791 research outputs found
Collision rates in the present-day Kuiper Belt and Centaur Regions: Applications to surface activation and modification on Comets, Kuiper Belt Objects, Centaurs, and Pluto-Charon
We extend previous results showing that the surfaces of Edgeworth-Kuiper Belt
objects are not primordial and have been moderately to heavily reworked by
collisions. Objects smaller than about km have collisional disruption
lifetimes less than 3.5 Gyr in the present-day collisional environment and have
been heavily damaged in their interiors by large collisions. In the 30--50 AU
region, impacts of 1 km radius comets onto individual 100 km radius objects
occur on -- yr timescales, cratering the surfaces of
the larger objects with 8--54 craters 6 km in diameter over 3.5 Gyr.
Collision time scales for impacts of 4 meter radius projectiles onto 1 km
radius comets range from 3--5 yr. The cumulative fraction of the
surface area of 1 and 100 km radius objects cratered by projectiles with radii
larger than 4 m ranges from a few to a few tens percent over 3.5 Gyr. The flux
of EKO projectiles onto Pluto and Charon is also calculated and is found to be
3--5 times that of previous estimates. Our impact model is also applied
to Centaur objects in the 5--30 AU region. We find the collisional/cratering
histories of Centaurs are dominated by the time spent in the Edgeworth-Kuiper
Belt rather than the time spent on planet-crossing orbits. Hence, the
predominant surface activity of Centaur objects like Chiron is almost certainly
not impact-induced.Comment: 17 pages, 8 figures. Icarus, 2000, in pres
Collisional evolution in the Vulcanoid region: Implications for present-day population constraints
We explore the effects of collisional evolution on putative Vulcanoid
ensembles in the region between 0.06 and 0.21 AU from the Sun, in order to
constrain the probable population density and population structure of this
region today. Dynamical studies have shown that the Vulcanoid Zone (VZ) could
be populated. However, we find that the frequency and energetics of collisional
evolution this close to the Sun, coupled with the efficient radiation transport
of small debris out of this region, together conspire to create an active and
highly intensive collisional environment which depletes any very significant
population of rocky bodies placed in it, unless the bodies exhibit orbits that
are circular to ~10^-3 or less, or highly lossy mechanical properties that
correspond to a fraction of impact energy significantly less than 10% being
imparted to ejecta. The most favorable locale for residual bodies to survive in
this region is in highly circular orbits near the outer edge of the dynamically
stable Vulcanoid Zone (i.e., near 0.2 AU), where collisional evolution and
radiation transport of small bodies and debris proceed most slowly. If the mean
random orbital eccentricity in this region exceeds ~10^-3, then our work
suggests it is unlikely that more than a few hundred objects with radii larger
than 1 km will be found in the entire VZ; assuming the largest objects have a
radius of 30 km, then the total mass of bodies in the VZ down to 0.1 km radii
is likely to be no more than ~10^-6Mearth, <10^-3 the mass of the asteroid
belt. Despite the dynamical stability of large objects in this region (Evans &
Tabachnik 1999), it is plausible that the entire region is virtually empty of
km-scale and larger objects.Comment: text plus 7 .ps figures, gzipped. Icarus, 2000, in pres
The Size-Frequency Distribution of the Zodiacal Cloud: Evidence from the Solar System Dust Bands
Recent observations of the size-frequency distribution of zodiacal cloud
particles obtained from the cratering record on the LDEF satellite (Love and
Brownlee 1993) reveal a significant large particle population (100 micron
diameter or greater) near 1 AU. Our previous modeling of the Solar System dust
bands (Grogan et al 1997), features of the zodiacal cloud associated with the
comminution of Hirayama family asteroids, has been limited by the fact that
only small particles (25 micron diameter or smaller) have been considered. This
was due to the prohibitively large amount of computing power required to
numerically analyze the dynamics of larger particles. The recent availability
of cheap, fast processors has finally made this work possible. Models of the
dust bands are created, built from individual dust particle orbits, taking into
account a size-frequency distribution of the material and the dynamical history
of the constituent particles. These models are able to match both the shapes
and amplitudes of the dust band structures observed by IRAS in multiple
wavebands. The size-frequency index, q, that best matches the observations is
approximately 1.4, consistent with the LDEF results in that large particles are
shown to dominate. However, in order to successfully model the `ten degree'
band, which is usually associated with collisional activity within the Eos
family, we find that the mean proper inclination of the dust particle orbits
has to be approximately 9.35 degrees, significantly different to the mean
proper inclination of the Eos family (10.08 degrees).Comment: 49 pages total, including 27 figure pages. Submitted to Icaru
The Cardiovascular Epidemiology and Genome-Wide Associations of Biomarkers of Innate and Adaptive Immunity: sCD163 and sIL2RA
Cardiovascular disease (CVD) is a major cause of morbidity and mortality in the U.S. and worldwide. Atherosclerosis, the buildup of plaque in the arteries, is a common cause of CVD. For many years, research in atherosclerosis was focused on lipid metabolism and the accumulation of low-density lipoprotein in the arteries. While this research set public health guidelines for lipid management, lipid concentration was not the only factor influencing atherosclerosis and CVD events. Many scientists, as far back as the 1850’s recognized the role of inflammation in the progression of atherosclerotic disease. The continuous low levels of immune activation in the body contribute to atherosclerosis. Research in animal models and epidemiologic studies have shown the involvement of both the innate and the adaptive immune systems in plaque development and to elucidate the roles of monocytes and T cells. In addition to animal studies and epidemiologic research, CVD and atherosclerotic research has extended to genetic analysis in the search for associations with risk factors and outcomes.
