Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...19410505s&link_type=abstract
American Astronomical Society Meeting 194, #105.05
Astronomy and Astrophysics
Astronomy
Scientific paper
A significant fraction of bright F stars within the evolved main sequence can actually be unidentified binaries with comparably bright components. We have tested the presence of such binaries using two independent approaches. First, we have compared absolute magnitudes obtained from HIPPARCOS parallaxes and TYCHO V_T magnitudes, MV_T, with those derived from uvby photometry, Mc_1. For unresolved binaries, absolute magnitudes from HIPPARCOS must be brighter because they represent the integrated flux of a binary stars while the uvby absolute magnitude is determined by the integrated spectrum and is the same for a single star and a binary with two identical components. This test has shown that at a given effective temperature the difference MV_T - Mc_1 gets increasingly brighter toward brighter MV_T, indicating the increasing fraction of binaries as one moves upward from the ZAMS. The second test compares the age--velocity relations of the stars above and below the loci of the turnoffs in the log T_e - MV_T diagram. The test has shown that the mean tangential velocities of the stars above the turnoff level are systematically larger, suggesting that many of these stars are in fact binaries with comparably bright components, with absolute magnitude of the primary much fainter than the observed MV_T. The actual age of such stars is older then that derived from MV_T, hence their velocities must be on average larger. The discrepancy between the age--velocity relations turns out to be much smaller if Mc_1 is used instead of MV_T, which additionally supports the first test: the result implies that the stars which are fainter in Mc_1 are mostly the same stars which are responisible for larger discrepancy between the age--velocity relations when age is derived from isochrone fitting to MV_T.
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