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A. Suchkov (STScI)
We compared the kinematics and metallicity of the main sequence binary and single uvby F stars from the HIPPARCOS catalog to see if the populations of these stars originate from the same statistical ensemble, and concluded that they do not. The velocity dispersions of the known unresolved binary F stars were found to be dramatically smaller than those of the single F~stars, suggesting that they are, on average, younger than the single stars. This inference was further supported by the difference in metal abundance: the binaries were found to be, on average, more metal rich than the single stars. So, we concluded that the population of unresolved binary F~stars is indeed younger than that of the single stars. Comparison of the single F~stars with the binary candidate (C binaries) identified in Suchkov & McMaster (ApJ, 524, L99,1999) showed that, unlike the known binaries, C binaries are, on average, older than single F~stars. We suggest that the age difference between the C binaries, single stars, and known binaries is due to dependence of stellar evolution in a binary system on binary components mass ratio and separation. We find evidence that a close companion prolongs the MS lifetime of the primary F~star. One can speculate that if the secondary is a close regular star, the primary can nevertheless prematurely cease to exist as a MS F~star because of mass loss/transfer caused by component interaction. This may explain young average age of known binaries. However, if the secondary is substellar, mass loss does not occur and the primary can stay within the MS much beyond the lifetime of a similar single star. This may explain old average age of C binaries provided they are dominated by systems with a substellar secondary.