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تسجيل مستخدم جديدGyr-timescale destruction of high-eccentricity asteroids by spin and why 2006 HY51 has been spared
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Asteroids and other small celestial bodies have markedly prolate shapes, and the perturbative triaxial torques which are applied during pericenter passages in highly eccentric orbits trigger and sustain a state of chaotic rotation. Because the prograde spin rate around the principal axis of inertia is not bounded from above, it can accidentally reach the threshold value corresponding to rotational break-up. Previous investigations of this process were limited to integrations of $sim 10^3$ orbits because of the stiff equation of motion. We present here a fast 1D simulation method to compute the evolution of this spin rate over $sim 10^9$ orbits. We apply the method to the most eccentric solar system asteroid known, 2006 HY51 (with $e = 0.9684$), and find that for any reasonably expected shape parameters, it can never be accelerated to break-up speed. However, primordial solar system asteroids on more eccentric orbits may have already broken up from this type of rotational fission. The method also represents a promising opportunity to investigate the long-term evolution of extremely eccentric triaxial exo-asteroids ($e > 0.99$), which are thought to be common in white dwarf planetary systems
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Few solar system asteroids and comets are found in high eccentricity orbits ($e > 0.9$) but in the primordial planetesimal disks and in exoplanet systems around dying stars such objects are believed to be common. For 2006 HY51, the main belt asteroid
with the highest known eccentricity 0.9684, we investigate the probable rotational states today using our computer-efficient chaotic process simulation method. Starting with random initial conditions, we find that this asteroid is inevitably captured into stable spin-orbit resonances typically within tens to a hundred Myr. The resonances are confirmed by direct integration of the equation of motion in the vicinity of end-points. Most resonances are located at high spin values above 960 times the mean motion (such as 964:1 or 4169:4), corresponding to rotation periods of a few days. We discover three types of resonance in the high-eccentricity regime: 1) regular circulation with weakly librating aphelion velocities and integer-number spin-orbit commensurabilities; 2) switching resonances of higher order with orientation alternating between aligned (0 or $pi$) and sidewise ($pi/2$) angles at aphelia and perihelia; 3) jumping resonances with aphelion spin alternating between two quantum states in the absence of spin-orbit commensurability. The islands of equilibrium are numerous at high spin rates but small in parameter space area, so that it takes millions of orbits of chaotic wandering to accidentally entrap in one of them. We discuss the implications of this discovery for the origins and destiny of high-eccentricity objects and the prospects of extending this analysis to the full 3D treatment.
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Context. The so-called Barbarian asteroids share peculiar, but common polarimetric properties, probably related to both their shape and composition. They are named after (234) Barbara, the first on which such properties were identified. As has been s
uggested, large scale topographic features could play a role in the polarimetric response, if the shapes of Barbarians are particularly irregular and present a variety of scattering/incidence angles. This idea is supported by the shape of (234) Barbara, that appears to be deeply excavated by wide concave areas revealed by photometry and stellar occultations. Aims. With these motivations, we started an observation campaign to characterise the shape and rotation properties of Small Main- Belt Asteroid Spectroscopic Survey (SMASS) type L and Ld asteroids. As many of them show long rotation periods, we activated a worldwide network of observers to obtain a dense temporal coverage. Methods. We used light-curve inversion technique in order to determine the sidereal rotation periods of 15 asteroids and the con- vergence to a stable shape and pole coordinates for 8 of them. By using available data from occultations, we are able to scale some shapes to an absolute size. We also study the rotation periods of our sample looking for confirmation of the suspected abundance of asteroids with long rotation periods. Results. Our results show that the shape models of our sample do not seem to have peculiar properties with respect to asteroids with similar size, while an excess of slow rotators is most probably confirmed.
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