A. Campo Bagatin

Wavy size distributions for collisional systems with a small-size cutoff

Abstract Dohnanyis [ J. geophys. Res . 74 , 2531–2554, 1969; in Physical Studies of Minor Planets (edited by T. Gehrels), pp. 263–295. NASA-SP 267, 1971] theory predicts that a collisional system such as the asteroid population should rapidly relax to a power-law equilibrium size distribution, provided all the collisional response parameters are independent of size. However, we have found that Dohnanyi did not include in a consistent way in the theory the possible occurrence of a small-size cutoff in the distribution. We have carried out a number of numerical simulations of the collisional evolution process, showing that the cutoff results in a wavy pattern superimposed on Dohnanyis equilibrium power law, which affects the distribution up to sizes of tens of km. The pattern arises because particles just above the cutoff are not removed by catastrophic impacts by smaller projectiles, and therefore are created by break-up of larger bodies faster than they are eliminated; larger particles are increasingly depleted up to the size where the smallest shattering projectile exceeds the cutoff, and beyond that the removal rate is reduced and the distribution flattens. Thus, to be effective in producing the waves, the cutoff (or any other persisting “discontinuity” in the particle properties) must be sharp over a size range corresponding to the threshold projectile-to-target ratio for fragmentation. The presence of a small-size cutoff in the real asteroid belt is an open question, since it may be generated by poorly known non-gravitational effects acting on μm-sized dust, and may be affected by influx of cometary debris. However, the observational evidence for a variable characteristic exponent of the size distribution of interplanetary bodies is now strong, and the cutoff effect may provide a simple explanation for this finding.

THE CANADA-FRANCE ECLIPTIC PLANE SURVEY—FULL DATA RELEASE: THE ORBITAL STRUCTURE OF THE KUIPER BELT*

We report the orbital distribution of the trans-Neptunian objects (TNOs) discovered during the Canada–France Ecliptic Plane Survey (CFEPS), whose discovery phase ran from early 2003 until early 2007. The follow-up observations started just after the first discoveries and extended until late 2009. We obtained characterized observations of 321 deg 2 of sky to depths in the range g ∼ 23.5–24.4 AB mag. We provide a database of 169 TNOs with high-precision dynamical classification and known discovery efficiency. Using this database, we find that the classical belt is a complex region with sub-structures that go beyond the usual splitting of inner (interior to 3:2 mean-motion resonance [MMR]), main (between 3:2 and 2:1 MMR), and outer (exterior to 2:1 MMR). The main classical belt (a = 40–47 AU) needs to be modeled with at least three components: the “hot” component with a wide inclination distribution and two “cold” components (stirred and kernel) with much narrower inclination distributions. The hot component must have a significantly shallower absolute magnitude (Hg) distribution than the other two components. With 95% confidence, there are 8000 +18001600 objects in the main belt with Hg 8.0, of which 50% are from the hot component, 40% from the stirred component, and 10% from the kernel; the hot component’s fraction drops rapidly with increasing Hg. Because of this, the apparent population fractions depend on the depth and ecliptic latitude of a trans-Neptunian survey. The stirred and kernel components are limited to only a portion of the main belt, while we find that the hot component is consistent with a smooth extension throughout the inner, main, and outer regions of the classical belt; in fact, the inner and outer belts are consistent with containing only hot-component objects. The Hg 8.0 TNO population estimates are 400 for the inner belt and 10,000 for the outer belt to within a factor of two (95% confidence). We show how the CFEPS Survey Simulator can be used to compare a cosmogonic model for the orbital element distribution to the real Kuiper Belt.

GRB 021004 modelled by multiple energy injections

GRB021004 is one of the best sampled gamma-ray bursis (GRB) to date. although the nature of its light curve is still being debated. Here we present 107 new optical, near-infrared (NIR) and millimetre observations, ranging from 2 h to more than a year after the burst. Fitting the inultiband data to a model based on multiple energy injections suggests that at least 7 refreshed shocks took place during the evolution of the afterglow, implying a total energy release (collimated within an angle of 1:8) of ∼8 x 10 51 erg. Analysis of the late photometry reveals that the GRB021004 host is a low extinction (A V ∼ 0.1) starburst galaxy with M B ≃ -22.0.

