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 eff ) and we measure bolometric luminosities (L bol ) by combining broad wavelength-coverage photometry with Gaia parallaxes. Radii are then computed by combining the T eff and L 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.

The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. “The Accident”)

“…Based on a collection of the nearest, brightest K and M subdwarfs in the northern two-thirds of the sky (Kesseli et al 2019), we find that some objects in the 20 pc census (e.g., sdM3 LHS 272 and the aforementioned Wolf 1130) have metallicities near −1, but more metal-poor objects are unlikely to be found with 20 pc. At larger distances, for example, Kesseli et al (2019) value of −1.5 < [M/H] < −1.0 for WISE 2005+5424, which is much lower than the measured −0.64 ± 0.17 value. Hence, the metallicity of WISE 1534−1043 is likely to be less extreme than the LOWZ models predict.…”

Moderate-resolution K-band Spectroscopy of Substellar Companion κ Andromedae b

“…Here, p(λ) is the mapping of the wavelength values to pixels, M[p(λ)] is the stellar atmosphere model parameterized by effective temperature (T eff ), surface gravity ( g log ), and metallicity ([M/H]), C is the dilution factor, (radius/distance) 2 , that scales the model to the observed fluxes (which is a measure of radius since the distance is known; e.g., Teague & West 2014;Kesseli et al 2019), and κ G (Δν inst ) is the line-spread function (LSF) calculated from the OSIRIS resolution of R=4000 to be 34.5 km s −1 . RV is the radial velocity that is used here only to account for wavelength calibration errors in the OSIRIS DRP, c is the speed of light, and C flux is an additive continuum correction to account for potential systematic offsets in the continuum.…”

A Super-Earth and Sub-Neptune Transiting the Late-type M Dwarf LP 791-18

“…Using their methodology, we find ∆(V − K S ) = 0.2, which implies [Fe/H] = −0.02 ± 0.21 (accounting for the relation's intrinsic scatter and our uncertainty on V ). Comparison to the (G R − J), M K color-magnitude diagram of Kesseli et al (2019) indicates a consistent metallicity of −0.5 ± 0.5. We report the weighted mean of these two independent estimates, [Fe/H] = −0.09 ± 0.19.…”