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

We present the results of a photometric BVI survey of 0.9deg² around the Wolf-Rayet binary γ² Vel and its early-type common proper motion companion γ¹ 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 X-ray 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 (MS) stars around γ Vel and to early-type stars of the wider Vela OB2 association of which γ² Vel is the brightest member. The ratio of MS stars to low-mass (0.1-0.6M_solar) PMS stars is consistent with a Kroupa mass function. MS fitting to stars around γ Vel gives an association distance modulus of 7.76 +/- 0.07mag, which is consistent with a similarly determined distance for Vela OB2 and also with interferometric distances to γ² Vel. High-mass stellar models indicate an age of 3-4Myr for γ² 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 this is a subcluster within the larger Vela OB2 association. We speculate that γ² 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.