The Opposition Effect of the Asteroid 4 Vesta

We present the results of photometric observations carried out with four small telescopes of the asteroid 4 Vesta in the $B$, $R_{rm C}$, and $z$ bands at a minimum phase angle of 0.1 $timeform{D}$. The magnitudes, reduced to unit distance and zero phase angle, were $M_{B}(1, 1, 0) = 3.83 pm 0.01, M_{R_{rm C}}(1, 1, 0) = 2.67 pm 0.01$, and $M_{z}(1, 1, 0) = 3.03 pm 0.01$ mag. The absolute magnitude obtained from the IAU $H$--$G$ function is $sim$0.1 mag darker than the magnitude at a phase angle of 0$timeform{D}$ determined from the Shevchenko function and Hapke models with the coherent backscattering effect term. Our photometric measurements allowed us to derive geometric albedos of 0.35 in the $B$ band, 0.41 in the $R_{rm C}$ band, and 0.31 in the $z$ bands by using the Hapke model with the coherent backscattering effect term. Using the Hapke model, the porosity of the optically active regolith on Vesta was estimated to be $rho$ = 0.4--0.7, yielding the bluk density of 0.9--2.0 $times$ $10^3$ kg $mathrm{m^{-3}}$. It is evident that the opposition effect for Vesta makes a contribution to not only the shadow-hiding effect, but also the coherent backscattering effect that appears from ca. $1timeform{D}$. The amplitude of the coherent backscatter opposition effect for Vesta increases with a brightening of reflectance. By comparison with other solar system bodies, we suggest that multiple-scattering on an optically active scale may contribute to the amplitude of the coherent backscatter opposition effect ($B_{C0}$).

Lightcurve Survey of V-type Asteroids in the Inner Asteroid Belt

We have observed the lightcurves of 13 V-type asteroids ((1933) Tinchen, (2011) Veteraniya, (2508) Alupka, (3657) Ermolova, (3900) Knezevic, (4005) Dyagilev, (4383) Suruga, (4434) Nikulin, (4796) Lewis, (6331) 1992 $mathrm{FZ_{1}}$, (8645) 1998 TN, (10285) Renemichelsen, and (10320) Reiland). Using these observations we determined the rotational rates of the asteroids, with the exception of Nikulin and Renemichelsen. The distribution of rotational rates of 59 V-type asteroids in the inner main belt, including 29 members of the Vesta family that are regarded as ejecta from the asteroid (4) Vesta, is inconsistent with the best-fit Maxwellian distribution. This inconsistency may be due to the effect of thermal radiation Yarkovsky--OKeefe--Radzievskii--Paddack (YORP) torques, and implies that the collision event that formed V-type asteroids is sub-billion to several billion years in age.

Lightcurve Survey of V-type Asteroids. I. Observations until Spring 2004

To examine the distribution of rotational rates for chips of asteroid 4 Vesta, lightcurve observation of seven V-type asteroids (2511 Patterson, 2640 Hallstorm, 2653 Principia, 2795 Lapage, 3307 Athabasca, 4147 Lennon, and 4977 Rauthgundis) were performed from fall 2003 to spring 2004. Distribution of spin rates of V-type main-belt asteroids from the past and our observations have three peaks. This result implies that age of catastrophic impact making Vesta family may be not as young as Karin and Iannini families but as old as Eos and Koronis families.