TITLE:  Detecting White Dwarf Companions of Blue Straggler Binaries in the Old Open Cluster NGC 188


ABSTRACT:  The discovery that the majority of blue stragglers in the old (7 Gyr) open cluster NGC 188 are in 
binaries with periods of order 1000 days constrains the possible formation mechanism(s) to: i) mass transfer 
in binary stars, ii) stellar collisions during dynamical encounters of multiple star systems, or iii) mergers of 
inner binaries in primordial triples driven by the Kozai mechanism.  A critical discriminant between these 
ideas are the secondary stars of the blue straggler binaries. The mass-transfer scenario predicts white dwarf 
companions, while the collision and merger scenarios predict mostly main-sequence companions. 
Ground-based spectra of the majority of blue stragglers in NGC 188 do not detect secondary star light, but 
analysis of the mass functions indicates that the blue straggler binaries with 1000-day period orbits have a 
secondary-mass distribution that is narrow and peaked near 0.5 Msolar, suggestive of white dwarfs or 
possibly low-mass main-sequence companions. With Cycle 19 HST/ACS/SBC FUV-imaging we will search 
for white dwarf companions of the blue stragglers in NGC 188.  We present spectral energy distribution 
models of blue straggler-white dwarf pairings.  We then combine these SEDs with blue straggler populations 
in predictive Monte Carlo models.  With these models we predict our white dwarf companion detection rates 
in NGC 188 as a function of blue straggler formation mechanism and age distribution. Support for Program 
number GO 12492 was provided by NASA through a grant from the Space Telescope Science Institute, 
which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA 
contract NAS5-26555.