The Galactic Chemical Evolution of Chlorine

We measured ${}^{35}$Cl abundances in 52 M giants with metallicities between -0.5 $<$ [Fe/H] $<$ 0.12. Abundances and atmospheric parameters were derived using infrared spectra from CSHELL on the IRTF and from optical echelle spectra. We measured Cl abundances by fitting a H${}^{35}$Cl molecular feature at 3.6985 $\mu$m with synthetic spectra. We also measured the abundances of O, Ca, Ti, and Fe using atomic absorption lines. We find that the [Cl/Fe] ratio for our stars agrees with chemical evolution models of Cl and the [Cl/Ca] ratio is broadly consistent with the solar ratio over our metallicity range. Both indicate that Cl is primarily made in core-collapse supernovae with some contributions from Type Ia SN. We suggest other potential nucleosynthesis processes, such as the $\nu$-process, are not significant producers of Cl. Finally, we also find our Cl abundances are consistent with H II and planetary nebular abundances at a given oxygen abundance, although there is scatter in the data.