Apsidal Motion Studies: DI Her

Maloney, Guinan, and student L. M. Barge are re-analyzing the puzzling eclipsing binary system DI Herculis. This system is rare among main sequence stars in that its apsidal motion is dominated by the effects of General Relativity. The GR contribution to its theoretically predicted apsidal motion is $2.34^\circ$/100 y., whereas the theoretically predicted classical contribution (due to tidal and rotational deformation of the component stars) is $1.93^\circ$/100 y. The interesting fact is that the observed apsidal motion, determined from timings of the stars' mutual eclipses, is anomalously low: $\sim 1^\circ$/100 y., well below the combined theoretical expectation of $.27^\circ$/100 y. DI Her consists of two main sequence stars (B5V and B6V) in a 10.55 day eccentric orbit ($e=0.489$). Observations of times of minima reveal the system's apsidal motion, computed from the changing displacement of the secondary eclipse from the primary eclipse. Four decades of photoelectric measurements show that the observed apsidal motion remains below that predicted. Various explanations for this discrepancy have been offered, with the most promising involving the presence of a third component of the system. In a highly inclined orbit, the third body would diminish the rate of apsidal advance of the close pair. Adding photometry recently taken with the 0.8 m Four College Automatic Photoelectric Telescope, we are computing a new determination of the apsidal motion for DI Her. With R. A. Mardling (Monash), we are also computing models utilizing a new formalism for studying three-body interactions in the DI Her system.

This research is supported by NSF/RUI grant AST00-71260, which we gratefully acknowledge.