316 research outputs found
Analytic orbit propagation for transiting circumbinary planets
The herein presented analytical framework fully describes the motion of
coplanar systems consisting of a stellar binary and a planet orbiting both
stars on orbital as well as secular timescales. Perturbations of the Runge-Lenz
vector are used to derive short period evolution of the system, while octupole
secular theory is applied to describe its long term behaviour. A post Newtonian
correction on the stellar orbit is included. The planetary orbit is initially
circular and the theory developed here assumes that the planetary eccentricity
remains relatively small (e_2<0.2). Our model is tested against results from
numerical integrations of the full equations of motion and is then applied to
investigate the dynamical history of some of the circumbinary planetary systems
discovered by NASA's Kepler satellite. Our results suggest that the formation
history of the systems Kepler-34 and Kepler-413 has most likely been different
from the one of Kepler-16, Kepler-35, Kepler-38 and Kepler-64, since the
observed planetary eccentricities for those systems are not compatible with the
assumption of initially circular orbits.Comment: Accepted for publication in Ap
Sublimation-induced orbital perturbations of extrasolar active asteroids and comets: application to white dwarf systems
The metal budgets in some white dwarf (WD) atmospheres reveal that
volatile-rich circumstellar bodies must both exist in extrasolar systems and
survive the giant branch phases of stellar evolution. The resulting behaviour
of these active asteroids or comets which orbit WDs is not well-understood, but
may be be strongly influenced by sublimation due to stellar radiation. Here we
develop a model, generally applicable to any extrasolar system with a main
sequence or WD star, that traces sublimation-induced orbital element changes in
approximately km-sized extrasolar minor planets and comets traveling within
hundreds of au. We derive evolution equations on orbital timescales and for
arbitrarily steep power-law sublimation dependencies on distance, and place our
model in a Solar system context. We also demonstrate the importance of coupling
sublimation and general relativity, and the orbital consequences of outgassing
in arbitrary directions. We prove that nongravitational accelerations alone
cannot result in orbit crossing with the WD disruption radius, but may shrink
or expand the orbit by up to several au after a single pericentre passage,
potentially affecting subsequent interactions with remnant debris and planets.
Our analysis suggests that extant planets must exist in polluted WD systems.Comment: Accepted for publication in MNRA
Detectability of Earth-like Planets in Circumstellar Habitable Zones of Binary Star Systems with Sun-like Components
Given the considerable percentage of stars that are members of binaries or
stellar multiples in the Solar neighborhood, it is expected that many of these
binaries host planets, possibly even habitable ones. The discovery of a
terrestrial planet in the alpha Centauri system supports this notion. Due to
the potentially strong gravitational interaction that an Earth-like planet may
experience in such systems, classical approaches to determining habitable
zones, especially in close S-Type binary systems, can be rather inaccurate.
Recent progress in this field, however, allows to identify regions around the
star permitting permanent habitability. While the discovery of alpha Cen Bb has
shown that terrestrial planets can be detected in solar-type binary stars using
current observational facilities, it remains to be shown whether this is also
the case for Earth analogues in habitable zones. We provide analytical
expressions for the maximum and RMS values of radial velocity and astrometric
signals, as well as transit probabilities of terrestrial planets in such
systems, showing that the dynamical interaction of the second star with the
planet may indeed facilitate the planets detection. As an example, we discuss
the detectability of additional Earth-like planets in the averaged, extended,
and permanent habitable zones around both stars of the alpha Centauri system.Comment: accepted for publication in The Astrophysical Journa
The orbital evolution of asteroids, pebbles and planets from giant branch stellar radiation and winds
The discovery of over 50 planets around evolved stars and more than 35 debris
discs orbiting white dwarfs highlight the increasing need to understand small
body evolution around both early and asymptotic giant branch (GB) stars.
Pebbles and asteroids are susceptible to strong accelerations from the intense
luminosity and winds of GB stars. Here, we establish equations that can model
time-varying GB stellar radiation, wind drag and mass loss. We derive the
complete three-dimensional equations of motion in orbital elements due to (1)
the Epstein and Stokes regimes of stellar wind drag, (2) Poynting-Robertson
drag, and (3) the Yarkovsky drift with seasonal and diurnal components. We
prove through averaging that the potential secular eccentricity and inclination
excitation due to Yarkovsky drift can exceed that from Poynting-Robertson drag
and radiation pressure by at least three orders of magnitude, possibly flinging
asteroids which survive YORP spin-up into a widely dispersed cloud around the
resulting white dwarf. The GB Yarkovsky effect alone may change an asteroid's
orbital eccentricity by ten per cent in just one Myr. Damping perturbations
from stellar wind drag can be just as extreme, but are strongly dependent on
the highly uncertain local gas density and mean free path length. We conclude
that GB radiative and wind effects must be considered when modelling the
post-main-sequence evolution of bodies smaller than about 1000 km.Comment: Corrected Fig. 3 and Eq. 14 (In Press, MNRAS
Adjoint-based mixing enhancement for binary fluids
Mixing is a fundamental fluid process that dominates {a} great many natural phenomena and is present in a wide variety of industrial applications. Therefore, studying the characteristics and optimisation of this process may lead to a significant impact in many fields.