The CFEPS Kuiper Belt Survey: Strategy and presurvey results

Abstract We present the data acquisition strategy and characterization procedures for the Canada–France Ecliptic Plane Survey (CFEPS), a sub-component of the Canada–France–Hawaii Telescope Legacy Survey. The survey began in early 2003 and as of summer 2005 has covered 430 square degrees of sky within a few degrees of the ecliptic. Moving objects beyond the orbit of Uranus are detected to a magnitude limit of m R = 23 – 24 (depending on the image quality). To track as large a sample as possible and avoid introducing followup bias, we have developed a multi-epoch observing strategy that is spread over several years. We present the evolution of the uncertainties in ephemeris position and orbital elements of a small 10-object sample of objects tracked through these epochs as part of a preliminary presurvey starting a year before the main CFEPS project. We describe the CFEPS survey simulator, to be released in 2006, which allows theoretical models of the Kuiper belt to be compared with the survey discoveries. The simulator utilizes the well-documented pointing history of CFEPS, with characterized detection efficiencies as a function of magnitude and rate of motion on the sky. Using the presurvey objects we illustrate the usage of the simulator in modeling the classical Kuiper belt. The primary purpose of this paper is to allow a user to immediately exploit the CFEPS data set and releases as they become available in the coming months.

Earth cratering record and impact energy flux in the last 150 Ma

Abstract Although the Earths cratering record is subject to strong bias (i.e. unknown craters yet to be discovered or obliterated by geological processes, geochronologic uncertainties of impact events) a compilation of the 33 best dated large impact craters on Earth with diameters larger than 5 km, and younger than 150 Ma, their diameters, geochronologic ages, and the corresponding unceertainties can be used to construct a diagram summarizing our current knowledge on the influx of impact energy onto the Earth versus time. From the crater diameters, we estimated the corresponding impact energies through suitable scaling laws. Then, we associated with each crater a gaussian (bell) function of time centered at its age, with a half-width consistent with the age uncertainty and a total area proportional to the impact energy. Finally, all the bell functions corresponding to different craters were summed up and the resulting curve (smoothed out by computing running averages over 4 Ma) was plotted on a semilogtarithmic scale. From this curve it is apparent that the recently discovered 144.7 Ma old Morokweng crater in South Afica, which is associated with the Jurassic/Cretaceous boundary and related mass extinction, corresponds to the highest energy influx peak, almost an order of magnitude larger than the Chicxulub crater in Yucatan Which on the other hand, is associated with the Cretaceous/Tertiary boundary mass extinction, the most severe in the marine record of the last 150 Ma. The third largest impact energy flux peak corresponds to the Late Eocene Popigai (Siberia) and Chesapeake Bay (U.S.A.) giant impact craters, which are also associated with a global biotic crisis. From the overall record examined herein it emerges that there is probably a threshold size of approximately 3 km for the smallest projectile capable of triggering large-scale extinctions. There is no convincing evidence for periodicities in the distribution of crater ages. A few groups of several craters appear to be more closely spaced in time than in a purely random distribution.

Possible ring material around centaur (2060) Chiron

We propose that several short duration events observed in past stellar occultations by Chiron were produced by rings material. From a reanalysis of the stellar occultation data in the literature we determined two possible orientations of the pole of Chirons rings, with ecliptic coordinates l=(352+/-10) deg, b=(37+/-10) deg or l=(144+/-10) deg, b=(24+/-10) deg . The mean radius of the rings is (324 +/- 10) km. One can use the rotational lightcurve amplitude of Chiron at different epochs to distinguish between the two solutions for the pole. Both imply lower lightcurve amplitude in 2013 than in 1988, when the rotational lightcurve was first determined. We derived Chirons rotational lightcurve in 2013 from observations at the 1.23-m CAHA telescope and indeed its amplitude is smaller than in 1988. We also present a rotational lightcurve in 2000 from images taken at CASLEO 2.15-m telescope that is consistent with our predictions. Out of the two poles the l=(144+/-10) deg, b=(24+/-10) deg solution provides a better match to a compilation of rotational lightcurve amplitudes from the literature and those presented here. We also show that using this preferred pole, Chirons long term brightness variations are compatible with a simple model that incorporates the changing brightness of the rings as the tilt angle with respect to the Earth changes with time. Also, the variability of the water ice band in Chirons spectra in the literature can be explained to a large degree by an icy ring system whose tilt angle changes with time and whose composition includes water ice, analogously to the case of Chariklo. We present several possible formation scenarios for the rings from qualitative points of view and speculate on the reasons why rings might be common in centaurs. We speculate on whether the known bimodal color distribution of centaurs could be due to presence of rings and lack of them.

