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Register a new userGalactic Tide and Local Stellar Perturbations on the Oort Cloud: Creation of Interstellar Comets
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Comets in the Oort cloud evolve under the influence of internal and external perturbations, such as giant planets, stellar passages, and the galactic tidal field. We aim to study the dynamical evolution of the comets in the Oort cloud, accounting for external perturbations (passing stars and the galactic tide). We first construct an analytical model of stellar encounters. We find that individual perturbations do not modify the dynamics of the comets in the cloud unless very close (< 0.5pc) encounters occur. Using proper motions, parallaxes, and radial velocities from Gaia DR2, we construct an astrometric catalogue of 14,659 stars that are within 50pc from the Sun. For all these stars we calculate the time and the closest distance to the Sun. We find that the cumulative effect of relatively distant ($leq1$ pc) passing stars can perturb the comets in the Oort cloud. Finally, we study the dynamical evolution of the comets in the Oort cloud under the influence of multiple stellar encounters within 2.5pc from the Sun and the galactic tidal field over $pm10$Myr. We considered two models for the Oort cloud, compact (a $leq$0.25 pc) and extended (a$ leq0.5$ pc). We find that the cumulative effect of stellar encounters is the major perturber of the Oort cloud for a compact configuration while for the extended, the galactic tide is the major perturber. In both cases, the effect of passing stars and the galactic tide raises the semi-major axis of $sim1.1$% of the comets at the edge of the cloud up to interstellar regions ($a >0.5$pc). This leads to the creation of transitional interstellar comets, which might become interstellar objects due to external perturbations. This raises the question about the existence of a cloud of objects in the interstellar space which might overlap with our Oort cloud if we consider that other planetary systems face similar processes for the ejection of comets.
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The interstellar comet 2I/Borisov bears a strong resemblance to Oort Cloud comets, judging from its appearance in images taken over the first six weeks of observation. To test the proposed affinity in more diagnostic terms, 2I is compared to Oort Cloud comets of similar perihelion distance, near 2 AU. Eight such objects are identified among the cataloged comets whose orbits have been determined with high accuracy. This work focuses on three particular characteristics: the light curve, the geometry of the dust tail, and the dust parameter Afrho. Unlike Oort Cloud comets with perihelia beyond the snow line, Oort Cloud comets with perihelia near 2 AU show strong evidence of the original halo of slowly accelerating, millimeter-sized and larger icy-dust grains only in early tail observations. The dust tail in later images is primarily the product of subsequent, water-sublimation driven activity nearer perihelion but not of activity just preceding observation, which suggests the absence of microscopic-dust ejecta. Comet 2I fits, in broad terms, the properties of the Oort Cloud comets with perihelia near 2 AU and of fairly low activity. Future tests of the preliminary conclusions are proposed.
A 2000-2017 set of long-period comets with high-quality orbits of perihelion distance <1 AU is used to show that the objects that perish shortly before perihelion are nearly exclusively the Oort Cloud comets, especially those with perihelia within 0.6 AU of the Sun, intrinsically fainter, and dust poor. Their propensity for disintegration is much higher than predicted by Bortles perihelion survival rule, prompting the author to propose a new synoptic index to be tested in future prognostication efforts. By their susceptibility to demise near the Sun, the nuclei of Oort Cloud comets differ dramatically from the nuclei of other long-period comets that almost always survive. In this scenario, `Oumuamua -- discovered after perihelion -- is in all probability a major piece of debris of an interstellar comet that was bound to perish near perihelion if it was similar to, though much fainter than, the known Oort Cloud comets. The nondetection of `Oumuamua by the Spitzer Space Telescope is compatible with optical data for pancake shape, but not for cigar shape, with the maximum dimension not exceeding 160 m (at an 0.1 albedo). Although the solar radiation pressure induced nongravitational acceleration requires very high porosity, `Oumuamuas estimated mass is orders of magnitude greater than for a cloud of unbound submicron-sized dust grains of equal cross section. The acceleration could have displaced `Oumuamua by 250,000 km in 50 days, scattering other potential debris over a large volume of space.
