461 research outputs found
Photometry and polarimetry of the nucleus of comet 2P/Encke
Broadband imaging photometry, and broadband and narrowband linear polarimetry
was measured for the nucleus of 2P/Encke over the phase-angle range 4 - 28 deg.
An analysis of the point spread function of the comet reveals only weak coma
activity, corresponding to a dust production of the order of 0.05 kg/s. The
nucleus displays a color independent photometric phase function of almost
linear slope. The absolute R filter magnitude at zero phase angle is 15.05 +/-
0.05, and corresponds to an equivalent radius for the nucleus of 2.43 +/- 0.06
km (for an adopted albedo of 0.047). The nucleus color V - R is 0.47 +/- 0.07,
suggesting a spectral slope of 11 +/- 8 %/100nm. The phase function of linear
polarimetry in the V and R filters shows a widely color independent linear
increase with phase angle (0.12 +/- 0.02%/deg). We find discrepancies in the
photometric and polarimetric parameters between 2P/Encke and other minor bodies
in the solar system, which may indicate significant differences in the surface
material properties and light-scattering behavior of the bodies.
The linear polarimetric phase function of 2P/Encke presented here is the
first ever measured for a cometary nucleus, and its analysis encourages future
studies of cometary nuclei in order to characterize the light-scattering
behavior of comets on firm empirical grounds and provide suitable input to a
comprehensive modeling of the light scattering by cometary surfaces.Comment: Accepted by A&
The 1990 MB: The first Mars Trojan
Asteroid 1990 MB was discovered by D. H. Levy and H. E. Holt during the course of the Mars and Earth Crossing Asteroid and Comet Survey. An orbit based on a 9 day arc and the asteroid's location near Mars' L5 (trailing Lagrangean) longitude led E. Boswell to speculate that it might be in 1:1 resonance with Mars, analogous to the Trojan asteroids of Jupiter. Subsequent observations strengthened the possibility, and later calculations confirmed it. Thus 1990 MB is the first known asteroid in 1:1 resonance with a planet other than Jupiter. The existence of 1990 MB (a small body most likely between 2 and 4 km in diameter) provides remarkable confirmation of computer simulations. These self consistent n-body simulations demonstrated this sort of stability for Trojans of all the terrestrial planets over at least a 2 million year time base. The discovery of 1990 MB suggests that others of similar or smaller diameter may be found. Using hypothetical populations of Mars Trojans, their possible sky plane distributions were modeled as a first step in undertaking a systematic observational search of Mars' L4 and L5 libration regions
Asteroid taxonomic signatures from photometric phase curves
We explore the correlation between an asteroid's taxonomy and photometric
phase curve using the H, G12 photometric phase function, with the shape of the
phase function described by the single parameter G12. We explore the usability
of G12 in taxonomic classification for individual objects, asteroid families,
and dynamical groups. We conclude that the mean values of G12 for the
considered taxonomic complexes are statistically different, and also discuss
the overall shape of the G12 distribution for each taxonomic complex. Based on
the values of G12 for about half a million asteroids, we compute the
probabilities of C, S, and X complex membership for each asteroid. For an
individual asteroid, these probabilities are rather evenly distributed over all
of the complexes, thus preventing meaningful classification. We then present
and discuss the G12 distributions for asteroid families, and predict the
taxonomic complex preponderance for asteroid families given the distribution of
G12 in each family. For certain asteroid families, the probabilistic prediction
of taxonomic complex preponderance can clearly be made. The Nysa-Polana family
shows two distinct regions in the proper element space with different G12
values dominating in each region. We conclude that the G12-based probabilistic
distribution of taxonomic complexes through the main belt agrees with the
general view of C complex asteroid proportion increasing towards the outer
belt. We conclude that the G12 photometric parameter cannot be used in
determining taxonomic complex for individual asteroids, but it can be utilized
in the statistical treatment of asteroid families and different regions of the
main asteroid belt.Comment: submitted to Icaru
Polarimetry of transneptunian objects (136472) Makemake and (90482) Orcus
Context. We study the surface properties of transneptunian populations of
Solar-system bodies. Aims. We investigate the surface characteristics of the
dwarf planet (136472) Makemake and the resonant object (90482) Orcus. Methods.
