IDENTIFICATION of A CLASS of LOW-MASS ASYMPTOTIC GIANT BRANCH STARS STRUGGLING to BECOME CARBON STARS in the MAGELLANIC CLOUDS

Abstract

We have identified a new class of Asymptotic Giant Branch (AGB) stars in the Small and Large Magellanic Clouds (SMC/LMC) using optical to infrared photometry, light curves, and optical spectroscopy. The strong dust production and long-period pulsations of these stars indicate that they are at the very end of their AGB evolution. Period-mass-radius relations for the fundamental-mode pulsators give median current stellar masses of in the LMC and in the SMC (with dispersions of 0.21 and 0.18, respectively), and models suggest initial masses of <1.5 and <1.25 , respectively. This new class of stars includes both O-rich and C-rich chemistries, placing the limit where dredge-up allows carbon star production below these masses. A high fraction of the brightest among them should show S star characteristics indicative of atmospheric C/O 1, and many will form O-rich dust prior to their C-rich phase. These stars can be separated from their less-evolved counterparts by their characteristically red colors.