234 research outputs found
Submillimeter Astronomy
For submillimeter astronomy, particularly at 200m, the ARENA working group has proposed a 25âm telescope at the For submillimeter astronomy, particularly at 200”m, the ARENA working group has proposed a 25âm telescope at the Concordia station on Dome C. Issues related to this suggestion are reviewed
Studying Turbulence using Doppler-broadened lines: Velocity Coordinate Spectrum
We discuss a new technique for studying astrophysical turbulence that
utilizes the statistics of Doppler-broadened spectral lines. The technique
relates the power Velocity Coordinate Spectrum (VCS), i.e. the spectrum of
fluctuations measured along the velocity axis in Position-Position-Velocity
(PPV) data cubes available from observations, to the underlying power spectra
of the velocity/density fluctuations. Unlike the standard spatial spectra, that
are function of angular wavenumber, the VCS is a function of the velocity wave
number k_v ~ 1/v. We show that absorption affects the VCS to a higher degree
for small k_v and obtain the criteria for disregarding the absorption effects
for turbulence studies at large k_v. We consider the retrieval of turbulence
spectra from observations for high and low spatial resolution observations and
find that the VCS allows one to study turbulence even when the emitting
turbulent volume is not spatially resolved. This opens interesting prospects
for using the technique for extragalactic research. We show that, while thermal
broadening interferes with the turbulence studies using the VCS, it is possible
to separate thermal and non-thermal contributions. This allows a new way of
determining the temperature of the interstellar gas using emission and
absorption spectral lines.Comment: 27 page, 3 figures, content extended and presentation reorganized to
correspond to the version accepted to Ap
Out-Of-Focus Holography at the Green Bank Telescope
We describe phase-retrieval holography measurements of the 100-m diameter
Green Bank Telescope using astronomical sources and an astronomical receiver
operating at a wavelength of 7 mm. We use the technique with parameterization
of the aperture in terms of Zernike polynomials and employing a large defocus,
as described by Nikolic, Hills & Richer (2006). Individual measurements take
around 25 minutes and from the resulting beam maps (which have peak signal to
noise ratios of 200:1) we show that it is possible to produce low-resolution
maps of the wavefront errors with accuracy around a hundredth of a wavelength.
Using such measurements over a wide range of elevations, we have calculated a
model for the wavefront-errors due to the uncompensated gravitational
deformation of the telescope. This model produces a significant improvement at
low elevations, where these errors are expected to be the largest; after
applying the model, the aperture efficiency is largely independent of
elevation. We have also demonstrated that the technique can be used to measure
and largely correct for thermal deformations of the antenna, which often exceed
the uncompensated gravitational deformations during daytime observing.
We conclude that the aberrations induced by gravity and thermal effects are
large-scale and the technique used here is particularly suitable for measuring
such deformations in large millimetre wave radio telescopes.Comment: 10 pages, 7 figures (accepted by Astronomy & Astrophysics
PyCOOL - a Cosmological Object-Oriented Lattice code written in Python
There are a number of different phenomena in the early universe that have to
be studied numerically with lattice simulations. This paper presents a graphics
processing unit (GPU) accelerated Python program called PyCOOL that solves the
evolution of scalar fields in a lattice with very precise symplectic
integrators. The program has been written with the intention to hit a sweet
spot of speed, accuracy and user friendliness. This has been achieved by using
the Python language with the PyCUDA interface to make a program that is easy to
adapt to different scalar field models. In this paper we derive the symplectic
dynamics that govern the evolution of the system and then present the
implementation of the program in Python and PyCUDA. The functionality of the
program is tested in a chaotic inflation preheating model, a single field
oscillon case and in a supersymmetric curvaton model which leads to Q-ball
