Gonzalo Tancredi

A ring system detected around the Centaur (10199) Chariklo

Hitherto, rings have been found exclusively around the four giant planets in the Solar System. Rings are natural laboratories in which to study dynamical processes analogous to those that take place during the formation of planetary systems and galaxies. Their presence also tells us about the origin and evolution of the body they encircle. Here we report observations of a multichord stellar occultation that revealed the presence of a ring system around (10199) Chariklo, which is a Centaur—that is, one of a class of small objects orbiting primarily between Jupiter and Neptune—with an equivalent radius of 124  9 kilometres (ref. 2). There are two dense rings, with respective widths of about 7 and 3 kilometres, optical depths of 0.4 and 0.06, and orbital radii of 391 and 405 kilometres. The present orientation of the ring is consistent with an edge-on geometry in 2008, which provides a simple explanation for the dimming of the Chariklo system between 1997 and 2008, and for the gradual disappearance of ice and other absorption features in its spectrum over the same period. This implies that the rings are partly composed of water ice. They may be the remnants of a debris disk, possibly confined by embedded, kilometre-sized satellites.

Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation

Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies. Pluto possesses an atmosphere whereas Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos. Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto, and is currently at a distance to the Sun intermediate between the two. Although Makemake’s size (1,420 ± 60 km) and albedo are roughly known, there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere. Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo pV = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4–12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm−3 is inferred from the data.

THE SIZE, SHAPE, ALBEDO, DENSITY, AND ATMOSPHERIC LIMIT OF TRANSNEPTUNIAN OBJECT (50000) QUAOAR FROM MULTI-CHORD STELLAR OCCULTATIONS

We present results derived from the first multi-chord stellar occultations by the transneptunian object (50000) Quaoar, observed on 2011 May 4 and 2012 February 17, and from a single-chord occultation observed on 2012 October 15. If the timing of the five chords obtained in 2011 were correct, then Quaoar would possess topographic features (crater or mountain) that would be too large for a body of this mass. An alternative model consists in applying time shifts to some chords to account for possible timing errors. Satisfactory elliptical fits to the chords are then possible, yielding an equivalent radius Requiv = 555±2.5 km and geometric visual albedo pV = 0.109±0.007. Assuming that Quaoar is a Maclaurin spheroid with an indeterminate polar aspect angle, we derive a true oblateness of � = 0.087 +0.0268 −0.0175 , an equatorial radius of 569 +2417 km, and a density of 1.99 ± 0.46 g cm −3 . The orientation of our preferred solution in the plane of the sky implies that Quaoar’s satellite Weywot cannot have an equatorial orbit. Finally, we detect no global atmosphere around Quaoar, considering a pressure upper limit of about 20 nbar for a pure methane atmosphere.

CHAOTIC DYNAMICS OF PLANET-ENCOUNTERING BODIES

The dynamics of two families of minor inner solar system bodies that suffer frequent close encounters with the planets is analyzed. These families are: Jupiter family comets (JF comets) and Near Earth Asteroids (NEAs).The motion of these objects has been considered to be chaotic in a short time scale,and the close encounters are supposed to be the cause of the fast chaos. For a better understanding of the chaotic behavior we have computed Lyapunov Characteristic Exponents (LCEs) for all the observed members of both populations. LCEs are a quantitative measure of the exponential divergence of initially close orbits. We have observed that most members of the two families show a concentration of Lyapunov times (inverse of LCE) around 50–100yr. The concentration is more pronounced for JF comets than for NEAs, among which a lesser spread is observed for those that actually cross the Earths orbit (mean perihelion distance q < 1.05 AU). It is also observed that a general correspondence exists between Lyapunov times and the time between consecutive encounters.A simple model is introduced to describe the basic characteristics of the dynamical evolution. This model considers an impulsive approach, where the particles evolve unperturbedly between encounters and suffer ‘kicks’ in semimajor axis at the encounters. It also reproduces successfully the short Lyapunov times observed in the numerical integrations and is able to estimate the dynamical lifetimes of comets during a stay in the Jupiter family in correspondence with previous estimates.It has been demonstrated with the model that the encounters with the largest effect on the exponential growth of the distance between initially nearby orbits are neither the infrequent deep encounters, nor the frequent and far ones; instead, the intermediate approaches have the most relevant contribution to the error growth. Such encounters are at a distance a few times the radius of the Hills sphere of the planet (e.g. 3).An even simpler model allows us to get analytical estimates of the Lyapunov times in good agreement with the values coming from the model above and the numerical integrations.The predictability of the medium‐term evolution and the hazard posed to the Earth by those objects are analysed in the Discussion section.

