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HD 259440: The Proposed Optical Counterpart of the gamma-ray Binary HESS J0632+057

HD 259440 is a B0pe star that was proposed as the optical counterpart to the gamma-ray source HESS J0632+057. Here we present optical spectra of HD 259440 acquired to investigate the stellar parameters, the properties of the Be star disk, and evidence of binarity in this system. Emission from the H-alpha line shows evidence of a spiral density wave in the nearly edge-on disk. We find a best fit stellar effective temperature of 27500-30000 K and a log surface gravity of 3.75-4.0, although our fits are somewhat ambiguous due to scattered light from the circumstellar disk. We derive a mass of 13.2-19.0 M_sun and a radius of 6.0-9.6 R_sun. By fitting the spectral energy distribution, we find a distance between 1.1-1.7 kpc. We do not detect any significant radial velocity shifts in our data, ruling out orbital periods shorter than one month. If HD 259440 is a binary, it is likely a long period (> 100 d) system.