Are sdAs helium core stars?

TL;DR

This paper investigates the nature of sdA stars, proposing they are helium core stars resulting from binary evolution rather than main sequence stars, supported by the discovery of new low-mass white dwarfs.

Contribution

The study suggests sdA stars are helium core stars from binary evolution, challenging their previous classification as main sequence stars, and reports the discovery of two new low-mass white dwarfs.

Findings

sdA stars are likely helium core stars, not main sequence stars.

Discovery of two new extremely-low mass white dwarfs among sdAs.

sdA stars have properties consistent with binary evolution outcomes.

Abstract

Evolved stars with a helium core can be formed by non-conservative mass exchange interaction with a companion or by strong mass loss. Their masses are smaller than 0.5 MSun. In the database of the Sloan Digital Sky Survey (SDSS), there are several thousand stars which were classified by the pipeline as dwarf O, B and A stars. Considering the lifetimes of these classes on the main sequence, and their distance modulus at the SDSS bright saturation, if these were common main sequence stars, there would be a considerable population of young stars very far from the galactic disk. Their spectra are dominated by Balmer lines which suggest effective temperatures around 8000-10000K. Several thousand have significant proper motions, indicative of distances smaller than 1kpc. Many show surface gravity in intermediate values between main sequence and white dwarf, 4.75 < log g < 6.5, hence they have been called sdA stars. Their physical nature and evolutionary history remains a puzzle. We propose they are not H-core main sequence stars, but helium core stars and the outcomes of binary evolution. We report the discovery of two new extremely-low mass white dwarfs among the sdAs to support this statement.