Using the FORS2 instrument of the ESO-VLT we have carried out linear
polarisation measurements of Makemake and Orcus. Results. Polarisation of Orcus
is similar to that of smaller size objects. The polarimetric properties of
Makemake are very close to those of Eris and Pluto. We have not found any
significant differences in the polarisation properties of objects from
different dynamical classes. However, there are significant differences in
polarisation of large and smaller size objects, and between large TNOs with
water-ice and methane-ice dominated surfaces. Conclusions. We confirm the
different types of polarisation phase behavior for the largest and smaller size
TNOs. To explain subtle surface polarisation of Pluto, Makemake and Eris we
assume that their surfaces are covered by a thin layer of hoarfrost masking the
surface structure
A revised asteroid polarization-albedo relationship using WISE/NEOWISE data
We present a reanalysis of the relationship between asteroid albedo and
polarization properties using the albedos derived from the Wide-field Infrared
Survey Explorer. We find that the function that best describes this relation is
a three-dimensional linear fit in the space of log(albedo)-log(polarization
slope)-log(minimum polarization). When projected to two dimensions the
parameters of the fit are consistent with those found in previous work. We also
define p* as the quantity of maximal polarization variation when compared with
albedo and present the best fitting albedo-p* relation. Some asteroid taxonomic
types stand out in this three-dimensional space, notably the E, B, and M Tholen
types, while others cluster in clumps coincident with the S- and C-complex
bodies. We note that both low albedo and small (D<30 km) asteroids are
under-represented in the polarimetric sample, and we encourage future
polarimetric surveys to focus on these bodies.Comment: 16 pages, Accepted to Ap
Inversion of HIPPARCOS and Gaia photometric data for asteroids Asteroid rotational properties from sparse photometric data
Context. Sparse photometric data can be used to determine the spin properties and infer information about the shapes of asteroids. The algorithm adopted for the inversion of Gaia photometric data assumes, for the sake of simplicity and to minimize CPU execution time, that the objects have triaxial ellipsoid shapes. In the past, this algorithm was tested against large sets of simulated data and small numbers of sparse photometric measurements obtained by HIPPARCOS. Aims. After the second Gaia data release, it is now possible to test the inversion algorithm against small samples of actual Gaia data for the first time. At the same time, we can attempt a new inversion of older HIPPARCOS measurements, using an updated version of the photometric inversion algorithm. Methods. The new version of our inversion algorithm includes the treatment of a Lommel-Seeliger scattering relation especially developed for the case of triaxial ellipsoid shapes. In addition, we also performed inversion attempts using a more refined shape model, based on the so-called cellinoid shapes. Results. With respect to the old inversion of HIPPARCOS data carried out in the past, we obtain only marginal improvements. In the case of Gaia data, however, we obtain very encouraging results. A successful determination of the rotation period is possible in most cases, in spite of the limited time span covered by data published in the second Gaia data release (GDR2), which makes the determination of the spin axis direction still uncertain. Even a small number of measurements, less than 30 in many cases, are sufficient to obtain a satisfactory inversion solution. Using the more realistic cellinoid shape model, we find further improvement in the determination of the spin period. Conclusions. This is a relevant validation of GDR2 photometry of asteroids, and proof of the satisfactory performances of the adopted inversion algorithm.Peer reviewe
Laboratory-based grain-shape models for simulating dust infrared spectra
Analysis of thermal dust emission spectra for dust mineralogy and physical
grain properties depends on laboratory-measured or calculated comparison
spectra. Often, the agreement between these two kinds of spectra is not
satisfactory because of the strong influence of the grain morphology on the
spectra. We investigate the ability of the statistical light-scattering model
with a distribution of form factors (DFF model) to reproduce experimentally
measured infrared extinction spectra for particles that are small compared to
the wavelength. We take advantage of new experimental spectra measured for free
particles dispersed in air with accompanying information on the grain
morphology. For the calculations, we used DFFs that were derived for aggregates
of spherical grains, as well as for compact grain shapes corresponding to
Gaussian random spheres. Irregular particle shapes require a DFF similar to
that of a Gaussian random sphere with sigma=0.3, whereas roundish grain shapes
are best fitted with that of a fractal aggregate of a fractal dimension
2.4-1.8. In addition we used a fitting algorithm to obtain the best-fit DFFs
for the various laboratory samples. In this way we can independently derive
information on the shape of the grains from their infrared spectra. For
anisotropic materials, different DFFs are needed for the different
crystallographic axes. This is due to a theoretical problem, which is inherent
to all models that are simply averaging the contributions of the
crystallographic directions.Comment: 8 pages, 8 figures, accepted by Astronomy and Astrophysic
Exploring the surface properties of Transneptunian Objects and Centaurs with polarimetric FORS1/VLT observations
Polarization is a powerful remote-sensing method to investigate solar system
bodies. It is an especially sensitive diagnostic tool to reveal physical
properties of the bodies whose observational characteristics are governed by
small scatterers (dust, regolith surfaces). For these objects, at small phase
angles, a negative polarization is observed, i.e., the electric vector E
oscillates predominantly in the scattering plane, contrary to what is typical
for rather smooth homogeneous surfaces. The behavior of negative polarization
with phase angle depends on the size, composition and packing of the
scatterers. These characteristics can be unveiled by modelling the light
scattering by the dust or regolith in terms of the coherent backscattering
mechanism.
We have investigated the surface properties of TNOs and Centaurs by means of
polarimetric observations with FORS1 of the ESO VLT.
TNOs Ixion and Quaoar, and Centaur Chiron show a negative polarization surge.
The Centaur Chiron has the deepest polarization minimum (-1.5 - 1.4%). The two
TNOs show differing polarization curves: for Ixion, the negative polarization
increases rapidly with phase; for Quaoar, the polarization is relatively small
(~ -0.6%), and nearly constant at the observed phase angles. For all three
objects, modelling results suggest that the surface contains an areal mixture
of at least two components with different single-scatterer albedos and photon
mean-free paths.Comment: 11 pages, 7 postscript figures, accepted by A&A; astro-ph abstract
has been replaced with a more complete on
Scattering Properties of Large Irregular Cosmic Dust Particles at Visible Wavelengths
The effect of internal inhomogeneities and surface roughness on the scattering behavior of large cosmic dust particles is studied by comparing model simulations with laboratory measurements. The present work shows the results of an attempt to model a dust sample measured in the laboratory with simulations performed by a ray-optics model code. We consider this dust sample as a good analogue for interplanetary and interstellar dust as it shares its refractive index with known materials in these media. Several sensitivity tests have been performed for both structural cases (internal inclusions and surface roughness). Three different samples have been selected to mimic inclusion/coating inhomogeneities: two measured scattering matrices of hematite and white clay, and a simulated matrix for water ice. These three matrices are selected to cover a wide range of imaginary refractive indices. The selection of these materials also seeks to study astrophysical environments of interest such as Mars, where hematite and clays have been detected, and comets. Based on the results of the sensitivity tests shown in this work, we perform calculations for a size distribution of a silicate-type host particle model with inclusions and surface roughness to reproduce the experimental measurements of a dust sample. The model fits the measurements quite well, proving that surface roughness and internal structure play a role in the scattering pattern of irregular cosmic dust particles.Peer reviewe
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