The long-period massive binary HD~54662 revisited

Abstract

HD54662 is an O-type binary star belonging to the CMa OB1 association. Due to its long-period orbit, this system is an interesting target to test the adiabatic wind shock model. The goal is to improve our knowledge of the orbital and stellar parameters of HD54662 and to analyze its X-ray emission to test the theoretical scaling of the X-ray emission with orbital separation for adiabatic wind shocks. We applied a spectral disentangling code to optical spectra to determine the radial velocities and the individual spectra of each star. The individual spectra were analyzed using the CMFGEN model atmosphere code. We fitted two X-ray spectra using a Markov Chain Monte Carlo algorithm and compared them to the emission expected from adiabatic shocks. We determine an orbital period of 2103.4days, a low orbital eccentricity of 0.11, and a mass ratio m2/m1=0.84. Combined with the orbital inclination inferred in a previous astrometric study, we obtain surprisingly low masses of 9.7 and 8.2Msun. From the individual spectra, we infer O6.5 spectral types for both stars and a brightness ratio of l1/l2~2. The softness of the X-ray spectra, the very small variation of spectral parameters, and the comparison of the X-ray-to-bolometric luminosity ratio with the canonical value for O-type stars allow us to conclude that X-ray emission from the wind interaction region is quite low. We cannot confirm the runaway status previously attributed to HD54662 and we find no X-ray emission associated with the bow shock detected in the infrared. The lack of hard X-ray emission from the wind-shock region suggests that the mass-loss rates are lower than expected and/or that the pre-shock wind velocities are much lower than the terminal wind velocities. The bow shock associated with HD54662 possibly corresponds to a wind-blown arc created by the interaction of the stellar winds with the ionized gas of CMa OB1. (abridged)Comment: Manuscript has been accepted. A&A, in pres