-band spectra of CH and red system bands of CN. The carbon isotope ratio drops abruptly from 12C/13C ~30-40, at MV ~+0.5 and (log g) ~2.0, to 12C/13C ~5-10, at MV>+1.0 and (log g)>2.5, as stars evolve up the RGB. Standard models of low mass RGB first dredge-up do not predict the low carbon isotope ratios that we observe in the more evolved (higher luminosity) stars in our sample, but more recent models that include other mixing mechanisms can explain our data. Our results lead us to suggest that the "excess" mixing in field halo giants begins shortly after the first dredge-up and then continues gradually as the stars evolve up the RGB. A second, distinct dredge-up event is not consistent with our data. These data are useful to developing a more complete understanding of Galactic chemical evolution and the history of our Galaxy as recorded in the chemical diversity between stellar populations.

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