The Physical Nature of Subdwarf A Stars: White Dwarf Impostors

TL;DR

This paper clarifies that most subdwarf A stars are actually metal-poor main sequence stars, not white dwarf impostors, based on spectroscopic analysis, binary studies, and their physical properties.

Contribution

It demonstrates that sdA stars are predominantly metal-poor main sequence stars, correcting previous misclassification as extremely low mass white dwarfs.

Findings

sdA stars are larger and more luminous than ELM WDs.

Surface gravity errors are due to metal line blanketing effects.

Most sdA stars are metal-poor A-F type main sequence stars.

Abstract

We address the physical nature of subdwarf A-type (sdA) stars and their possible link to extremely low mass (ELM) white dwarfs (WDs). The two classes of objects are confused in low-resolution spectroscopy. However, colors and proper motions indicate that sdA stars are cooler and more luminous, and thus larger in radius, than published ELM WDs. We demonstrate that surface gravities derived from pure hydrogen models suffer a systematic ~1 dex error for sdA stars, likely explained by metal line blanketing below 9000 K. A detailed study of five eclipsing binaries with radial velocity orbital solutions and infrared excess establishes that these sdA stars are metal-poor ~1.2 Msun main sequence stars with ~0.8 Msun companions. While WDs must exist at sdA temperatures, only ~1% of a magnitude-limited sdA sample should be ELM WDs. We conclude that the majority of sdA stars are metal-poor A-F type stars in the halo, and that recently discovered pulsating ELM WD-like stars with no obvious radial velocity variations may be SX Phe variables, not pulsating WDs.