Variable Blue Stragglers in M67 Zhang Xiao-Bin NAOC

Abstract A long-term time-series CCD photometry on variable blue stragglers (BSs) was performed, aiming on the understanding of the properties and physical nature of the BSs based on the comprehensive photometric study. I report here the progress and some results drawn from this project.

Blue stragglers Bluer and brighter than the turn-off of the stellar population; noticed first by Sandage (1953) in the GC M3; BSs related: yellow or red stragglers; up and under-loop stars; Origin: complicated proceedings; many theories: Binary merger; Multiple epochs of star formation; Star-life extend due to internal mixing; Mass-transfer within binary stars; Stellar collisions;

All relying on the complete understanding of the properties and evolutionary status of BSs; Variable BSs (photometric and/or spectroscopic): the most ideal samples;

BSs in M67 M67: with the age similar to the Sun; extensively observed; More than 30 BSs identified; More than 10 BSs are proved to be photometric and/or spectroscopic variables; Well-classified photometric variable BSs: 1 RS CVn star: S1082; 2 W Uma systems: S1036 and S1282; 2 oscillation BSs: S1280 and S1284;

The long-term photometry BATC: Feb. 2001; 85 cm reflector: Dec. 2003 – Jan. 2004; Dec. 2004 – Jan. 2004; Complete photometric study has been finished for S1282, S1280 and S1284; physical parameters for the 3 variable BSs are determined, evolutionary status and probable formation mechanism are discussed; S1036 and S1082; waiting for more observations.

S1282, an example W Uma system discovered early in 1960; orbital and physical parameters are very uncertain; almost at the turn-off in the CMD

Light curve analysis: Physical parameters: adopting the distance modulus of the cluster

Discussions: The primary: un-evolved normal MS star; the deduced mass matches its position in the CMD (the turn-off mass of M67: 1.25 solar mass); The secondary: over-radius, over-luminosity, obviously evolved; Mass transfer and mass ratio inverse undergone; To be a BS…