The first chapter is a review of the literature studying the immune system’s involvement in atherosclerosis. Beginning with an examination of the impact of CVD and atherosclerosis, the basic pathophysiology, and the involvement of the innate and adaptive immune systems through animal models and epidemiology. Some of the significant cohort studies in CVD and genome wide association studies are also discussed.
Chapter 2 examines the associations of soluble interleukin 2 receptor alpha (sIL-2Rα) with clinical events in the Cardiovascular Health Study and genetic variants. Interleukin 2 (IL-2) and its receptor regulate both tolerance and immunity, IL-2 induces the proliferation and differentiation of T cells, part of the adaptive immune system. The results showed an association between sIL-2Rα and CVD events. The genome-wide association study found 52 variants to be significantly associated with sIL-2Rα in European Americans.
Chapter 3 assesses the involvement of the innate immune system in atherosclerosis through the associations of soluble CD163 (sCD163). CD163 is a marker of macrophage activation, specifically associated with M2 macrophages. In CHS, sCD163 levels were analyzed for associations with cardiovascular events and genetic variants. sCD163 was found to be associated with CVD risk factors and with cardiovascular events. In a genome-wide association study six variants in European Americans and three variants in African Americans were found to be significant.
Chapter 4 summarizes the results and discusses some bench to bedside translational science already seen in atherosclerosis treatment and prevention. Continued investigation of markers of T-cell and monocyte differentiation in animal models and cohort studies may lead to opportunities for the prevention of atherosclerosis and/or treatment through an increased understanding of the biology and genetics of the innate and adaptive immune
Collisional Formation and Modeling of Asteroid Families
In the last decade, thanks to the development of sophisticated numerical
codes, major breakthroughs have been achieved in our understanding of the
formation of asteroid families by catastrophic disruption of large parent
bodies. In this review, we describe numerical simulations of asteroid
collisions that reproduced the main properties of families, accounting for both
the fragmentation of an asteroid at the time of impact and the subsequent
gravitational interactions of the generated fragments. The simulations
demonstrate that the catastrophic disruption of bodies larger than a few
hundred meters in diameter leads to the formation of large aggregates due to
gravitational reaccumulation of smaller fragments, which helps explain the
presence of large members within asteroid families. Thus, for the first time,
numerical simulations successfully reproduced the sizes and ejection velocities
of members of representative families. Moreover, the simulations provide
constraints on the family dynamical histories and on the possible internal
structure of family members and their parent bodies.Comment: Chapter to appear in the (University of Arizona Press) Space Science
Series Book: Asteroids I
Collisional and dynamic evolution of dust from the asteroid belt
The size and spatial distribution of collisional debris from main belt asteroids is modeled over a 10 million year period. The model dust and meteoroid particles spiral toward the Sun under the action of Poynting-Robertson drag and grind down as they collide with a static background of field particles
Identifying Near Earth Object Families
The study of asteroid families has provided tremendous insight into the
forces that sculpted the main belt and continue to drive the collisional and
dynamical evolution of asteroids. The identification of asteroid families
within the NEO population could provide a similar boon to studies of their
formation and interiors. In this study we examine the purported identification
of NEO families by Drummond (2000) and conclude that it is unlikely that they
are anything more than random fluctuations in the distribution of NEO
osculating orbital elements. We arrive at this conclusion after examining the
expected formation rate of NEO families, the identification of NEO groups in
synthetic populations that contain no genetically related NEOs, the orbital
evolution of the largest association identified by Drummond (2000), and the
decoherence of synthetic NEO families intended to reproduce the observed
members of the same association. These studies allowed us to identify a new
criterion that can be used to select real NEO families for further study in
future analyses, based on the ratio of the number of pairs and the size of
strings to the number of objects in an identified association.Comment: Accepted for publication in Icarus. 19 pages including 11 figure
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