The M dwarf GJ 436 and its Neptune-mass planet

We determine stellar parameters for the M dwarf GJ 436, which hosts a Neptune-mass planet. We employ primarily spectral modeling at low and high resolution, examining the agreement between model and observed optical spectra of five comparison stars of type M0-M3. The modeling of high-resolution optical spectra suffers from uncertainties in TiO transitions, affecting the predicted strengths of both atomic and molecular lines in M dwarfs. The determination of T_eff, gravity, and metallicity from optical spectra remains at ∼10%. As molecules provide opacity both in lines and as an effective continuum, determining molecular transition parameters remains a challenge facing models such as the PHOENIX series, best verified with high resolution and spectro-photometric spectra. Our analysis of GJ 436 yields an effective temperature of T_eff = 3350 ± 300 K and a mass of 0.44 M _⊙. New Doppler measurements of GJ 436 with a precision of 3 m s^-1 taken during 6 years improve the Keplerian model of the planet, giving it a minimum mass M sin i = 0.0713M_Jup = 22.6 M _⊙, period P = 2.6439 days, and eccentricity e = 0.16 ±0.02. The noncircular orbit contrasts with the tidally circularized orbits of all close-in exoplanets, implying either ongoing pumping of eccentricity by a more distant companion, or a higher Q value for this low-mass planet. The velocities indeed reveal a long-term trend, indicating a possible distant companion.