冕状星团中的极低质量天体：过渡区
Very Low-Mass Objects in the Coronet Cluster: The Realm of the Transition Disks
摘要：
我们目前的光学和红外光谱的一组低质量恒星和棕矮星的冠状星团（年龄~ 100万年），获得了多光纤光谱仪火焰/VLT和IRS/斯皮策。光谱显示了M1和M7.5之间的光谱类型，证实了物体的年轻（通过Li 6708 A吸收），并显示了吸积（通过Halpha）和冲击（通过禁止线发射）的存在。IRS光谱，以及斯皮策和2 MASS上IRAC/MIPS仪器的红外测光，证实了约70%的物体存在红外过量特征。一半的磁盘没有表现出任何硅酸盐排放，或目前的大颗粒的特点，平面。其余的圆盘显示硅酸盐发射典型的无定形和结晶硅酸盐颗粒的尺寸为几微米。大约50%的天体与磁盘不显示近红外过量发射，有“过渡”磁盘，根据其经典的定义。对于这样一个年轻的星系团来说，这是一个非常高的分数。大量的“过渡”磁盘表明寿命相当于典型的光学厚磁盘的寿命。因此，这些磁盘可能不会处于短暂的阶段，介于II类和III类对象之间。冕状星团中的圆盘的光谱能量分布中值也比在类似年龄的区域中观察到的太阳型恒星周围的圆盘更接近于平坦的圆盘。冕状星团中的星盘形态和演化的差异可能与这样一个事实有关，即与其他星团研究中的太阳型恒星相比，这些天体具有非常晚的光谱类型。最后，光谱学显示其中一个X射线源是由云中的赫比格·哈罗天体产生的。
英文摘要：
We present optical and IR spectra of a set of low-mass stars and brown dwarfs in the Coronet cluster (aged ~1Myr), obtained with the multifiber spectrograph FLAMES/VLT and IRS/Spitzer. The optical spectra reveal spectral types between M1 and M7.5, confirm the youth of the objects (via Li 6708 A absorption), and show the presence of accretion (via Halpha) and shocks (via forbidden line emission). The IRS spectra, together with IR photometry from the IRAC/MIPS instruments on Spitzer and 2MASS, confirm the presence of IR excesses characteristic of disks around ~70% of the objects. Half of the disks do not exhibit any silicate emission, or present flat features characteristic of large grains. The rest of the disks show silicate emission typical of amorphous and crystalline silicate grains a few microns in size. About 50% of the objects with disks do not show near-IR excess emission, having "transitional" disks, according to their classical definition. This is a very high fraction for such a young cluster. The large number of "transitional" disks suggests lifetimes comparable to the lifetimes of typical optically thick disks. Therefore, these disks may not be in a short-lived phase, intermediate between Class II and Class III objects. The median spectral energy distribution of the disks in the Coronet cluster is also closer to a flat disk than observed for the disks around solar-type stars in regions with similar age. The differences in the disk morphology and evolution in the Coronet cluster could be related to fact that these objects have very late spectral types compared to the solar-type stars in other cluster studies. Finally, the optical spectroscopy reveals that one of the X-ray sources is produced by a Herbig Haro object in the cloud.
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