The Hipparcos distance determination of the Wolf-Rayet system γ2 Velorum (WC8+O) and its ramifications

Hipparcos parallax measurements give a distance to the Wolf-Rayet WC8+O spectroscopic binary γ² Vel of d = 258 ^-31+41 pc and a distance to the O4I(n)f star ξ Pup of d = 429 _-77⁺¹²⁰pc. Adopting for γ² Vel an interstellar extinction of A_ν = 0.06 mag, this implies an absolute magnitude M _ν = - 5.4 mag for the WC8+O binary system. Given that the binary components have a magnitude difference Δm = 1.4 mag, we derive M _ν(WC8) = -3.7 and M _v(O) = -5.0 mag. The latter indicates an O8.5III rather than an O9I companion, as was adopted during the last 25 years. Apparently γ² Vel is not a member of, but a foreground object before the open cluster Cr 173 and the association Vel OB2. Given a re-assessment of the distance of the Gum Nebula, γ² Vel is still one of its ionizing sources, while ξ Pup appears to be located at the back of the Gum Nebula. Consequences of the Hipparcos distance determination of γ² Vel for its mass, mass loss rate, luminosities at various wavelengths, and, briefly, its association with the Gum Nebula, are discussed.