2,464 research outputs found
Exploring the Use of Virtual Worlds as a Scientific Research Platform: The Meta-Institute for Computational Astrophysics (MICA)
We describe the Meta-Institute for Computational Astrophysics (MICA), the
first professional scientific organization based exclusively in virtual worlds
(VWs). The goals of MICA are to explore the utility of the emerging VR and VWs
technologies for scientific and scholarly work in general, and to facilitate
and accelerate their adoption by the scientific research community. MICA itself
is an experiment in academic and scientific practices enabled by the immersive
VR technologies. We describe the current and planned activities and research
directions of MICA, and offer some thoughts as to what the future developments
in this arena may be.Comment: 15 pages, to appear in the refereed proceedings of "Facets of Virtual
Environments" (FaVE 2009), eds. F. Lehmann-Grube, J. Sablating, et al., ICST
Lecture Notes Ser., Berlin: Springer Verlag (2009); version with full
resolution color figures is available at
http://www.mica-vw.org/wiki/index.php/Publication
Effects of Magnetic Braking and Tidal Friction on Hot Jupiters
Tidal friction is thought to be important in determining the long-term
spin-orbit evolution of short-period extrasolar planetary systems. Using a
simple model of the orbit-averaged effects of tidal friction Eggleton, Kiseleva
& Hut (1998), we analyse the effects of the inclusion of stellar magnetic
braking on the evolution of such systems. A phase-plane analysis of a
simplified system of equations, including only the stellar tide together with a
model of the braking torque proposed by Verbunt & Zwaan (1981), is presented.
The inclusion of stellar magnetic braking is found to be extremely important in
determining the secular evolution of such systems, and its neglect results in a
very different orbital history. We then show the results of numerical
integrations of the full tidal evolution equations, using the misaligned spin
and orbit of the XO-3 system as an example, to study the accuracy of simple
timescale estimates of tidal evolution. We find that it is essential to
consider coupled evolution of the orbit and the stellar spin in order to model
the behaviour accurately. In addition, we find that for typical Hot Jupiters
the stellar spin-orbit alignment timescale is of the same order as the inspiral
time, which tells us that if a planet is observed to be aligned, then it
probably formed coplanar. This reinforces the importance of Rossiter-McLaughlin
effect observations in determining the degree of spin-orbit alignment in
transiting systems.Comment: 6 pages, 2 figures, to appear in IAU 259 Conference Proceeding
Astrophysics on the GRAPE Family of Special Purpose Computers
The GRAPE-4, the world's fastest computer in 1995-1997, has produced some
major scientific results, through a wide diversity of large-scale simulations
in astrophysics. Applications have ranged from planetary formation, through the
evolution of star clusters and galactic nuclei, to the formation of galaxies
and clusters of galaxies.Comment: 15 pages, to apper in Scienc
A stochastic Monte Carlo approach to model real star cluster evolution, III. Direct integrations of three- and four-body interactions
Spherically symmetric equal mass star clusters containing a large amount of
primordial binaries are studied using a hybrid method, consisting of a gas
dynamical model for single stars and a Monte Carlo treatment for relaxation of
binaries and the setup of close resonant and fly-by encounters of single stars
with binaries and binaries with each other (three- and four-body encounters).
What differs from our previous work is that each encounter is being integrated
using a highly accurate direct few-body integrator which uses regularized
variables. Hence we can study the systematic evolution of individual binary
orbital parameters (eccentricity, semi-major axis) and differential and total
cross sections for hardening, dissolution or merging of binaries (minimum
distance) from a sampling of several ten thousands of scattering events as they
occur in real cluster evolution including mass segregation of binaries,
gravothermal collapse and reexpansion, binary burning phase and ultimately
gravothermal oscillations. For the first time we are able to present empirical
cross sections for eccentricity variation of binaries in close three- and
four-body encounters. It is found that a large fraction of three-body and
four-body encounters results in merging. Previous cross sections obtained by
Spitzer and Gao for strong encounters can be reproduced, while for weak
encounters non-standard processes like formation of hierarchical triples occur.Comment: 16 pages, 19 figures, Latex in the MN style, submitted to MNRA
On the tidal evolution of the orbits of low-mass satellites around black holes
Low-mass satellites, like asteroids and comets, are expected to be present
around the black hole at the Galactic center. We consider small bodies orbiting
a black hole, and we study the evolution of their orbits due to tidal
interaction with the black hole. In this paper we investigate the consequences
of the existence of plunging orbits when a black hole is present. We are
interested in finding the conditions that exist when capture occurs. The main
difference between the Keplerian and black hole cases is in the existence of
plunging orbits. Orbital evolution, leading from bound to plunging orbits, goes
through a final unstable circular orbit. On this orbit, tidal energy is
released on a characteristic black hole timescale. This process may be relevant
for explaining how small, compact clumps of material can be brought onto
plunging orbits, where they may produce individual short duration accretion
events. The available energy and the characteristic timescale are consistent
with energy released and the timescale typical of Galactic flares.Comment: 7 pages, 6 figure
Collisional Hardening of Compact Binaries in Globular Clusters
We consider essential mechanisms for orbit-shrinkage or "hardening" of
compact binaries in globular clusters to the point of Roche-lobe contact and
X-ray emission phase, focussing on the process of collisional hardening due to
encounters between binaries and single stars in the cluster core. The interplay
between this kind of hardening and that due to emission of gravitational
radiation produces a characteristic scaling of the orbit-shrinkage time with
the single-star binary encounter rate in the cluster which we
introduce, clarify, and explore. We investigate possible effects of this
scaling on populations of X-ray binaries in globular clusters within the
framework of a simple "toy" scheme for describing the evolution of pre-X-ray
binaries in globular clusters. We find the expected qualitative trends
sufficiently supported by data on X-ray binaries in galactic globular clusters
to encourage us toward a more quantitative study.Comment: 8 pages, 4 figures. Accepted for publication in MNRA
A Parallel Tree-SPH code for Galaxy Formation
We describe a new implementation of a parallel Tree-SPH code with the aim to
simulate Galaxy Formation and Evolution. The code has been parallelized using
SHMEM, a Cray proprietary library to handle communications between the 256
processors of the Silicon Graphics T3E massively parallel supercomputer hosted
by the Cineca Supercomputing Center (Bologna, Italy). The code combines the
Smoothed Particle Hydrodynamics (SPH) method to solve hydro-dynamical equations
with the popular Barnes and Hut (1986) tree-code to perform gravity calculation
with a NlogN scaling, and it is based on the scalar Tree-SPH code developed by
Carraro et al(1998)[MNRAS 297, 1021]. Parallelization is achieved distributing
particles along processors according to a work-load criterion. Benchmarks, in
terms of load-balance and scalability, of the code are analyzed and critically
discussed against the adiabatic collapse of an isothermal gas sphere test using
20,000 particles on 8 processors. The code results balanced at more that 95%
level. Increasing the number of processors, the load-balance slightly worsens.
The deviation from perfect scalability at increasing number of processors is
almost negligible up to 32 processors. Finally we present a simulation of the
formation of an X-ray galaxy cluster in a flat cold dark matter cosmology,
using 200,000 particles and 32 processors, and compare our results with Evrard
(1988) P3M-SPH simulations. Additionaly we have incorporated radiative cooling,
star formation, feed-back from SNae of type II and Ia, stellar winds and UV
flux from massive stars, and an algorithm to follow the chemical enrichment of
the inter-stellar medium. Simulations with some of these ingredients are also
presented.Comment: 19 pages, 14 figures, accepted for publication in MNRA
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