9,845 research outputs found
On the Survivability and Metamorphism of Tidally Disrupted Giant Planets: the Role of Dense Cores
A large population of planetary candidates in short-period orbits have been
found through transit searches. Radial velocity surveys have also revealed
several Jupiter-mass planets with highly eccentric orbits. Measurements of the
Rossiter-McLaughlin effect indicate some misaligned planetary systems. This
diversity could be induced by post-formation dynamical processes such as
planet-planet scattering, the Kozai effect, or secular chaos which brings
planets to the vicinity of their host stars. In this work, we propose a novel
mechanism to form close-in super-Earths and Neptune-like planets through the
tidal disruption of giant planets as a consequence of these dynamical
processes. We model the core-envelope structure of giant planets with composite
polytropes. Using three-dimensional hydrodynamical simulations of close
encounters between planets and their host stars, we find that the presence of a
core with a mass more than ten Earth masses can significantly increase the
fraction of envelope which remains bound to it. After the encounter, planets
with cores are more likely to be retained by their host stars in contrast with
previous studies which suggested that coreless planets are often ejected. As a
substantial fraction of their gaseous envelopes is preferentially lost while
the dense incompressible cores retain most of their original mass, the
resulting metallicity of the surviving planets is increased. Our results
suggest that some gas giant planets can be effectively transformed into either
super-Earths or Neptune-like planets after multiple close stellar passages.
Finally, we analyze the orbits and structure of known planets and Kepler
candidates and find that our model is capable producing some of the
shortest-period objects.Comment: Accepted for publication in ApJ. 15 pages, 9 figures, 3 tables. Two
movies at http://youtu.be/jHxPKAEgFic and http://youtu.be/QXqkS0vDi5
Embryo impacts and gas giant mergers II: Diversity of Hot Jupiters' internal structure
We consider the origin of compact, short-period, Jupiter-mass planets. We
propose that their diverse structure is caused by giant impacts of embryos and
super-Earths or mergers with other gas giants during the formation and
evolution of these hot Jupiters. Through a series of numerical simulations, we
show that typical head-on collisions generally lead to total coalescence of
impinging gas giants. Although extremely energetic collisions can disintegrate
the envelope of gas giants, these events seldom occur. During oblique and
moderately energetic collisions, the merger products retain higher fraction of
the colliders' cores than their envelopes. They can also deposit considerable
amount of spin angular momentum to the gas giants and desynchronize their spins
from their orbital mean motion. We find that the oblateness of gas giants can
be used to infer the impact history. Subsequent dissipation of stellar tide
inside the planets' envelope can lead to runaway inflation and potentially a
substantial loss of gas through Roche-lobe overflow. The impact of super-Earths
on parabolic orbits can also enlarge gas giant planets' envelope and elevates
their tidal dissipation rate over 100 Myr time scale. Since giant
impacts occur stochastically with a range of impactor sizes and energies, their
diverse outcomes may account for the dispersion in the mass-radius relationship
of hot Jupiters.Comment: 19 pages, 7 figures, 7 tables. Accepted for publication in MNRA
Experimental Comparisons of Derivative Free Optimization Algorithms
In this paper, the performances of the quasi-Newton BFGS algorithm, the
NEWUOA derivative free optimizer, the Covariance Matrix Adaptation Evolution
Strategy (CMA-ES), the Differential Evolution (DE) algorithm and Particle Swarm
Optimizers (PSO) are compared experimentally on benchmark functions reflecting
important challenges encountered in real-world optimization problems.
Dependence of the performances in the conditioning of the problem and
rotational invariance of the algorithms are in particular investigated.Comment: 8th International Symposium on Experimental Algorithms, Dortmund :
Germany (2009
Collapse of Vacuum Bubbles in a Vacuum
Motivated by the discovery of a plenitude of metastable vacua in a string
landscape and the possibility of rapid tunneling between these vacua, we
revisit the dynamics of a false vacuum bubble in a background de Sitter
spacetime. We find that there exists a large parameter space that allows the
bubble to collapse into a black hole or to form a wormhole. This may have
interesting implications to inflationary physics.Comment: 8 pages including 6 figures, LaTex; references adde
Regression Depth and Center Points
We show that, for any set of n points in d dimensions, there exists a
hyperplane with regression depth at least ceiling(n/(d+1)). as had been
conjectured by Rousseeuw and Hubert. Dually, for any arrangement of n
hyperplanes in d dimensions there exists a point that cannot escape to infinity
without crossing at least ceiling(n/(d+1)) hyperplanes. We also apply our
approach to related questions on the existence of partitions of the data into
subsets such that a common plane has nonzero regression depth in each subset,
and to the computational complexity of regression depth problems.Comment: 14 pages, 3 figure
Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz)(ClO)
Cu(pz)(ClO) (with pz denoting pyrazine CHN) is a
two-dimensional spin-1/2 square-lattice antiferromagnet with =
4.24 K. Due to a persisting focus on the low-temperature magnetic properties,
its room-temperature structural and physical properties caught no attention up
to now. Here we report a study of the structural features of
Cu(pz)(ClO) in the paramagnetic phase, up to 330 K. By employing
magnetization, specific heat, Cl nuclear magnetic resonance, and neutron
diffraction measurements, we provide evidence of a second-order phase
transition at = 294 K, not reported before. The absence of a
magnetic ordering across in the magnetization data, yet the
presence of a sizable anomaly in the specific heat, suggest a structural
order-to-disorder type transition. NMR and neutron-diffraction data corroborate
our conjecture, by revealing subtle angular distortions of the pyrazine rings
and of ClO counteranion tetrahedra, shown to adopt a configuration of
higher symmetry above the transition temperature.Comment: 10 pages, 12 figure
Constraints on the Abundance of Highly Ionized Proto-Cluster Regions from the Absence of Large Voids in the Lyman Alpha Forest
Energetic feedback processes during the formation of galaxy clusters may have
heated and ionized a large fraction of the intergalactic gas in proto-cluster
regions. When such a highly ionized hot ``super-bubble'' falls along the
sightline to a background quasar, it would be seen as a large void, with little
or no absorption, in the Lyman alpha forest. We examine the spectra of 137
quasars in the Sloan Digital Sky Survey, to search for such voids, and find no
clear evidence of their existence. The size distribution of voids in the range
5-70 Angstrom (corresponding to physical sizes of approximately 3-35 comoving
Mpc/h) is consistent with the standard model for the Lyman alpha forest without
additional hot bubbles. We adapt a physical model for HII bubble growth during
cosmological reionization (Furlanetto, Zaldarriaga and Hernquist 2004), to
describe the expected size-distribution of hot super-bubbles at redshift around
z = 3. This model incorporates the conjoining of bubbles around individual
neighboring galaxies. Using the non-detection of voids, we find that models in
which the volume filling factor of hot bubbles exceeds approximately 20 percent
at z=3 can be ruled out, primarily because they overproduce the number of large
(40-50 Angstrom) voids. We conclude that any pre-heating mechanism that
explains galaxy cluster observations must avoid heating the low-density gas in
the proto-cluster regions, either by operating relatively recently (z<3) or by
depositing entropy in the high-density regions.Comment: submitted to ApJ, 9 emulateapj pages with 3 figure
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