22,877 research outputs found
Chemical Modelling of Young Stellar Objects, I. Method and Benchmarks
Upcoming facilities such as the Herschel Space Observatory or ALMA will
deliver a wealth of molecular line observations of young stellar objects
(YSOs). Based on line fluxes, chemical abundances can then be estimated by
radiative transfer calculations. To derive physical properties from abundances,
the chemical network needs to be modeled and fitted to the observations. This
modeling process is however computationally exceedingly demanding, particularly
if in addition to density and temperature, far UV (FUV) irradiation, X-rays,
and multi-dimensional geometry have to be considered.
We develop a fast tool, suitable for various applications of chemical
modeling in YSOs. A grid of the chemical composition of the gas having a
density, temperature, FUV irradiation and X-ray flux is pre-calculated as a
function of time. A specific interpolation approach is developed to reduce the
database to a feasible size. Published models of AFGL 2591 are used to verify
the accuracy of the method. A second benchmark test is carried out for FUV
sensitive molecules. The novel method for chemical modeling is more than
250,000 times faster than direct modeling and agrees within a mean factor of
1.35. The tool is distributed for public use.
In the course of devloping the method, the chemical evolution is explored: We
find that X-ray chemistry in envelopes of YSOs can be reproduced by means of an
enhanced cosmic-ray ionization rate. We further find that the abundance of CH+
in low-density gas with high ionization can be enhanced by the recombination of
doubly ionized carbon (C++) and suggest a new value for the initial abundance
of the main sulphur carrier in the hot-core.Comment: Accepted by ApJS. 24 pages, 15 figures. A version with higher
resolution images is available from
http://www.astro.phys.ethz.ch/staff/simonbr/papgridI.pdf . Online data
available at http://www.astro.phys.ethz.ch/chemgrid.html . Second paper of
this series of papers available at arXiv:0906.058
Cheating for Problem Solving: A Genetic Algorithm with Social Interactions
We propose a variation of the standard genetic algorithm that incorporates
social interaction between the individuals in the population. Our goal is to
understand the evolutionary role of social systems and its possible application
as a non-genetic new step in evolutionary algorithms. In biological
populations, ie animals, even human beings and microorganisms, social
interactions often affect the fitness of individuals. It is conceivable that
the perturbation of the fitness via social interactions is an evolutionary
strategy to avoid trapping into local optimum, thus avoiding a fast convergence
of the population. We model the social interactions according to Game Theory.
The population is, therefore, composed by cooperator and defector individuals
whose interactions produce payoffs according to well known game models
(prisoner's dilemma, chicken game, and others). Our results on Knapsack
problems show, for some game models, a significant performance improvement as
compared to a standard genetic algorithm.Comment: 7 pages, 5 Figures, 5 Tables, Proceedings of Genetic and Evolutionary
Computation Conference (GECCO 2009), Montreal, Canad
Classical novae and type I X-ray bursts: challenges for the 21st century
Classical nova explosions and type I X-ray bursts are the most frequent types
of thermonuclear stellar explosions in the Galaxy. Both phenomena arise from
thermonuclear ignition in the envelopes of accreting compact objects in close
binary star systems. Detailed observations of these events have stimulated
numerous studies in theoretical astrophysics and experimental nuclear physics.
