High-precision orbital parameters for the short-period spectroscopic binary HD 159656 – an eccentric orbit confirmed for an old but active system

Abstract

HD 159656 is a seventh magnitude short-period (P ∼ 10 d) binary system with a solar-type (G2/3V) primary component. The single-lined spectroscopic orbital solution has been recomputed with high precision. This work confirms the orbit's elliptical nature (e = 0.116 16 ± 0.000 20) which had been brought into doubt based on an earlier less precise solution (Barker, Evans & Laing 1967) and the comprehensive study conducted by Lucy & Sweeney (1971). Short-period binary orbits of this eccentricity are uncommon, particularly when the system has the considerable age that we estimate to be τise = 9.3 ± 0.6 Gyr. Thus HD 159656 may be a system that challenges our present theories of circularization time-scales. Alternatively, HD 159656 could be considered to be suitable for future studies aiming to establish whether or not a third star is interfering with the circularization process. Apsidal motion, undetectable based on this and the previous solution, may also be revealed in the future, this possibility being greatly increased by the high precision of our results. Attempts to detect any companions directly have so far been unsuccessful. The age claimed provides indirect support for the proposition that the system is pseudo-synchronized. This in turn gives us reason to claim an estimate for the inclination, i = 32 +14 -12 degrees with a corresponding mass ratio q ∼ 0.3 that implies that the secondary is an M3 dwarf, or an unusually low-mass white dwarf, since M 2 ∼ 0.3 M ⊙ . HD 159656 is a standard star in many photometric systems. However, a small sample of our spectra indicates that the Ca II H&K line emission, that we demonstrate arises principally from the primary, is variable. Furthermore, ROSAT data of an X-ray source probably coincident with HD 159656 provide evidence of flare activity. Such behaviour is more typical of M-dwarf stars and so may be more properly associated with the secondary, if it is of this spectral type. If the binary is tidally locked, this could explain the activity of this system, which implies a chromospheric age an order of magnitude less than the isochrone age. The spectra reveal a very low lithium abundance for the primary component, together with many other spectral characteristics and photometric colours that are typical of solar analogues. Thus the primary star represents an example of the behaviour of solar analogues in this astrophysical context.