2,176 research outputs found
The secular evolution of the Kuiper belt after a close stellar encounter
We show the effects of the perturbation caused by a passing by star on the
Kuiper belt objects (KBOs) of our Solar System. The dynamics of the Kuiper belt
(KB) is followed by direct -body simulations. The sampling of the KB has
been done with up to , setting the KBOs on initially nearly
circular orbits distributed in a ring of surface density .
This modelization allowed us to investigate the secular evolution of the KB
upon the encounter with the perturbing star. Actually, the encounter itself
usually leads toward eccentricity and inclination distributions similar to
observed ones, but tends also to excite the low-eccentricity population ( around \, from the Sun), depleting this region of
low eccentricities. The following long-term evolution shows a "cooling" of the
eccentricities repopulating the low-eccentricity area. In dependence on the
assumed KBO mass spectrum and sampled number of bodies, this repopulation takes
place in a time that goes from 0.5 Myr to 100 Myr. Due to the unavoidable
limitation in the number of objects in our long-term simulations (), we could not consider a detailed KBO mass spectrum, accounting for low
mass objects, thus our present simulations are not reliable in constraining
correlations among inclination distribution of the KBOs and other properties,
such as their size distribution. However, our high precision long term
simulations are a starting point for future larger studies on massively
parallel computational platforms which will provide a deeper investigation of
the secular evolution (Myr) of the KB over its whole mass spectrum.Comment: 13 pages, 12 figures, 5 table
Fun for Two
We performed populations synthesis calculations of single stars and binaries
and show that binary evolution is extremely important for Galactic astronomy.
We review several binary evolution models and conclude that they give quite
different results. These differences can be understood from the assumptions
related to how mass is transfered in the binary systems. Most important are 1)
the fraction of mass that is accreted by the companion star during mass
transfer, 2) the amount of specific angular momentum which is carried away with
the mass that leaves the binary system.Comment: 7 pages, 0 figures to appear in the proceeding of the IAU Symposium
200, "The Formation of Binary Stars" eds. H. Zinnecker and R. Mathie
Monte-Carlo Simulations of Globular Cluster Evolution - I. Method and Test Calculations
We present a new parallel supercomputer implementation of the Monte-Carlo
method for simulating the dynamical evolution of globular star clusters. Our
method is based on a modified version of Henon's Monte-Carlo algorithm for
solving the Fokker-Planck equation. Our code allows us to follow the evolution
of a cluster containing up to 5x10^5 stars to core collapse in < 40 hours of
computing time. In this paper we present the results of test calculations for
clusters with equal-mass stars, starting from both Plummer and King model
initial conditions. We consider isolated as well as tidally truncated clusters.
Our results are compared to those obtained from approximate, self-similar
analytic solutions, from direct numerical integrations of the Fokker-Planck
equation, and from direct N-body integrations performed on a GRAPE-4
special-purpose computer with N=16384. In all cases we find excellent agreement
with other methods, establishing our new code as a robust tool for the
numerical study of globular cluster dynamics using a realistic number of stars.Comment: 35 pages, including 8 figures, submitted to ApJ. Revised versio
A pilgrimage to gravity on GPUs
In this short review we present the developments over the last 5 decades that
have led to the use of Graphics Processing Units (GPUs) for astrophysical
simulations. Since the introduction of NVIDIA's Compute Unified Device
Architecture (CUDA) in 2007 the GPU has become a valuable tool for N-body
simulations and is so popular these days that almost all papers about high
precision N-body simulations use methods that are accelerated by GPUs. With the
GPU hardware becoming more advanced and being used for more advanced algorithms
like gravitational tree-codes we see a bright future for GPU like hardware in
computational astrophysics.Comment: To appear in: European Physical Journal "Special Topics" : "Computer
Simulations on Graphics Processing Units" . 18 pages, 8 figure
Comparing compact binary parameter distributions I: Methods
Being able to measure each merger's sky location, distance, component masses,
and conceivably spins, ground-based gravitational-wave detectors will provide a
extensive and detailed sample of coalescing compact binaries (CCBs) in the
local and, with third-generation detectors, distant universe. These
measurements will distinguish between competing progenitor formation models. In
this paper we develop practical tools to characterize the amount of
experimentally accessible information available, to distinguish between two a
priori progenitor models. Using a simple time-independent model, we demonstrate
the information content scales strongly with the number of observations. The
exact scaling depends on how significantly mass distributions change between
similar models. We develop phenomenological diagnostics to estimate how many
models can be distinguished, using first-generation and future instruments.
