274 research outputs found
Migration of extrasolar planets to large orbital radii
Observations of structure in circumstellar debris discs provide
circumstantial evidence for the presence of massive planets at large (several
tens of au) orbital radii, where the timescale for planet formation via core
accretion is prohibitively long. Here, we investigate whether a population of
distant planets can be produced via outward migration subsequent to formation
in the inner disc. Two possibilities for significant outward migration are
identified. First, cores that form early at radii of around 10 au can be
carried to larger radii via gravitational interaction with the gaseous disc.
This process is efficient if there is strong mass loss from the disc - either
within a cluster or due to photoevaporation from a star more massive than the
Sun - but does not require the extremely destructive environment found, for
example, in the core of the Orion Nebula. We find that, depending upon the disc
model, gas disc migration can yield massive planets (several Jupiter masses) at
radii of around 20-50 au. Second, interactions within multiple planet systems
can drive the outer planet into a large, normally highly eccentric orbit. A
series of scattering experiments suggests that this process is most efficient
for lower mass planets within systems of unequal mass ratio. This mechanism is
a good candidate for explaining the origin of relatively low mass giant planets
in eccentric orbits at large radii.Comment: MNRAS, in pres
Dynamical Interactions of Planetary Systems in Dense Stellar Environments
We study dynamical interactions of star--planet binaries with other single
stars. We derive analytical cross sections for all possible outcomes, and
confirm them with numerical scattering experiments. We find that a wide mass
ratio in the binary introduces a region in parameter space that is inaccessible
to comparable-mass systems, in which the nature of the dynamical interaction is
fundamentally different from what has traditionally been considered in the
literature on binary scattering. We study the properties of the planetary
systems that result from the scattering interactions for all regions of
parameter space, paying particular attention to the location of the
"hard--soft" boundary. The structure of the parameter space turns out to be
significantly richer than a simple statement of the location of the
"hard--soft" boundary would imply. We consider the implications of our
findings, calculating characteristic lifetimes for planetary systems in dense
stellar environments, and applying the results to previous analytical studies,
as well as past and future observations. Recognizing that the system PSR
B1620-26 in the globular cluster M4 lies in the "new" region of parameter
space, we perform a detailed analysis quantifying the likelihood of different
scenarios in forming the system we see today.Comment: Accepted for publication in ApJ. Minor changes to reflect accepted
version. 14 pages, 14 figure
Head-On Collision of Neutron Stars As A Thought Experiment
The head-on collision of identical neutron stars from rest at infinity
requires a numerical simulation in full general relativity for a complete
solution. Undaunted, we provide a relativistic, analytic argument to suggest
that during the collision, sufficient thermal pressure is always generated to
support the hot remnant in quasi-static stable equilibrium against collapse
prior to slow cooling via neutrino emission. Our conclusion is independent of
the total mass of the progenitors and holds even if the remnant greatly exceeds
the maximum mass of a cold neutron star.Comment: to appear in Physical Review D (revtex, 3 figs, 5 pgs
LISA Sources in Globular Clusters
Globular clusters house a population of compact binaries that will be
interesting gravitational wave sources for LISA. We provide estimates for the
numbers of sources of several categories and discuss the sensitivity of LISA to
detecting these sources. The estimated total number of detectable sources
ranges from about 10 to about 1000 with gravitational wave frequencies above 1
mHz. These sources are typically undetectable by any other means and thus offer
an opportunity for doing true gravitational-wave astronomy. The detection of
these sources would provide information about both binary star evolution and
the dynamics of globular clusters.Comment: Contribution to Proceedings of 3rd LISA Symposium 7 pages, added
reference
Simulations for Multi-Object Spectrograph Planet Surveys
Radial velocity surveys for extra-solar planets generally require substantial
amounts of large telescope time in order to monitor a sufficient number of
stars. Two of the aspects which can limit such surveys are the single-object
capabilities of the spectrograph, and an inefficient observing strategy for a
given observing window. In addition, the detection rate of extra-solar planets
using the radial velocity method has thus far been relatively linear with time.
