Atomic diffusion and mixing in old stars VII. Abundances of Mg, Ti, and Fe in M30

TL;DR

This study constrains the efficiency of mixing processes in old stars by analyzing Mg, Ti, and Fe abundances in M30, revealing a narrow range for the transport efficiency parameter and its impact on surface abundances.

Contribution

It introduces a method to constrain stellar mixing efficiency parameters using spectral synthesis and abundance trends, improving upon previous estimates.

Findings

The efficiency parameter T_0 is constrained between 6.09 and 6.2 in log scale.

Surface abundances at the main sequence turn-off are reduced compared to the red giant branch.

Systematic errors are accounted for in the abundance estimation process.

Abstract

We attempt to constrain the efficiency of additional transport or mixing processes that reduce the effect of atomic diffusion in stellar atmospheres. We apply spectral synthesis methods to spectra observed with the GIRAFFE spectrograph on the VLT to estimate abundances of Mg, Ti, Fe, and Ba in stars in the metal-poor globular cluster M30. To the abundances we fit trends of abundances predicted by stellar evolution models assuming different efficiencies of additional transport or mixing processes. The fitting procedure attempts to take into account the effects of parameter-dependent systematic errors in the derived abundances. We find that the parameter $T_0$, which describes the efficiency of additional transport or mixing processes, can almost certainly be constrained to the narrow range $\log_{10}{\left( T_0 / \left[ \mathrm{K} \right] \right)}$ between $6.09$ and $6.2$. This corresponds to decreased abundances for stars at the main sequence turn-off point compared to the red giant branch by $0.2\,\mathrm{dex}$ for Mg, $0.1\,\mathrm{dex}$ for Fe, and $0.07\,\mathrm{dex}$ for Ti. We also find that while our estimates do have non-negligible systematic errors stemming from the continuum placement and the assumed microturbulence, our method can take them into account. Our results partly amend the results of an earlier paper in this article series, that tentatively used a value of $\log_{10}{\left( T_0 / \left[ \mathrm{K} \right] \right)} = 6.0$ when modelling the Spite plateau of lithium. To more easily distinguish physical effects from systematic errors, we recommend that studies of this kind focus on elements for which the expected surface abundances as functions of effective temperature have a distinct structure and cover a wide range.