317 research outputs found
The role of organic polymers in the structure of cometary dust
Several phenomena observed in P/Halley and other comets indicate additional fragmentation of dust particles or dust aggregates in cometary atmospheres. The disintegration of dust aggregates may be explained by sublimation of polymerized formaldehyde - POM - which play a role as binding material between submicron individual particles
Polarimetry and photometry of the peculiar main-belt object 7968 = 133P/Elst-Pizarro
133P/Elst-Pizarro is an object that has been described as either an active
asteroid or a cometary object in the main asteroid belt. Here we present a
photometric and polarimetric study of this object in an attempt to infer
additional information about its origin.
With the FORS1 instrument of the ESO VLT, we have performed during the 2007
apparition of 133P/Elst-Pizarro quasi-simultaneous photometry and polarimetry
of its nucleus at nine epochs in the phase angle range 0 - 20 deg. For each
observing epoch, we also combined all available frames to obtain a deep image
of the object, to seek signatures of weak cometary activity. Polarimetric data
were analysed by means of a novel physical interference modelling.
The object brightness was found to be highly variable over timescales <1h, a
result fully consistent with previous studies. Using the albedo-polarization
relationships for asteroids and our photometric results, we found for our
target an albedo of about 0.06-0.07 and a mean radius of about 1.6 km.
Throughout the observing epochs, our deep imaging of the comet detects a tail
and an anti-tail. Their temporal variations are consistent with an activity
profile starting around mid May 2007 of minimum duration of four months. Our
images show marginal evidence of a coma around the nucleus. The overall light
scattering behaviour (photometry and polarimetry) resembles most closely that
of F-type asteroids.Comment: Accepted by Astronomy and Astrophysic
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&
Imaging the Dust Trail and Neckline of 67P/Churyumov-Gerasimenko
We report on the results of nearly 10 hours of integration of the dust trail
and neckline of comet 67P/Churyumov-Gerasimenko (67P henceforth) using the Wide
Field Imager at the ESO/MPG 2.2m telescope in La Silla. The data was obtained
in April 2004 when the comet was at a heliocentric distance of 4.7 AU outbound.
67P is the target of the Rosetta spacecraft of the European Space Agency.
Studying the trail and neckline can contribute to the quantification of
mm-sized dust grains released by the comet. We describe the data reduction and
derive lower limits for the surface brightness. In the processed image, the
angular separation of trail and neckline is resolved. We do not detect a coma
of small, recently emitted grains.Comment: 4 pages, 3 figures, to be published in the proceedings book of the
conference "Dust in Planetary Systems 2005", Calibration updated in Section
Spitzer Observations of Comet 67P/Churyumov-Gerasimenko at 5.5-4.3 AU From the Sun
We report Spitzer Space Telescope observations of comet
67P/Churyumov-Gerasimenko at 5.5 and 4.3 AU from the Sun, post-aphelion. Comet
67P is the primary target of the European Space Agency's Rosetta mission. The
Rosetta spacecraft will rendezvous with the nucleus at heliocentric distances
similar to our observations. Rotationally resolved observations at 8 and 24
microns (at a heliocentric distance, rh, of 4.8 AU) that sample the size and
color-temperature of the nucleus are combined with aphelion R-band light curves
observed at the Very Large Telescope (VLT) and yield a mean effective radius of
2.04 +/- 0.11 km, and an R-band geometric albedo of 0.054 +/- 0.006. The
amplitudes of the R-band and mid-infrared light curves agree, which suggests
that the variability is dominated by the shape of the nucleus. We also detect
the dust trail of the comet at 4.8 and 5.5 AU, constrain the grain sizes to be
less than or similar to 6 mm, and estimate the impact hazard to Rosetta. We
find no evidence for recently ejected dust in our images. If the activity of
67P is consistent from orbit to orbit, then we may expect the Rosetta
spacecraft will return images of an inactive or weakly active nucleus as it
rendezvous with the comet at rh = 4 AU in 2014.Comment: 19 pages, 2 tables, 10 figures. Accepted for publication in the
Astronomical Journa
Spectro-polarimetry of the bright side of Saturn's moon Iapetus
Measurements of the polarized reflected sunlight from atmosphereless solar
system bodies, over a range of phase angles, provide information about the
surface structure and composition. With this work, we provide analysis of the
polarimetric observations of the bright side of Iapetus at five different phase
angles, and over the full useful wavelength range (400-800nm), so as to assess
the light scattering behaviour of a typical surface water ice. Using FORS2 of
the ESO VLT, we have performed linear spectro-polarimetric observations of
Iapetus' bright side from 2009 to 2011 at five different phase angles, in the
range from 0.80-5.20^{0}, along with circular spectro-polarimetric observations
at one phase angle. By measuring, with high accuracy (~0.1% per spectral bin
for each Stokes parameter), the spectral polarization of the bright trailing
hemisphere of Saturn's moon Iapetus, we have identified the polarimetric
characteristics of water ice, and found that its linear degree of negative
polarization decreases with increasing phase angle of observation (varying from
-0.9% to -0.3%), with a clear dependence on wavelengths of observation.Comment: Accepted for publication on Astonomy and Astrophysics Journal,
8-Pages and 6-figure
Reopening the TNOs Color Controversy: Centaurs Bimodality and TNOs Unimodality
We revisit the Trans-Neptunian Objects (TNOs) color controversy allegedly
solved by Tegler and Romanishin 2003. We debate the statistical approach of the
quoted work and discuss why it can not draw the claimed conclusions, and
reanalyze their data sample with a more adequate statistical test. We find
evidence for the existence of two color groups among the Centaurs. Therefore,
mixing both centaurs and TNOs populations lead to the erroneous conclusion of a
global bimodality, while there is no evidence for two color groups in the TNOs
population alone. We use quasi-simultaneous visible color measurements
published for 20 centaurs (corresponding to about half of the identified
objects of this class), and conclude on the existence of two groups. With the
surface evolution model of Delsanti et al. (2003) we discuss how the existence
of two groups of Centaurs may be compatible with a continuous TNOs color
distribution.Comment: 4 pages, 4 figures, accepted for publication in Astronomy and
Astrophysics Letter
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
Rosetta target comet 67P/Churyumov-Gerasimenko. Postperihelion gas and dust production rates
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
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