336 research outputs found
Understanding the assembly of Kepler's compact planetary systems
The Kepler mission has recently discovered a number of exoplanetary systems,
such as Kepler-11 and Kepler-32, in which ensembles of several planets are
found in very closely packed orbits (often within a few percent of an AU of one
another). These compact configurations present a challenge for traditional
planet formation and migration scenarios. We present a dynamical study of the
assembly of these systems, using an N-body method which incorporates a
parametrized model of planet migration in a turbulent protoplanetary disc. We
explore a wide parameter space, and find that under suitable conditions it is
possible to form compact, close-packed planetary systems via traditional
disc-driven migration. We find that simultaneous migration of multiple planets
is a viable mechanism for the assembly of tightly-packed planetary systems, as
long as the disc provides significant eccentricity damping and the level of
turbulence in the disc is modest. We discuss the implications of our preferred
parameters for the protoplanetary discs in which these systems formed, and
comment on the occurrence and significance of mean-motion resonances in our
simulations.Comment: 12 pages, 4 figures, 2 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Zero Lattice Sound
We study the N_f-flavor Gross-Neveu model in 2+1 dimensions with a baryon
chemical potential mu, using both analytical and numerical methods. In
particular, we study the self-consistent Boltzmann equation in the Fermi liquid
framework using the quasiparticle interaction calculated to O(1/N_f), and find
solutions for zero sound propagation for almost all mu > mu_c, the critical
chemical potential for chiral symmetry restoration. Next we present results of
a numerical lattice simulation, examining temporal correlation functions of
mesons defined using a point-split interpolating operator, and finding evidence
for phonon-like behaviour characterised by a linear dispersion relation in the
long wavelength limit. We argue that our results provide the first evidence for
a collective excitation in a lattice simulation.Comment: 18 pages, 6 figure
Stability of the Black Hole Horizon and the Landau Ghost
The stability of the black hole horizon is demanded by both cosmic censorship
and the generalized second law of thermodynamics. We test the consistency of
these principles by attempting to exceed the black hole extremality condition
in various process in which a U(1) charge is added to a nearly extreme
Reissner--Nordstr\"om black hole charged with a {\it different\/} type of U(1)
charge. For an infalling spherical charged shell the attempt is foiled by the
self--Coulomb repulsion of the shell. For an infalling classical charge it
fails because the required classical charge radius exceeds the size of the
black hole. For a quantum charge the horizon is saved because in order to avoid
the Landau ghost, the effective coupling constant cannot be large enough to
accomplish the removal.Comment: 12 pages, RevTe
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