The sdA problem I: Physical Properties

TL;DR

This study analyzes the properties of sdA stars to determine their evolutionary origins, suggesting a significant fraction are likely (pre-)ELMs or metal-poor A/F stars, based on their colors, motions, and velocities.

Contribution

It introduces a method to classify sdA stars' evolutionary status using colors, proper motions, and velocities, providing new insights into their nature.

Findings

7% of sdAs are likely (pre-)ELMs.

Over 35% of sdAs cannot be explained as single metal-poor A/F stars.

Analysis suggests diverse origins for sdA stars, including binary evolution.

Abstract

The so-called sdA stars are defined by having H-rich spectra and surface gravities similar to hot subdwarf stars, but effective temperature below the zero-age horizontal branch. Their evolutionary history is an enigma: their surface gravity is too high for main sequence stars, but too low for single evolution white dwarfs. They are most likely byproducts of binary evolution, including blue-stragglers, extremely-low mass white dwarf stars (ELMs) and their precursors (pre-ELMs). A small number of ELMs with similar properties to sdAs is known. Other possibilities include metal-poor A/F dwarfs, second generation stars, or even stars accreted from dwarf galaxies. In this work, we analyse colours, proper motions and spacial velocities of a sample of sdAs from the Sloan Digital Sky Survey to assess their nature and evolutionary origin. We define a probability of belonging to the main sequence and a probability of being a (pre-)ELM based on these properties. We find that 7 per cent of the sdAs are more likely to be (pre-)ELMs than main sequence stars. However, the spacial velocity distribution suggests that over 35 per cent of them cannot be explained as single metal-poor A/F stars.