563 research outputs found
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
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
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
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&
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
Polarimetric survey of main-belt asteroids: IV New results from the first epoch of the CASLEO survey
Aims: We present results of a polarimetric survey of main-belt asteroids at Complejo Astronómico El Leoncito (CASLEO), San Juan, Argentina. The aims of this survey are to increase the database of asteroid polarimetry, to estimate diversity in polarimetric properties of asteroids that belong to different taxonomic classes, and to search for objects that exhibit anomalous polarimetric properties.
Methods: The data were obtained using the Torino and CASPROF polarimeters at the 2.15 m telescope. The Torino polarimeter is an instrument that allows simultaneous measurement of polarization in five different bands, and the CASPROF polarimeter is a two-hole aperture polarimeter with rapid modulation.
Results: The survey began in 1995, and until 2012 data on a large sample of asteroids were obtained. We here present and analyze the unpublished results for 129 asteroids of different taxonomic types, 56 which were polarimetrically observed for the first time. We find that the asteroids (402) Chloe and (729) Watsonia are Barbarians, and asteroid (269) Justitia shows a phase – polarization curve that seems to have a small inversion angle. Data obtained in UBVRI colors allow us to sketch an analysis of the wavelength dependence of the degree of linear polarization for 31 asteroids, in spite of some large error bars in some cases.Fil: Gil Hutton, Ricardo Alfredo. Consejo Nacional de Investigaciones CientÃÂÂficas y Técnicas. Centro CientÃÂÂfico Tecnológico San Juan. Complejo Astronómico "El Leoncito"; Argentina. Universidad Nacional de San Juan; ArgentinaFil: Cellino, A.. Osservatorio Astrofisico di Torino; ItaliaFil: Bendjoya, Ph.. Université de Nice Sophia Antipolis. Observatoire de la Côte d’Azur; Franci
Unusual polarimetric properties of (101955) Bennu: similarities with F-class asteroids and cometary bodies
We have obtained polarimetric measurements of asteroid (101955) Bennu, a
presumably primitive near-Earth object (NEO) that is the target of NASA's
sample return mission OSIRIS-REx. During our observing campaign, Bennu was
visible from Earth under a wide range of illumination conditions, with
phase-angle in the range 16 to 57 degrees. Together with (3200) Phaethon and
(152679) 1998 KU2, observed very recently, Bennu is the only existing example
of a primitive NEO observed in polarimetric mode over a wide interval of phase
angles. Based on our polarimetric data, we propose that Bennu belongs to the
unusual F taxonomic class defined in the 80s. According to previous works, the
F-class includes objects with cometary features. This fact can be of great
importance for the interpretation of the results of the exploration of this
object by OSIRIS-REx. From polarimetry we also derive an estimate of the
geometric albedo of Bennu: 0.059 +/- 0.003.Comment: Accepted by MNRAS Letter
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