600 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
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
Origin of Tidal Dissipation in Jupiter: II. the Value of Q
The process of tidal dissipation inside Jupiter is not yet understood. Its
tidal quality factor () is inferred to lie between and . We
examine effects of inertial-modes on tidal dissipation in a neutrally bouyant,
core-less, uniformly rotating planet. The rate of dissipation caused by
resonantly excited inertial-modes depends on the following three parameters:
how well they are coupled to the tidal potential, how strongly they are
dissipated (by the turbulent viscosity), and how densely distributed they are
in frequency. We find that as a function of tidal frequency, the value
exhibits large fluctuations, with its maximum value set by the group of
inertial-modes that have a typical offset from an exact resonance of order
their turbulent damping rates. In our model, inertial-modes shed their tidally
acquired energy very close to the surface within a narrow latitudinal zone (the
'singularity belt'), and the tidal luminosity escapes freely out of the planet.
Strength of coupling between the tidal potential and inertial-modes is
sensitive to the presence of density discontinuities inside Jupiter. In the
case of a discreet density jump (as may be caused by the transition between
metallic and molecular hydrogen), we find a time-averaged . Even
though it remains unclear whether tidal dissipation due to resonant
inertial-modes is the correct answer to the problem, it is impressive that our
simple treatment here already leads to three to five orders of magnitude
stronger damping than that from the equilibrium tide. Moreover, our conclusions
are not affected by the presence of a small solid core, a different
prescription for the turbulent viscosity, or nonlinear mode coupling, but they
depend critically on the static stability in the upper atmosphere of Jupiter.Comment: 27 pages, incl. 11 figures, ApJ in print, expanded discussions
(nonlinearity, radiative envelope
The New Transiting Planet OGLE-TR-56b: Orbit and Atmosphere
Motivated by the identification of the very close-in extrasolar giant planet
OGLE-TR-56b, we explore the implications of its existence on problems of tidal
dissipation, planet migration, and atmospheric stability. The small orbit of
OGLE-TR-56b makes the planet an interesting test particle case for tidal
dissipation in stellar convection zones. We show that it favors prescriptions
of suppressed convective eddy viscosity. Precise timing of the transits of
OGLE-TR-56b might place interesting constraints on stellar convection theory,
if orbital period change is detected in the near future.Comment: 12 pages, 1 figure, submitted to ApJ
The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation
We assess models for the assembly of supermassive black holes (SMBHs) at the
center of galaxies that trace their hierarchical build-up far up in the dark
halo `merger tree'. We assume that the first `seed' black holes (BHs) formed in
(mini)halos collapsing at z=20 from high-sigma density fluctuations. As these
pregalactic holes become incorporated through a series of mergers into larger
and larger halos, they sink to the center owing to dynamical friction, accrete
a fraction of the gas in the merger remnant to become supermassive, form a
binary system, and eventually coalesce. The merger history of dark matter halos
and associated BHs is followed by cosmological Monte Carlo realizations of the
merger hierarchy. A simple model, where quasar activity is driven by major
mergers and SMBHs accrete at the Eddington rate a mass that scales with the
fifth power of the velocity dispersion, is shown to reproduce the optical LF of
quasars in the redshift range 1<z<4. Binary and triple BH interactions are
followed in our merger tree. The assumptions underlying our scenario lead to
the prediction of a population of massive BHs wandering in galaxy halos and the
intergalactic medium at the present epoch, and contributing <10% to the total
BH mass density. At all epochs the fraction of binary SMBHs in galaxy nuclei is
of order 10%, while the fraction of binary quasars is less than 0.3%Comment: revised version, accepted for publication in the ApJ, emulateapj, 15
pages, 16 figure
From Canonical to Enhanced Extra Mixing in Low-Mass Red Giants: Tidally Locked Binaries
Stellar models which incorporate simple diffusion or shear induced mixing are
used to describe canonical extra mixing in low mass red giants of low and solar
metallicity. These models are able to simultaneously explain the observed Li
and CN abundance changes along upper red giant branch (RGB) in field
low-metallicity stars and match photometry, rotation and carbon isotopic ratios
for stars in the old open cluster M67. The shear mixing model requires that
main sequence (MS) progenitors of upper RGB stars possessed rapidly rotating
radiative cores and that specific angular momentum was conserved in each of
their mass shells during their evolution. We surmise that solar-type stars will
not experience canonical extra mixing on the RGB because their more efficient
MS spin-down resulted in solid-body rotation, as revealed by helioseismological
data for the Sun. Thus, RGB stars in the old, high metallicity cluster NGC 6791
should show no evidence for mixing in their carbon isotopic ratios.
