(Proposal ID) S15B-084

(PI) Ferraro, Francesco

(Proposal Title)
The final word on the mass-transfer process as BSS formation mechanism

(Abstract)
We propose to dedicate just 2 nights of SUBARU observing time to finally get a 
full and coherent picture of the mass transfer (MT) formation process of Blue 
Straggler Stars (BSSs), a 50-yr long open issue. The UV excess recently 
observed for 3 BSSs in the open cluster NGC188 (Gosnell et al. 2014, ApJ 783, 
L8) has been interpreted as the photometric signature of hot and young white 
dwarf (WD) companions, thus suggesting that these 3 BSSs have been recently 
formed through MT activity in binary systems. On the other hand, Ferraro et al.
 (2006, ApJ 467, L56) discovered that a sub-sample of BSSs in 47 Tucanae shows 
C and O depletion with respect to normal cluster stars. This feature is 
considered as the chemical signature of the MT process. Here we propose to use 
HDS to search for C and O depletion on the surface of the 3 UV-excess BSSs in 
NGC188, thus combining for the first time the two (photometric and 
spectroscopic) observational proofs of the MT-BSS formation mechanism. Indeed, 
at the cost of just 2 observing nights, these observations offer the unique 
opportunity to fully characterize one of the most promising channels of BSS 
formation.