Hot Gaseous Stellar Disks Avoid Regions of Low Interstellar Densities

Abstract

I surveyed spectra of 170 bright A-type stars with large rotational velocities (V sin i≥200 km s-1). The spectra of 20 A stars showed sharp lines of excited Ti II, which is assumed to arise from hot gaseous disks, not interstellar atoms. The fact that the lines come from Ti II and not other singly-ionized iron-peak elements is because among the iron-peak elements, only Ti II has lines from low-lying states in the optical region. However, disk lines due to other iron-peak elements are found in IUE spectra. Therefore, there is no evidence that the stars or disks have abnormal compositions. Whereas the 150 stars without disks are well distributed with 200 pc, the ones with disks avoid the Local Cavity where the density is 5% of that outside the bubble. We conclude that the disks are accreted by rapidly rotating stars (V≥200 km s-1) from the interstellar medium where the density is moderate or large.