This thesis presents an analytical and computational framework for optimising fluid mixing processes using embedded stirrers based on a non-linear direct-adjoint looping approach. The governing equations are the non-linear Navier-Stokes equations, augmented by an evolution equation for a passive scalar, which are solved by a Fourier-based spectral method. Stirrers are embedded in the computational domain by a Brinkman-penalisation technique, and shape and path gradients for the stirrers are computed from the adjoint solution.
The relationship between this penalisation approach and the adjoint will be examined through the derivation of a dual system of equations, and three different optimisation scenarios of increasing complexity, each focusing on different optimisation parameters, are considered.
Within the limits of the parameterisations of the geometry and the externally imposed bounds, significant improvements in mixing efficiency are achieved in all cases.Open Acces
Impact flux of asteroids and water transport to the habitable zone in binary star systems
By now, observations of exoplanets have found more than 50 binary star
systems hosting 71 planets. We expect these numbers to increase as more than
70% of the main sequence stars in the solar neighborhood are members of binary
or multiple systems. The planetary motion in such systems depends strongly on
both the parameters of the stellar system (stellar separation and eccentricity)
and the architecture of the planetary system (number of planets and their
orbital behaviour). In case a terrestrial planet moves in the so-called
habitable zone (HZ) of its host star, the habitability of this planet depends
on many parameters. A crucial factor is certainly the amount of water. We
investigate in this work the transport of water from beyond the snow-line to
the HZ in a binary star system and compare it to a single star system
Disc-protoplanet interaction Influence of circumprimary radiative discs on self-gravitating protoplanetary bodies in binary star systems
Context. More than 60 planets have been discovered so far in systems that
harbour two stars, some of which have binary semi-major axes as small as 20 au.
It is well known that the formation of planets in such systems is strongly
influenced by the stellar components, since the protoplanetary disc and the
particles within are exposed to the gravitational influence of the binary.
However, the question on how self-gravitating protoplanetary bodies affect the
evolution of a radiative, circumprimary disc is still open. Aims. We present
our 2D hydrodynamical GPU-CPU code and study the interaction of several
thousands of self-gravitating particles with a viscous and radiative
circumprimary disc within a binary star system. To our knowledge this program
is the only one at the moment that is capable to handle this many particles and
to calculate their influence on each other and on the disc. Methods. We
performed hydrodynamical simulations of a circumstellar disc assuming the
binary system to be coplanar. Our gridbased staggered mesh code relies on ideas
from ZEUS-2D, where we implemented the FARGO algorithm and an additional energy
equation for the radiative cooling according to opacity tables. To treat
particle motion we used a parallelised version of the precise Bulirsch - Stoer
algorithm. Four models in total where computed taking into account (i) only
N-body interaction, (ii) N-body and disc interaction, (iii) the influence of
computational parameters (especially smoothing) on N-body interaction, and (iv)
the influence of a quiet low-eccentricity disc while running model (ii). The
impact velocities where measured at two different time intervals and were
compared. Results. We show that the combination of disc- and N-body
self-gravity can have a significant influence on the orbit evolution of roughly
Moon sized protoplanets
Spectral properties of near-Earth and Mars-crossing asteroids using Sloan photometry
The nature and origin of the asteroids orbiting in near-Earth space,
including those on a potentially hazardous trajectory, is of both scientific
interest and practical importance. We aim here at determining the taxonomy of a
large sample of near-Earth (NEA) and Mars-crosser (MC) asteroids and analyze
the distribution of these classes with orbit. We use this distribution to
identify their source regions and to study the strength of planetary encounters
to refresh asteroid surfaces. We measure the photometry of these asteroids over
four filters at visible wavelengths on images taken by the SDSS. These colors
are used to classify the asteroids into a taxonomy consistent with the widely
used Bus-DeMeo taxonomy based on spectroscopy. We report here on the taxonomic
classification of 206 NEAs and 776 MCs determined from SDSS photometry,
representing an increase of 40% and 663% of known taxonomy classifications in
these populations. Using the source region mapper by Greenstreet et al. (2012),
we compare the taxonomic distribution among NEAs and main-belt asteroids of
similar diameters. Both distributions agree at the few percent level for the
inner part of the Main Belt and we confirm this region as a main source of
near-Earth objects. The effect of planetary encounters on asteroid surfaces are
also studied by developing a simple model of forces acting on a surface grain
during planetary encounter, which provides the minimum distance at which a
close approach should occur to trigger resurfacing events. By integrating
numerically the orbit of the 519 S-type and 46 Q-type asteroids back in time
and monitoring their encounter distance with planets, we seek to understand the
conditions for resurfacing events. The population of Q-type is found to present
statistically more encounters with Venus and the Earth than S-types, although
both types present the same amount of encounters with Mars.Comment: Accepted for publication in Icarus. 45 pages, 11 figures, 4 tables, 2
tables in appendix (supplementary material
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