1,304 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
Why did Supernova 1054 shine at late times?
The Crab nebula is the remnant of supernova 1054 (SN 1054). The progenitor of
this supernova has, based on nucleosynthesis arguments, been modeled as an 8-10
solar mass star. Here we point out that the observations of the late light
curve of SN 1054, from the historical records, are not compatible with the
standard scenario, in which the late time emission is powered by the
radioactive decay of small amounts of Ni-56. Based on model calculations we
quantify this discrepancy. The rather large mass of Ni-56 needed to power the
late time emission, 0.06[-0.03,+0.02] solar masses, seems inconsistent with
abundances in the Crab nebula. The late light curve may well have been powered
by the pulsar, which would make SN 1054 unique in this respect. Alternatively,
the late light curve could have been powered by circumstellar interaction, in
accordance with scenarios in which 8-10 solar mass stars are progenitors to
`dense wind' supernovae.Comment: 5 pages, 2 figures. Accepted for publication in A&
SN 1998bw at late phases
We present observations of the peculiar supernova SN 1998bw, which was
probably associated with GRB 980425. The photometric and spectroscopic
evolution is monitored up to 500 days past explosion. We also present modeling
based on spherically symmetric, massive progenitor models and very energetic
explosions. The models allow line identification and clearly show the
importance of mixing. From the late light curves we estimate that about 0.3-0.9
solar masses of ejected Nickel-56 is required to power the supernova.Comment: With 3 figures Accepted for ApJ Letter
A liaison between mTOR signaling, ribosome biogenesis and cancer
The ability to translate genetic information into functional proteins is considered a landmark in evolution. Ribosomes have evolved to take on this responsibility and, although there are some differences in their molecular make-up, both prokaryotes and eukaryotes share a common structural architecture and similar underlying mechanisms of protein synthesis. Understanding ribosome function and biogenesis has been the focus of extensive research since the early days of their discovery. In the last decade however, new and unexpected roles have emerged that place deregulated ribosome biogenesis and protein synthesis at the crossroads of pathological settings, particularly cancer, revealing a set of novel cellular checkpoints. Moreover, it is also becoming evident that mTOR signaling, which regulates an array of anabolic processes, including ribosome biogenesis, is often exploited by cancer cells to sustain proliferation through the upregulation of global protein synthesis. The use of pharmacological agents that interfere with ribosome biogenesis and mTOR signaling has proven to be an effective strategy to control cancer development clinically. Here we discuss the most recent findings concerning the underlying mechanisms by which mTOR signaling controls ribosome production and the potential impact of ribosome biogenesis in tumor development
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&
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
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