160 research outputs found
Polarization of asteroid (387) Aquitania: the newest member of a class of large inversion angle asteroids
We present new imaging polarimetric observations of two Main Belt asteroids,
(234) Barbara and (387) Aquitania, taken in the first half of 2008 using the
Dual-Beam Imaging Polarimeter on the University of Hawaii 2.2 meter telescope,
located on Mauna Kea, Hawaii. Barbara had been previously shown to exhibit a
very unusual polarization-phase curve by Cellino, et al. (2006). Our
observations confirm this result and add Aquitania to the growing class of
large inversion angle objects. Interestingly, these asteroids show spinel
features in their IR spectra suggesting a mineralogical origin to the phase
angle-dependent polarimetric features. As spinel is associated with
calcium-aluminum-rich inclusions and carbonaceous chondrites, these large
inversion angle asteroids may represent some of the oldest surfaces in the
solar system. Circular as well as linear polarization measurements were
obtained but circular polarization was not detected.Comment: 12 pages, 2 figures, accepted to Icaru
A comparison between families obtained from different proper elements
Using the hierarchical method of family identification developed by Zappala et al., the results coming from the data set of proper elements computed by Williams (about 2100 numbered + about 1200 PLS 2 asteroids) and by Milani and Knezevic (5.7 version, about 4200 asteroids) are compared. Apart from some expected discrepancies due to the different data sets and/or low accuracy of proper elements computed in peculiar dynamical zones, a good agreement was found in several cases. It follows that these high reliability families represent a sample which can be considered independent on the methods used for their proper elements computation. Therefore, they should be considered as the best candidates for detailed physical studies
The possible role of ground-based support for a better determination of asteroid physical parameters based on Gaia data.
International audienceThe determination of bulk physical properties for the largest possible number of asteroids which will be observed by Gaia is an important task which can be made easier by performing some dedicated groundbased observing campaigns. The proposed actions, consisting of applications of different observing techniques, primarily photometry and polarimetry, are not expected to be triggered by Gaia alarms, nor do they require in principle prompt reaction times triggered by special circumstances. We deal here, conversely, with the problem of improving the amount and quality of data at our disposal concerning some properties of a suitable sample of asteroids, to be used for the purposes of a better calibration of some fundamental relations which are commonly used in asteroid science
Simulated families: A test for different methods of family identification
A set of families generated in fictitious impact events (leading to a wide range of 'structure' in the orbital element space have been superimposed to various backgrounds of different densities in order to investigate the efficiency and the limitations of the methods used by Zappala et al. (1990) and by Bendjoya et al. (1990) for identifying asteroid families. In addition, an evaluation of the expected interlopers at different significance levels and the possibility of improving the definition of the level of maximum significant of a given family were analyzed
From asteroid clusters to families: A proposal for a new nomenclature
Some confusion on the number, reliability, and characteristics of asteroid families is the result of using the single word 'family' for naming asteroid groupings identified in very different ways. Here we propose a new terminology which in our opinion would alleviate this problem
Is the Eureka cluster a collisional family of Mars Trojan asteroids?
