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We determine stellar parameters for the M dwarf GJ 436 that hosts aNeptune-mass planet. We employ primarily spectral modeling at low and highresolution, examining the agreement between model and observed optical spectraof five comparison stars of type, M0-M3. Modeling high resolution opticalspectra suffers from uncertainties in TiO transitions, affecting the predictedstrengths of both atomic and molecular lines in M dwarfs. The determination ofTeff, gravity, and metallicity from optical spectra remains at ~10%. Asmolecules provide opacity both in lines and as an effective continuum,determing molecular transition parameters remains a challenge facing modelssuch as the PHOENIX series, best verified with high resolution andspectrophotometric spectra. Our analysis of GJ 436 yields an effectivetemperature of Teff = 3350 +/- 300 K and a mass of 0.44 Msun. New Dopplermeasurements for GJ 436 with a precision of 3 m/s taken during 6 years improvethe Keplerian model of the planet, giving a minimum mass, M sin i = 0.0713 Mjup= 22.6 Mearth, period, P = 2.6439 d, and e = 0.16 +/- 0.02. The noncircularorbit contrasts with the tidally circularized orbits of all close-inexoplanets, implying either ongoing pumping of eccentricity by a more distantcompanion, or a higher Q value for this low-mass planet. The velocities indeedreveal a long term trend, indicating a possible distant companion.Comment: 27 pages, 7 figures, accepted to PAS

The M Dwarf GJ 436 and its Neptune‐Mass Planet