1. Variable Blue Stragglers in M67
Zhang Xiao-Bin
NAOC

2. 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.

3. 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;

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

5. 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;

7. The long-term photometry
BATC: Feb. 2001;
85 cm reflector:
Dec – Jan. 2004;
Dec – 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.

8. 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

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

10. 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…