ASTEROID SPIN-RATE STUDY USING the INTERMEDIATE PALOMAR TRANSIENT FACTORY

Two dedicated asteroid rotation-period surveys have been carried out in the R band with ∼20 minute cadence using the intermediate Palomar Transient Factory (iPTF) during 2014 January 6-9 and February 20-23. The total survey area covered 174 deg² in the ecliptic plane. Reliable rotation periods for 1438 asteroids are obtained from a larger data set of 6551 mostly main-belt asteroids, each with detections. Analysis of 1751, PTF-based, reliable rotation periods clearly shows the spin barrier at ∼2 hr for rubble-pile asteroids. We found a new large super-fast rotator, 2005 UW163, and another five candidates as well. For asteroids of km, our spin-rate distribution shows a number decrease along with frequency after 5 rev day^-1, which is consistent with the results of the Asteroid Light Curve Database. The discrepancy between our work and that of Pravec et al. (update 2014 April 20) comes mainly from asteroids with mag, which could be the result of different survey strategies. For asteroids with km, we see a significant number drop at f = 6 rev day^-1. The relatively short YORP effect timescale for small asteroids could have spun up those elongated objects to reach their spin-rate limit resulting in breakup to create such a number deficiency. We also see that the C-type asteroids show a smaller spin-rate limit than the S-type, which agrees with the general impression that C-type asteroids have a lower bulk density than S-type asteroids.