1,450 research outputs found
Aqueous alteration on main belt primitive asteroids: results from visible spectroscopy
This work focuses on the study of the aqueous alteration process which acted
in the main belt and produced hydrated minerals on the altered asteroids. The
aqueous alteration is particularly important for unraveling the processes
occurring during the earliest times of the Solar System history, as it can give
information both on the asteroids thermal evolution and on the localization of
water sources in the asteroid belt. We present new spectral observations in the
visible region of 80 asteroids belonging to the primitive classes C, G, F, B
and P. We combine the present observations with the visible spectra of
asteroids available in the literature for a total of 600 primitive main belt
asteroids. Our analysis shows that the aqueous alteration sequence starts from
the P-type objects, practically unaltered, and increases through the F, B, C,
and G asteroids. Around 50% of the observed C-type asteroids show absorption
features in the vis. range due to hydrated silicates, implying that more than
70% of them will have a 3 m absorption band and thus hydrated minerals on
their surfaces. The process dominates in primitive asteroids located between
2.3 and 3.1 AU, that is at smaller heliocentric distances than previously
suggested. The aqueous alteration process dominates in the 50--240 km sized
primitive asteroids, while it is less effective for bodies smaller than 50 km.
No correlation is found between the aqueous alteration process and the
asteroids albedo or orbital elements. Comparing the 0.7 m band
parameters of hydrated silicates and CM2 carbonaceous chondrites, we see that
the band center of meteorites is at longer wavelengths than that of asteroids.
This difference on center positions may be attributed to different minerals
abundances, and to the fact that CM2 available on Earth might not be
representative of the whole aqueous altered asteroids population.Comment: Icarus, accepted for publication on 28 January 2014 Manuscript pages:
38; Figures: 13 ; Tables:
Colors and taxonomy of Centaurs and Trans-Neptunian Objects
The study of the surface properties of Centaurs and Trans-Neptunian Objects
(TNOs) provides essential information about the early conditions and evolution
of the outer Solar System. Due to the faintness of most of these distant and
icy bodies, photometry currently constitutes the best technique to survey a
statistically significant number of them. Our aim is to investigate color
properties of a large sample of minor bodies of the outer Solar System, and set
their taxonomic classification. We carried out visible and near-infrared
photometry of Centaurs and TNOs, making use, respectively, of the FORS2 and
ISAAC instruments at the Very Large Telescope (European Southern Observatory).
Using G-mode analysis, we derived taxonomic classifications according to the
Barucci et al. (2005a) system. We report photometric observations of 31
objects, 10 of them have their colors reported for the first time ever. 28
Centaurs and TNOs have been assigned to a taxon. We combined the entire sample
of 38 objects taxonomically classified in the framework of our programme (28
objects from this work; 10 objects from DeMeo et al. 2009a) with previously
classified TNOs and Centaurs, looking for correlations between taxonomy and
dynamics. We compared our photometric results to literature data, finding hints
of heterogeneity for the surfaces of 4 objects.Comment: 7 pages, 4 figures. To be published in Astronomy and Astrophysic
The Rise of Complex Beliefs Dynamics
We prove that complex beliefs dynamics may emerge in linear stochastic models as the outcome of bounded rationality learning. If agents believe in a misspecified law of motion (which is correctly specified at the Rational Expectations Equilibria of the model) and update their beliefs observing the evolving economy, their beliefs can follow in the limit a beliefs cycle which is not a self-fulfilling solution of the model. The stochastic process induced by the learning rule is analyzed by means of an associated ordinary differential equation (ODE). The existence of a uniformly asymptotically stable attractor for the ODE implies the existence of a beliefs attractor, to which the learning process converges. We prove almost sure convergence by assuming that agents employ a projection facility and convergence with positive probability dropping this assumption. The rise of a limit cycle and of even more complex attractors is established in some monetary economics models assuming that agents update their beliefs with the Recursive Ordinary Least Squares and the Least Mean Squares algorithm
First albedo determination of 2867 Steins, target of the Rosetta mission
We present the first albedo determination of 2867 Steins, the asteroid target
o f the Rosetta space mission together with 21 Lutetia. The data were obtained
in polarimetric mode at the ESO-VLT telescope with the FORS1 instrument in the
V and R filters. Observations were carried out from Jun e to August 2005
covering the phase angle range from 10.3 deg. to 28.3 deg., allowing the
determination of the asteroid albedo by the well known experimenta l
relationship between the albedo and the slope of the polarimetric curve at th e
inversion angle. The measured polarization values of Steins are small,
confirming an E-type cla ssification for this asteroid, as already suggested
from its spectral propertie s. The inversion angle of the polarization curve in
the V and R filters is resp ectively of 17.3 +/-1.5deg. and 18.4+/-1.0 deg.,
and the corresponding sl ope parameter is of 0.037+/-0.003 %/deg and
0.032+/-0.003 %/deg. On the basis of its polarimetric slope value, we have
derived an albedo of 0.45 +/-0.1, that gives an estimated diameter of 4.6 km,
assuming an absolute V ma gnitude of 13.18 mag.Comment: 4 pages, 4 figures, letter accepted for pubblication on A&
The Maximal Denumerant of a Numerical Semigroup
Given a numerical semigroup S = and n in S, we
consider the factorization n = c_0 a_0 + c_1 a_1 + ... + c_t a_t where c_i >=
0. Such a factorization is maximal if c_0 + c_1 + ... + c_t is a maximum over
all such factorizations of n. We provide an algorithm for computing the maximum
number of maximal factorizations possible for an element in S, which is called
the maximal denumerant of S. We also consider various cases that have
connections to the Cohen-Macualay and Gorenstein properties of associated
graded rings for which this algorithm simplifies.Comment: 13 Page
Spectral variability on primitive asteroids of the Themis and Beagle families: space weathering effects or parent body heterogeneity?
