1,425 research outputs found
Ipopv2: Photoionization of Ni XIV -- a test case
Several years ago, M. Asplund and coauthors (2004) proposed a revision of the
Solar composition. The use of this new prescription for Solar abundances in
standard stellar models generated a strong disagreement between the predictions
and the observations of Solar observables. Many claimed that the Standard Solar
Model (SSM) was faulty, and more specifically the opacities used in such
models. As a result, activities around the stellar opacities were boosted. New
experiments (J. Bailey at Sandia on Z-Pinch, The OPAC consortium at LULI200) to
measure directly absorbtion coefficients have been realized or are underway.
Several theoretical groups (CEA-OPAS, Los Alamos Nat. Lab., CEA-SCORCG, The
Opacity Project - The Iron Project (IPOPv2)) have started new sets of
calculations using different approaches and codes. While the new results seem
to confirm the good quality of the opacities used in SSM, it remains important
to improve and complement the data currently available. We present recent
results in the case of the photoionization cross sections for Ni XIV (Ni13+ )
from IPOPv2 and possible implications on stellar modelling.Comment: 10 pages, 3 figures, Conf. on New Advances in Stellar Physics: From
Microscopic to Macroscopic Processe
Radiative transition rates and collision strengths for Si II
Aims. This work reports radiative transition rates and electron impact
excitation collision strengths for levels of the 3s23p, 3s3p2, 3s24s, and 3s23d
configurations of Siii. Methods. The radiative data were computed using the
Thomas-Fermi-Dirac-Amaldi central potential, but with the modifications
introduced by Bautista (2008) that account for the effects of electron-electron
interactions. We also introduce new schemes for the optimization of the
variational parameters of the potential. Additional calculations were carried
out with the Relativistic Hartree-Fock and the multiconfiguration Dirac-Fock
methods. Collision strengths in LS-coupling were calculated in the close
coupling approximation with the R-matrix method. Then, fine structure collision
strengths were obtained by means of the intermediate-coupling frame
transformation (ICFT) method which accounts for spin-orbit coupling effects.
Results. We present extensive comparisons between the results of different
approximations and with the most recent calculations and experiment available
in the literature. From these comparisons we derive a recommended set of gf-
values and radiative transition rates with their corresponding estimated
uncertainties. We also study the effects of different approximations in the
representation of the target ion on the electron-impact collision strengths.
Our most accurate set of collision strengths were integrated over a Maxwellian
distribution of electron energies and the resulting effective collision
strengths are given for a wide range of temperatures. Our results present
significant differences from recent calculations with the B-spline
non-orthogonal R-matrix method. We discuss the sources of the differences.Comment: 6 figures, 5 tables within text, 2 electronic table
Thermal denaturation of fluctuating finite DNA chains: the role of bending rigidity in bubble nucleation
Statistical DNA models available in the literature are often effective models
where the base-pair state only (unbroken or broken) is considered. Because of a
decrease by a factor of 30 of the effective bending rigidity of a sequence of
broken bonds, or bubble, compared to the double stranded state, the inclusion
of the molecular conformational degrees of freedom in a more general mesoscopic
model is needed. In this paper we do so by presenting a 1D Ising model, which
describes the internal base pair states, coupled to a discrete worm like chain
model describing the chain configurations [J. Palmeri, M. Manghi, and N.
Destainville, Phys. Rev. Lett. 99, 088103 (2007)]. This coupled model is
exactly solved using a transfer matrix technique that presents an analogy with
the path integral treatment of a quantum two-state diatomic molecule. When the
chain fluctuations are integrated out, the denaturation transition temperature
and width emerge naturally as an explicit function of the model parameters of a
well defined Hamiltonian, revealing that the transition is driven by the
difference in bending (entropy dominated) free energy between bubble and
double-stranded segments. The calculated melting curve (fraction of open base
pairs) is in good agreement with the experimental melting profile of
polydA-polydT. The predicted variation of the mean-square-radius as a function
of temperature leads to a coherent novel explanation for the experimentally
observed thermal viscosity transition. Finally, the influence of the DNA strand
length is studied in detail, underlining the importance of finite size effects,
even for DNA made of several thousand base pairs.Comment: Latex, 28 pages pdf, 9 figure
Radiative rates and opacity calculations in Ce II-IV Multiconfiguration Dirac-Hartree-Fock radiative parameters for emission lines in Ce II-IV ions and cerium opacity calculations for kilonovae
Large-scale calculations of atomic structures and radiative properties have
been carried out for singly, doubly- and trebly ionized cerium. For this
purpose, the purely relativistic multiconfiguration Dirac-Hartree-Fock (MCDHF)
method was used, taking into account the effects of valence-valence and
core-valence electronic correlations in detail. The results obtained were then
used to calculate the expansion opacities characterizing the kilonovae observed
as a result of neutron star mergers. Comparisons with previously published
experimental and theoretical studies have shown that the results presented in
this work are the most complete currently available, in terms of quantity and
quality, concerning the atomic data and monochromatic opacities for Ce II, Ce
