THE SPECTROSCOPIC ORBIT AND SUBSYNCHRONOUS ROTATION OF THE HERBIG AE/BE STAR TY CRA

Eighty-nine high-resolution spectra of the Herbig Ae/Be eclipsing binary TY CrA have been obtained with the CTIO 1.5 m telescope, from which high-precision radial velocities (sigma almost-equal-to 1.5 km s-1) have been measured. We find TY CrA to be a single-lined, spectroscopic binary having a circular orbit with a period of 2.88873 days, supporting a previous photometric period of 2.888777 days. We also place an upper limit of 15 km s-1 on the v sin i of the primary. Such a low rotational velocity corresponds to highly subsynchronous rotation, presuming the stellar rotation axis to have an inclination angle near 90-degrees. We argue that this remarkably slow rotation velocity in a circular orbit cannot be explained in the context of stellar evolution and tidal interactions alone. Presuming TY CrA to be a pre-main-sequence star, the origin of its subsynchronism must lie in a braking mechanism early in the life of the primary. Finally, we note that the lack of near-infrared excess emission indicates that no optically thick disk material is present within a few orbital separations of the binary. In addition, the extinction of only 3 mag toward TY CrA places an upper limit on circumbinary disk mass that is several orders of magnitude below disk masses detected around T Tauri stars. However, this conclusion is sensitive to the precise geometry of the disk-binary system, so that a massive disk with a central hole in which the binary resides is not excluded.