The formation of subdwarf A-type stars

TL;DR

This study uses stellar evolution models and population synthesis to explore the origins of subdwarf A-type stars, concluding that most are metal-poor main sequence stars with a smaller fraction being low-mass white dwarfs, depending on stellar age.

Contribution

It provides a detailed analysis of the formation pathways of sdAs using MESA simulations and population synthesis, clarifying their population proportions over time.

Findings

Most sdAs are metal-poor main sequence stars (>95%).

The proportion of ELM WDs among sdAs increases with stellar age.

Population proportions vary significantly from 5 to 13.7 Gyr.

Abstract

Subdwarf A-type stars (sdAs) are objects that have hydrogen-rich spectra with surface gravity similar to that of hot subdwarf stars but effective temperature below the zero-age horizontal branch (ZAHB). They are considered to be metal-poor main sequence (MS) stars or extremely low-mass white dwarfs (ELM WDs). In this work, using the stellar evolution code, Modules for Experiments in Stellar Astrophysics (MESA), we investigate the sdAs formed both by the evolution of (pre-)ELM WDs in double degenerate systems (DDs) and metal-poor main sequence with the single evolution models. We find that both of the evolutionary tracks of ELM WDs and metal-poor MSs can explain the observation properties of sdAs. However, the proportion between these two populations are uncertain. In this work, we adopt the method of binary population synthesis of both ELM WDs in the disk and metal-poor MS in the halo to obtain the populations of ELM WDs and metal-poor MSs at different stellar population ages and calculate their proportions. We find that the proportion of metal-poor MSs to sdA population for a stellar population of 10 Gyr is $\sim$98.5 percent, which is consistent with the conclusion that most sdAs ($>$95 percent) are metal-poor MSs. And the proportion of ELM WDs (metal-poor MSs) to sdA population increases (decreases) from 0.1\% (99.9\%) to 20\% (80\%) with the stellar population ages from 5 to 13.7 Gyr.