1,606 research outputs found
Monte-Carlo methods for NLTE spectral synthesis of supernovae
We present JEKYLL, a new code for modelling of supernova (SN) spectra and
lightcurves based on Monte-Carlo (MC) techniques for the radiative transfer.
The code assumes spherical symmetry, homologous expansion and steady state for
the matter, but is otherwise capable of solving the time-dependent radiative
transfer problem in non-local-thermodynamic-equilibrium (NLTE). The method used
was introduced in a series of papers by Lucy, but the full time-dependent NLTE
capabilities of it have never been tested. Here, we have extended the method to
include non-thermal excitation and ionization as well as charge-transfer and
two-photon processes. Based on earlier work, the non-thermal rates are
calculated by solving the Spencer-Fano equation. Using a method previously
developed for the SUMO code, macroscopic mixing of the material is taken into
account in a statistical sense. In addition, a statistical Markov-chain model
is used to sample the emission frequency, and we introduce a method to control
the sampling of the radiation field. Except for a description of JEKYLL, we
provide comparisons with the ARTIS, SUMO and CMFGEN codes, which show good
agreement in the calculated spectra as well as the state of the gas. In
particular, the comparison with CMFGEN, which is similar in terms of physics
but uses a different technique, shows that the Lucy method does indeed converge
in the time-dependent NLTE case. Finally, as an example of the time-dependent
NLTE capabilities of JEKYLL, we present a model of a Type IIb SN, taken from a
set of models presented and discussed in detail in an accompanying paper. Based
on this model we investigate the effects of NLTE, in particular those arising
from non-thermal excitation and ionization, and find strong effects even on the
bolometric lightcurve. This highlights the need for full NLTE calculations when
simulating the spectra and lightcurves of SNe.Comment: Accepted for publication by Astronomy & Astrophysic
An -expansion for Small-World Networks
I construct a well-defined expansion in for diffusion
processes on small-world networks. The technique permits one to calculate the
average over disorder of moments of the Green's function, and is used to
calculate the average Green's function and fluctuations to first non-leading
order in , giving results which agree with numerics. This technique
is also applicable to other problems of diffusion in random media.Comment: 7 pages Europhysics style, 3 figure
Evolution equation for a model of surface relaxation in complex networks
In this paper we derive analytically the evolution equation of the interface
for a model of surface growth with relaxation to the minimum (SRM) in complex
networks. We were inspired by the disagreement between the scaling results of
the steady state of the fluctuations between the discrete SRM model and the
Edward-Wilkinson process found in scale-free networks with degree distribution
for [Pastore y Piontti {\it et al.},
Phys. Rev. E {\bf 76}, 046117 (2007)]. Even though for Euclidean lattices the
evolution equation is linear, we find that in complex heterogeneous networks
non-linear terms appear due to the heterogeneity and the lack of symmetry of
the network; they produce a logarithmic divergency of the saturation roughness
with the system size as found by Pastore y Piontti {\it et al.} for .Comment: 9 pages, 2 figure
Consensus formation on coevolving networks: groups' formation and structure
We study the effect of adaptivity on a social model of opinion dynamics and
consensus formation. We analyze how the adaptivity of the network of contacts
between agents to the underlying social dynamics affects the size and
topological properties of groups and the convergence time to the stable final
state. We find that, while on static networks these properties are determined
by percolation phenomena, on adaptive networks the rewiring process leads to
different behaviors: Adaptive rewiring fosters group formation by enhancing
communication between agents of similar opinion, though it also makes possible
the division of clusters. We show how the convergence time is determined by the
characteristic time of link rearrangement. We finally investigate how the
adaptivity yields nontrivial correlations between the internal topology and the
size of the groups of agreeing agents.Comment: 10 pages, 3 figures,to appear in a special proceedings issue of J.
