ROTATIONAL PROPERTIES OF TRANSITIONAL SIZE MAIN BELT ASTEROIDS: PHOTOMETRIC OBSERVATIONS OF THREE OBJECTS

We present lightcurves and analysis of rotational periods for three main belt asteroids in the 30 -50 km diameter range, which places them near or just above the estimated lower bound diameter of the “large asteroid” group (D > 40 km) on a basis of statistical analysis of asteroid rotation properties: 446 Aeternitas (D = 45.4 km), 1013 Tombecka (D = 31.9 km), and 1304 Arosa (D = 41.9 km). Photometric observations obtained using the 0.4m Ritchey-Chretien telescope at Florida Gulf Coast University's Egan Observatory during the period Dec. 2005 – Dec. 2006 produced lightcurves which yielded the rotational periods for the three asteroids. We derive refined rotational periods for the three objects: 446 Aeternitas (Psyn = 15.748 ± 0.002 h with an amplitude of ~ 0.40 mag), 1013 Tombecka (Psyn = 6.051 ± 0.001 h with an amplitude of ~ 0.50 mag), and 1304 Arosa (Psyn = 7.7478 ± 0.0001 h with an amplitude of ~ 0.375 mag). The rotational period of 1304 Arosa was derived in a collaborative effort between observatories in France, Italy, and the United States. Below approximately D = 40 km, larger deviations of the geometric mean spin rate from a Maxwellian distribution start to occur in the 30 – 40 km diameter range. Despite a factor of two range in rotational periods for the D > 40 km asteroids in this study - 446 Aeternitas (Psyn = 15.748 h) and 1304 Arosa (Psyn = 7.7478 h), the objects nonetheless plot in a Maxwellian distribution for large asteroids. As asteroid diameters decrease to between 40 and 10 km, a steep increase in the mean spin rate occurs. 1013 Tombecka (Psyn = 6.051 h) is then considered in the transitional region where the large and small asteroid groups overlap.