Binary and multiple systems with pulsating variable components offer a unique opportunity of coupling the information obtained by astrometric means (association type - parallax - total mass) to the astrophysical quantities gained from the photometry /spectroscopy (luminosity ratio - colours - pulsation characteristics) (see Lampens & Boffin 2000 for a review of Scuti stars in stellar systems). The detailed investigation of the differences in variability and simultaneously in physical properties between two components of a binary system may provide clues with respect to the pulsation: differences in origin and age can be ruled out as well as differences in overall chemical composition. Stronger constraints exist for the determination of the position of the components in the H-R diagram, there is therefore less ambiguity in determining the evolutionary status and the mass than in the case of single variable stars. This is important when one of the components is located in the zone where evolutionary tracks are bent (e.g. near the end of the core hydrogen burning phase).
A relevant question is what factors determine the pulsation characteristics (the amplitudes and the modes) in the Scuti instability strip? We addressed this from the point-of-view of two bright A/F-type stars that are both located in the Scuti instability strip and that are shown to be physically associated, i.e. they either form a common origin pair or they are the components of a true wide binary system. In this case, evolution (and mass) is the most pronounced physical difference between both stars and it is very probable that this is the cause for the observed difference in variability behaviour. Further observations are needed, the more that, since there is no evidence for any metal lines in the spectra, a comprehensive variability analysis of this system might also help explaining the presence of non-variable, non-metallic stars in the instability strip.
In the light of the discussion by Solano & Fernley (1997) on the relation between rotational velocity and amplitude, we noted the remarkable similarity of the projected rotational velocities: both stars are rather fast rotators. If fast rotation favours pulsation of the Scuti type, we expect to find short-period variability for the B-component as well! Since it is less evolved than its brighter companion, smaller amplitudes are expected. This is another reason why intensive monitoring of this southern system is certainly worthwhile. In our example it was very easy to identify the short-period pulsating component and the information obtained from the astrometry could be coupled to the astrophysical parameters of each component individually. Even better would be to investigate these characteristics in a close visual binary for which information on the orbital motion can also be derived. This will allow to obtain a direct estimation of the stellar mass, independent from the choice of modelisation. The derivation of the pulsation constant will be more straightforward (the error on the mass defines the accuracy of Q). More cases like this one should be looked into (see Frandsen et al. 1995).
With this application in mind, we made a crossidentification between the Annex of Variable Stars and the Annex of Double and Multiple Stars from the Hipparcos Catalogue (ESA 1997). Some 2500 systems with at least one variable component have been identified. But the description of the variability or the light curve in the Annex always refer to the combined magnitudes. Additional observations should help identify which component is variable and which are the binaries that offer the opportunity of coupling the information obtained by astrometric means to the physical properties in order to obtain a consistent picture of the system and its components.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000