Very Low Mass Objects In The Coronet Cluster: The Realm Of The Transition Disks

摘要

We present optical and IR spectra of a set of low-mass stars and brown dwarfs in the Coronet cluster (aged ～1 Myr), obtained with the multifiber spectrograph FLAMES on the VLT and with the IRS instrument on Spitzer. Most of the objects had been selected via their X-ray emission in a deep Chandra survey. The optical spectra reveal spectral types between Ml and M7.5, confirm the youth of the objects (via Li λ6708 absorption), and show the presence of accretion (via Hα) 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 (> 6 μm). The rest of the disks show silicate emission with indications 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, corresponding to the presence of "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 SED 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 the 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.