Physical model of near-earth asteroid 6489 golevka (1991 JX) from optical and infrared observations.

Abstract

In 1995 asteroid 6489 Golevka (1991 IX) had a close encounter with the Earth at a distance of 0.034 AU, providing a good opportunity for a detailed study of a small Solar System object. In this paper we report the results of an extensive international observing campaign aimed at determining Golevka's rotational and physical properties from optical photometry and thermal W radiometry. From the analysis of photometric light curves we derive a spin axis model whose coordinates are λ<SUB>p</SUB> (J2000)=347°±10° and β<SUB>p</SUB> (J2000)=350°±10°. The strict periodicity observed in light curves taken during virtually constant illumination and observing conditions argues against free precession of the spin vector. The rotation is prograde with a sidereal period P<SUB>sid</SUB>=0.251101 d (6.02640±0.00024 hr). The derived ellipsoidal shape model results in a/b 1.4±0.2 while not satisfactorily constraining the b/c ratio. No systematic V-R color index variation larger than 0.005 mag (1σ) was observed, suggesting a spectrally uniform surface on a hemispheric scale. The integral phase curve is moderately well represented by the H-G parameter system that fails, however, to reproduce the observed opposition spike. We derive mean H, G values of 19.074±0.029 and 0.138±0.013, respectively. The application of the more accurate Hapke photometric function produces a better fit and allows us to describe the surface scattering properties in terms of model parameters: σ‾=7°±7°,g=-0.435±0.001, ω<SUB>0</SUB>=0.58±0.03, h=0.0114±0.0004, B<SUB>0</SUB>=0.758±0.014. Infrared measurements reveal that the flux at 10.6 μm is unusually small in view of the object's optical brightness. The combined application of a modified Fast Rotating Thermal Model and of the photometric model suggests that Golevka is a small body with approximate dimensions (0.35×0.25×0.25 km) and a very high geometric albedo P<SUB>v</SUB>∼0.6. However, the possible effect of model assumptions on these latter results still has to be explored.