The stellar association around Gamma Velorum and its relationship with Vela OB2
Monthly Notices of the Royal Astronomical Society
Oxford University Press (OUP) / Royal Astronomical Society
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.
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.
Vol. 393, pp. 538 - 556