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Astron. Astrophys. 322, 785-800 (1997)

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1. Introduction

So far, the phenomenon of coronal activity has been studied almost exclusively for stars belonging to the Galactic disk. These are metal-rich objects with low space-velocities, which have been formed during later stages of Galactic evolution. Hence, our present knowledge about stellar coronae refers to high-metallicity, relatively young systems. The reason for this concentration on Pop I objects is that the majority of nearby stars belong to the disk component. Stars of the Galactic halo have weak metallic lines and large space velocities. They have formed during the collapse of the Galaxy, long before the disk configuration has settled. Halo stars differ significantly from disk objects with respect to metallicity, age, and, presumably, in the nature of their coronae. The halo component consists of globular clusters and field stars. As globular clusters are rather distant, field stars are far better suited to study the coronae of Pop II objects.

Extremely old single stars are expected to be slow rotators and, hence, weak X-ray emitters, as a result of the permanent loss of angular momentum due to magnetic braking. Extremely old binary systems can still possess high rotation rates due to their synchronous rotation. If Pop II coronal activity, similar to that of Pop I systems, essentially depends on the rotation rate, Pop II binaries may be vigorous X-ray emitters. Therefore, one can study the effect of metallicity and age on the activity of stellar coronae by comparing the X-ray luminosities of Pop II with those of Pop I binaries. Since the bulk of coronal radiation is emitted in lines from elements such as Fe, it is clearly a function of metallicity. The effect of stellar age on main-sequence, short-period binary systems is not yet clear; it might be a decrease in the production rate of magnetic flux, or a dynamical effect due to the longer times of tidal interaction.

The survey of about 900 proper motion stars by Carney and Latham (1987) - together with a similar program by Sandage and Fouts (1987) - provided the first extended photometric database of halo field stars. For part of this sample, Laird et al. (1988) determined photometric reddenings and parallaxes, and spectroscopic metallicities. Further, Latham et al. (1988, 1992) obtained orbits for 80 spectroscopic binaries of the sample by radial-velocity measurements. This proper-motion survey provided the basis for a number of subsequent studies, aimed at investigating the atmospheric activity of Pop II field binaries. Pasquini et al. (1991) found evidence for strong chromospheric activity in 4 short-period Pop II binaries, from which one (HD 85091) was detected by ROSAT as the first Pop II coronal source. Next, Pasquini and Lindgren (1994) analyzed Ca II and H [FORMULA] observations of a sample of 27 Pop II binaries, and found systems with periods less than 10 days to possess active chromospheres. Finally, while modelling the spectra of 16 metal-poor binaries, Spite et al. (1994) found about half of them to be slightly evolved. While these studies are concerned mainly with the optical properties of Pop II binaries, their X-ray properties are largely unknown. So far, X-ray studies have been performed only for two Pop II binaries: Besides the X-ray detection of HD 85091, only the extremely metal-poor system HD 89499 (Gehren 1982, Hartmann and Gehren 1988) has been studied by ASCA and ROSAT (Fleming 1996). With regard to the wealth of X-ray data available for Pop I binaries, it is clear that further X-ray observations of Pop II binaries are required to enable a reliable comparison between halo and disk binaries.

The ROSAT all-sky survey is ideally suited for providing X-ray data for a large sample of Pop II binaries, because it yields a complete and flux-limited coverage of the entire sky. The ROSAT survey data allow the determination of X-ray luminosities and hardness ratios for each Pop II binary. From the X-ray parameters of the individual objects, mean X-ray properties, characteristic for the class of Pop II binaries, can be derived by means of statistical methods. By a comparison between the X-ray properties of halo and disk binaries, conclusions about the effects of metallicity and age on the coronal activity can be derived.

In Sect. 2, we define our input sample of Pop II binaries and compile its optical properties. In Sect. 3, we describe the X-ray observations and the applied source detection technique. Sect. 4 contains the results of our X-ray analysis. In Sect. 5, the X-ray luminosity function of the Pop II binaries is computed. In Sect. 6, we investigate which stellar properties determine coronal activity in Pop II binaries. Finally, a summary and discussion on the effects of metallicity and age is given in Sect. 7.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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