With the development of various multi-object Doppler survey instruments, there
is growing potential to dramatically increase the detection rate using the
Doppler method. Several of these instruments have already begun usage in large
scale surveys for extra-solar planets, such as FLAMES on the VLT and Keck ET on
the Sloan 2.5m wide-field telescope.
In order to plan an effective observing strategy for such a program, one must
examine the expected results based on a given observing window and target
selection. We present simulations of the expected results from a generic
multi-object survey based on calculated noise models and sensitivity for the
instrument and the known distribution of exoplanetary system parameters. We
have developed code for automatically sifting and fitting the planet candidates
produced by the survey to allow for fast follow-up observations to be
conducted. The techniques presented here may be applied to a wide range of
multi-object planet surveys.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Formation of Short-Period Binary Pulsars in Globular Clusters
We present a new dynamical scenario for the formation of short-period binary
millisecond pulsars in globular clusters. Our work is motivated by the recent
observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc,
most neutron stars acquire binary companions through exchange interactions with
primordial binaries. The resulting systems have semimajor axes in the range
\~0.1-1 AU and neutron star companion masses ~1-3 Msun. For many of these
systems we find that, when the companion evolves off the main sequence and
fills its Roche lobe, the subsequent mass transfer is dynamically unstable.
This leads to a common envelope phase and the formation of short-period neutron
star - white dwarf binaries. For a significant fraction of these binaries, the
decay of the orbit due to gravitational radiation will be followed by a period
of stable mass transfer driven by a combination of gravitational radiation and
tidal heating of the companion. The properties of the resulting short-period
binaries match well those of observed binary pulsars in 47 Tuc.Comment: To appear in ApJ Letters, slightly abbreviated version with only
minor change
Migration of giant planets in planetesimal discs
Planets orbiting a planetesimal circumstellar disc can migrate inward from
their initial positions because of dynamical friction between planets and
planetesimals. The migration rate depends on the disc mass and on its time
evolution. Planets that are embedded in long-lived planetesimal discs, having
total mass of , can migrate inward a large distance and
can survive only if the inner disc is truncated or because of tidal interaction
with the star. In this case the semi-major axis, a, of the planetary orbit is
less than 0.1 AU. Orbits with larger are obtained for smaller value of the
disc mass or for a rapid evolution (depletion) of the disc. This model may
explain several of the orbital features of the giant planets that were
discovered in last years orbiting nearby stars as well as the metallicity
enhancement found in several stars associated with short-period planets.Comment: 21 pages; 6 encapsulated figures. Accepted by MNRA
Equilibrium, Stability and Orbital Evolution of Close Binary Systems
We present a new analytic study of the equilibrium and stability properties
of close binary systems containing polytropic components. Our method is based
on the use of ellipsoidal trial functions in an energy variational principle.
We consider both synchronized and nonsynchronized systems, constructing the
compressible generalizations of the classical Darwin and Darwin-Riemann
configurations. Our method can be applied to a wide variety of binary models
where the stellar masses, radii, spins, entropies, and polytropic indices are
all allowed to vary over wide ranges and independently for each component. We
find that both secular and dynamical instabilities can develop before a Roche
limit or contact is reached along a sequence of models with decreasing binary
separation. High incompressibility always makes a given binary system more
susceptible to these instabilities, but the dependence on the mass ratio is
more complicated. As simple applications, we construct models of double
degenerate systems and of low-mass main-sequence-star binaries. We also discuss
the orbital evolution of close binary systems under the combined influence of
fluid viscosity and secular angular momentum losses from processes like
gravitational radiation. We show that the existence of global fluid
instabilities can have a profound effect on the terminal evolution of
coalescing binaries. The validity of our analytic solutions is examined by
means of detailed comparisons with the results of recent numerical fluid
calculations in three dimensions.Comment: 37 pages, plain TeX, postscript figures and tables in separate
uufile, IAS AST 93/4
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