| 1. |
- Perryman, M.A.P., et al.
(författare)
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The Hipparcos Catalogue
- 2009
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Ingår i: Astronomy & Astrophysics. ; 500, s. 501-504
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Tidskriftsartikel (refereegranskat)
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| 2. |
- Fouchard, M., et al.
(författare)
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Distribution of long-period comets : comparison between simulations and observations
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 604
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Tidskriftsartikel (refereegranskat)abstract
- Aims: This paper is devoted to a comparison between observations and simulations of the so-called Oort spike formed by the "new" observable long-period comets.Methods: The synthetic distributions of observable comets come from the propagation of a huge sample of objects during the age of the solar system that were initially in a proto-Oort cloud, which was flattened around the ecliptic and had perihelia in the region of Uranus and Neptune. For the known new long-period comets, two samples were used, one that is assumed to be complete, and the comets of the other exclusively come from the Warsaw catalog of comets. The original orbital energy of the comets in this catalog is more reliable.Results: Considering comets with a perihelion distance smaller than 4 AU, for which one of our samples of known comets can be assumed to be complete, the comparison shows small but significant differences in the orbital energy distribution and in the proportion of retrograde comets. When we extend the limiting perihelion distance to 10 AU, the observed samples are obviously strongly incomplete. The synthetic distribution shows that the number of observable comets per year and per perihelion distance unit is proportional to q(1.09) for q < 4 AU and proportional to q(2.13) for 6 < q < 10 AU. The increase for q > 6 AU comes from comets that were already within the Jupiter-Saturn barrier (q < 15 AU) at their previous perihelion passage (which we call creepers and Kaib and Quinn creepers), with original semi-major axes generally smaller than 20 000 AU.Conclusions: To explain the small but significant differences between our synthetic sample and the known comets for a perihelion distance smaller than 4 AU, different hypotheses are proposed: a still erroneous value of the original orbital energy in the observed sample, a higher density of low-mass stars in the actual solar neighborhood, a ninth planet, and obviously the initial population of objects from which the synthetic distributions are derived.
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| 3. |
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| 4. |
- Fouchard, M., et al.
(författare)
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On the present shape of the Oort cloud and the flux of "new" comets
- 2017
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 292, s. 218-233
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Fouchard, M., et al.
(författare)
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Planetary perturbations for Oort cloud comets : II. Implications for the origin of observable comets
- 2014
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 231, s. 110-121
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Tidskriftsartikel (refereegranskat)abstract
- We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b] Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. We concentrate on the production of observable comets, reaching for the first time a perihelion within 5 AU of the Sun. We distinguish between four categories, depending on whether the comet jumps across, or creeps through, the Jupiter-Saturn barrier (perihelion distances between 5 and 15 AU), and whether the orbit leading to the observable perihelion is preceded by a major planetary perturbation or not. For reasons explained in the paper, we call the strongly perturbed comets "Kaib-Quinn comets". We thus derive a synthetic picture of the Oort spike, from which we draw two main conclusions regarding the full dynamical model. One is that 2/3 of the observable comets are injected with the aid of a planetary perturbation at the previous perihelion passage, and about half of the observable comets are of the Kaib-Quinn type. The other is that the creepers dominate over the jumpers. Due to this fact, the spike peaks at only 31000 AU, and the majority of new comets have semi-major axes less than this value. The creepers show a clear preference for retrograde orbits as a consequence of the need to avoid untimely, planetary ejection before becoming observable. Thus, the new comets should have a 60/40 preference for retrograde against prograde orbits in apparent conflict with observations. However, both these and other results depend on our model assumptions regarding the initial structure of the Oort cloud, which is isotropic in shape and has a relatively steep energy distribution. We also find that they depend on the details of the past history of external perturbations including GMC encounters, and we provide special discussions of those issues.
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| 6. |
- Fouchard, M., et al.
(författare)
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Planetary perturbations for Oort cloud comets : III. Evolution of the cloud and production of centaurs and Halley type comets
- 2014
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 231, s. 99-109
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Tidskriftsartikel (refereegranskat)abstract
- We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b] Icarus, in press), the planetary perturbations experienced each time the comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled comets, which evolve into semi-major axes less than 1000 AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of comets within the cloud. Thus, comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled comets, which form a source of centaurs and Halley type comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.
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| 7. |
- Fouchard, M., et al.
(författare)
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Planetary perturbations for Oort Cloud comets. I. Distributions and dynamics
- 2013
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 222:1, s. 20-31
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Tidskriftsartikel (refereegranskat)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 < 15 AU 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.
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| 8. |
- Fouchard, M., et al.
(författare)
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The key role of massive stars in Oort cloud comet dynamics
- 2011
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 214:1, s. 334-347
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Tidskriftsartikel (refereegranskat)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 100 Myr 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.
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| 9. |
- Fouchard, M., et al.
(författare)
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The last revolution of new comets : the role of stars and their detectability
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 535, s. A86-
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Tidskriftsartikel (refereegranskat)abstract
- Context. This work is a follow-up of a previous study, where we simulated the dynamical evolution of the Oort Cloud over 5 Gyr with special attention to the injection of comets into observable orbits.Aims. We wish to clarify how comet injection operates with two types of perturbers: Galactic tides and passing stars. We illustrate why attempts to identify the stars that might have played an important role in injecting the observed new Oort Cloud comets are as yet unlikely to succeed, and investigate how large an improvement can be expected from the Gaia mission.Methods. We simulate a 5 Gyr time span, concentrating on the injections found during the last 3 Gyr by extracting detailed information about the last revolution of the injected comets. We analyse the contributions of both the Galactic tides and the stars separately, and assess their importance as a function of the semi-major axis of the comets. We also compute the distances and motions of the perturbing stars at the time the comets reach their perihelia and thus estimate their observability.Results. By studying more than 20 000 injected comets, we determine how the likelihood of tidal and stellar injections varies with the semi-major axis. We establish the range of semi-major axis for which a real-time synergy between stellar and tidal perturbations is important. We find how many perturbing stars could be identified using HIPPARCOS and Gaia data, and how the dynamics of injections would change, if only the observable stars were acting.Conclusions. The number of injected comets peaks at a semi-major axis (a) of about 33 000 AU but the comets spread over a wide range around this value. The tides are unable to inject any comets at a < 23 000 AU but would be able to inject almost all of them at a > 50 000 AU. The real-time synergy is found to extend between a similar to 15 000 AU and a similar to 45 000 AU and to be the main contributor at a similar to 25 000 AU. Stellar perturbations make important contributions at all semi-major axes. On the basis of HIPPARCOS data, only a minority of the stars that may contribute to comet injections are detectable, since most stars have escaped to distances beyond the HIPPARCOS detection limit. For Gaia, on the other hand, a large majority of the perturbing stars will be both identifiable and measurable.
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| 10. |
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| 11. |
- Rickman, Hans, et al.
(författare)
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Gaia and the new comets from the Oort cloud
- 2012
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 73:1, s. 124-129
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Tidskriftsartikel (refereegranskat)abstract
- We use Oort cloud simulations covering a time span of 5 Gyr, 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.
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