Efficient spin sense determination of Flora-region asteroids via the epoch method

Abstract

The Flora asteroid family’s size and location on the inner edge of the main belt make it a likely source of NEOs and terrestrial planet impactors; however, reliable determination of Flora membership is inhibited by the family’s age and the presence of a high density of background objects. Dykhuis et al. (Dykhuis et al. [2014]. Icarus 243, 111–128) identified the Flora family as the product of a 950-My-old collision dispersed in semimajor axis as a result of the Yarkovsky effect, and defined the family’s membership and extent in orbital parameter space. The observed preponderance of prograde rotators at semimajor axes greater than that of (8) Flora is consistent with the predictions of the single-collision Yarkovsky dispersion model.Here we extend the available rotational property data for the Flora family via a survey of 21 Flora-region asteroids, using a time-efficient modification of the “epoch method” to determine prograde/retrograde spin sense. Five of the survey asteroids are shown to be prograde; five are shown to be retrograde; six are shown to have spin axes in or near their orbital planes; and five represent other cases for which spin axis information was not determined. The high-semimajor axis component of the Flora family is found to have only prograde and in-plane rotators, consistent with model predictions of Yarkovsky dispersion. Moreover, we confirm a wide range of ecliptic latitudes of the spin axes among these prograde rotators, consistent with models of family evolution in which a significant fraction of the members are captured in spin–orbit resonance. Near the “center” of the family (near the semimajor axis location of (8) Flora), the spin directions are mixed, with a slight preference for retrograde rotators, placing constraints on the efficiency of YORP-cycle spin reorientation for the family.In addition to our spin sense survey, we also report new measurements of the Sloan colors of a number of large inner main belt asteroids.