121 research outputs found
Towards Initial Mass Functions for Asteroids and Kuiper Belt Objects
Our goal is to understand primary accretion of the first planetesimals. The
primitive meteorite record suggests that sizeable planetesimals formed in the
asteroid belt over a period longer than a million years, each composed entirely
of an unusual, but homogeneous, mixture of mm-size particles. We sketch a
scenario in which primary accretion of 10-100km size planetesimals proceeds
directly, if sporadically, from aerodynamically-sorted mm-size particles
(generically "chondrules"). These planetesimal sizes are in general agreement
with the currently observed asteroid mass peak near 100km diameter, which has
been identified as a "fossil" property of the pre-erosion, pre-depletion
population. We extend our primary accretion theory to make predictions for
outer solar system planetesimals, which may also have a preferred size in the
100km diameter range. We estimate formation rates of planetesimals and assess
the conditions needed to match estimates of both asteroid and Kuiper Belt
Object (KBO) formation rates. For nebula parameters that satisfy observed mass
accretion rates of Myr-old protoplanetary nebulae, the scenario is roughly
consistent with not only the "fossil" sizes of the asteroids, and their
estimated production rates, but also with the observed spread in formation ages
of chondrules in a given chondrite, and with a tolerably small radial diffusive
mixing during this time between formation and accretion (the model naturally
helps explain the peculiar size distribution of chondrules within such
objects). The scenario also produces 10-100km diameter primary KBOs. The
optimum range of parameters, however, represents a higher gas density and
fractional abundance of solids, and a smaller difference between keplerian and
pressure-supported orbital velocities, than "canonical" models of the solar
nebula. We discuss several potential explanations for these differences.Comment: Icarus, in pres
Origin of the Near-Ecliptic Circumsolar Dust Band
The zodiacal dust bands are bright infrared (IR) strips produced by thermal emission from circumsolar rings of particles. Two of the three principal dust bands, known as β and γ, were previously linked to the recent asteroid collisions that produced groups of fragments, so-called asteroid families, near the orbits of (832) Karin and (490) Veritas. The origin of the third, near-ecliptic α band has been unknown until now. Here we report the discovery of a recent breakup of a >20 km diameter asteroid near α's originally suspected source location in the Themis family. Numerical modeling and observations of the α-band thermal emission from the Spitzer Space Telescope indicate that the discovered breakup is the source of α-band particles. The recent formation of all principal dust bands implies a significant time variability of the circumstellar debris disks
Debiased population of very young asteroid families
We use observations from the Catalina Sky Survey (CSS) to determine the
bias-corrected population of small members in four very young families down to
sizes equivalent to several hundred meters. Using the most recent catalog of
known asteroids, we identified members from four young families for which the
population has grown appreciably over recent times. A large fraction of these
bodies have also been detected by CSS. We used synthetic populations of
asteroids, with their magnitude distribution controlled by a small number of
parameters, as a template for the bias-corrected model of these families.
