802 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
Linear spectro-polarimetry: a new diagnostic tool for the classification and characterisation of asteroids
We explore the use of spectro-polarimetry as a remote sensing tool for
asteroids in addition to traditional reflectance measurements. In particular we
are interested in possible relationships between the wavelength-dependent
variation of linear polarization and the properties of the surfaces, including
albedo and composition.
We have obtained optical spectro-polarimetric measurements of a dozen
asteroids of different albedo and taxonomic classes and of two small regions at
the limb of the Moon.
We found that objects with marginally different relative reflectance spectra
(in the optical) may have totally different polarization spectra. This suggests
that spectro-polarimetry may be used to refine the classification of asteroids.
We also found that in some cases the Umov law may be violated, that is, in
contrast to what is expected from basic physical considerations, the fraction
of linear polarization and the reflectance may be positively correlated. In
agreement with a few previous studies based on multi-colour broadband
polarimetry, we found that the variation of linear polarization with wavelength
and with phase-angle is correlated with the albedo and taxonomic class of the
objects. Finally, we have serendipitously discovered that spinel-rich asteroid
(599) Luisa, located very close to the Watsonia family, is a member of the rare
class of Barbarian asteroids.
We suggest that future modelling attempts of the surface structure of
asteroids should be aimed at explaining both reflectance and polarization
spectra.Comment: Accepted for publication in MNRAS Letter
Albedo heterogeneity on the surface of (1943) Anteros
We have investigated the effect of rotation on the polarization of scattered
light for the near-Earth asteroid (1943) Anteros using the Dual Beam Imaging
Polarimeter on the University of Hawaii's 2.2 m telescope. Anteros is an L-type
asteroid that has not been previously observed polarimetrically. We find weak
but significant variations in the polarization of Anteros as a function of
rotation, indicating albedo changes across the surface. Specifically, we find
that Anteros has a background albedo of p_v = 0.18 +/- 0.02 with a dark spot of
p_v < 0.09 covering < 2% of the surface.Comment: Accepted to Icarus, 15 pages, 3 fig
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
Recent collisional jet from a primitive asteroid
Here we show an example of a young asteroid cluster located in a dynamically
stable region, which was produced by partial disruption of a primitive body
about 30 km in size. We estimate its age to be only 1.9 +/- 0.3 Myr, thus its
post-impact evolution should have been very limited. The large difference in
size between the largest object and the other cluster members means that this
was a cratering event. The parent body had a large orbital inclination, and was
subject to collisions with typical impact speeds higher by a factor of 2 than
in the most common situations encountered in the main belt. For the first time
we have at disposal the observable outcome of a very recent event to study
high-speed collisions involving primitive asteroids, providing very useful
constraints to numerical simulations of these events and to laboratory
experiments.Comment: Accepted for publication by MNRA
Testing the inversion of asteroids' Gaia photometry combined with ground-based observations
We investigated the reliability of the genetic algorithm which will be used
to invert the photometric measurements of asteroids collected by the European
Space Agency Gaia mission. To do that, we performed several sets of simulations
for 10 000 asteroids having different spin axis orientations, rotational
periods and shapes. The observational epochs used for each simulation were
extracted from the Gaia mission simulator developed at the Observatoire de la
C\^{o}te d'Azur, while the brightness was generated using a Z-buffer standard
graphic method. We also explored the influence on the inversion results of
contaminating the data set with Gaussian noise with different values.
The research enabled us to determine a correlation between the reliability of
the inversion method and the asteroid's pole latitude. In particular, the
results are biased for asteroids having quasi-spherical shapes and low pole
latitudes. This effect is caused by the low lightcurve amplitude observed under
such circumstances, as the periodic signal can be lost in the photometric
random noise when both values are comparable, causing the inversion to fail.
Such bias might be taken into account when analysing the inversion results, not
to mislead it with physical effects such as non-gravitational forces. Finally,
we studied what impact on the inversion results has combining a full lightcurve
and Gaia photometry collected simultaneously. Using this procedure we have
shown that it is possible to reduce the number of wrong solutions for asteroids
having less than 50 data points. The latter will be of special importance for
planning ground-based observations of asteroids aiming to enhance the
scientific impact of Gaia on Solar system science.Comment: Accepted in MNRA
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
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
La trama de la droga : migraciones y representaciones en la crónica Si me querés, quéreme transa, de Cristián Alarcón
En Si me querés, quereme transa (2010), Cristian Alarcón organiza la trama que une la villa, la violencia y el narcotráfico en Villa del Señor, Argentina. En este artículo, indagaremos en una serie de relaciones -económicas, culturales y sociales- que se despliegan en la crónica a partir de las voces de los propios protagonistas, especialmente, los vínculos entre lo rural y lo urbano, las migraciones, y las representaciones que desde el melodrama al gore se configuran en torno a la lógica de la droga. Analizaremos principalmente dichas articulaciones a partir de la figura de la transa Alcira.A Si me querés, quereme transa (2010), Cristina Alarcón organitza la trama que uneix una vila, la violència i el narcotràfic a Villa del Señor, Argentina. En aquest article, indagarem en una sèrie de relacions -econòmiques, culturals i socials- que es despleguen a la crònica a partir de les veus dels mateixos protagonistes, especialment, els vincles entre el rural i l'urbà, les migracions, i les representacions que des del melodrama al gore es configuren entorn a la lògica de la droga. Analitzarem principalment aquestes qüestions a partir de la figura de la transa Alcira.In the book named Si me querés, quereme transa (2010), Cristian Alarcón organizes a plot that joins the slum, violence and drug trafficking in Villa del Señor, Argentina. In this article, we will investigate in a series of connections -economics, cultural and socials- that are displayed in the chronicle from the voices of the typical protagonists, especially, the link between the rural and the urban areas, migrations, and the representations that go from melodrama to gore being configured around the drugs's logic. Mainly, we will analyze those articulations from the figures of "transa", Alcira
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