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arXiv · 1810.07702

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

Aurora Y. Kesseli·J. Davy Kirkpatrick·Sergio B. Fajardo-Acosta·Matthew T. Penny·B. Scott Gaudi·Mark Veyette·Patricia C. Boeshaar·Calen B. Henderson·Michael C. Cushing·Sebastiano Calchi-Novati·Yossi Shvartzvald·Philip S. Muirhead
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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.

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Aurora Y. Kesseli, J. Davy Kirkpatrick, Sergio B. Fajardo-Acosta, Matthew T. Penny, B. Scott Gaudi, Mark Veyette, Patricia C. Boeshaar, Calen B. Henderson, Michael C. Cushing, Sebastiano Calchi-Novati, Yossi Shvartzvald, Philip S. Muirhead. 2018-10-17. Radii of 88 M Subdwarfs and Updated Radius Relations for Low-Metallicity M Dwarf Stars. https://doi.org/10.3847/1538-3881%2Faae982

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