Radii of Rapidly-Rotating Stars, with Application to Transiting-Planet Hosts

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Abstract
The currently favored method for estimating radii and other parameters of transiting-planet host stars is to match theoretical models to observations of the stellar mean density rho_*, the effective temperature T_eff, and the composition parameter [Z]. This explicitly model-dependent approach is based on readily-available observations, and results in small formal errors. Here I use two calibration samples of stars (eclipsing binaries and stars for which asteroseismic analyses are available) having well-determined masses and radii to estimate the accuracy and systematic errors inherent in the rho_* method. When matching to the Yonsei-Yale stellar evolution models, I find the most important systematic error results from selection bias favoring rapidly-rotating (hence probably magnetically active) stars among the eclipsing binary sample. If unaccounted for, this bias leads to a mass-dependent underestimate of stellar radii by as much as 4% for stars of 0.4 M_sun, decreasing to zero for masses above about 1.4 M_sun. The asteroseismic sample suggests (albeit with significant uncertainty) that systematic errors are small for slowly-rotating, inactive stars. Systematic errors arising from failings of the Yonsei-Yale models of inactive stars probably exist, but are difficult to assess because of the small number of well-characterized comparison stars having low mass and slow rotation. Poor information about [Z] is an important source of random error, and may be a minor source of systematic error as well. With suitable corrections for rotation, it is likely that systematic errors in the rho_* method can be comparable to or smaller than the random errors, yielding radii that are accurate to about 2% for most stars.

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