WIDE BINARIES AMONG HIGH-VELOCITY AND METAL-POORSTARS

CHRISTINE ALLEN, M.A. HERRERA and A. POVEDAInstitute of Astronomy, Universidad Nacional Autónoma de México, México, D.F.

The properties of old disk and halo binaries are of interest for the understandingof the processes of formation and early dynamical evolution of the Galaxy. Theluminosity function of the components of wide binaries and multiples, their massfunction, the fraction of halo or old disk stars that are members of wide binaries,and the distribution of their separations are some of the basic properties that arepoorly understood, mainly because of the paucity of known wide binaries amonghalo and old disk stars.

The present work is an attempt to ameliorate this situation. We have elaborateda list of 133 wide binaries mostly belonging to the halo or high-velocity disk, bysearching for common-proper-motion companions to the high-velocity and metal-poor stars studied by Schuster and collaborators (1988, 1993). Based on Stromgrenphotometry, these authors have derived distances, metallicities and ages for theirstars. Since each star has a large and well determined proper motion it was possibleto compare it with that of the NLTT stars of its vicinity. In this way we were able toidentify over 100 high-velocity and metal-poor common-proper-motion binary sys-tems. Each system was carefully checked to avoid misidentifications, and in mostof the cases distances were improved using the Hipparcos trigonometric parallaxes.

We have determined the distribution of angular separations for our wide binar-ies. Reliable distances are available for all of our systems, so this distribution canbe converted into a separation distribution in AU. We find 11 systems that have projected separations in excess of 10000 AU, or 16 systems with expected semiaxeslarger than 10000 AU; we note that their existence poses interesting dynamicalproblems.

The analysis of the frequency distribution of the expected major semiaxes showsthat they closely follow Oepik’s relation, namely,f (a) ∝ 1/a, for a ≤ 10000 AU.A comparison of the frequency distribution of major semiaxes in the present list ofwide binaries with that of the wide binaries in the solar vicinity, shows that althoughboth groups of binaries closely follow Oepik’s law, they begin to depart from it atvery different separations. In the solar vicinity we found the wideyoungerpairs tofollow Oepik’s law for major semiaxes of up to 8000 AU, whereas the wideolderpairs follow this law only up to 2400 AU (Povedaet al., 1997). This difference wasinterpreted as due to disruptive effects caused by massive perturbers which, withthe passage of time, tend to eliminate the widest systems. It is worth noting that

Astrophysics and Space Scienceis the original source of publication of this article. It is recom-mended that this article is cited as:Astrophysics and Space Science265: 233–234, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

234 C. ALLEN ET AL.

a frequency distribution of Oepik’s form has been found also for very young stars(see Allenet al., 1997 and references therein).

In the present group of wide binaries, the departure from Oepik’s law occurs atabout 10000 AU. This much larger value can be interpreted as a result of the smallerfraction of their lifetimes that the binaries spend in the galactic disk, where thevolume density of the perturbers is highest, as well as to their large space velocities;both effects will tend to decrease the rate of energy exchanges of the binary withexternal perturbers, and hence to preserve the widest systems. Even when thesesystems finally become unbound, their motions will follow parallel tracks for along time, causing them to be identified as common-proper-motion pairs.

Since many systems also have known radial velocities, space velocities for themwere be determined, and galactic orbits computed and characterized. The second-aries of these wide binaries are interesting in themselves, since they represent asampling of the very faint end of the main sequence of old disk and halo stars.

Acknowledgements

Our sincere thanks go to G. Cordero, A. Nigoche and A. Hernández.

References

Allen, C., Poveda, A. and Herrera, M.A.: 1997, in: Docoboet al. (eds.),Visual Double Stars:Formation, Dynamics and Evolutionary Tracks, Dordrecht, Kluwer, p. 133.

Poveda, A., Allen, C. and Herrera, M.A.: 1997, in: Docoboet al. (eds.),Visual Double Stars:Formation, Dynamics and Evolutionary Tracks, Dordrecht, Kluwer, p. 191.

Schuster, W.J. and Nissen, P.E.: 1988,Astron. Astrophys. Suppl.73, 225.Schuster, W.J., Parrao, L. and Contreras, M.E.: 1993,Astron. Astrophys. Suppl.97, 951.