ABSTRACT

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 spectrophotometric spectra. Our analysis of GJ 436 yields an effective temperature of T_eff = 3350 ± 300 K and a mass of 0.44 . 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_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.