Constraining intrinsic S-type AGB masses and third dredge-up using pulsation

TL;DR

This study estimates initial masses of intrinsic S-type AGB stars using pulsation analysis, Gaia data, and stellar models, revealing a mass distribution peaking at 1.3-1.4 solar masses and challenging current models for lower masses.

Contribution

It provides new mass estimates for S-type AGB stars and highlights discrepancies with existing stellar models, especially for stars with masses near 1 solar mass.

Findings

Mass distribution peaks at 1.3-1.4 solar masses.

Stars with initial masses down to 1 solar mass found, conflicting with models.

Fundamental mode Mira variables pose observational and theoretical challenges.

Abstract

The lowest mass at which the third dredge-up (TDU) occurs for thermally-pulsing asymptotic giant branch (TP-AGB) stars remains a key uncertainty in detailed stellar models. S-type AGB stars are an important constraint on this uncertainty as they have C/O ratios between 0.5 and 1, meaning they have only experienced up to a few episodes of TDU. AGB stars are also long-period variable stars, pulsating in low order radial pulsation modes. In this paper we estimate the initial masses of a large literature sample of intrinsic S-type AGB stars, by analysing their visual light curves, estimating their luminosities with Gaia DR3 parallax distances and finally comparing to a grid of detailed stellar models combined with linear pulsation models. We find that the initial mass distribution of intrinsic S-type stars peaks at 1.3 to 1.4 \Msun, depending on model assumptions. There also appear to be stars with initial masses down to 1 solar mass, which is in conflict with current detailed stellar models. Additionally, we find that though the mass estimates for semiregular variable stars pulsating in higher order radial modes are precise, the Mira variables pulsating in the fundamental mode present challenges observationally from uncertain parallax distances, and theoretically from the onset of increased mass-loss and the necessity of non-linear pulsation models.