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

Abstract

We present the results of a photometric BVI survey of 0.9deg<SUP>2</SUP> around the Wolf-Rayet binary gamma<SUP>2</SUP> Vel and its early-type common proper motion companion gamma<SUP>1</SUP> Vel (together referred to as the gamma 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, Halpha emission and high levels of X-ray activity. We show that the PMS stars are kinematically coherent and spatially concentrated around gamma Vel. The PMS stars have similar proper motions to gamma Vel, to main-sequence (MS) stars around gamma Vel and to early-type stars of the wider Vela OB2 association of which gamma<SUP>2</SUP> Vel is the brightest member. The ratio of MS stars to low-mass (0.1-0.6M<SUB>solar</SUB>) PMS stars is consistent with a Kroupa mass function. MS fitting to stars around gamma 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 gamma<SUP>2</SUP> Vel. High-mass stellar models indicate an age of 3-4Myr for gamma<SUP>2</SUP> 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 gamma Vel, and this is a subcluster within the larger Vela OB2 association. We speculate that gamma<SUP>2</SUP> 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.