Journal Article

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

R. D. Jeffries, Tim Naylor, F. M. Walter, M. P. Pozzo and C. R. Devey

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 393, issue 2, pages 538-556
Published in print February 2009 | ISSN: 0035-8711
Published online February 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.14162.x
The stellar association around Gamma Velorum and its relationship with Vela OB2

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We present the results of a photometric BVI survey of 0.9 deg2 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 (0.1–0.6 M) PMS stars is consistent with a Kroupa mass function. MS fitting to stars around γ Vel gives an association distance modulus of 7.76 ± 0.07 mag, 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 3–4 Myr for γ2 Vel, but the low-mass PMS stars have ages of ≃10 Myr according to low-mass evolutionary models and 5–10 Myr 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.

Keywords: stars: formation; stars: pre-main-sequence; stars: Wolf–Rayet; open clusters and associations: individual: Vela OB2

Journal Article.  15059 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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