Physical model of near-Earth Asteroid (6849) Golevka from optical and infrared observations.

S. Mottola, A. Erikson, A. W. Harris, G. Hahn, G. Neukum, M. W. Buie, W. D. Sears, A. W. Harris, D. J. Tholen, R. J. Whitely, P. Magnusson, J. Piironen, T. Kwiatkowski, W. Borczyk, E. S. Howell, M. Hicks, R. Fevig, Yu. N. Krugly, F. P. Velichko, V. G. Chiorny, N. M. Gaftonyuk, M. Di Martino, P. Pravec, L. Sarounova, M. Wolf, W. Worman, J. K. Davies, H.-J. Schober, and W. Pych. Astron. J. 114, 1234-1245 (1997) .


In 1995 asteroid 6489 Golevka (1991 JX) 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 IR radiometry. From the analysis of photometric light curves we derive a spin axis model whose coordinates are lambda_p (J2000) = 347 +/- 10 degrees and beta_p (J2000) = 35 +/- 10 degrees. 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_sid = 0.25110 +/- 0.00001 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 (1-sigma) 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 funnction produces a better fit and allows us to descrive the surface scattering properties in terms of model parameters: thetabar = 7 +/- 7 degrees, g = -0.435 +/- 0.001, wbar_0 = 0.58 +/- 0.03, h = 0.0114 +/- 0.0004, B_0 = 0.758 +/- 0.014. Infrared measurements reveal that the flux at 10.6 microns 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 x 0.25 x 0.25 km) and a very high geometric albedo p_v ~ 0.6. However, the possible effect of model assumptions on these latter results still has to be explored.

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