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Explores the range of applicability of Monte Carlo modelling by reconsidering in depth the Monte Carlo simulations of Froeschle & Rickman aimed at the study of the orbital evolution of Jupiter family comets. Analysis of the computation of perturbation distributions; Necessity of introducing the Spurious Perturbations Removal procedure; Celestial mechanics.

Long term evolution of an initial set of 10 7 Oort cloud comets is performed for the age of the solar system taking into account the action of passing stars using 10 different sequences of stellar encounters, Galactic tides and the gravity of the giant planets. The initial conditions refer to a disk-shaped Oort cloud precursor, concentrated toward the ecliptic with perihelia in the region of Uranus and Neptune. Our results show that the shape of the Oort cloud quickly reach a kind of steady state beyond a semi-major axis greater than about 2000 AU (this threshold depending on the evolution time-span), with a Boltzmann distribution of the orbital energy. The stars act in an opposite way to what was found in previous papers, that is they emptied an initial Tidal Active Zone that is overfilled with respect to the isotropic case. Consequently, the inclusion of stellar perturbations strongly affect the shape of the Oort spike. On the contrary, the Oort spike shape appears to be poorly dependent on the stellar sequences used, whereas the total flux of observable comets and the proportion of retrograde comets for the inner part of the spike are significantly dependent of it. Then it has been highlighted that the total flux, the shape of the Oort spike and the shape of the final Oort cloud are almost independent of the initial distribution of orbital energy considered. [ABSTRACT FROM AUTHOR]

Abstract: This paper is the first in a series, where we aim to model the injection of comets from the Oort Cloud so well that the shape of the energy distribution of long-period comets (i.e., the distribution of reciprocal semi-major axis) together with the observed rate of perihelion passages can be used to make serious inferences about the population size and energy distribution of the cloud. Here we explore the energy perturbations caused by the giant planets on long-period comets with perihelia inside or near the planetary system. We use a simplified dynamical model to integrate such perturbations for large samples of fictitious comets and analyse the statistics of the outcomes. After demonstrating the sensitivity of derived parameters to the sample size, when close encounters are involved, we derive a map of the RMS energy perturbation as a function of perihelion distance (q) and the cosine of the inclination (i), which compares well with the results of previous papers. We perform a critical analysis of the loss cone concept by deriving the “opacity” (chance of leaving the Oort spike by planetary perturbations per perihelion passage) as a function of q and cos i, concluding that the often made assumption of full opacity for q <15AU is seriously in error. While such a conclusion may also have been drawn from earlier studies, we provide the first full, quantitative picture. Moreover, we make a preliminary investigation of the long-term evolution of long-period comet orbits under the influence of planetary perturbations, neglecting the external effects of Galactic tides and stellar encounters. This allows us to make predictions about the production of decoupled objects like Halley-type comets and Centaurs from the injection of Oort Cloud comets, as well as of a related population of transneptunians deriving from the Oort Cloud with perihelia well detached from the planets. [Copyright &y& Elsevier]

Abstract: We use Oort cloud simulations covering a time span of 5Gyr, including the Galactic tides and stellar encounters and focussing on the last revolution of comets as they get injected into observable orbits, in order to analyze in detail the role of stars in those injections. We find this role to be very important in all parts of the cloud, so that most injected comets require the intervention of a star. Characterizing the stellar influence by the decrease of the perihelion distance, projected to the time of the next perihelion by means of tidal evolution, we identify the most efficient stars and study the properties of the corresponding encounters. We also judge the detectability of the culprit stars, responsible for the current arrival of new comets, by the Hipparcos and Gaia missions based on the magnitudes of the stars. Our main result is that the chances to detect and identify those culprits will be revolutionized by the Gaia data independent of which region of the cloud the comets come from. [Copyright &y& Elsevier]

Abstract: The effects of a sample of 1300 individual stellar encounters spanning a wide range of parameter values (mass, velocity and encounter distance) are investigated. Power law fits for the number of injected comets demonstrate the long range effect of massive stars, whereas light stars affect comets mainly along their tracks. Similarly, we show that the efficiency of a star to fill the phase space region of the Oort cloud where the Galactic tides are able to inject comets into the observable region – the so-called “tidally active zone” (TAZ) – is also strongly dependent on the stellar mass. Power laws similar to those for direct injection are obtained for the efficiency of stars to fill the TAZ. This filling of the tidally active zone is crucial for the long term flux of comets from the Oort cloud. Based on long-term Monte Carlo simulations using a constant Galactic tide and a constant flux of stellar encounters, but neglecting the detailed effects of planetary perturbations, we show that this flux essentially results from a two step mechanism: (i) the stellar injection of comets into the TAZ; and (ii) the tidal injection of TAZ comets into the loss cone. We find that single massive stars are able to induce “comet drizzles” – corresponding to an increase of the cometary flux of about 40% – which may last for more than 100Myr by filling the TAZ to a higher degree than normal. It appears that the stars involved in this process are the same that cause comet showers. [Copyright &y& Elsevier]