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

arXiv commentAccepted to AJ

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