Finally, we emphasize that multi-observable distributions can be fully
exploited only with very precisely calibrated detectors, search pipelines,
parameter estimation, and Bayesian model inference
Face-on accretion onto a protoplanetary disc
Globular clusters (GCs) are known to harbor multiple stellar populations. To
explain these observations Bastian et al. suggested a scenario in which a
second population is formed by the accretion of enriched material onto the
low-mass stars in the initial GC population. The idea is that the low-mass,
pre-main sequence stars sweep up gas expelled by the massive stars of the same
generation into their protoplanetary disc as they move through the GC core. We
perform simulations with 2 different smoothed particle hydrodynamics codes to
investigate if a low-mass star surrounded by a protoplanetary disc can accrete
the amount of enriched material required in this scenario. We focus on the gas
loading rate onto the disc and star as well as on the lifetime of the disc. We
find that the gas loading rate is a factor of 2 smaller than the geometric
rate, because the effective cross section of the disc is smaller than its
surface area. The loading rate is consistent for both codes, irrespective of
resolution. The disc gains mass in the high resolution runs, but loses angular
momentum on a time scale of 10^4 yrs. Two effects determine the loss of
(specific) angular momentum in our simulations: 1) continuous ram pressure
stripping and 2) accretion of material with no azimuthal angular momentum. Our
study and previous work suggest that the former, dominant process is mainly
caused by numerical rather than physical effects, while the latter is not. The
latter process causes the disc to become more compact, increasing the surface
density profile at smaller radii. The disc size is determined in the first
place by the ram pressure when the flow first hits the disc. Further evolution
is governed by the decrease in the specific angular momentum of the disc. We
conclude that the size and lifetime of the disc are probably not sufficient to
accrete the amount of mass required in Bastian et al.'s scenario.Comment: Accepted for publication in A&A, 15 pages, 5 figures, 4 table
Simple Stellar Population Models as probed by the Large Magellanic Cloud Star Cluster ESO 121-SC03
The presence of blue straggler stars (BSs) in star clusters has proven a
challenge to conventional simple stellar population (SSP) models. Conventional
SSP models are based on the evolution theory of single stars. Meanwhile, the
typical locations of BSs in the colour-magnitude diagram of a cluster are
brighter and bluer than the main sequence turn-off point. Such loci cannot be
predicted by single-star evolution theory. However, stars with such properties
contribute significantly to the integrated light of the cluster. In this paper,
we reconstruct the integrated properties of the Large Magellanic Cloud cluster
ESO 121-SC03, based on a detailed exploration of the individual cluster stars,
and with particular emphasis on the cluster's BSs. We find that the integrated
light properties of ESO 121-SC03 are dramatically modified by its BS component.
The integrated spectral energy distribution (ISED) flux level is significantly
enhanced toward shorter wavelengths, and all broad-band colours become bluer.
When fitting the fully integrated ISED of this cluster based on conventional
SSP models, the best-fitting values of age and metallicity are significantly
underestimated compared to the true cluster parameters. The age underestimate
is per cent if we only include the BSs within the cluster's half-light
radius and per cent if all BSs are included. The corresponding
underestimates of the cluster's metallicity are and per cent,
respectively. The populous star clusters in the Magellanic Clouds are ideal
objects to explore the potential importance of BSs for the integrated light
properties of more distant unresolved star clusters in a statistically robust
manner, since they cover a large range in age and metallicity.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in MNRA
Risk of Oxidative Damage to Bone from Increased Iron Stores During Space Flight
Iron stores are increased secondary to neocytolysis of red blood cells and a high dietary intake of iron during space flight. This raises concerns about the risk of excess iron causing oxidative damage in many tissues, including bone. Biomarkers of iron status, oxidative damage, and bone resorption during space flight were analyzed for 23 (16 M/7 F) International Space Station crewmembers as part of the Nutrition SMO project. Up to 5 in-flight blood samples and 24-h urine pools were collected over the course of the 4-6 month missions. Serum iron increased slightly during space flight and was decreased at landing (P < 0.0004). An increase in serum ferritin early in flight (217% in women and 68% in men, P < 0.0004), returning to preflight concentrations at landing, and a decrease in transferrin and transferrin receptors during flight indicated that a transient increase in iron stores occurred. No inflammatory response was observed during flight. The oxidative damage markers 8-hydroxy-2'-deoxyguanosine and prostaglandin F(sub 2(alpha)) were positively correlated (both P < 0.001) with serum ferritin. A greater area under the curve for ferritin during flight was correlated with greater changes in bone mineral density of several bone regions after flight (1). In a separate study (2), a ground-based investigation was conducted that examined the combined effects of radiation exposure and iron overload on sensitivity to radiation injury in several physiological systems in 12-wk male Sprague-Dawley rats. The rats were acclimated to an adequate iron diet (45 mg iron (ferric citrate)/kg diet) for 3 wk and then assigned to one of four groups: adequate iron (Fe) diet/no radiation, adequate Fe diet/ radiation, moderately high Fe diet (650 mg Fe (ferric citrate)/kg diet)/no radiation, and moderately high Fe diet/radiation. Animals remained on the assigned diet for 4 wk. Starting on day 14 of experimental diet treatment, animals were exposed to a fractionated dose (0.375 Gy) of Cs-137 every other day (3 Gy total dose). On day 29 (24 h after last radiation exposure), animals were euthanized. Oxidative stress markers in the liver, bone, eyes, and serum were assessed. There was evidence that the iron diet contributed to DNA damage as well as radiation exposure in the liver, eyes, and bone. Together, the results suggest that increased iron stores do constitute a risk factor for oxidative damage and bone resorption, during space flight and on Earth. Funded by the Human Health and Countermeasures Element of the NASA Human Research Program
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