Transcript Variable Blue Stragglers in M67

Variable Blue Stragglers in
M67
Zhang Xiao-Bin
NAOC
Abstract
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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
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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 underloop stars;
Origin: complicated proceedings; many theories:
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Binary merger;
Multiple epochs of star formation;
Star-life extend due to internal mixing;
Mass-transfer within binary stars;
Stellar collisions;
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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
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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
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BATC: Feb. 2001;
85 cm reflector:
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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
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W Uma system discovered early in 1960; orbital and physical
parameters are very uncertain; almost at the turn-off in the
CMD
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Light curve analysis:
Physical parameters:
adopting the
distance modulus
of the cluster
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Discussions:
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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…