Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The stellar association around Gamma Velorum and its relationship with
  Vela OB2   We present the results of a photometric BVI survey of 0.9 square degrees
around the Wolf-Rayet binary gamma^2 Vel and its early-type companion gamma^1
Vel. Several hundred PMS stars are identified and the youth of a subset of
these is confirmed by the presence of lithium, H-alpha emission and 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 stars around gammaVel and to early-type stars of
the wider Vela OB2 association of which gamma^2 Vel is the brightest member.
The ratio of main-sequence stars to low-mass (0.1-0.6 Msun) PMS stars is
consistent with a Kroupa mass function. Main-sequence fitting to stars around
gamma Vel gives a distance modulus of 7.76+/-0.07 mag, consistent with a
similarly-determined distance for Vela OB2 and with interferometric distances
to gamma^2 Vel. High-mass stellar models indicate an age of 3-4 Myr for gamma^2
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 gamma
Vel and that this is a subcluster within the larger Vela OB2 association. We
speculate that gamma^2 Vel formed after 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 Vela OB2. Instead, star formation must have started
at several sites within a molecular cloud, either sequentially or,
simultaneously after some triggering event [abridged].

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The categories of this paper are: eess
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The categories of this paper are: mtrl-th
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The categories of this paper are: comp-gas
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The categories of this paper are: math
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The categories of this paper are: atom-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: quant-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: cond-mat
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The categories of this paper are: cs
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The categories of this paper are: alg-geom
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The categories of this paper are: supr-con
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The categories of this paper are: q-alg
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The categories of this paper are: hep-ph
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The categories of this paper are: hep-ex