The stellar association around Gamma Velorum and its relationship with Vela OB2

We present the results of a photometric BV I survey of 0.9 square degrees around the Wolf-Rayet binary γ Vel and its early-type common proper motion companion γ Vel (together referred to as the γ Vel system). Several hundred pre-main-sequence (PMS) stars are identified and the youth of a subset of these is spectroscopically confirmed by the presence of lithium in their atmospheres, Hα emission and high levels of Xray activity. We show that the PMS stars are kinematically coherent and spatially concentrated around γ Vel. The PMS stars have similar proper motions to γ Vel, to main-sequence stars around γ Vel and to early-type stars of the wider Vela OB2 association of which γ Vel is the brightest member. The ratio of main-sequence stars to low-mass (0.1–0.6M⊙) PMS stars is consistent with a Kroupa (2001) mass function. Main-sequence fitting to stars around γ Vel gives an association distance modulus of 7.76±0.07 mag, which is consistent with a similarly-determined distance for Vela OB2 and also with interferometric distances to γ Vel. High-mass stellar models indicate an age of 3–4Myr for γ Vel, but the low-mass PMS stars have ages of ≃ 10Myr according to low-mass evolutionary models and 5–10Myr by empirically placing them in an age sequence with other clusters based on colour-magnitude diagrams and lithium depletion. We conclude that the low-mass PMS stars form a genuine association with γ Vel and that this is a subcluster within the larger Vela OB2 association. We speculate that γ Vel formed after the bulk of the low-mass stars, expelling gas, terminating star formation and unbinding the association. The velocity dispersion of the PMS stars is too low for this star forming event to have produced all the stars in the extended Vela OB2 association. Instead, star formation must have been initiated at several sites within a molecular cloud, either sequentially or, simultaneously after some triggering event.