The Extreme Kuiper Belt Binary 2001 QW322

The study of binary Kuiper Belt objects helps to probe the dynamic conditions present during planet formation in the solar system. We report on the mutual-orbit determination of 2001 QW322, a Kuiper Belt binary with a very large separation whose properties challenge binary-formation and -evolution theories. Six years of tracking indicate that the binarys mutual-orbit period is ≈25 to 30 years, that the orbit pole is retrograde and inclined 50° to 62° from the ecliptic plane, and, most surprisingly, that the mutual orbital eccentricity is <0.4. The semimajor axis of 105,000 to 135,000 kilometers is 10 times that of other near-equal-mass binaries. Because this weakly bound binary is prone to orbital disruption by interlopers, its lifetime in its present state is probably less than 1 billion years.

Fragment ejection velocities and the collisional evolution of asteroids

Abstract We have applied the algorithm developed by Petit and Farinella (Celest. Mech. 57, 1–28, 1993) to model the outcomes of impacts between asteroids of different sizes, to show that a crucial feature of these models is the assumed relationship between velocity and mass of fragments ejected after a shattering impact. Not only how the mean velocity depends upon mass is important to determine the extent of fragment reaccumulation, but also the distribution of velocities about the mean values. The available experimental evidence on this issue is still sparse, and does not constrain the collisional models well enough to allow us to make reliable predictions on the outcomes of impacts between bodies of size much larger than the laboratory targets. As a consequence, when the collisional outcome models are used as an input for simulations of the asteroid collisional history since the origin of the solar system, the results show a strong sensitivity to the assumed velocity vs mass relationship. This sensitivity is stronger in the diameter range (a few tens to a few hundreds of km) where the self-gravitational reaccumulation of fragments is most effective, but may also extend to much smaller sizes.

Rotational fission of trans-Neptunian objects: the case of Haumea

This research was partially supported by Spanish grants AYA2008-06202-C03-01, AYA-06202-C03-02, AYA2008-06202-C03-03, P07-FQM-02998 and European FEDER funds. RD acknowledges financial support from the MICINN (contract Ramon y Cajal). DCR acknowledges support from the National Aeronautics and Space Administration under grant No. NNX08AM39G issued through the Office of Space Science.

Statistics of encounters in the trans-Neptunian region

The inventory of the populations of trans-Neptunian objects (TNO) has grown considerably over the last decade. As for other groups of small bodies in our solar system, TNOs are expected to have experienced a collisional evolution owing to their mutual impacts. The knowledge of the statistics of collisions, including determination of the rate of mutual collisions and the distribution of the impact velocity, is indeed a fundamental prerequisite for developing models of collisional evolution. We revised the evaluation of those statistical parameters for TNOs provided more than ten years ago on the basis of a much more limited sample of objects than currently available. We used the Canada-France Ecliptic Plane Survey (CFEPS) L7 model to extract an unbiased sample of orbits for TNOs, while the statistical parameters of impact are computed using a statistical tool. We investigated the statistics of impacts among TNOs for the whole population and for different dynamical subgroups. Moreover, we investigated the statistics of collisions between subgroups with crossing orbits. The peculiar dynamical behavior of objects in resonant orbits is taken into account. Our present computation of the probabilities of collision are 20% to 50% lower than previous estimates, while mean impact velocities turn out to be about 70% higher. For instance, the rate of collisions among Plutinos, expressed in terms of the so-called mean intrinsic probability of collision, results to be (3.90 ± 0.01) × 10 −22 km −2 yr −1 and the mean impact velocity is 2.46 ± 0.01 km s −1 .W e also fi nd that the distributions of impact velocities seem to be quite different from pure Maxwellian distributions. These results can be useful in developing models of the collisional evolution in the trans-Neptunian region.