385 research outputs found
A Region Void of Irregular Satellites Around Jupiter
An interesting feature of the giant planets of our solar system is the
existence of regions around these objects where no irregular satellites are
observed. Surveys have shown that, around Jupiter, such a region extends from
the outermost regular satellite Callisto, to the vicinity of Themisto, the
innermost irregular satellite. To understand the reason for the existence of
such a satellite-void region, we have studied the dynamical evolution of Jovian
irregulars by numerically integrating the orbits of several hundred test
particles, distributed in a region between 30 and 80 Jupiter-radii, for
different values of their semimajor axes, orbital eccentricities, and
inclinations. As expected, our simulations indicate that objects in or close to
the influence zones of the Galilean satellites become unstable because of
interactions with Ganymede and Callisto. However, these perturbations cannot
account for the lack of irregular satellites in the entire region between
Callisto and Themisto. It is suggested that at distances between 60 and 80
Jupiter-radii, Ganymede and Callisto may have long-term perturbative effects,
which may require the integrations to be extended to times much longer than 10
Myr. The interactions of irregular satellites with protosatellites of Jupiter
at the time of the formation of Jovian regulars may also be a destabilizing
mechanism in this region. We present the results of our numerical simulations
and discuss their applicability to similar satellite void-regions around other
giant planets.Comment: 21 pages, 9 figures, 2 tables, accepted for publication in the
Astronomical Journa
Footprints of a possible Ceres asteroid paleo-family
Ceres is the largest and most massive body in the asteroid main belt.
Observational data from the Dawn spacecraft reveal the presence of at least two
impact craters about 280~km in diameter on the Ceres surface, that could have
expelled a significant number of fragments. Yet, standard techniques for
identifying dynamical asteroid families have not detected any Ceres family. In
this work, we argue that linear secular resonances with Ceres deplete the
population of objects near Ceres. Also, because of the high escape velocity
from Ceres, family members are expected to be very dispersed, with a
considerable fraction of km-sized fragments that should be able to reach the
pristine region of the main belt, the area between the 5J:-2A and 7J:-3A
mean-motion resonances, where the observed number of asteroids is low. Rather
than looking for possible Ceres family members near Ceres, here we propose to
search in the pristine region. We identified 156 asteroids whose taxonomy,
colors, albedo could be compatible with being fragments from Ceres. Remarkably,
most of these objects have inclinations near that of Ceres itself.Comment: 12 pages, 6 figures, 1 table. Accepted for publication in MNRA
Did the Hilda collisional family form during the late heavy bombardment?
We model the long-term evolution of the Hilda collisional family located in
the 3/2 mean-motion resonance with Jupiter. Its eccentricity distribution
evolves mostly due to the Yarkovsky/YORP effect and assuming that: (i) impact
disruption was isotropic, and (ii) albedo distribution of small asteroids is
the same as for large ones, we can estimate the age of the Hilda family to be
. We also calculate collisional activity in the J3/2
region. Our results indicate that current collisional rates are very low for a
200\,km parent body such that the number of expected events over Gyrs is much
smaller than one.
The large age and the low probability of the collisional disruption lead us
to the conclusion that the Hilda family might have been created during the Late
Heavy Bombardment when the collisions were much more frequent. The Hilda family
may thus serve as a test of orbital behavior of planets during the LHB. We
tested the influence of the giant-planet migration on the distribution of the
family members. The scenarios that are consistent with the observed Hilda
family are those with fast migration time scales to
, because longer time scales produce a family that is depleted
and too much spread in eccentricity. Moreover, there is an indication that
Jupiter and Saturn were no longer in a compact configuration (with period ratio
) at the time when the Hilda family was created
On the oldest asteroid families in the main belt
Asteroid families are groups of minor bodies produced by high-velocity
collisions. After the initial dispersions of the parent bodies fragments, their
orbits evolve because of several gravitational and non-gravitational
effects,such as diffusion in mean-motion resonances, Yarkovsky and YORP
effects, close encounters of collisions, etc. The subsequent dynamical
evolution of asteroid family members may cause some of the original fragments
to travel beyond the conventional limits of the asteroid family. Eventually,
the whole family will dynamically disperse and no longer be recognizable.
