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Variable Blue Stragglers in M67

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

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Variable Blue Stragglers in M67

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  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;

  6. 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.

  7. 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

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

  9. 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…

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