Themis is an old and statistically robust asteroid family populating the
outer main belt, and resulting from a catastrophic collision that took place
2.51.0 Gyr ago. Within the old Themis family a young sub-family, Beagle,
formed less than 10 Myr ago, has been identified. We present the results of a
spectroscopic survey in the visible and near infrared range of 22 Themis and 8
Beagle families members. The Themis members investigated exhibit a wide range
of spectral behaviors, while the younger Beagle family members look spectrally
bluer with a smaller spectral slope variability. The best meteorite spectral
analogues found for both Themis and Beagle families members are carbonaceous
chondrites having experienced different degrees of aqueous alteration,
prevalently CM2 but also CV3 and CI, and some of them are chondrite samples
being unusual or heated. We extended the spectral analysis including the data
available in the literature on Themis and Beagle families members, and we
looked for correlations between spectral behavior and physical parameters using
the albedo and size values derived from the WISE data. The analysis of this
larger sample confirm the spectral diversity within the Themis family and that
Beagle members tend to be bluer and to have an higher albedo. The differences
between the two family may be partially explained by space weathering
processes, which act on these primitive surfaces in a similar way than on
S-type asteroids, i.e. producing reddening and darkening. However we see
several Themis members having albedos and spectral slopes similar to the young
Beagle members. Alternative scenarios are proposed including heterogeneity in
the parent body having a compositional gradient with depth, and/or the survival
of projectile fragments having a different composition than the parent body.Comment: Manuscript pages: 40; Figures: 15 ; Tables: 4 Icarus (2016),in pres
Inhomogeneities on the surface of 21 Lutetia, the asteroid target of the Rosetta mission
CONTEXT: In July 2010 the ESA spacecraft Rosetta will fly-by the main belt
asteroid 21 Lutetia. Several observations of this asteroid have been so far
performed, but its surface composition and nature are still a matter of debate.
For long time Lutetia was supposed to have a metallic nature due to its high
IRAS albedo. Later on it has been suggested to have a surface composition
similar to primitive carbonaceous chondrite meteorites, while further
observations proposed a possible genetic link with more evolved enstatite
chondrite meteorites. AIMS: In order to give an important contribution in
solving the conundrum of the nature of Lutetia, in November 2008 we performed
visible spectroscopic observations of this asteroid at the Telescopio Nazionale
Galileo (TNG, La Palma, Spain). METHODS: Thirteen visible spectra have been
acquired at different rotational phases. RESULTS: We confirm the presence of a
narrow spectral feature at about 0.47-0.48 micron already found by Lazzarin et
al. (2009) on the spectra of Lutetia. We also find a spectral feature at about
0.6 micron, detected by Lazzarin et al. (2004) on one of their Lutetia's
spectra. More importantly, our spectra exhibit different spectral slopes
between 0.6 and 0.75 micron and, in particular, we found that up to 20% of the
Lutetia surface could have flatter spectra. CONCLUSIONS: We detected a
variation of the spectral slopes at different rotational phases that could be
interpreted as possibly due to differences in the chemical/mineralogical
composition, as well as to inhomogeneities of the structure of the Lutetia's
surface (e.g., the presence of craters or albedo spots) in the southern
hemisphere.Comment: 3 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysics. Updated on 25 March 2010
Spectroscopic survey of M--type asteroids
M-type asteroids, as defined in the Tholen taxonomy (Tholen, 1984), are
medium albedo bodies supposed to have a metallic composition and to be the
progenitors both of differentiated iron-nickel meteorites and enstatite
chondrites. We carried out a spectroscopic survey in the visible and near
infrared wavelength range (0.4-2.5 micron) of 30 asteroids chosen from the
population of asteroids initially classified as Tholen M -types, aiming to
investigate their surface composition. The data were obtained during several
observing runs during the years 2004-2007 at the TNG, NTT, and IRTF telescopes.
We computed the spectral slopes in several wavelength ranges for each observed
asteroid, and we searched for diagnostic spectral features. We confirm a large
variety of spectral behaviors for these objects as their spectra are extended
into the near-infrared, including the identification of weak absorption bands,
mainly of the 0.9 micron band tentatively attributed to orthopyroxene, and of
the 0.43 micron band that may be associated to chlorites and Mg-rich
serpentines or pyroxene minerals such us pigeonite or augite. A comparison with
previously published data indicates that the surfaces of several asteroids
belonging to the M-class may vary significantly. We attempt to constrain the
asteroid surface compositions of our sample by looking for meteorite spectral
analogues in the RELAB database and by modelling with geographical mixtures of
selected meteorites/minerals. We confirm that iron meteorites, pallasites, and
enstatite chondrites are the best matches to most objects in our sample, as
suggested for M-type asteroids. The presence of subtle absorption features on
several asteroids confirms that not all objects defined by the Tholen M-class
have a pure metallic composition.Comment: 10 figures, 6 tables; Icarus, in pres
Analytically unramified one-dimensional semilocal rings and their value semigroups
AbstractIn a one-dimensional local ring R with finite integral closure each nonzerodivisor has a value in Nd, where d is the number of maximal ideals in the integral closure. The set of values constitutes a semigroup, the value semigroup of R. We investigate the connection between the value semigroup and the ring. There is a particularly close connection for some classes of rings, e.g. Gorenstein rings, Arf rings, and rings of small multiplicity. In many respects, the Arf rings and the Gorenstein rings turn out to be opposite extremes. We give applications to overrings, intersection numbers, and multiplicity sequences in the blow-up sequences studied by Lipman
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