58 research outputs found
Effects of perturbing forces on the orbital stability of planetary systems
We consider dynamical effects of additional perturbative forces due to the
non-point mass nature of stars and planets: effects such as quadrupolar
distortion and tidal friction in the systems of exo-planets. It is shown that
these forces should not be neglected while modelling the dynamics of planetary
systems, especially taking into account the undefined real masses of the
planets due to unknown orbital inclinations and the unsatisfactory application
of Keplerian fits to the radial velocity data in multiple planetary systems.Comment: 5 pages, 2 figures, accepted by ApJ Le
Global dynamics and stability limits for planetary systems around HD 12661, HD 38529, HD 37124 and HD 160691
In order to distinguish between regular and chaotic planetary orbits we apply
a new technique called MEGNO in a wide neighbourhood of orbital parameters
determined using standard two-body Keplerian fits for HD 12661, HD 38529, HD
37124 and HD 160691 planetary systems. We show that the currently announced
orbital parameters place these systems in very different situations from the
point of view of dynamical stability. While HD 38529 and HD 37124 are located
within large stability zones in the phase space around their determined orbits,
the preliminary orbits in HD 160691 are highly unstable. The orbital parameters
of the HD 12661 planets are located in a border region between stable and
unstable dynamical regimes, so while its currently determined orbital
parameters produce stable regular orbits, a minor change within the margin of
error of just one parameter may result in a chaotic dynamical system.Comment: 12 pages, 3 figures, accepted ApJ, revised version following the
referee's repor
Towards Multiple-Star Population Synthesis
The multiplicities of stars, and some other properties, were collected
recently by Eggleton & Tokovinin, for the set of 4559 stars with Hipparcos
magnitude brighter than 6.0 (4558 excluding the Sun). In this paper I give a
numerical recipe for constructing, by a Monte Carlo technique, a theoretical
ensemble of multiple stars that resembles the observed sample. Only
multiplicities up to 8 are allowed; the observed set contains only
multiplicities up to 7. In addition, recipes are suggested for dealing with the
selection effects and observational uncertainties that attend the determination
of multiplicity. These recipes imply, for example, that to achieve the observed
average multiplicity of 1.53, it would be necessary to suppose that the real
population has an average multiplicity slightly over 2.0.
This numerical model may be useful for (a) comparison with the results of
star and star cluster formation theory, (b) population synthesis that does not
ignore multiplicity above 2, and (c) initial conditions for dynamical cluster
simulations
Improved equations for eccentricity generation in hierarchical triple systems
In a series of papers, we developed a technique for estimating the inner
eccentricity in hierarchical triple systems, with the inner orbit being
initially circular. However, for certain combinations of the masses and the
orbital elements, the secular part of the solution failed. In the present
paper, we derive a new solution for the secular part of the inner eccentricity,
which corrects the previous weakness. The derivation applies to hierarchical
triple systems with coplanar and initially circular orbits. The new formula is
tested numerically by integrating the full equations of motion for systems with
mass ratios from 10^(-3) to 10^(3). We also present more numerical results for
short term eccentricity evolution, in order to get a better picture of the
behaviour of the inner eccentricity.Comment: Accepted for publication in MNRA
Conditions of Dynamical Stability for the HD 160691 Planetary System
The orbits in the HD 160691 planetary system at first appeared highly
unstable, but using the MEGNO and FLI techniques of global dynamics analysis in
the orbital parameter space we have found a stabilizing mechanism that could be
the key to its existence. In order to be dynamically stable, the HD 160691
planetary system has to satisfy the following conditions: (1) a 2:1 mean motion
resonance, combined with (2) an apsidal secular resonance in (3) a
configuration where the two apsidal lines are
anti-aligned, and (4) specific conditions on the respective sizes of the
eccentricities (high eccentricity for the outer orbit is in particular the most
probable necessary condition). More generally, in this original orbital
topology, where the resonance variables and librate
about while librates about , the HD
160691 system and its mechanism have revealed aspects of the 2:1 orbital
resonances that have not been observed nor analyzed before. The present
topology combined with the 2:1 resonance is indeed more wide-ranging than the
particular case of the HD 160691 planetary system. It is a new theoretical
possibility suitable for a stable regime despite relatively small semi-major
axes with respect to the important masses in interactions.Comment: 21 pages, 8 figures, 1 table, accepted version to ApJ (31 Jul 2003
Planets in binary systems: is the present configuration indicative of the formation process?
The present dynamical configuration of planets in binary star systems may not
reflect their formation process since the binary orbit may have changed in the
past after the planet formation process was completed. An observed binary
system may have been part of a former hierarchical triple that became unstable
after the planets completed their growth around the primary star.
Alternatively, in a dense stellar environment even a single stellar encounter
between the star pair and a singleton may singificantly alter the binary orbit.
In both cases the planets we observe at present would have formed when the
dynamical environment was different from the presently observed one.
We have numerically integrated the trajectories of the stars (binary plus
singleton) and of test planets to investigate the abovementioned mechanisms.
Our simulations show that the circumstellar environment during planetary
formation around the primary was gravitationally less perturbed when the binary
was part of a hierarchical triple because the binary was necessarely wider and,
possibly, less eccentric. This circumstance has consequences for the planetary
system in terms of orbital spacing, eccentricity, and mass of the individual
planets. Even in the case of a single stellar encounter the present appearance
of a planetary system in a binary may significantly differ from what it had
while planet formation was ongoing. However, while in the case of instability
of a triple the trend is always towards a tighter and more eccentric binary
system, when a single stellar encounter affects the system the orbit of the
binary can become wider and be circularized.Comment: 5 pages, 5 figures Accepted for publication on A&
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