Carbon Isotope Ratios in M10 Giants

Abstract

We measured carbon abundances and the ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ ratio in 31 giant branch stars with previous CN and CH band measurements that span -2.33 $<$ M${}_{\mathrm{V}}$ $<$ 0.18 in the globular cluster M10 (NGC 6254). Abundances were determined by comparing CO features at $\sim 2.3\mu \mathrm{m}$ and specifically the ${}^{13}$CO bandhead at $2.37\mu \mathrm{m}$, to synthetic spectra generated with MOOG. The observed spectra were obtained with GNIRS on Gemini North with a resolution of R $\approx 3500$. The carbon abundances derived from the IR spectra agree with previous [C/Fe] measurements found using CN and CH features at the near-UV/blue wavelength range. We found an average carbon isotope ratio of ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ = 5.10${}_{-0.17}^{+0.18}$ for first generation stars (CN-normal; 13 stars total) and ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ = 4.84${}_{-0.22}^{+0.27}$ for second generation stars (CN-enhanced; 15 stars). We therefore find no statistically significant difference in ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ ratio between stars in either population for the observed magnitude range. Finally, we created models of the expected carbon, nitrogen, and ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ surface abundance evolution on the red giant branch due to thermohaline mixing using the MESA stellar evolution code. The efficiency of the thermohaline mixing must be increased to a factor of $\approx 60$ to match [C/Fe] abundances, and by a factor of $\approx 666$ to match ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ ratios. We could not simultaneously fit the evolution of both carbon and the ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ ratio with models using the same thermohaline efficiency parameter.

17 pages, 8 figures, accepted for publication in The Astrophysical Journal

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