364 research outputs found
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
On the calibration of the relation between geometric albedo and polarimetric properties for the asteroids
We present a new extensive analysis of the old problem of finding a
satisfactory calibration of the relation between the geometric albedo and some
measurable polarization properties of the asteroids. To achieve our goals, we
use all polarimetric data at our disposal. For the purposes of calibration, we
use a limited sample of objects for which we can be confident to know the
albedo with good accuracy, according to previous investigations of other
authors. We find a new set of updated calibration coefficients for the
classical slope - albedo relation, but we generalize our analysis and we
consider also alternative possibilities, including the use of other
polarimetric parameters, one being proposed here for the first time, and the
possibility to exclude from best-fit analyzes the asteroids having low albedos.
We also consider a possible parabolic fit of the whole set of data.Comment: Accepted by MNRA
The Orbital and Absolute Magnitude Distributions of Main Belt Asteroids
We have developed a model-independent analytical method for debiasing the
four-dimensional (a,e,i,H) distribution obtained in any asteroid observation
program and have applied the technique to results obtained with the 0.9m
Spacewatch Telescope. From 1992 to 1995 Spacewatch observed ~3740 deg^2 near
the ecliptic and made observations of more than 60,000 asteroids to a limiting
magnitude of V~21. The debiased semi-major axis and inclination distributions
of Main Belt asteroids in this sample with 11.5<= H <16 match the distributions
of the known asteroids with H <11.5. The absolute magnitude distribution was
studied in the range 8< H <17.5. We have found that the set of known asteroids
is complete to about absolute magnitudes 12.75, 12.25 and 11.25 in the inner,
middle and outer regions of the belt respectively. The number distribution as a
function of absolute magnitude cannot be represented by a single power-law
(10^{alpha H}) in any region. We were able to define broad ranges in H in each
part of the belt where alpha was nearly constant. Within these ranges of H the
slope does not correspond to the value of 0.5 expected for an equilibrium
cascade in self-similar collisions (Dohnanyi, 1971). The value of alpha varies
with absolute magnitude and shows a `kink' in all regions of the belt for H~13.
This absolute magnitude corresponds to a diameter ranging from about 8.5 to
12.5 km depending on the albedo or region of the belt.Comment: 33 pages, 6 figures, 6 tables. published in Icaru
Evidence for a colour dependence in the size distribution of main belt asteroids
We present the results of a project to detect small (~1 km) main-belt
asteroids with the 3.6 meter Canada-France-Hawaii Telescope (CFHT). We observed
in 2 filters (MegaPrime g' and r') in order to compare the results in each
band. Owing to the observational cadence we did not observe the same asteroids
through each filter and thus do not have true colour information. However
strong differences in the size distributions as seen in the two filters point
to a colour-dependence at these sizes, perhaps to be expected in this regime
where asteroid cohesiveness begins to be dominated by physical strength and
composition rather than by gravity. The best fit slopes of the cumulative size
distributions (CSDs) in both filters tend towards lower values for smaller
asteroids, consistent with the results of previous studies. In addition to this
trend, the size distributions seen in the two filters are distinctly different,
with steeper slopes in r' than in g'. Breaking our sample up according to
semimajor axis, the difference between the filters in the inner belt is found
to be somewhat less pronounced than in the middle and outer belt, but the CSD
of those asteroids seen in the r' filter is consistently and significantly
steeper than in g' throughout. The CSD slopes also show variations with
semimajor axis within a given filter, particularly in r'. We conclude that the
size distribution of main belt asteroids is likely to be colour dependent at
kilometer sizes and that this dependence may vary across the belt.Comment: 28 pages, 5 figures, submitted to the Astronomical Journa
Shape models and physical properties of asteroids
Despite the large amount of high quality data generated in recent space
encounters with asteroids, the majority of our knowledge about these objects
comes from ground based observations. Asteroids travelling in orbits that are
potentially hazardous for the Earth form an especially interesting group to be
studied. In order to predict their orbital evolution, it is necessary to
investigate their physical properties. This paper briefly describes the data
requirements and different techniques used to solve the lightcurve inversion
problem. Although photometry is the most abundant type of observational data,
models of asteroids can be obtained using various data types and techniques. We
describe the potential of radar imaging and stellar occultation timings to be
combined with disk-integrated photometry in order to reveal information about
physical properties of asteroids.Comment: From Assessment and Mitigation of Asteroid Impact Hazards boo
The Size Distributions of Asteroid Families in the SDSS Moving Object Catalog 4
Asteroid families, traditionally defined as clusters of objects in orbital
parameter space, often have distinctive optical colors. We show that the
separation of family members from background interlopers can be improved with
the aid of SDSS colors as a qualifier for family membership. Based on an
~88,000 object subset of the Sloan Digital Sky Survey Moving Object Catalog 4
with available proper orbital elements, we define 37 statistically robust
asteroid families with at least 100 members using a simple Gaussian
distribution model in both orbital and color space. The interloper rejection
rate based on colors is typically ~10% for a given orbital family definition,
with four families that can be reliably isolated only with the aid of colors.
About 50% of all objects in this data set belong to families, and this fraction
varies from about 35% for objects brighter than an H magnitude of 13 and rises
to 60% for objects fainter than this. The fraction of C-type objects in
families decreases with increasing H magnitude for H > 13, while the fraction
of S-type objects above this limit remains effectively constant. This suggests
that S-type objects require a shorter timescale for equilibrating the
background and family size distributions via collisional processing. The size
distributions for 15 families display a well-defined change of slope and can be
modeled as a "broken" double power-law. Such "broken" size distributions are
twice as likely for S-type familes than for C-type families, and are dominated
by dynamically old families. The remaining families with size distributions
that can be modeled as a single power law are dominated by young families. When
size distribution requires a double power-law model, the two slopes are
correlated and are steeper for S-type families.Comment: 50 pages, 16 figures, accepted for publication in Icaru
Recent advances in asteroid polarimetry
Asteroid polarimetry has experienced important advancements in recent years. This includes the discovery of new classes of objects, a new assessment of the relation between geometric albedo and a variety of different polarimetric parameters, the first attempt to use in situ analyses of asteroid (4) Vesta to better understand the relation between local surface properties and disk-integrated polarimetric measurements, and the first applications of spectro-polarimetry to the physical characterization of the asteroids. The most recent results in the above topics are briefly summarized
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