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Shepard M.K.,Bloomsburg University | Taylor P.A.,NAIC Arecibo Observatory | Nolan M.C.,NAIC Arecibo Observatory | Howell E.S.,NAIC Arecibo Observatory | And 12 more authors.
Icarus | Year: 2015

Using the S-band radar at Arecibo Observatory, we observed thirteen X/M-class asteroids; nine were previously undetected and four were re-observed, bringing the total number of Tholen X/M-class asteroids observed with radar to 29. Of these 29M-class asteroids, 13 are also W-class, defined as M-class objects that also display a 3-μm absorption feature which is often interpreted as the signature of hydrated minerals (Jones, T.D., Lebofsky, L.A., Lewis, J.S., Marley, M.S. [1990]. Icarus 88, 172-192; Rivkin, A.S., Howell, E.S., Britt, D.T., Lebofsky, L.A., Nolan, M.C., Branston, D.D. [1995]. Icarus 117, 90-100; Rivkin, A.S., Howell, E.S., Lebofsky, L.A., Clark, B.E., Britt, D.T. [2000]. Icarus 145, 351-368).Consistent with our previous work (Shepard, M.K. et al. [2008]. Icarus 195, 184-205; Shepard, M.K., Harris, A.W., Taylor, P.A., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M. [2011]. Icarus 215, 547-551), we find that 38% of our sample (11 of 29) have radar albedos consistent with metal-dominated compositions. With the exception of 83 Beatrix and 572 Rebekka, the remaining objects have radar albedos significantly higher than the mean S- or C-class asteroid (Magri, C., Nolan, M.C., Ostro, S.J., Giorgini, J.D. [2007]. Icarus 186, 126-151).Seven of the eleven high-radar-albedo asteroids, or 64%, also display a 3-μm absorption feature (W-class) which is thought to be inconsistent with the formation of a metal dominated asteroid. We suggest that the hydration absorption could be a secondary feature caused by low-velocity collisions with hydrated asteroids, such as CI or CM analogs, and subsequent implantation of the hydrated minerals into the upper regolith. There is recent evidence for this process on Vesta (Reddy, V. et al. [2012]. Icarus 221, 544-559; McCord, T.B. et al. [2012]. Nature 491, 83-86; Prettyman, T.H. et al. [2012]. Science 338, 242-246; Denevi, B.W. et al. [2012]. Science 338, 246-249).Eleven members of our sample show bifurcated radar echoes at some rotation phases; eight of these are high radar albedo targets. One interpretation of a bifurcated echo is a contact binary, like 216 Kleopatra, and several of our sample are contact binary candidates. However, evidence for other targets indicates they are not contact binaries. Instead, we hypothesize that these asteroids may have large-scale variations in surface bulk density, i.e. isolated patches of metal-rich and silicate-rich regions at the near-surface, possibly the result of collisions between metal and silicate-rich asteroids. © 2014 Elsevier Inc.

Becker T.M.,University of Central Florida | Howell E.S.,Arecibo Observatory Universities Space Research Association | Nolan M.C.,Arecibo Observatory Universities Space Research Association | Magri C.,University of Maine at Farmington | And 17 more authors.
Icarus | Year: 2015

We report radar observations (2380-MHz, 13-cm) by the Arecibo Observatory and optical light curves observed from eight different observatories and collected at the Ondřejov Observatory of the triple near-Earth asteroid system (153591) 2001 SN263. The radar observations were obtained over the course of ten nights spanning February 12-26, 2008 and the light curve observations were made throughout January 12 - March 31, 2008. Both data sets include observations during the object's close approach of 0.06558AU on February 20th, 2008. The delay-Doppler images revealed the asteroid to be comprised of three components, making it the first known triple near-Earth asteroid. Only one other object, (136617) 1994 CC is a confirmed triple near-Earth asteroid.We present physical models of the three components of the asteroid system. We constrain the primary's pole direction to an ecliptic longitude and latitude of (309°, -80°)±15°. We find that the primary rotates with a period 3.4256±0.0002h and that the larger satellite has a rotation period of 13.43±0.01h, considerably shorter than its orbital period of approximately 6days. We find that the rotation period of the smaller satellite is consistent with a tidally locked state and therefore rotates with a period of 0.686±0.002 days (Fang et al. [2011]. Astron. J. 141, 154-168). The primary, the larger satellite, and the smaller satellite have equivalent diameters of 2.5±0.3km, 0.77±0.12km, 0.43±0.14km and densities of 1.1±0.2g/cm3, 1.0±0.4g/cm3, 2.3±1.3g/cm3, respectively. © 2014 The Authors.