Applying the known detection probability of the CSS observations, we could
adjust these model parameters to match the observed (biased) populations in the
young families. In the case of three families, Datura, Adelaide, and Rampo, we
find evidence that the magnitude distribution transitions from steep to shallow
slopes near to meters. Conversely, the Hobson family population may
be represented by a single power-law model. The Lucascavin family has a limited
population; no new members have been discovered over the past two decades. We
consider a model of parent body rotational fission with the escaping secondary
tidally split into two components (thereby providing three members within this
family). In support of this idea, we find that no other asteroid with absolute
magnitude accompanies the known three members in the Lucascavin
family. A similar result is found for the archetypal asteroid pair
Rheinland--Kurpfalz.Comment: 32 pages, 27 figures, accepted for publication in Astronomy and
Astrophysic
Considerations on the magnitude distributions of the Kuiper belt and of the Jupiter Trojans
By examining the absolute magnitude (H) distributions (hereafter HD) of the
cold and hot populations in the Kuiper belt and of the Trojans of Jupiter, we
find evidence that the Trojans have been captured from the outer part of the
primordial trans-Neptunian planetesimal disk. We develop a sketch model of the
HDs in the inner and outer parts of the disk that is consistent with the
observed distributions and with the dynamical evolution scenario known as the
`Nice model'. This leads us to predict that the HD of hot population should
have the same slope of the HD of the cold population for 6.5 < H < 9, both as
steep as the slope of the Trojans' HD. Current data partially support this
prediction, but future observations are needed to clarify this issue. Because
the HD of the Trojans rolls over at H~9 to a collisional equilibrium slope that
should have been acquired when the Trojans were still embedded in the
primordial trans-Neptunian disk, our model implies that the same roll-over
should characterize the HDs of the Kuiper belt populations, in agreement with
the results of Bernstein et al. (2004) and Fuentes and Holman (2008). Finally,
we show that the constraint on the total mass of the primordial trans-Neptunian
disk imposed by the Nice model implies that it is unlikely that the cold
population formed beyond 35 AU.Comment: Icarus (2009) in pres
Observed Binary Fraction Sets Limits on the Extent of Collisional Grinding in the Kuiper Belt
The size distribution in the cold classical Kuiper belt can be approximated
by two idealized power laws: one with steep slope for radii R>R* and one with
shallow slope for R<R*, where R*~25-50 km. Previous works suggested that the
SFD roll-over at R* can be the result of extensive collisional grinding in the
Kuiper belt that led to the catastrophic disruption of most bodies with R<R*.
Here we use a new code to test the effect of collisions in the Kuiper belt. We
find that the observed roll-over could indeed be explained by collisional
grinding provided that the initial mass in large bodies was much larger than
the one in the present Kuiper belt, and was dynamically depleted. In addition
to the size distribution changes, our code also tracks the effects of
collisions on binary systems. We find that it is generally easier to dissolve
wide binary systems, such as the ones existing in the cold Kuiper belt today,
than to catastrophically disrupt objects with R~R*. Thus, the binary survival
sets important limits on the extent of collisional grinding in the Kuiper belt.
We find that the extensive collisional grinding required to produce the SFD
roll-over at R* would imply a strong gradient of the binary fraction with R and
separation, because it is generally easier to dissolve binaries with small
components and/or those with wide orbits. The expected binary fraction for R<R*
is <0.1. The present observational data do not show such a gradient. Instead,
they suggest a large binary fraction of ~0.4 for R=30-40 km. This may indicate
that the roll-over was not produced by disruptive collisions, but is instead a
fossil remnant of the KBO formation process.Comment: The Astronomical Journal, in pres
Implications of Jupiter Inward Gas-Driven Migration for the Inner Solar System
The migration history of Jupiter in the sun's natal disk remains poorly
constrained. Here we consider how Jupiter's migration affects small-body
reservoirs and how this constrains its original orbital distance from the Sun.
We study the implications of large-scale and inward radial migration of Jupiter
for the inner solar system while considering the effects of collisional
evolution of planetesimals. We use analytical prescriptions to simulate the
growth and migration of Jupiter in the gas disk. We assume the existence of a
planetesimal disk inside Jupiter's initial orbit. This planetesimal disk
received an initial total mass and size-frequency distribution (SFD).
Planetesimals feel the effects of aerodynamic gas drag and collide with one
another, mostly while shepherded by the migrating Jupiter. Our main goal is to
measure the amount of mass in planetesimals implanted into the main asteroid
belt (MAB) and the SFD of the implanted population. We also monitor the amount
of dust produced during planetesimal collisions. We find that the SFD of the
planetesimal population implanted into the MAB tends to resemble that of the
original planetesimal population interior to Jupiter. We also find that unless
very little or no mass existed between 5 au and Jupiter's original orbit, it
would be difficult to reconcile the current low mass of the MAB with the
possibility that Jupiter migrated from distances beyond 15 au. This is because
the fraction of the original disk mass that gets implanted into the MAB is very
large. Finally, we discuss the implications of our results in terms of dust
production to the so-called NC-CC isotopic dichotomy.Comment: Accepted for publication in The Astrophysical Journal Letters; In
pres
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