Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007dps....39.3513s&link_type=abstract
American Astronomical Society, DPS meeting #39, #35.13; Bulletin of the American Astronomical Society, Vol. 39, p.484
Astronomy and Astrophysics
Astronomy
Scientific paper
Lightcurve attenuation features characteristic of mutual eclipse events in asynchronous binary asteroid systems (Pravec et al., 2006) have been observed for (8534) Knutsson and (8581) Johnen, two asteroids among a sample of ten 5- to 10-km members of the Koronis asteroid family. Observations were made under excellent conditions on two consecutive nights using the Perkins 1.8-m telescope of Lowell Observatory. Eight-hour spans of brightness measurements sampled at a rate of six points per hour definitively show that both objects' lightcurves are doubly-periodic with amplitudes near 0.1 mag. and periods near 3 hours; the sampling is thus about 18 data points per rotation. The 0.1-mag. depth of the attenuation features is significantly greater than the 0.01-mag. error bars on the brightness measurements. Although there are not enough data to establish periodicity of the attenuations, the data available resemble those for NEA binaries reported by Pravec et al. (2006) for which spheroidal primaries seem to be spinning close to their breakup limits. If binary asteroids can indeed be created by mass loss from small asteroids spun up by YORP thermal reradiation effects (Bottke et al., 2002; Cuk, 2007), then these two objects are strong candidates because Koronis family members of this size likely are reaccumulated objects (Michel et al., 2003), and observed alignments of spin vectors among larger members of the family (Slivan, 2002) independently suggest that the Koronis family is a YORP-evolved population (Vokrouhlický et al., 2003). The binary fraction of 20% inferred from the new observations also appears to be consistent with that found among small Koronis family members by Merline et al. (2004), and with the photometric survey of small Main Belt binaries by Pravec and Harris (2006). Telescope time was provided by Lowell Observatory, Boston University, and NSF PREST.
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