Chlorine Isotope Ratios in M Giants

Abstract: We have measured the chlorine isotope ratio in six M giant stars using HCl 1-0 P8 features at 3.7 microns with R $\sim$ 50,000 spectra from Phoenix on Gemini South. The average Cl isotope ratio for our sample of stars is 2.66 $\pm$ 0.58 and the range of measured Cl isotope ratios is 1.76 $<$ $^{35}$Cl/$^{37}$Cl $<$ 3.42. The solar system meteoric Cl isotope ratio of 3.13 is consistent with the range seen in the six stars. We suspect the large variations in Cl isotope ratio are intrinsic to the stars in our sample given the uncertainties. Our average isotopic ratio is higher than the value of 1.80 for the solar neighborhood at solar metallicity predicted by galactic chemical evolution models. Finally the stellar isotope ratios in our sample are similar to those measured in the interstellar medium.