AAS 207th Meeting, 8-12 January 2006
Session 104 Binary and Variable Stars
Oral, Tuesday, 2:00-3:30pm, January 10, 2006, Balcony B

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[104.03] An Observational Exploration of the Evolution of Angular Momentum in Close Detached Binary Stars of Solar Type.

S Meibom (University of Wisconsin - Madison)

We have observationally studied the impact of a close binary companion on the evolution of rotational and orbital angular momentum of solar-type stars. We present results on tidal circularization, tidal synchronization, and on non-tidal effects of a close secondary on the rotation of the primary star.

We present a population of close binaries (a ~< 0.2 AU) in the 150Myr open cluster M35 and introduce a diagnostic to determine the orbital period at which binaries with the most frequent initial orbital eccentricity circularizes at the age of the population (the circularization period). We determine the circularization period for M35 and 7 additional populations of close binaries and compare the distribution of circularization periods with population age to the predictions by tidal theory. We conclude that models of tidal circularization cannot account for the observed circularization periods.

From an extensive time-series photometric survey of stars in the field of M35 we have determined rotation periods for 310 late-type members, including 13 solar-type primaries in close binaries with known orbital parameters. We have investigated the degree of tidal synchronization in each binary by comparing the angular rotation velocity of the primaries to the (pseudo-) synchronization angular velocity. The results offer interesting challenges to tidal theories.

Finally, we find by comparison of the rotation period distribution from 17 primaries in close (a ~< 5 AU), but non-synchronized, binaries to the distribution from 53 single stars or primaries in wide binaries (a >~ 5 AU), that the primaries in close binaries rotate on average faster than the single stars. The difference between the two period distributions suggest that a close companion can indirectly affect the rotational evolution of a solar-type star.

This work was supported by NSF grant AST-0453442 and by a Ph.D fellowship from the Danish Research Agency (Forskningstyrelsen) to S.M.

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