A natural question that may arise concerns the timescales for dispersion of
large families. In particular, what is the oldest still recognizable family in
the main belt? Are there any families that may date from the late stages of the
Late Heavy Bombardment and that could provide clues on our understanding of the
primitive Solar System? In this work, we investigate the dynamical stability of
seven of the allegedly oldest families in the asteroid main belt. Our results
show that none of the seven studied families has a nominally mean estimated age
older than 2.7 Gyr, assuming standard values for the parameters describing the
strength of the Yarkovsky force. Most "paleo-families" that formed between 2.7
and 3.8 Gyr would be characterized by a very shallow size-frequency
distribution, and could be recognizable only if located in a dynamically less
active region (such as that of the Koronis family). V-type asteroids in the
central main belt could be compatible with a formation from a paleo-Eunomia
family.Comment: 9 pages, 5 figures, 5 tables. Accepted for publication in MNRA
Constraining the cometary flux through the asteroid belt during the late heavy bombardment
In the Nice model, the late heavy bombardment (LHB) is related to an orbital
instability of giant planets which causes a fast dynamical dispersion of a
transneptunian cometary disk. We study effects produced by these hypothetical
cometary projectiles on main-belt asteroids. In particular, we want to check
whether the observed collisional families provide a lower or an upper limit for
the cometary flux during the LHB.
We present an updated list of observed asteroid families as identified in the
space of synthetic proper elements by the hierarchical clustering method,
colour data, albedo data and dynamical considerations and we estimate their
physical parameters. We selected 12 families which may be related to the LHB
according to their dynamical ages. We then used collisional models and N-body
orbital simulations to gain insight into the long-term dynamical evolution of
synthetic LHB families over 4 Gyr. We account for the mutual collisions, the
physical disruptions of comets, the Yarkovsky/YORP drift, chaotic diffusion, or
possible perturbations by the giant-planet migration.
Assuming a "standard" size-frequency distribution of primordial comets, we
predict the number of families with parent-body sizes D_PB >= 200 km which
seems consistent with observations. However, more than 100 asteroid families
with D_PB >= 100 km should be created at the same time which are not observed.
This discrepancy can be nevertheless explained by the following processes: i)
asteroid families are efficiently destroyed by comminution (via collisional
cascade), ii) disruptions of comets below some critical perihelion distance (q
<~ 1.5 AU) are common.
Given the freedom in the cometary-disruption law, we cannot provide stringent
limits on the cometary flux, but we can conclude that the observed distribution
of asteroid families does not contradict with a cometary LHB.Comment: accepted in Astronomy and Astrophysic
A multi-domain approach to asteroid families identification
Previous works have identified families halos by an analysis in proper
elements domains, or by using Sloan Digital Sky Survey-Moving Object Catalog
data, fourth release (SDSS-MOC4) multi-band photometry to infer the asteroid
taxonomy, or by a combination of the two methods. The limited number of
asteroids for which geometric albedo was known until recently discouraged in
the past the extensive use of this additional parameter, which is however of
great importance in identifying an asteroid taxonomy. The new availability of
geometric albedo data from the Wide-field Infrared Survey Explorer (WISE)
mission for about 100,000 asteroids significantly increased the sample of
objects for which such information, with some errors, is now known.
In this work we proposed a new method to identify families halos in a
multi-domain space composed by proper elements, SDSS-MOC4 (a*,i-z) colors, and
WISE geometric albedo for the whole main belt (and the Hungaria and Cybele
orbital regions). Assuming that most families were created by the breakup of an
undifferentiated parent body, they are expected to be homogeneous in colors and
albedo. The new method is quite effective in determining objects belonging to a
family halo, with low percentages of likely interlopers, and results that are
quite consistent in term of taxonomy and geometric albedo of the halo members.Comment: 23 pages, 18 figures, 6 tables. Accepted for publication in MNRA
Radar detectability studies of slow and small zodiacal dust cloud particles. I. The case of Arecibo 430 MHz meteor head echo observations
Recent model development of the Zodiacal Dust Cloud (ZDC) argues that the incoming flux of meteoric material into the Earth's upper atmosphere is mostly undetected by radars because they cannot detect small extraterrestrial particles entering the atmosphere at low velocities due to the relatively small production of electrons. In this paper, we present a new methodology utilizing meteor head echo radar observations that aims to constrain the ZDC physical model by ground-based measurements. In particular, for this work, we focus on Arecibo 430 MHz observations since this is the most sensitive radar utilized for this type of observations to date. For this, we integrate and employ existing comprehensive models of meteoroid ablation, ionization, and radar detection to enable accurate interpretation of radar observations and show that reasonable agreement in the hourly rates is found between model predictions and Arecibo observations when (1) we invoke the lower limit of the model predicted flux (∼16 t d-1) and (2) we estimate the ionization probability of ablating metal atoms using laboratory measurements of the ionization cross sections of high-speed metal atom beams, resulting in values up to two orders of magnitude lower than the extensively utilized figure reported by Jones for low-speed meteors. However, even at this lower limit, the model overpredicts the slow portion of the Arecibo radial velocity distributions by a factor of three, suggesting that the model requires some revision
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