We determine stellar parameters for the M dwarf GJ 436 that 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 .
Modeling 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 \sim 10 % .
As molecules provide opacity both in lines and as an effective continuum , determing 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 \pm 300 K and a mass of 0.44 M _ { \odot } .
New Doppler measurements for GJ 436 with a precision of 3 m s ^ { -1 } ~ { } taken during 6 years improve the Keplerian model of the planet , giving a minimum mass , M \sin i = 0.0713 M _ { JUP } ~ { } = 22.6 M _ { Earth } ~ { } , period , P = 2.6439 d , and e = 0.16 \pm 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 .