8,047 research outputs found
On the Importance of the Interclump Medium for Superionization: O VI Formation in the Wind of Zeta Pup
We have studied superionization and X-ray line formation in the spectra of
Zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to
simultaneously analyze optical, UV, and X-ray observations. Here, we present
results on the formation of the O VI ll1032, 1038 doublet. Our simulations,
supported by simple theoretical calculations, show that clumped wind models
that assume void in the interclump space cannot reproduce the observed O VI
profiles. However, enough O VI can be produced if the voids are filled by a low
density gas. The recombination of O VI is very efficient in the dense material
but in the tenuous interclump region an observable amount of O VI can be
maintained. We also find that different UV resonance lines are sensitive to
different density regimes in Zeta Pup : C IV is almost exclusively formed
within the densest regions, while the majority of O VI resides between clumps.
N V is an intermediate case, with contributions from both the tenuous gas and
clumps.Comment: Accepted for publication in ApJL, 4 pages with 3 figure
Synthetic line and continuum linear-polarisation signatures of axisymmetric type II supernova ejecta
We present synthetic single-line and continuum linear-polarisation signatures
due to electron scattering in axially-symmetric Type II supernovae (SNe) which
we calculate using a Monte Carlo and a long-characteristic radiative-transfer
code. Aspherical ejecta are produced by prescribing a latitudinal scaling or
stretching of SN ejecta inputs obtained from 1-D non-LTE time-dependent
calculations. We study polarisation signatures as a function of inclination,
shape factor, wavelength, line identity, post-explosion time. At early times,
cancellation and optical-depth effects make the polarisation intrinsically low,
causing complicated sign reversals with inclination or continuum wavelength,
and across line profiles. While the line polarisation is positive (negative)
for an oblate (prolate) morphology at the peak and in the red wing, the
continuum polarisation may be of any sign. These complex polarisation
variations are produced not just by the asymmetric distribution of scatterers
but also of the flux. Our early-time signatures are in contradiction with
predictions for a centrally illuminated aspherical nebula, although this
becomes a better approximation at nebular times. For a fixed asymmetry, our
synthetic continuum polarisation is generally low, may evolve non-monotonically
during the plateau phase, but it systematically rises as the ejecta become
optically thin. Changes in polarization over time do not necessarily imply a
change in the asymmetry of the ejecta. The SN structure (e.g.,
density/ionization) critically influences the level of polarisation.
Importantly, a low polarisation (<0.5%) at early times does not necessarily
imply a low degree of asymmetry as usually assumed. Asphericity influences
line-profile morphology and the luminosity, which may compromise the accuracy
of SN characteristics inferred from these.Comment: 25 pages, 23 figures, accepted to MNRA
Time Dependent Radiative Transfer Calculations for Supernovae
In previous papers we discussed results from fully time-dependent radiative
transfer models for core-collapse supernova (SN) ejecta, including the Type
II-peculiar SN 1987A, the more "generic" SN II-Plateau, and more recently Type
IIb/Ib/Ic SNe. Here we describe the modifications to our radiative modeling
code, CMFGEN, which allowed those studies to be undertaken. The changes allow
for time-dependent radiative transfer of SN ejecta in homologous expansion. In
the modeling we treat the entire SN ejecta, from the innermost layer that does
not fall back on the compact remnant out to the progenitor surface layers. From
our non-LTE time-dependent line-blanketed synthetic spectra, we compute the
bolometric and multi-band light curves: light curves and spectra are thus
calculated simultaneously using the same physical processes and numerics. These
upgrades, in conjunction with our previous modifications which allow the
solution of the time dependent rate equations, will improve the modeling of SN
spectra and light curves, and hence facilitate new insights into SN ejecta
properties, the SN progenitors and the explosion mechanism(s). CMFGEN can now
be applied to the modeling of all SN typesComment: 20 pages, 10 figures, to appear in MNRA
Type-I superconductivity in noncentrosymmetric superconductor AuBe
The noncentrosymmetric superconductor AuBe have been investigated using the
magnetization, resistivity, specific heat, and muon-spin relaxation/rotation
measurements. AuBe crystallizes in the cubic FeSi-type B20 structure with
superconducting transition temperature observed at = 3.2 0.1 K.
The low-temperature specific heat data, (T), indicate a weakly-coupled
fully gapped BCS superconductivity with an isotropic energy gap
2 = 3.76, which is close to the BCS value of 3.52.
Interestingly, type-I superconductivity is inferred from the SR
measurements, which is in contrast with the earlier reports of type-II
superconductivity in AuBe. The Ginzburg-Landau parameter is = 0.4
1/. The transverse-field SR data transformed in the maximum
entropy spectra depicting the internal magnetic field probability distribution,
P(H), also confirms the absence of the mixed state in AuBe. The thermodynamic
critical field, , calculated to be around 259 Oe. The zero-field SR
results indicate that time-reversal symmetry is preserved and supports a
spin-singlet pairing in the superconducting ground state.Comment: 9 pages, 9 figure
Scaling and Universality in City Space Syntax: between Zipf and Matthew
We report about universality of rank-integration distributions of open spaces
in city space syntax similar to the famous rank-size distributions of cities
(Zipf's law). We also demonstrate that the degree of choice an open space
represents for other spaces directly linked to it in a city follows a power law
statistic. Universal statistical behavior of space syntax measures uncovers the
universality of the city creation mechanism. We suggest that the observed
universality may help to establish the international definition of a city as a
specific land use pattern.Comment: 24 pages, 5 *.eps figure
A one-dimensional Chandrasekhar-mass delayed-detonation model for the broad-lined Type Ia supernova 2002bo
We present 1D non-local thermodynamic equilibrium (non-LTE) time-dependent
radiative-transfer simulations of a Chandrasekhar-mass delayed-detonation model
which synthesizes 0.51 Msun of 56Ni, and confront our results to the Type Ia
supernova (SN Ia) 2002bo over the first 100 days of its evolution. Assuming
only homologous expansion, this same model reproduces the bolometric and
multi-band light curves, the secondary near-infrared (NIR) maxima, and the
optical and NIR spectra. The chemical stratification of our model qualitatively
agrees with previous inferences by Stehle et al., but reveals significant
quantitative differences for both iron-group and intermediate-mass elements. We
show that +/-0.1 Msun (i.e., +/-20 per cent) variations in 56Ni mass have a
modest impact on the bolometric and colour evolution of our model. One notable
exception is the U-band, where a larger abundance of iron-group elements
results in less opaque ejecta through ionization effects, our model with more
56Ni displaying a higher near-UV flux level. In the NIR range, such variations
in 56Ni mass affect the timing of the secondary maxima but not their magnitude,
in agreement with observational results. Moreover, the variation in the I, J,
and K_s magnitudes is less than 0.1 mag within ~10 days from bolometric
maximum, confirming the potential of NIR photometry of SNe Ia for cosmology.
Overall, the delayed-detonation mechanism in single Chandrasekhar-mass white
dwarf progenitors seems well suited for SN 2002bo and similar SNe Ia displaying
a broad Si II 6355 A line. Whatever multidimensional processes are at play
during the explosion leading to these events, they must conspire to produce an
ejecta comparable to our spherically-symmetric model.Comment: Accepted for publication in MNRAS. The hydrodynamical input and
synthetic spectra are available at https://www-n.oca.eu/supernova/home.html .
Minor changes from v1: corrected several typos and updated acknowledgement
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
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