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

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 s...

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Veröffentlicht in:Monthly notices of the Royal Astronomical Society 2009, Vol.393 (2), p.538-556
Hauptverfasser: Jeffries, R. D., Naylor, Tim, Walter, F. M., Pozzo, M. P., Devey, C. R.
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Sprache:eng
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Zusammenfassung: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.
ISSN:0035-8711
1365-2966
DOI:10.1111/j.1365-2966.2008.14162.x