          Astron. Astrophys. 361, 770-780 (2000)

## 2. The Hipparcos double stars

In the Hipparcos Catalogue (ESA 1997) binary solutions are divided into five different categories, to which a sixth is added below. A full description is given in the Catalogue, so only a brief outline is given here, cf. Lindegren et al. (1997). It should be recalled that the astrometric solution for a single star uses five parameters: two components of the position, two components of the proper motion, and the trigonometric parallax. Binary solutions require additional parameters.

### 2.1. Component solutions (C)

The `C' solutions consist of double and multiple stars for which a 5-parameter solution could be applied to each component, usually assuming a common parallax. They basically include all resolved objects ( arcsec, mag) with orbital periods much greater than the mission life-time (3.3 years).

### 2.2. Acceleration solutions (G)

For binaries with separations less than about 0.1 arcsec, the components were not resolved by Hipparcos. The binary nature could however sometimes be detected through the non-linear motion of the photocentre. With a mission lifetime of 3.3 years, the satellite was sensitive to orbital periods up to a few decades. For periods less than a few years, the complete orbit could sometimes be determined, giving a solution of type `O' (see below). For systems with longer periods, only a fraction of the orbit was observed and the motion was fitted by low-order polynomials in time. Two or four terms were added to the standard 5-parameter model, representing quadratic or cubic polynomials of time in each coordinate ( , ). Although these terms express a physical acceleration of the photocentre, they are not uniquely related to the orbital elements and therefore provide only indirect information about binary properties.

### 2.3. Orbital solutions (O)

These objects have solutions for the seven Keplerian elements of the orbit of the photocentre around the centre of mass, in addition to the five astrometric parameters for the centre of mass.

### 2.4. VIM solutions (V)

A new category of double stars, the `variability-induced movers' (VIM), was introduced in the course of the Hipparcos data reductions (Wielen 1996). They are unresolved systems, in which non-linear motions of the photocentre are induced by variability in one or both of the components.

### 2.5. Stochastic solutions (X)

These represent about 1 per cent of the Hipparcos objects for which no good double-star solution could be found. Since the standard 5-parameter fit gave too large residuals to be explained by observational errors, some sort of multiplicity is probably implied. In these solutions, the effects of multiplicity were simply treated as an added noise source.

### 2.6. Delta-mu binaries ( )

This category was not introduced in the Hipparcos Catalogue but results from comparing the quasi-instantaneous proper motion measured by Hipparcos with the long-term averaged, ground-based proper motion or with the proper motion derived from old ground-based positions and the Hipparcos position (Wielen et al. 1999). This method may be quite sensitive for detecting binaries with periods up to 100 yr and more. Statistical predictions of the effect were included already in the model by Söderhjelm (1985).    © European Southern Observatory (ESO) 2000

Online publication: October 2, 2000 