Hot Inner Disks that Appear and Disappear Around Rapidly Rotating A‐Type Dwarfs

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Abstract
At any one time, approximately one-quarter of the most rapidly rotating normal A-type dwarfs (V sin i ≥ 200 km s-1) show shell lines of Ti II in the near-ultraviolet. Our observations during 22 years show that the lines appear and disappear on timescales of decades but do not display significant changes within 1 year. This implies that they are not remnants of the star formation but rather are probably caused by sporadic mass-loss events. A working hypothesis is that all A-type stars that are rotating near their limits have these shells, but for only one-quarter of the time. Because these lines do not appear in stars with smaller sin i, the shells must be disks. These are hot inner disks that may or may not be related to the cool outer disks seen by Smith and Terrile around β Pic or through infrared excesses around Vega and other A-type dwarfs. The similar, limited line widths indicate that the disks are ~7 R* above the stellar surfaces.

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