We develop the idea that canonical extra mixing in a giant component of a
binary system may be switched to its enhanced mode with much faster and
somewhat deeper mixing as a result of the giant's tidal spin-up. This scenario
can explain photometric and composition peculiarities of RS CVn binaries. The
tidally enforced enhanced extra mixing might contribute to the star-to-star
abundance variations of O, Na and Al in globular clusters. This idea may be
tested with observations of carbon isotopic ratios and CN abundances in RS CVn
binaries.Comment: 47 pages, 19 figures, accepted for publication in Ap
Eccentric binaries: Tidal flows and periastron events
A number of binary systems present evidence of enhanced activity around
periastron passage, suggesting a connection between tidal interactions and
these periastron effects. The aim of this investigation is to study the
time-dependent response of a star's surface as it is perturbed by a binary
companion. We derive expressions for the rate of dissipation, , of the
kinetic energy by the viscous flows driven by tidal interactions on the surface
layer. The method is tested by comparing the results from a grid of model
calculations with the analytical predictions of Hut (1981) and the
synchronization timescales of Zahn (1977, 2008). Our results for the orbital
cycle averaged energy dissipation on orbital separation are consistent with
those of Hut for model binaries with orbital separations at periastron >8
stellar radii. The model also reproduces the predicted pseudo-synchronization
angular velocity for moderate eccentricities and the same scaling of
synchronization timescales for circular orbits with separation as given by
Zahn. The computations gives the distribution of over the stellar
surface, and show that it is generally concentrated at the equatorial latitude,
with maxima generally located around four clearly defined longitudes,
corresponding to the fastest azimuthal velocity perturbations. Maximum
amplitudes occur around periastron passage or slightly thereafter for
supersynchronously rotating stars. In very eccentric binaries, the distribution
of over the surface changes significantly as a function of orbital
phase, with small spatial structures appearing after periastron. An exploratory
calculation for the highly eccentric binary system delta Sco suggests that the
sudden and large amplitude variations in surface properties around periastron
may contribute toward the activity observed around this orbital phase.Comment: Accepted for publication in A&
Gonads or body?:Differences in gonadal and somatic photoperiodic growth response in two vole species
To optimally time reproduction, seasonal mammals use a photoperiodic neuroendocrine system (PNES) that measures photoperiod and subsequently drives reproduction. To adapt to late spring arrival at northern latitudes, a lower photoperiodic sensitivity and therefore a higher critical photoperiod for reproductive onset is necessary in northern species to arrest reproductive development until spring onset. Temperature-photoperiod relationships, and hence food availability-photoperiod relationships, are highly latitude dependent. Therefore, we predict PNES sensitivity characteristics to be latitude dependent. Here, we investigated photoperiodic responses at different times during development in northern (tundra or root vole, Microtus oeconomus) and southern vole species (common vole, Microtus arvalis) exposed to constant short (SP) or long photoperiod (LP). Although the tundra vole grows faster under LP, no photoperiodic effect on somatic growth is observed in the common vole. In contrast, gonadal growth is more sensitive to photoperiod in the common vole, suggesting that photoperiodic responses in somatic and gonadal growth can be plastic, and might be regulated through different mechanisms. In both species, thyroid-stimulating hormone β-subunit (Tshβ) and iodothyronine deiodinase 2 (Dio2) expression is highly increased under LP, whereas Tshr and Dio3 decrease under LP. High Tshr levels in voles raised under SP may lead to increased sensitivity to increasing photoperiods later in life. The higher photoperiodic-induced Tshr response in tundra voles suggests that the northern vole species might be more sensitive to thyroid-stimulating hormone when raised under SP. In conclusion, species differences in developmental programming of the PNES, which is dependent on photoperiod early in development, may form different breeding strategies as part of latitudinal adaptation
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