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

TL;DR

This study identifies a new class of late-stage AGB stars in the Magellanic Clouds, characterized by strong dust production and pulsations, providing insights into the mass limits for carbon star formation.

Contribution

It introduces a new class of low-mass AGB stars with detailed characterization, expanding understanding of the transition to carbon star phase in the Magellanic Clouds.

Findings

Median stellar masses of 1.14 M_sun in LMC and 0.94 M_sun in SMC.

Stars include both O-rich and C-rich chemistries, indicating the dredge-up limit.

Many stars show characteristics of atmospheric C/O ~ 1 and O-rich dust formation.

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 1.14 M_sun in the LMC and 0.94 M_sun in the SMC (with dispersions of 0.21 and 0.18 M_sun, respectively), and models suggest initial masses of <1.5 M_sun and <1.25 M_sun, 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.