Orbit computation for transneptunian objects

Using statistical orbital ranging, we systematically study the orbit computation problem for transneptunian objects (TNOs). We have automated orbit computation for large numbers of objects, and, more importantly, we are able to obtain orbits even for the most sparsely observed objects (observational arcs of a few days). For such objects, the resulting orbit distributions include a large number of high-eccentricity orbits, in which TNOs can be perturbed by close encounters with Neptune. The stability of bodies on the computed orbits has therefore been ascertained by performing a study of close encounters with the major planets. We classify TNO orbit distributions statistically, and we study the evolution of their ephemeris uncertainties. We find that the orbital element distributions for the most numerous single-apparition TNOs do not support the existence of a postulated sharp edge to the belt beyond 50 AU. The technique of statistical ranging provides ephemeris predictions more generally than previously possible also for poorly observed TNOs.

The Cometary Contribution to Planetary Impact Rates

We use statistics on the nuclear magnitudes of Jupiter Family comets to derive the number of km-sized members with perihelion distances less than 2 AU. When coupled with impact rates per comet taken from Levison et al. (2000), and with our estimate of the number of dormant comets in similar orbits yielding a dormant/active ratio of about 2, we find a terrestrial impact rate of about 1.10−6 per year. This is already significant (~ 20 – 50%) compared with the total estimated impact rate by km-sized bodies from cratering statistics, and we estimate that further contributions by Halley-type and long-period comets are also substantial. Thus, in broad agreement with a number of earlier investigators, we find that comets yield a large, perhaps dominant, contribution to km-sized terrestrial impactors.

A photometric search for active Main Belt asteroids

It is well known that some Main Belt asteroids show comet-like features. A representative example is the first known Main Belt comet 133P/(7968) Elst-Pizarro. If the mechanisms causing this activity are too weak to develop visually evident comae or tails, the objects stay unnoticed. We are presenting a novel way to search for active asteroids, based on looking for objects with deviations from their expected brightnesses in a database. Just by using the MPCAT-OBS Observation Archive we have found five new candidate objects that possibly show a type of comet-like activity, and the already known Main Belt comet 133P/(7968) Elst-Pizarro. Four of the new candidates, (315) Constantia, (1026) Ingrid, (3646) Aduatiques, and (24 684) 1990 EU4, show brightness deviations independent of the object’s heliocentric distance, while (35 101) 1991 PL16 shows deviations dependent on its heliocentric distance, which could be an indication of a thermal triggered mechanism. The method could be implemented in future sky survey programmes to detect outbursts on Main Belt objects almost simultaneously with their occurrence.

High-Performance Computing of Self-Gravity for Small Solar System Bodies

To study the evolution of small solar system bodies like asteroids and comets, a fast method is needed to compute the self-gravity of systems composed of millions of particles. A proposed fully parallel shared-memory algorithm for dense, self-gravitating agglomerates scales efficiently with the number of particles as well as the number of computational resources.

Physical and dynamical evolution of Jupiter family comets : simulations based on the observed sample

Abstract We describe a method to simulate the combined physical and dynamical evolution of Jupiter family comets. We integrate all the observed Jupiter family comets and three variant orbits until they leave the family. Regarding the physical modelling, the parameters are the nuclear mass (or radius) and the fraction of the surface area covered by dust mantles formed during previous orbital evolution. This picture is motivated by recent findings concerning the coupled orbital and physical evolution of cometary nuclei. I present results concerning the lifetime of comets, both in the active and dormant phase, and snapshot distributions of the number of comets vs perihelion distance.

A Parallel Multithreading Algorithm for Self-gravity Calculation on Agglomerates

This article presents the application of parallel multithreading strategies for the calculation of the self-gravity force field in astronomical small bodies. Efficient and accurate algorithms are needed to simulate the dynamic of asteroids and comets, which are formed by the agglomeration of many (i.e. in the order of million) small objects. Parallel high performance computing comes to help researchers to perform the required simulations of large systems in reasonable execution times. In this article, we describe several strategies for the computation on shared-memory high performance computing infrastructures and a experimental analysis studying the execution time, speedup and computational efficiency are reported. Promising results are reported for the strategy that applies a smart isolation lineal approach for dividing the calculation work to be performed by each computing element. The experimental results demonstrate that this strategy achieves almost-linear speedup, allowing researchers to perform accurate simulations in reduced execution times, even for those cases where very large systems are studied.