Reddy V.,Planetary Science Institute | Li J.-Y.,Planetary Science Institute | Sanchez J.A.,Planetary Science Institute | Stephens R.D.,Center for Solar System Studies | And 5 more authors.
Icarus | Year: 2015

The dwarf planet Ceres is likely differentiated similar to the terrestrial planets but with a water/ice dominated mantle and an aqueously altered crust. Detailed modeling of Ceres' phase function has never been performed to understand its surface properties. The Dawn spacecraft began orbital science operations at the dwarf planet in April 2015. We observed Ceres with flight spares of the seven Dawn Framing Camera color filters mounted on ground-based telescopes over the course of three years to model its phase function versus wavelength. Our analysis shows that the modeled geometric albedos derived from both the IAU HG model and the Hapke model are consistent with a flat and featureless spectrum of Ceres, although the values are ~10% higher than previous measurements. Our models also suggest a wavelength dependence of Ceres' phase function. The IAU G-parameter and the Hapke single-particle phase function parameter, g, are both consistent with decreasing (shallower) phase slope with increasing wavelength. Such a wavelength dependence of phase function is consistent with reddening of spectral slope with increasing phase angle, or phase-reddening. This phase reddening is consistent with previous spectra of Ceres obtained at various phase angles archived in the literature, and consistent with the fact that the modeled geometric albedo spectrum of Ceres is the bluest of all spectra because it represents the spectrum at 0° phase angle. Ground-based FC color filter lightcurve data are consistent with HST albedo maps confirming that Ceres' lightcurve is dominated by albedo and not shape. We detected a positive correlation between 1.1-μm absorption band depth and geometric albedo suggesting brighter areas on Ceres have absorption bands that are deeper. We did not see the "extreme" slope values measured by Perna et al. (Perna, D., et al. [2015]. Astron. Astrophys. 575 (L1-6)), which they have attributed to "resurfacing episodes" on Ceres. © 2015 Elsevier Inc.

Hanus J.,Charles University | Broz M.,Charles University | Durech J.,Charles University | Warner B.D.,Palmer Divide Observatory | And 8 more authors.
Astronomy and Astrophysics | Year: 2013

Context. The current number of ∼500 asteroid models derived from the disk-integrated photometry by the lightcurve inversion method allows us to study the spin-vector properties of not only the whole population of main-belt asteroids, but also of several individual collisional families. Aims. We create a data set of 152 asteroids that were identified by the hierarchical clustering method (HCM) as members of ten collisional families, among which are 31 newly derived unique models and 24 new models with well-constrained pole-ecliptic latitudes of the spin axes. The remaining models are adopted from the DAMIT database or a few individual publications. Methods. We revised the preliminary family membership identification by the HCM according to several additional criteria: taxonomic type, color, albedo, maximum Yarkovsky semi-major axis drift, and the consistency with the size-frequency distribution of each family, and consequently we remove interlopers. We then present the spin-vector distributions for asteroidal families Flora, Koronis, Eos, Eunomia, Phocaea, Themis, Maria, and Alauda. We use a combined orbital-and spin-evolution model to explain the observed spin-vector properties of objects among collisional families. Results. In general, for studied families we observe similar trends in (ap, β) space (proper semi-major axis vs. ecliptic latitude of the spin axis): (i) larger asteroids are situated in the proximity of the center of the family; (ii) asteroids with β > 0 are usually found to the right of the family center; (iii) on the other hand, asteroids with β < 0 to the left of the center; (iv) the majority of asteroids have large pole-ecliptic latitudes (|β| â‰30); and finally (v) some families have a statistically significant excess of asteroids with β > 0 or β < 0. Our numerical simulation of the long-term evolution of a collisional family is capable of reproducing the observed spin-vector properties well. Using this simulation, we also independently constrain the age of families Flora (1.0 ± 0.5 Gyr) and Koronis (2.5-4 Gyr). © 2013 ESO.

French L.M.,Illinois Wesleyan University | Stephens R.D.,Center for Solar System Studies | Coley D.,Center for Solar System Studies | Wasserman L.H.,Lowell Observatory | Sieben J.,Illinois Wesleyan University
Icarus | Year: 2015

Several lines of evidence support a common origin for, and possible hereditary link between, cometary nuclei and jovian Trojan asteroids. Due to their distance and low albedos, few comet-sized Trojans have been studied. We present new lightcurve information for 19 Trojans. ≲. 30. km in diameter, more than doubling the number of objects in this size range for which some rotation information is known. The minimum densities for objects with complete lightcurves are estimated and are found to be comparable to those measured for cometary nuclei. A significant fraction (~40%) of this observed small Trojan population rotates slowly (P> 24 h), with measured periods as long as 375. h (Warner, B.D., Stephens, R.D. [2011]. Minor Planet Bull. 38, 110-111). The excess of slow rotators may be due to the YORP effect. Results of the Kolmogorov-Smirnov test suggest that the distribution of Trojan rotation rates is dissimilar to those of Main Belt Asteroids of the same size. Concerted observations of a large number of Trojans could establish the spin barrier (Warner, B.D., Harris, A.W., Pravec, P. [2009]. Icarus 202, 134-146), making it possible to estimate densities for objects near the critical period. © 2015 Elsevier Inc.

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