We discuss observational features of these phenomena and theoretical efforts to
better understand the energy production and nucleosynthesis in these
explosions. We also examine and summarize studies directed at identifying
nuclear physics quantities with uncertainties that significantly affect model
predictions.Comment: 40 pages, accepted for AIP Advances: Stardust - Progress and Problems
in Nuclear Astrophysic
Anisotropic winds from close-in extra-solar planets
We present two-dimensional hydrodynamic models of thermally driven winds from
highly irradiated, close-in extra-solar planets. We adopt a very simple
treatment of the radiative heating processes at the base of the wind, and
instead focus on the differences between the properties of outflows in
multidimensions in comparison to spherically symmetric models computed with the
same methods. For hot (T > 2 x 10^{4} K) or highly ionized gas, we find strong
(supersonic) polar flows are formed above the planet surface which produce weak
shocks and outflow on the night-side. In comparison to a spherically symmetric
wind with the same parameters, the sonic surface on the day-side is much closer
to the planet surface in multidimensions, and the total mass loss rate is
reduced by almost a factor of four. We also compute the steady-state structure
of interacting planetary and stellar winds. Both winds end in a termination
shock, with a parabolic contact discontinuity which is draped over the planet
separating the two shocked winds. The planetary wind termination shock and the
sonic surface in the wind are well separated, so that the mass loss rate from
the planet is essentially unaffected. However, the confinement of the planetary
wind to the small volume bounded by the contact discontinuity greatly enhances
the column density close to the planet, which might be important for the
interpretation of observations of absorption lines formed by gas surrounding
transiting planets.Comment: ApJ accepte
Invariant Manifolds and Rate Constants in Driven Chemical Reactions
Reaction rates of chemical reactions under nonequilibrium conditions can be
determined through the construction of the normally hyperbolic invariant
manifold (NHIM) [and moving dividing surface (DS)] associated with the
transition state trajectory. Here, we extend our recent methods by constructing
points on the NHIM accurately even for multidimensional cases. We also advance
the implementation of machine learning approaches to construct smooth versions
of the NHIM from a known high-accuracy set of its points. That is, we expand on
our earlier use of neural nets, and introduce the use of Gaussian process
regression for the determination of the NHIM. Finally, we compare and contrast
all of these methods for a challenging two-dimensional model barrier case so as
to illustrate their accuracy and general applicability.Comment: 28 pages, 13 figures, table of contents figur
Coupling hydrodynamics with comoving frame radiative transfer: II. Stellar wind stratification in the high-mass X-ray binary Vela X-1
CONTEXT: Vela X-1, a prototypical high mass X-ray binary (HMXB), hosts a
neutron star (NS) in a close orbit around an early-B supergiant donor star.
Accretion of the donor star's wind onto the NS powers its strong X-ray
luminosity. To understand the physics of HMXBs, detailed knowledge about the
donor star winds is required. AIMS: To gain a realistic picture of the donor
star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model
describing the wind stratification while properly reproducing the observed
donor spectrum. To investigate how X-ray illumination affects the stellar wind,
we calculated additional models for different X-ray luminosity regimes.
METHODS: We use the recently updated version of the PoWR code to consistently
solve the hydrodynamic equation together with the statistical equations and the
radiative transfer. RESULTS: The wind flow in Vela X-1 is driven by ions from
various elements with Fe III and S III leading in the outer wind. The
model-predicted mass-loss rate is in line with earlier empirical studies. The
mass-loss rate is almost unaffected by the presence of the accreting NS in the
wind. The terminal wind velocity is confirmed at km/s.
On the other hand, the wind velocity in the inner region where the NS is
located is only km/s, which is not expected on the basis of a
standard -velocity law. In models with an enhanced level of X-rays, the
velocity field in the outer wind can be altered. If the X-ray flux is too high,
the acceleration breaks down because the ionization increases. CONCLUSIONS:
Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model
reveals a low wind speed at the NS location, and it provides quantitative
information on wind driving in this important HMXB.Comment: 19 pages, 10 figures, accepted for publication in Astronomy &
Astrophysic
Temporal-varying failures of nodes in networks
We consider networks in which random walkers are removed because of the
failure of specific nodes. We interpret the rate of loss as a measure of the
importance of nodes, a notion we denote as failure-centrality. We show that the
degree of the node is not sufficient to determine this measure and that, in a
first approximation, the shortest loops through the node have to be taken into
account. We propose approximations of the failure-centrality which are valid
for temporal-varying failures and we dwell on the possibility of externally
changing the relative importance of nodes in a given network, by exploiting the
interference between the loops of a node and the cycles of the temporal pattern
of failures. In the limit of long failure cycles we show analytically that the
escape in a node is larger than the one estimated from a stochastic failure
with the same failure probability. We test our general formalism in two
real-world networks (air-transportation and e-mail users) and show how
communities lead to deviations from predictions for failures in hubs.Comment: 7 pages, 3 figure
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