III and Ce IV ions.Comment: Submitted to MNRA
Recommended from our members
Transition Probabilities Of Astrophysical Interest In The Niobium Ions Nb+ And Nb2+
Aims. We attempt to derive accurate transition probabilities for astrophysically interesting spectral lines of Nb II and Nb III and determine the niobium abundance in the Sun and metal-poor stars rich in neutron-capture elements. Methods. We used the time-resolved laser-induced fluorescence technique to measure radiative lifetimes in Nb II. Branching fractions were measured from spectra recorded using Fourier transform spectroscopy. The radiative lifetimes and the branching fractions were combined yielding transition probabilities. In addition, we calculated lifetimes and transition probablities in Nb II and Nb III using a relativistic Hartree-Fock method that includes core polarization. Abundances of the sun and five metal-poor stars were derived using synthetic spectra calculated with the MOOG code, including hyperfine broadening of the lines. Results. We present laboratory measurements of 17 radiative lifetimes in Nb II. By combining these lifetimes with branching fractions for lines depopulating the levels, we derive the transition probabilities of 107 Nb II lines from 4d(3)5p configuration in the wavelength region 2240-4700 angstrom. For the first time, we present theoretical transition probabilities of 76 Nb III transitions with wavelengths in the range 1430-3140 angstrom. The derived solar photospheric niobium abundance log epsilon(circle dot) = 1.44 +/- 0.06 is in agreement with the meteoritic value. The stellar Nb/Eu abundance ratio determined for five metal-poor stars confirms that the r-process is a dominant production method for the n-capture elements in these stars.Integrated Initiative of Infrastructure RII3-CT-2003-506350Swedish Research CouncilKnut and Alice Wallenberg FoundationBelgian FRS-FNRSFRIAUS National Science Foundation AST-0607708, AST-0908978Astronom
Nitrogen K-shell photoabsorption
Reliable atomic data have been computed for the spectral modeling of the
nitrogen K lines, which may lead to useful astrophysical diagnostics. Data sets
comprise valence and K-vacancy level energies, wavelengths, Einstein
-coefficients, radiative and Auger widths and K-edge photoionization cross
sections. An important issue is the lack of measurements which are usually
employed to fine-tune calculations so as to attain spectroscopic accuracy. In
order to estimate data quality, several atomic structure codes are used and
extensive comparisons with previous theoretical data have been carried out. In
the calculation of K photoabsorption with the Breit--Pauli -matrix method,
both radiation and Auger damping, which cause the smearing of the K edge, are
taken into account. This work is part of a wider project to compute atomic data
in the X-ray regime to be included in the database of the popular {\sc xstar}
modeling code
Updated opacities from the opacity project
Using the code autostructure, extensive calculations of inner-shell atomic data have been made for the chemical elements He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used to obtain updated opacities from the Opacity Project (OP). A number of other improvements on earlier work have also been included. Rosseland-mean opacities from the OP are compared with those from OPAL. Differences of 5-10 per cent occur. The OP gives the 'Z-bump', at log(T) 5.2, to be shifted to slightly higher temperatures. The opacities from the OP, as functions of temperature and density, are smoother than those from OPAL. The accuracy of the integrations used to obtain mean opacities can depend on the frequency mesh used. Tests involving variation of the numbers of frequency points show that for typical chemical mixtures the OP integrations are numerically correct to within 0.1 per cent. The accuracy of the interpolations used to obtain mean opacities for any required values of temperature and density depends on the temperature-density meshes used. Extensive tests show that, for all cases of practical interest, the OP interpolations give results correct to better than 1 per cent. Prior to a number of recent investigations which have indicated a need for downward revisions in the solar abundances of oxygen and other elements, there was good agreement between properties of the Sun deduced from helioseismology and from stellar evolution models calculated using OPAL opacities. The revisions destroy that agreement. In a recent paper, Bahcall et al. argue that the agreement would be restored if opacities for the regions of the Sun with 2 × 106T 5 × 106 K (0.7-0.4 R) were larger than those given by OPAL by about 10 per cent. In the region concerned, the present results from the OP do not differ from those of OPAL by more than 2.5 per cent
A Quantitative Comparison of Opacities Calculated Using the Distorted- Wave and -Matrix Methods
The present debate on the reliability of astrophysical opacities has reached
a new climax with the recent measurements of Fe opacities on the Z-machine at
the Sandia National Laboratory \citep{Bailey2015}. To understand the
differences between theoretical results, on the one hand, and experiments on
the other, as well as the differences among the various theoretical results,
detailed comparisons are needed. Many ingredients are involved in the
calculation of opacities; deconstructing the whole process and comparing the
differences at each step are necessary to quantify their importance and impact
on the final results. We present here such a comparison using the two main
approaches to calculate the required atomic data, the -Matrix and
distorted-wave methods, as well as sets of configurations and coupling schemes
to quantify the effects on the opacities for the and ions.Comment: 10 pages, 2 figure
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