The solar abundance problem and eMSTOs in clusters

TL;DR

This paper investigates how accretion of metal-rich or metal-poor material from protoplanetary discs influences stellar evolution and observed properties, potentially explaining scatter in cluster color-magnitude diagrams.

Contribution

It introduces a model combining disc accretion scenarios with stellar evolution, showing significant effects on stellar chemical composition and evolutionary tracks, which was not previously accounted for.

Findings

Accretion alters stellar surface metallicity and evolution.

Model explains scatter in young cluster CMDs.

Solar composition affected modestly by accretion.

Abstract

We study the impact of accretion from protoplanetary discs on stellar evolution of AFG-type stars. We use a simplified disc model computed using the Two-Pop-Py code that contains the growth and drift of dust particles in the protoplanetary disc. It is used to model the accretion scenarios for a range of physical conditions of protoplanetary discs. Two limiting cases are combined with the evolution of stellar convective envelopes computed using the Garstec stellar evolution code. We find that the accretion of metal-poor (gas) or metal-rich (dust) material has a significant impact on the chemical composition of the stellar convective envelope. As a consequence, the evolutionary track of the star diverts from the standard scenario predicted by canonical stellar evolution models, which assume a constant and homogeneous chemical composition after the assembly of the star has finished. In the case of the Sun, we find a modest impact on the solar chemical composition. Accretion of metal-poor material indeed reduces the overall metallicity of the solar atmosphere, and it is consistent, within the uncertainty, with the solar Z reported by Caffau et al. (2011), but our model is not consistent with the measurement by Asplund et al. (2009). Another effect is the change of the position of the star in the colour-magnitude diagram. We compare our predictions to a set of open clusters from the Gaia DR2 and show that it is possible to produce a scatter close to the turn-off of young clusters that could contribute to explain the observed scatter in CMDs. Detailed measurements of metallicities and abundances in the nearby open clusters will provide a stringent observational test of our proposed scenario.