Radii of 88 M Subdwarfs and Updated Radius Relations for Low-Metallicity M Dwarf Stars

TL;DR

This study measures the radii of 88 low-metallicity M subdwarfs, revealing that metal-poor stars are significantly smaller at a given temperature, and provides updated relations to estimate their radii.

Contribution

It offers the first comprehensive analysis of radii across a wide metallicity range for M subdwarfs, improving radius estimation methods for low-metallicity stars.

Findings

Ultra-subdwarfs can be up to five times smaller than solar-metallicity stars at the same temperature.

Developed color-radius and color-surface brightness relations down to [Fe/H] = -2.0 dex.

Results are crucial for exoplanet surveys like WFIRST targeting low-metallicity stars.

Abstract

M subdwarfs are low-metallicity M dwarfs that typically inhabit the halo population of the Galaxy. Metallicity controls the opacity of stellar atmospheres; in metal poor stars, hydrostatic equilibrium is reached at a smaller radius, leading to smaller radii for a given effective temperature. We compile a sample of 88 stars that span spectral classes K7 to M6 and include stars with metallicity classes from solar-metallicity dwarf stars to the lowest metallicity ultra-subdwarfs to test how metallicity changes the stellar radius. We fit models to Palomar Double Spectrograph (DBSP) optical spectra to derive effective temperatures ($T_\mathrm{eff}$) and we measure bolometric luminosities ($L_\mathrm{bol}$) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the $T_\mathrm{eff}$ and $L_\mathrm{bol}$ using the Stefan-Boltzman law. We find that for a given temperature, ultra-subdwarfs can be as much as five times smaller than their solar-metallicity counterparts. We present color-radius and color-surface brightness relations that extend down to [Fe/H] of $-$2.0 dex, in order to aid the radius determination of M subdwarfs, which will be especially important for the WFIRST exoplanetary microlensing survey.