We explore the hypothesis that the Eureka family of sub-km asteroids in the
L5 region of Mars could have formed in a collision. We estimate the size
distribution index from available information on family members; model the
orbital dispersion of collisional fragments; and carry out a formal calculation
of the collisional lifetime as a function of size. We find that, as initially
conjectured by Rivkin et al (2003), the collisional lifetime of objects the
size of (5261) Eureka is at least a few Gyr, significantly longer than for
similar-sized Main Belt asteroids. In contrast, the observed degree of orbital
compactness is inconsistent with all but the least energetic family-forming
collisions. Therefore, the family asteroids may be ejecta from a cratering
event sometime in the past ~1 Gyr if the orbits are gradually dispersed by
gravitational diffusion and the Yarkovsky effect (Cuk et al, 2015). The
comparable sizes of the largest family members require either negligible target
strength or a particular impact geometry under this scenario (Durda et al,
2007; Benavidez et al, 2012). Alternatively, the family may have formed by a
series of YORP-induced fission events (Pravec.et.al, 2010). The shallow size
distribution of the family is similar to that of small MBAs (Gladman et al,
2009) interpreted as due to the dominance of this mechanism for
Eureka-family-sized asteroids (Jacobson et al, 2014). However, our population
index estimate is likely a lower limit due to the small available number of
family asteroids and observational incompleteness. Future searches for fainter
family members, further observational characterisation of the known Trojans'
physical properties as well as orbital and rotational evolution modelling will
help distinguish between different formation models.Comment: 3 Tables, 13 Figures, Accepted for publication in Icaru
Asteroid families classification: exploiting very large data sets
The number of asteroids with accurately determined orbits increases fast. The
catalogs of asteroid physical observations have also increased, although the
number of objects is still smaller than in the orbital catalogs. We developed a
new approach to the asteroid family classification by combining the
Hierarchical Clustering Method (HCM) with a method to add new members to
existing families. This procedure makes use of the much larger amount of
information contained in the proper elements catalogs, with respect to
classifications using also physical observations for a smaller number of
asteroids. Our work is based on the large catalog of the high accuracy
synthetic proper elements (available from AstDyS). We first identify a number
of core families; to these we attribute the next layer of smaller objects.
Then, we remove all the family members from the catalog, and reapply the HCM to
the rest. This gives both halo families which extend the core families and new
independent families, consisting mainly of small asteroids. These two cases are
discriminated by another step of attribution of new members and by merging
intersecting families. By using information from absolute magnitudes, we take
advantage of the larger size range in some families to analyze their shape in
the proper semimajor axis vs. inverse diameter plane. This leads to a new
method to estimate the family age (or ages). The results from the previous
steps are then analyzed, using also auxiliary information on physical
properties including WISE albedos and SDSS color indexes. This allows to solve
some difficult cases of families overlapping in the proper elements space but
generated by different collisional events. We analyze some examples of
cratering families (Massalia, Vesta, Eunomia) which show internal structures,
interpreted as multiple collisions. We also discuss why Ceres has no family
THE STATISTICAL ASTEROID MODEL. I. THE MAIN-BELT POPULATION FOR DIAMETERS GREATER THAN 1 KILOMETER
We describe the creation of a model of the main asteroid belt whose purpose is to describe the main-belt asteroid size frequency distribution and simulate the number of main-belt asteroids and their fluxes at visual through mid-infrared (~0.3–70 μm) wavelengths in any area of sky for an arbitrary date. This model is based on a population of ~1.9 × 106 asteroids obtained from the complete known asteroid sample, plus extrapolation of the size-frequency distributions of 15 asteroid dynamical families and three background populations, to a diameter limit of 1 km. The model is compared with data and other models, example applications are given, planned refinements and extensions to the model are presented, and some implications of the resulting size frequency distribution are discussed
Optical spectropolarimetry of large C-complex asteroids: polarimetric evidence for heterogeneous surface compositions
This study presents the first optical spectropolarimetric study of large
C-complex asteroids. A total of 64 C-complex asteroids of different subclasses
are analyzed using archival polarimetric and reflectance data to refine the
link between polarimetric parameters and surface properties of the asteroids.
We find a consistent difference in the polarization spectra between asteroids
containing phyllosilicates and those without, which correlates with the overall
morphology of the reflectance spectrum. They exhibit broad similarities in
polarization-phase curves; nonetheless, we observe a gradual enhancement of the
negative polarization branch in the ascending order of F-B-T-Ch types, along
with an increase in reflectance curvature around 500 nm. Our observations
suggest at least for large C-complex asteroids a common mechanism underlies the
diversity in optical properties. The observed trends would be explained by the
surface composition of the asteroids, particularly optical heterogeneity caused
by carbon's varying levels of optical influence, primarily regulated by aqueous
alteration of the surfaces.Comment: Accepted for publication in Astronomy & Astrophysic
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