We present a chronology of the formation and early evolution of the Oort cloud by simulations. These simulations start with the Solar System being born with planets and asteroids in a stellar cluster orbiting the Galactic center. Upon ejection from its birth environment, we continue to follow the evolution of the Solar System while it navigates the Galaxy as an isolated planetary system. We conclude that the range in semi-major axis between 100au and several 10$^3$,au still bears the signatures of the Sun being born in a 1000MSun/pc$^3$ star cluster, and that most of the outer Oort cloud formed after the Solar System was ejected. The ejection of the Solar System, we argue, happened between 20Myr and 50Myr after its birth. Trailing and leading trails of asteroids and comets along the Suns orbit in the Galactic potential are the by-product of the formation of the Oort cloud. These arms are composed of material that became unbound from the Solar System when the Oort cloud formed. Today, the bulk of the material in the Oort cloud ($sim 70$%) originates from the region in the circumstellar disk that was located between $sim 15$,au and $sim 35$,au, near the current location of the ice giants and the Centaur family of asteroids. According to our simulations, this population is eradicated if the ice-giant planets are born in orbital resonance. Planet migration or chaotic orbital reorganization occurring while the Solar System is still a cluster member is, according to our model, inconsistent with the presence of the Oort cloud. About half the inner Oort cloud, between 100 and $10^4$,au, and a quarter of the material in the outer Oort cloud, $apgt 10^4$,au, could be non-native to the Solar System but was captured from free-floating debris in the cluster or from the circumstellar disk of other stars in the birth cluster.
In this paper, we present a study about the dynamical effects of the Galaxy on the external region of the Oort Cloud. The aims of this paper are: i) to determine an outer limit for the Oort Cloud; and ii) to analyse the dynamical behaviour of the most external objects of the Cloud and how they are ejected from the Solar System. This is undertaken by following the temporal evolution of massless test particles in the Galactic environment of the solar neighbourhood. Here we show that the effect of the perturbations from the Galactic tide in the particles is similar to that find for the evolution of wide binary stars population. Moreover, in the Oort Cloud we found a dynamical structure around 10 5 au conformed by objects unbound of the Sun. This structure allows us to define a transition region of stability and an outer boundary for the Oort Cloud, and it is also in agreement with previous results about the disruption of wide binary stars.
It is possible that the formation of the Oort Cloud dates back to the earliest epochs of solar system history. At that time, the Sun was almost certainly a member of the stellar cluster, where it was born. Since the solar birth cluster is likely to have been massive (1000--10 000 Msol), and therefore long-lived, an issue concerns the survival of such a primordial Oort Cloud. We have investigated this issue by simulating the orbital evolution of Oort Cloud comets for several hundred Myr, assuming the Sun to start its life as a typical member of such a massive cluster. We have devised a synthetic representation of the relevant dynamics, where the cluster potential is represented by a King model, and about 20 close encounters with individual cluster stars are selected and integrated based on the solar orbit and the cluster structure. Thousands of individual simulations are made, each including 3 000 comets with orbits with three different initial semi-major axes. Practically the entire initial Oort Cloud is found to be lost for our choice of semi-major axes (5 000--20 000 au), independent of the cluster mass, although the chance of survival is better for the smaller cluster, since in a certain fraction of the simulations the Sun orbits at relatively safe distances from the dense cluster centre. For the range of birth cluster sizes that we investigate, a primordial Oort Cloud will likely survive only as a small inner core with semi-major axes < 3 000 au. Such a population of comets would be inert to orbital diffusion into an outer halo and subsequent injection into observable orbits. Some mechanism is therefore needed to accomplish this transfer, in case the Oort Cloud is primordial and the birth cluster did not have a low mass. From this point of view, our results lend some support to a delayed formation of the Oort Cloud, that occurred after the Sun had left its birth cluster.
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