DETECTION OF WHITE DWARF COMPANIONS TO BLUE STRAGGLERS IN THE OPEN CLUSTER NGC 188: DIRECT EVIDENCE FOR RECENT MASS TRANSFER

Abstract

Several possible formation pathways for blue straggler stars have been\ndeveloped recently, but no one pathway has yet been observationally\nconfirmed for a specific blue straggler. Here we report the first\nfindings from a Hubble Space Telescope Advanced Camera for Surveys/Solar\nBlind Channel far-UV photometric program to search for white dwarf\ncompanions to blue straggler stars. We find three hot and young white\ndwarf companions to blue straggler stars in the 7 Gyr open cluster NGC\n188, indicating that mass transfer in these systems ended less than 300\nMyr ago. These companions are direct and secure observational evidence\nthat these blue straggler stars were formed through mass transfer in\nbinary stars. Their existence in a well-studied cluster environment\nallows for observational constraints of both the current binary system\nand the progenitor binary system, mapping the entire mass transfer\nhistory.