Evidence for prolonged main sequence stellar evolution of F stars in closebinaries

Binary F stars exhibit large brightness anomaly; which is defined here as t he difference between. the absolute magnitude from the uvby photometry and tile actual absolute magnitude of the star. we have found that the anomaly inversely correlates with thf binary components separation. There is eviden ce that the correlation reflects actual population differences between clos e and wide binary pairs, in which case it indicates that the anomaly is som ehow associated with the interaction of binary's components, The anomaly ha s also been found to correlate with both kinematics and metallicity. The se nse of the correlations implies that the anomaly increases as the star evol ves, suggesting a peculiar evolution of a primary F star in a tight binary pair. This conclusion has further been supported by the study of the age-ve locity relation (AVR) of F stars that are cataloged in the HIPPARCOS as sin gle. Among these stars, those with brightness anomaly were previously shown to be most likely unidentified close binaries. We have found that the AVR of these binary; candidates is different from that of the "truly single": F stars. The discrepancy between the two AVRs indicates that the putative bi naries are, on average; older than similar normal single F stars at the sam e effective temperature and luminosity, which is consistent with the inferr ed peculiar evolution in close binaries. It al,pears that this peculiarity is caused by the impact of tile components interaction in a tight pair on s tellar evolution. which results in the prolonged main sequence lifetime of the primary F star.