production. We have also compared the performance of a consumer graphics card
to a professional Tesla compute card in these simulations. We find that the
program is not only accurate but also very fast. To further increase the
usefulness of the program we have equipped it with numerous post-processing
functions that provide useful information about the cosmological model. These
include various spectra and statistics of the fields. The program can be
additionally used to calculate the generated curvature perturbation. The
program is publicly available under GNU General Public License at
https://github.com/jtksai/PyCOOL . Some additional information can be found
from http://www.physics.utu.fi/tiedostot/theory/particlecosmology/pycool/ .Comment: 23 pages, 12 figures; some typos correcte
MYRIAD: A new N-body code for simulations of Star Clusters
We present a new C++ code for collisional N-body simulations of star
clusters. The code uses the Hermite fourth-order scheme with block time steps,
for advancing the particles in time, while the forces and neighboring particles
are computed using the GRAPE-6 board. Special treatment is used for close
encounters, binary and multiple sub-systems that either form dynamically or
exist in the initial configuration. The structure of the code is modular and
allows the appropriate treatment of more physical phenomena, such as stellar
and binary evolution, stellar collisions and evolution of close black-hole
binaries. Moreover, it can be easily modified so that the part of the code that
uses GRAPE-6, could be replaced by another module that uses other
accelerating-hardware like the Graphics Processing Units (GPUs). Appropriate
choice of the free parameters give a good accuracy and speed for simulations of
star clusters up to and beyond core collapse. Simulations of Plummer models
consisting of equal-mass stars reached core collapse at t~17 half-mass
relaxation times, which compares very well with existing results, while the
cumulative relative error in the energy remained below 0.001. Also, comparisons
with published results of other codes for the time of core collapse for
different initial conditions, show excellent agreement. Simulations of King
models with an initial mass-function, similar to those found in the literature,
reached core collapse at t~0.17, which is slightly smaller than the expected
result from previous works. Finally, the code accuracy becomes comparable and
even better than the accuracy of existing codes, when a number of close binary
systems is dynamically created in a simulation. This is due to the high
accuracy of the method that is used for close binary and multiple sub-systems.Comment: 24 pages, 29 figures, accepted for publication to Astronomy &
Astrophysic
A modified parallel tree code for N-body simulation of the Large Scale Structure of the Universe
N-body codes to perform simulations of the origin and evolution of the Large
Scale Structure of the Universe have improved significantly over the past
decade both in terms of the resolution achieved and of reduction of the CPU
time. However, state-of-the-art N-body codes hardly allow one to deal with
particle numbers larger than a few 10^7, even on the largest parallel systems.
In order to allow simulations with larger resolution, we have first
re-considered the grouping strategy as described in Barnes (1990) (hereafter
B90) and applied it with some modifications to our WDSH-PT (Work and Data
SHaring - Parallel Tree) code. In the first part of this paper we will give a
short description of the code adopting the Barnes and Hut algorithm
\cite{barh86} (hereafter BH), and in particular of the memory and work
distribution strategy applied to describe the {\it data distribution} on a
CC-NUMA machine like the CRAY-T3E system. In the second part of the paper we
describe the modification to the Barnes grouping strategy we have devised to
improve the performance of the WDSH-PT code. We will use the property that
nearby particles have similar interaction list. This idea has been checked in
B90, where an interaction list is builded which applies everywhere within a
cell C_{group} containing a little number of particles N_{crit}. B90 reuses
this interaction list for each particle in the cell in turn.
We will assume each particle p to have the same interaction list.
Thus it has been possible to reduce the CPU time increasing the performances.
