2003ApJS..147..103G

We have conducted a radial-velocity survey of bright (V<9), overluminous, F stars within 80 pc. The criterion of overluminosity is ΔM_c0=M_c0-M_V≥0.5, where M_Vis the absolute magnitude based on the Hipparcos parallax and M_c0 is the absolute magnitude from uvby photometry. For F stars, overluminosity at a level of ΔM_c0>0.15 has been argued to be indicative of the presence of a bright companion, so many of the overluminous ``single'' F stars are expected to be, in fact, binaries with comparably bright components. Therefore, the main goals of the survey have been to gain insight into the nature of overluminous F stars and to test specifically the hypothesis of the duplicity of overluminous F stars by searching for previously unrecognized binary stars among them. Other goals of this project included the determination, wherever possible, of the orbits of the binaries discovered as a result of the survey and comparing the properties of the binaries with those of the visually single stars with constant radial velocity. The program list comprised 111 stars that appear in the Hipparcos catalog as single, plus seven members of multiple systems whose components have individual entries in the catalog.

Of the total of 118 stars selected, the radial velocities of 77 have been measured with the Cambridge CORAVEL in the present survey. Among the remainder, six have not allowed us to determine their radial velocities because of the weakness or lack of a ``dip'' in their radial-velocity traces, and one was optically inaccessible to the telescope. All the rest proved to be sufficiently well known already either as binary or as constant-radial-velocity (CRV) stars. The survey discovered 25 new binary systems, not counting two that were not generally known as such but were already under observation by R. F. G. and two that were discovered by Hipparcos to show eclipses but had never been recognized as SBs. With the previously known binaries, there is a total of 61 definite plus three probable binaries out of a total of 111 stars upon which we can adjudicate–a binary fraction of ∼58%, just double the ∼29% found in a sample of randomly selected F stars by Nordström et al. (1997A&AS..126...21N). Of the binaries, 52 are SB2 and 12 are SB1. Orbits (some of them still rather preliminary) have been determined for the first time for 28 binaries, 21 of which are SB2.

Comparison of the CRV stars (a total of 47) with the SB2 binaries has shown that the two groups have different kinematics. The difference implies that on average the CRV stars are 3 to 4 Gyr older than the SB2s, although several of the CRVs appear to be very young. At the same time the SB2 stars turn out to have about the same kinematics, hence the same average age, as both binary and normal single F stars from a control sample drawn from the Hipparcos catalog. The much older age of the bulk of the overluminous CRV F stars may be indicative of anomalous stellar evolution that results in anomalously long main-sequence lifetimes for those stars. The SB1 stars appear to be closer to the CRVs than to the SB2s and are probably also very old. The properties of the ``no-dip'' stars suggest very young age; some of the stars are probably pre-main-sequence (PMS). A few PMS candidates have been identified also among the CRV stars. Thus, overluminous F stars comprising our sample appear to consist of three distinct groups: mostly young SB2 binaries with comparably bright components, mostly very old CRV and SB1 stars, and very young no-dip stars.