| Through study and understanding of the rotational behaviour of main belt asteroids, information relating to their physical and collisional evolution may be determined. For small asteroids, particularly those found in the inner main belt, the YORP rotational timescales are shorter than those for collisions and thus we expect YORP to be the dominant effect. This makes the small body population ideal in looking for evidence in lightcurves of spin-axis orientation due to YORP. We have so far studied the lightcurve variations of >10,000 asteroids as obtained by Pan-STARRS 1. Asteroids were only considered with a formal uncertainty in w-band of 0.02 magnitudes or better. The majority of asteroids so far observed by Pan-STARRS have too few observations to allow lightcurve periods and amplitudes to be derived as proposed by Warner and Harris (2011). Hence we have analysed the cumulative distribution functions (CDF) of the rate of magnitude change between pairs of observations separated by 15 minutes. This was carried out for a range of asteroid diameters to observe the effect of size on magnitude variation in population samples in the inner and outer main belt. Analysis up to this point has shown that the rates of change in magnitude decreases toward lower rates with decreasing diameter for objects with diameters < 8km in both the inner main belt (2.0 AU < a < 2.5 AU) and the outer main belt (3.0 AU < a < 3.5 AU). Our current findings indicate that the observed CDF is indicative of significant YORP-induced spin-pole alignments for larger bodies, but with an increasing influence of collisions towards smaller diameters. |
