疏散星团Praesepe中心区的亚恒星质量函数的深LBT观测
The substellar mass function in the central region of the open cluster Praesepe from deep LBT observations
摘要：
对不同年龄疏散星系团质量函数（MF）的研究使我们能够探测棕矮星从星系团中蒸发到磁场中的效率。对较老的星团（年龄> 1亿年）的观测不会受到年轻星团中遇到的一些问题的严重影响，例如星团内灭绝和褐矮星模型中的大不确定性。我们提出了一个深测光调查的结果，以研究中心区域的老疏散星团Praesepe（年龄~590 $^{+150}_{-120}$百万年，距离~190 $^{+6.0}_{-5.8}$ pc）的MF，到亚恒星制度。我们使用大双筒望远镜照相机对普雷塞普进行了光学（$riz$和$Y$波段）光度测量，覆盖了星团中心约0.59度$^2$的区域，从$i$ ~19.0等（~100 M_Jup）下降到5个$σ$检测限（$i$ ~ 25.6等（~40 M_Jup））。在$i=23.8$ mag和$z=22.0$ mag（~55 M_Jup）下，测量完成约95%。我们确定了59个集群成员的候选人，其中37个是亚恒星，通过比较与尘埃大气模型的预测。这些候选者的MF从亚恒星边界开始上升，直到~67 M_Jup，然后下降。这与其他年龄大于5000万年的疏散星团的形式有很大的不同，但似乎与非常年轻的疏散星团中发现的相似，其MFs的峰值约为10 M_Jup。要么Praesepe真的有一个不同的MF从其他集群或他们有类似的初始MF，但不同的动力学演化。由于大多数候选者都很微弱，我们缺乏天体测量或光谱跟踪来测试它们的成员资格。然而，污染的领域矮星，星系，或巨人被发现对MF的形状几乎没有影响，因此MF的“真正的”集群成员应该有类似的特点。
英文摘要：
Studies of the mass function (MF) of open clusters of different ages allow us to probe the efficiency with which brown dwarfs evaporate from clusters to populate the field. Surveys of older clusters (age >100 Myr) are not affected so severely by several problems encountered in young clusters, such as intra-cluster extinction and large uncertainties in brown dwarf models. We present the results of a deep photometric survey to study the MF of the central region of the old open cluster Praesepe (age~590$^{+150}_{-120}$ Myr, distance ~190$^{+6.0}_{-5.8}$ pc), down to the substellar regime. We performed an optical ($riz$ and $Y$-band) photometric survey of Praesepe using the Large Binocular Telescope Camera covering an area of ~0.59 deg$^2$ in the cluster centre from $i$~19.0 mag (~100 M_Jup) down to a 5$σ$ detection limit at $i$~25.6mag (~40 M_Jup). The survey is approximately 95% complete at $i=23.8$ mag and $z=22.0$ mag (~55 M_Jup). We identify 59 cluster member candidates, of which 37 are substellar, by comparing with the predictions of a dusty atmosphere model. The MF of those candidates rises from the substellar boundary until ~67 M_Jup and then declines. This is quite different from the form inferred for other open clusters older than 50 Myr, but seems to be similar to those found in very young open clusters, the MFs of which peak at ~10 M_Jup. Either Praesepe really does have a different MF from other clusters or they had similar initial MFs but a different dynamical evolution. Since most of the candidates are faint, we lack astrometric or spectroscopic follow-ups to test their memberships. However, the contaminations by field dwarfs, galaxies, or giants are found to have little effect on the shape of MF and therefore the MF of `real' cluster members should have similar characteristics.
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