Chlorine Isotope Ratios in M Giants

TL;DR

This study measures chlorine isotope ratios in six M giant stars, finding values comparable to the solar system and interstellar medium, with variations likely intrinsic to the stars, challenging existing galactic chemical evolution models.

Contribution

First measurement of chlorine isotope ratios in M giant stars using high-resolution infrared spectroscopy, revealing unexpected variations and similarities to interstellar medium ratios.

Findings

Average Cl isotope ratio is 2.66 ± 0.58.

Range of ratios is 1.76 to 3.42.

Ratios are consistent with solar system and interstellar medium values.

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.