Phys. A covering the "Complex Networks: from Biology to Information
Technology" conference (Pula, Italy, 2007
Three-dimensional modeling of Type Ia supernovae - The power of late time spectra
Late time synthetic spectra of Type Ia supernovae, based on three-dimensional
deflagration models, are presented. We mainly focus on one
model,"c3_3d_256_10s", for which the hydrodynamics (Roepke 2005) and
nucleosynthesis (Travaglio et al. 2004) was calculated up to the homologous
phase of the explosion. Other models with different ignition conditions and
different resolution are also briefly discussed. The synthetic spectra are
compared to observed late time spectra. We find that while the model spectra
after 300 to 500 days show a good agreement with the observed Fe II-III
features, they also show too strong O I and C I lines compared to the observed
late time spectra. The oxygen and carbon emission originates from the
low-velocity unburned material in the central regions of these models. To get
agreement between the models and observations we find that only a small mass of
unburned material may be left in the center after the explosion. This may be a
problem for pure deflagration models, although improved initial conditions, as
well as higher resolution decrease the discrepancy. The relative intensity from
the different ionization stages of iron is sensitive to the density of the
emitting iron-rich material. We find that clumping, with the presence of low
density regions, is needed to reproduce the observed iron emission, especially
in the range between 4000 and 6000 AA. Both temperature and ionization depend
sensitively on density, abundances and radioactive content. This work therefore
illustrates the importance of including the inhomogeneous nature of realistic
three-dimensional explosion models. We briefly discuss the implications of the
spectral modeling for the nature of the explosion.Comment: 20 pages, 9 figures, resolution of Fig 1 is reduced to meet astro-ph
file size restriction, submitted to A&
No trace of a single-degenerate companion in late spectra of SNe 2011fe and 2014J
Left-over, ablated material from a possible non-degenerate companion can
reveal itself after about one year in spectra of Type Ia SNe (SNe Ia). We have
searched for such material in spectra of SN 2011fe (at 294 days after the
explosion) and for SN 2014J (315 days past explosion). The observations are
compared with numerical models simulating the expected line emission. The
spectral lines sought for are H-alpha, [O I] 6300 and [Ca II] 7291,7324, and
the expected width of these lines is about 1000 km/s. No signs of these lines
can be traced in any of the two supernovae. When systematic uncertainties are
included, the limits on hydrogen-rich ablated gas in SNe 2011fe and 2014J are
0.003 M_sun and 0.0085 M_sun, respectively, where the limit for SN 2014J is the
second lowest ever, and the limit for SN 2011fe is a revision of a previous
limit. Limits are also put on helium-rich ablated gas. These limits are used,
in conjunction with other data, to argue that these supernovae can stem from
double-degenerate systems, or from single-degenerate systems with a spun
up/spun down super-Chandrasekhar white dwarf. For SN 2011fe, other types of
hydrogen-rich donors can likely be ruled out, whereas for SN 2014J a
main-sequence donor system with large intrinsic separation is still possible.
Helium-rich donor systems cannot be ruled out for any of the two supernovae,
but the expected short delay time for such progenitors makes this possibility
less likely, especially for SN 2011fe. The broad [Ni II] 7378 emission in SN
2014J is redshifted by about +1300 km/s, as opposed to the known blueshift of
roughly -1100 km/s for SN 2011fe. [Fe II] 7155 is also redshifted in SN 2014J.
SN 2014J belongs to a minority of SNe Ia that both have a nebular redshift of
[Fe II] 7155 and [Ni II] 7378, and a slow decline of the Si II 6355 absorption
trough just after B-band maximum.Comment: 13 pages, submitted to A&
Synchronization in Weighted Uncorrelated Complex Networks in a Noisy Environment: Optimization and Connections with Transport Efficiency
Motivated by synchronization problems in noisy environments, we study the
Edwards-Wilkinson process on weighted uncorrelated scale-free networks. We
consider a specific form of the weights, where the strength (and the associated
cost) of a link is proportional to with and
being the degrees of the nodes connected by the link. Subject to the
constraint that the total network cost is fixed, we find that in the mean-field
approximation on uncorrelated scale-free graphs, synchronization is optimal at
-1. Numerical results, based on exact numerical diagonalization
of the corresponding network Laplacian, confirm the mean-field results, with
small corrections to the optimal value of . Employing our recent
connections between the Edwards-Wilkinson process and resistor networks, and
some well-known connections between random walks and resistor networks, we also
pursue a naturally related problem of optimizing performance in queue-limited
communication networks utilizing local weighted routing schemes.Comment: Papers on related research can be found at
http://www.rpi.edu/~korniss/Research
Genetic diversity among twelve grape cultivars indigenous to the Carpathian Basin revealed by RAPD markers
Twelve cultivars (Vitis vinifera L.) were subjected to RAPD analysis in order to estimate the genetic diversity among these genotypes and to analyse their genetic relationships. The study was performed using 28 primers that generated 120 polymorphic fragments. There was genetic variation among the cultivars with values of genetic diversity ranging from 0.419 to 0.642 using the Jaccard coefficient. UPGMA analysis of distance matrix resulted in a dendrogram with three clusters. The dendrogram shows that the cultivars of our study can be distinguished to a relatively high degree. Results were compared with the taxonomic classification and with the synonyms of the cultivars. The RAPD technique was useful for identification and discrimination of these grape cultivars
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