天增一的星协及其与船帆座OB2的关系
The stellar association around Gamma Velorum and its relationship with Vela OB2
摘要：
我们给出了沃尔夫-拉叶双星γ Vel及其早期伴星γ Vel周围0.9平方度的光度BVI测量结果。数百颗PMS恒星被确认，其中一个子集的年轻被锂，H-α发射和X射线活动的存在所证实。我们发现PMS星是运动学相干的，空间上集中在伽马Vel周围。PMS星的自行与Vel γ星、Vel γ星周围的主序星以及船帆座OB 2星协中最亮的成员--Vel γ星--的早型星相似。主序星与低质量（0.1-0.6 Msun）PMS星的比率与Kroupa质量函数一致。对Vel γ周围恒星的主序星拟合得到的距离模数为7.76+/-0.07等，这与船帆座OB 2的类似测定距离以及Vel γ ^2的干涉测量距离一致。大质量恒星模型表明γ ^2 Vel的年龄为300 ~ 400万年，但根据低质量演化模型，低质量PMS星的年龄约为1000万年，根据颜色-星等图和锂的消耗将它们与其他星团经验性地放在一个年龄序列中，则为500 ~ 1000万年。我们的结论是，低质量PMS星形成一个真正的协会与伽马船帆座，这是一个子集群内的更大的船帆座OB 2协会。我们推测，γ ^2 Vel是在低质量恒星之后形成的，它排出了气体，终止了恒星的形成，并解除了星协的束缚。PMS星的速度色散太低，不可能在这次恒星形成事件中产生船帆座OB 2中的所有恒星。相反，恒星的形成必须在分子云内的几个地点开始，或者顺序地，或者在某些触发事件之后同时发生。
英文摘要：
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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