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

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.