Photometric observations and modelling of the asteroid 85 Io in conjunction with data from an occultation event during the 1995-96 apparition

The asteroid 85 Io has been observed using CCD and photoelectric photometry on 18 nights during its 1995-96 and 1997 apparitions. We present the obser ved lightcurves, determined colour indices and modelling of the asteroid sp in vector and shape. The colour indices (U-B = 0.35 +/- 0.02, B-V = 0.66 +/ - 0.02, V-R = 0.34 +/- 0.02, R-I = 0.36 0.02) are as expected for a C-type asteroid. The allowed spin vector solutions have the pole co-ordinates lamb da(0) = 285 +/- 4 degrees, beta(0) = -52+/-9 degrees or beta(0), = 108 +/- 10 degrees, beta(0) = -46 +/- 10 degrees and lambda(0) = 290 +/- 10 degrees , beta(0) = - 16 +/- 10 degrees with a retrograde sense of rotation and a s idereal period P-sid = 0(d).286463 +/- 0(d).000001. During the 1995-96 appa rition the International Occultation Time Association (IOTA) observed an oc cultation event by 85 Io. The observations and modelling presented here wer e analysed together with the occultation data to develop improved constrain ts on the size of the asteroid. The derived value of 164 km is about 5% lar ger than the IRAS diameter. (C) 1999 Elsevier Science Ltd. All rights reser ved.