This leads us to run simulations with a large number of particles (N ~
10^7/10^9) in non-prohibitive times.Comment: 13 pages and 7 Figure
Modification of Projected Velocity Power Spectra by Density Inhomogeneities in Compressible Supersonic Turbulence
(Modified) The scaling of velocity fluctuation, dv, as a function of spatial
scale L in molecular clouds can be measured from size-linewidth relations,
principal component analysis, or line centroid variation. Differing values of
the power law index of the scaling relation dv = L^(g3D) in 3D are given by
these different methods: the first two give g3D=0.5, while line centroid
analysis gives g3D=0. This discrepancy has previously not been fully
appreciated, as the variation of projected velocity line centroid fluctuations
(dv_{lc} = L^(g2D)) is indeed described, in 2D, by g2D=0.5. However, if
projection smoothing is accounted for, this implies that g3D=0. We suggest that
a resolution of this discrepancy can be achieved by accounting for the effect
of density inhomogeneity on the observed g2D obtained from velocity line
centroid analysis. Numerical simulations of compressible turbulence are used to
show that the effect of density inhomogeneity statistically reverses the effect
of projection smoothing in the case of driven turbulence so that velocity line
centroid analysis does indeed predict that g2D=g3D=0.5. Using our numerical
results we can restore consistency between line centroid analysis, principal
component analysis and size-linewidth relations, and we derive g3D=0.5,
corresponding to shock-dominated (Burgers) turbulence. We find that this
consistency requires that molecular clouds are continually driven on large
scales or are only recently formed.Comment: 28 pages total, 20 figures, accepted for publication in Ap
Not an open cluster after all: the NGC 6863 asterism in Aquila
Shortly after birth, open clusters start dissolving; gradually losing stars
into the surrounding star field. The time scale for complete disintegration
depends both on their initial membership and location within the Galaxy. Open
clusters undergoing the terminal phase of cluster disruption or open cluster
remnants (OCRs) are notoriously difficult to identify. From an observational
point, a combination of low number statistics and minimal contrast against the
general stellar field conspire to turn them into very challenging objects. To
make the situation even worst, random samples of field stars often display
features that may induce to classify them erroneously as extremely evolved open
clusters. In this paper, we provide a detailed study of the stellar content and
kinematics of NGC 6863, a compact group of a few stars located in Aquila and
described by the POSS as a non existent cluster. Nonetheless, this object has
been recently classified as OCR. The aim of the present work is to either
confirm or disprove its OCR status by a detailed star-by-star analysis. The
analysis is performed using wide-field photometry in the UBVI pass-band, proper
motions from the UCAC3 catalogue, and high resolution spectroscopy as well as
results from extensive -body calculations. Our results show that the four
brightest stars commonly associated to NGC 6863 form an asterism, a group of
non-physically associated stars projected together, leading to the conclusion
that NGC 6863 is not a real open cluster.Comment: 10 pages, 8 eps figure, in press in Astronomy and Astrophysis.
Abstract shortened to fit i
Habitable Zones and UV Habitable Zones around Host Stars
Ultraviolet radiation is a double-edged sword to life. If it is too strong,
the terrestrial biological systems will be damaged. And if it is too weak, the
synthesis of many biochemical compounds can not go along. We try to obtain the
continuous ultraviolet habitable zones, and compare the ultraviolet habitable
zones with the habitable zones of host stars. Using the boundary ultraviolet
radiation of ultraviolet habitable zone, we calculate the ultraviolet habitable
zones of host stars with masses from 0.08 to 4.00 \mo. For the host stars with
effective temperatures lower than 4,600 K, the ultraviolet habitable zones are
closer than the habitable zones. For the host stars with effective temperatures
higher than 7,137 K, the ultraviolet habitable zones are farther than the
habitable zones. For hot subdwarf as a host star, the distance of the
ultraviolet habitable zone is about ten times more than that of the habitable
zone, which is not suitable for life existence.Comment: 5 pages, 3 figure
Hydra: A Parallel Adaptive Grid Code
We describe the first parallel implementation of an adaptive
particle-particle, particle-mesh code with smoothed particle hydrodynamics.
Parallelisation of the serial code, ``Hydra'', is achieved by using CRAFT, a
Cray proprietary language which allows rapid implementation of a serial code on
a parallel machine by allowing global addressing of distributed memory.
The collisionless variant of the code has already completed several 16.8
million particle cosmological simulations on a 128 processor Cray T3D whilst
the full hydrodynamic code has completed several 4.2 million particle combined
gas and dark matter runs. The efficiency of the code now allows parameter-space
explorations to be performed routinely using particles of each species.
A complete run including gas cooling, from high redshift to the present epoch
requires approximately 10 hours on 64 processors.
In this paper we present implementation details and results of the
performance and scalability of the CRAFT version of Hydra under varying degrees
of particle clustering.Comment: 23 pages, LaTex plus encapsulated figure
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