Identification of a Class of Low-Mass AGB Stars Struggling to Become Carbon Stars in the Magellanic Clouds

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 1.14 solar masses in the LMC and 0.94 solar masses in the SMC (with dispersions of 0.21 and 0.18 solar masses, respectively), and models suggest initial masses of <1.5 solar masses and <1.25 solar masses, 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 J-[8] colors.