The N - rich Mass Donor Star of the Microquasar LS 5039

We present an investigation of new optical and ultraviolet spectra of the mass donor star in the massive X-ray binary LS 5039. The optical band spectral line strengths indicate that the atmosphere is N-rich and C-poor, and we classify the stellar spectrum as type ON6.5 V((f)). The N-strong and C-weak pattern is also found in the stellar wind P Cygni lines of N V λ1240 and C IV λ1550 (narrow absorption components in the former indicate that the wind terminal velocity is V ∞ = 2440 ± 190 km s −1). We suggest that the N-enrichment may result from internal mixing if the O-star was born as a rapid rotator, or the O-star may have accreted N-rich gas during a common envelope interaction with the progenitor of the supernova. We re-evaluated the orbital elements to find an orbital period – 2 – of P = 4.4267 ± 0.0010 d. We compared the spectral line profiles with new non-LTE, line-blanketed model spectra from Lanz & Hubeny (2003), from which we derive an effective temperature T eff = 37.5 ±1.7 kK, gravity log g = 4.0 ±0.1, and projected rotational velocity V sin i = 140±8 km s −1. We fit the UV, optical, and IR flux distribution using a model spectrum and extinction law with parameters E(B − V) = 1.28 ± 0.04 and R = 3.25 ± 0.15. We confirm the co-variability of the observed X-ray flux and stellar wind mass loss rate derived from the Hα profile (Reig et al. 2003), which supports the wind accretion scenario for the X-ray production in LS 5039. Wind accretion models indicate that the compact companion has a mass M X /M ⊙ = 1.4 ± 0.4, consistent with its identification as a neutron star. We argue that the O-star has a mass in the range 20 – 35M ⊙ (based upon a lower limit for the distance and the lack of eclipses). The observed eccentricity and runaway velocity of the binary can only be reconciled if the neutron star received a modest kick velocity due to a slight asymmetry in the supernova explosion.