First Evidence of Circumstellar Disks around Blue Straggler Stars

Abstract

We present an analysis of optical Hubble Space Telescope Space Telescope Imaging Spectrograph and Faint Object Spectrograph spectroscopy of six blue stragglers found in the globular clusters M3, NGC 6752, and NGC 6397. These stars are a subsample of a set of ~50 blue stragglers and stars above the main-sequence turnoff in four globular clusters that will be presented in a forthcoming paper. All but the six stars presented here can be well fitted with non-LTE model atmospheres. The six misfits, on the other hand, possess Balmer jumps that are too large for the effective temperatures implied by their Paschen continua. We find that our data for these stars are consistent with models only if we account for extra absorption of stellar Balmer photons by an ionized circumstellar disk. Column densities of H I and Ca II are derived as are the the disks' thicknesses. This is the first time that a circumstellar disk is detected around blue stragglers. The presence of magnetically locked disks attached to the stars has been suggested as a mechanism to lose the large angular momentum imparted by the collision event at the birth of these stars. The disks implied by our study might not be massive enough to constitute such an angular momentum sink, but they could be the leftovers of once larger disks.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-2655.