First Evidence of Circumstellar Disks around Blue Straggler Stars

We present an analysis of optical HST/STIS and HST/FOS spectroscopy of 6 bluestragglers found in the globular clusters M3, NGC6752 and NGC6397. These starsare a subsample of a set of ~50 blue stragglers and stars above the mainsequence turn-off in four globular clusters which will be presented in anforthcoming paper. All but the 6 stars presented here can be well fitted withnon-LTE model atmospheres. The 6 misfits, on the other hand, possess Balmerjumps which are too large for the effective temperatures implied by theirPaschen continua. We find that our data for these stars are consistent withmodels only if we account for extra absorption of stellar Balmer photons by anionized circumstellar disk. Column densities of HI and CaII are derived as arethe the disks' thicknesses. This is the first time that a circumstellar disk isdetected around blue stragglers. The presence of magnetically-locked disksattached to the stars has been suggested as a mechanism to lose the largeangular 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 suchan angular momentum sink, but they could be the leftovers of once larger disks.Comment: Accepted by ApJ Letters 10 pages, 2 figure