1,721,238 research outputs found
Origing, Aging, and Death of Asteroid F amilies.
No full textWe have investigated the creation and destruction of asteroid families using a collisional code which follows the evolution of the size distributions of both the main-belt background population and individual families which are produced by collisions within the population. Starting with a small-mass initial belt, the number of families produced by collisional breakup of parent bodies larger than 100 km and surviving to the present time is in good agreement with the observed number of families. Increasing the mass of the initial belt increases the number of families in the model; hence the number of families that we see today provides a significant constraint on the overall collisional history of asteroids. Most families formed by disruption of large (D,>200 km) parent bodies are still recognizable today. Families formed from smaller parent bodies early in Solar System history have been eroded away, and there is a general trend in our models to have younger ages associated with families formed from smaller parent bodies. Today about one-third of the overall population should be in families, consistent with the findings of V. Zappalà et al. (1995, Icarus 116, 291-314)
Collisional evolution of asteroid families
No full textMost asteroid dynamical families are thought to be the outcomes of collisional disruption of parent asteroids destroyed by high-velocity impacts with other asteroids. However, subsequent collisions modify both the sizes and the orbits of family members, so the distributions that we see today may be very different from those following the breakup of the parent body. We study the post-breakup evolution of family asteroids with a numerical model which keeps track of both the sizes and the orbits of the fragments as they collisionally interact with the field population of asteroids. Using this model we visualize how the family appears at different evolutionary stages. In particular we find that the size distribution of a family becomes less steep with time. We have simulated the possible evolutionary history of the three most populous Hirayama families, Koronis, Eos, and Themis. By matching the distribution of sizes and orbits with those observed for the families, we obtain significant constraints on the properties of their parent bodies and on some collisional response parameters, together with the evolutionary ages of the families. The Themis family appears as the outcome of the catastrophic disruption of one of the largest asteroids, probably a unique event over the history of the Solar System. On the other hand, the Koronis and Eos families appear to have been formed from smaller parent bodies, but peculiar features may require specific processes or events. Koronis' size distribution has several bodies of comparable size at the large diameter end, which can be explained if the largest fragment of the initial breakup underwent subsequent fragmentation. The 'anistrophic' orbital distribution of the Eos family requires either a peculiar fragment velocity field or the action of poorly understood dynamical processes on the orbits of its members. For both the Koronis and the Themis families we derive an estimate of the age of the order of 2 Byr. The uncertainties affecting our estimates of family ages and of the properties of the parent bodies are mainly due to the present limited understanding of collisional breakup processes for bodies hundreds of kilometers in size and to the poor knowledge of the size distribution of small asteroids
The Psyche Family: Collisionally destroyed or never formed?
No full textAsteroid 16 Psyche, the largest M-type asteroid, is widely considered to be the collisionally exposed core of an ~500-km diameter differentiated parent body which was similar to asteroid 4 Vesta. However, there is no dynamical family associated with Psyche nor are there spectroscopic data for the existence of the mantle or crustal material from the parent body. The usual explanation for the missing material requires that the Psyche parent body was collisionally disrupted early in solar system history, followed by collisional grinding of the family down to below the current observational limit sizes. We test the exposed core hypothesis for the origin of Psyche using a numerical code that simultaneously calculates both the collisional evolution of the asteroid belt and the model family formed by the breakup of the Psyche parent body (PPB). We find that it would take a projectile about 300-350 km in size to thoroughly disrupt a 500-km asteroid (the estimated size of the PPB), and that the probability of such an event occurring in the first 500 Ma of solar system history is only about 1%. While the Psyche model family is found to have been significantly ground down subsequent to its formation, there should be several tens of survivors from the mantle and crust larger than ~10 km that should be spectroscopically detectable by current technology. Although only a small fraction of the asteroids larger than 10 km have been discovered and observed spectroscopically to date, none have been identified as potential survivors from the PPB (Burbine et al. 1996, Meteor. Planet. Sci. 31, 607-620). Given the low probability of the disruption of a Vesta-like body and the lack of either dynamical or observational confirmation of a family or material from the parent body, we think it more likely that Psyche has possibly been shattered by impacts but not catastrophically disrupted. In this case, it would be a plausible candidate parent body for the mesosiderites. The exposed-core scenario more probably applies to other, smaller (diameter <~100 km) M-type asteroids, which could be the parent bodies of the iron meteorites. However, this interpretation raises the interesting problem of why among the larger asteroids only Vesta and the PPB would have been fully differentiated
Catastrophic disruption of asteriods and satellites; Proceedings of the International Workshop, Pisa, Italy, July 30-August 2, 1985
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Accretional evolution of a planetesimal swarm: 1. A new simulation
No full textThis novel simulation of planetary accretion simultaneously treats many interacting heliocentric distance zones and characterizes planetesimals via Keplerian elements. The numerical code employed, in addition to following the size distribution and the orbit-element distribution of a planetesimal swarm from arbitrary size and orbit distributions, treats a small number of the largest bodies as discrete objects with individual orbits. The accretion algorithm used yields good agreement with the analytic solutions; agreement is also obtained with the results of Weatherill and Stewart (1989) for gravitational accretion of planetesimals having equivalent initial conditions
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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