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

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 successful search for hidden Barbarians in the Watsonia asteroid family

Barbarians, so named after the prototype of this class (234) Barbara, are a rare class of asteroids exhibiting anomalous polarimetric properties. Their very distinctive feature is that they show negative polarization at relatively large phase-angles, where all 'normal' asteroids show positive polarization. The origin of the Barbarian phenomenon is unclear, but it seems to be correlated with the presence of anomalous abundances of spinel, a mineral usually associated with the so-called Calcium Aluminum-rich inclusions (CAIs) on meteorites. Since CAIs are samples of the oldest solid matter identified in our solar system, Barbarians are very interesting targets for investigations. Inspired by the fact that some of the few known Barbarians are members of, or very close to the dynamical family of Watsonia, we have checked whether this family is a major repository of Barbarians, in order to obtain some hints about their possible collisional origin. We have measured the linear polarization of a sample of nine asteroids which are members of the Watsonia family within the phase-angle range 17-21 DEG. We found that seven of them exhibit the peculiar Barbarian polarization signature, and we conclude that the Watsonia family is a repository of Barbarian asteroids. The new Barbarians identified in our analysis will be important to confirm the possible link between the Barbarian phenomenon and the presence of spinel on the surface.Comment: Accepted by MNRA

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

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

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 $\sigma$ 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

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

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