The transiting exoplanet host star GJ 436: A test of stellar evolution models in the lower main sequence, and revised planetary parameters

Knowledge of the stellar parameters for the parent stars of transiting exoplanets is a prerequisite for establishing the planet properties themselves, and often relies on stellar evolution models. GJ 436, which is orbited by a transiting Neptune-mass object, presents a difficult case because it is an M dwarf. Stellar models in this mass regime are not as reliable as for higher mass stars, and tend to underestimate the radius. Here we use constraints from published transit light curve solutions for GJ 436 along with other spectroscopic quantities to show how the models can still be used to infer the mass and radius accurately, and at the same time allow the radius discrepancy to be estimated. Similar systems should be found during the upcoming Kepler mission, and could provide in this way valuable constraints to stellar evolution models in the lower main sequence. The stellar mass and radius of GJ 436 are M-* = 0.452(-0.012)(+0.014) M-circle dot and R-* = 0.464(-0.011)(+0.009) R-circle dot, and the radius is 10% larger than predicted by the standard models, in agreement with previous results from well-studied double-lined eclipsing binaries. We obtain an improved planet mass and radius of M-p = 23.17 +/- 0.79 M-circle dot and R-p = 4.22(-0.10)(+0.09) R-circle plus, a density of rho(p) = 1.69(-0.12)(+0.14) g cm(-3), and an orbital semimajor axis of a = 0.02872 +/- 0.00027 AU.