128 research outputs found
Collision strengths and transition probabilities for Co II infrared forbidden lines
We calculate collision strengths and their thermally-averaged Maxwellian
values for electron excitation and de-excitation between the fifteen lowest
levels of singly-ionised cobalt, Co+, which give rise to emission lines in the
near- and mid-infrared. Transition probabilities are also calculated and
relative line intensities predicted for conditions typical of supernova ejecta.
The diagnostic potential of the 10.52, 15.46 and 14.74 micro-metre transition
lines is briefly discussed.Comment: 9 pages, 4 figures, 7 table
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
Atomic data from the Iron Project.XLIII. Transition probabilities for Fe V
An extensive set of dipole-allowed, intercombination, and forbidden
transition probabilities for Fe V is presented. The Breit-Pauli R-matrix (BPRM)
method is used to calculate 1.46 x 10^6 oscillator strengths for the allowed
and intercombination E1 transitions among 3,865 fine-structure levels dominated
by configuration complexes with n <= 10 and l <= 9. These data are complemented
by an atomic structure configuration interaction (CI) calculation using the
SUPERSTRUCTURE program for 362 relativistic quadrupole (E2) and magnetic dipole
(M1) transitions among 65 low-lying levels dominated by the 3d^4 and 3d^ 4s
configurations. Procedures have been developed for the identification of the
large number of fine-structure levels and transitions obtained through the BPRM
calculations. The target ion Fe VI is represented by an eigenfunction expansion
of 19 fine-structure levels of 3d^3 and a set of correlation configurations. Fe
V bound levels are obtained with angular and spin symmetries SL\pi and J\pi of
the (e + Fe VI) system such that 2S+1 = 5,3,1, L <= 10, J <= 8 of even and odd
parities. The completeness of the calculated dataset is verified in terms of
all possible bound levels belonging to relevant LS terms and transitions in
correspondence with the LS terms. The fine-structure averaged relativistic
values are compared with previous Opacity Project LS coupling data and other
works. The 362 forbidden transition probabilities considerably extend the
available data for the E2 and M1 transtions, and are in good agreement with
those computed by Garstang for the 3d^4 transitions.Comment: 19 pages, 1 figure. This paper marks the beginning of a large-scale
effort of ab initio atomic calculations that should eventually lead to
re-calculation of accurate iron opacities. Astron. Astrophys. Suppl. Ser. (in
press
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
Studying the Variation of the Fine Structure Constant Using Emission Line Multiplets
As an extension of the method by Bahcall et al. (2004) to investigate the
time dependence of the fine structure constant, we describe an approach based
on new observations of forbidden line multiplets from different ionic species.
We obtain optical spectra of fine structure transitions in [Ne III], [Ne V], [O
III], [OI], and [SII] multiplets from a sample of 14 Seyfert 1.5 galaxies in
the low-z range 0.035 < z < 0.281. Each source and each multiplet is
independently analyzed to ascertain possible errors. Averaging over our sample,
we obtain a conservative value alpha^2(t)/\alpha^2(0) = 1.0030+-0.0014.
However, our sample is limited in size and our fitting technique simplistic as
we primarily intend to illustrate the scope and strengths of emission line
studies of the time variation of the fine structure constant. The approach can
be further extended and generalized to a "many-multiplet emission line method"
analogous in principle to the corresponding method using absorption lines. With
that aim, we note that the theoretical limits on emission line ratios of
selected ions are precisely known, and provide well constrained selection
criteria. We also discuss several other forbidden and allowed lines that may
constitute the basis for a more rigorous study using high-resolution
instruments on the next generation of 8 m class telescopes.Comment: 20 pages, 4 figures, sumbitted to A
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
A theoretical study of the C- 4So_3/2 and 2Do_{3/2,5/2} bound states and C ground configuration: fine and hyperfine structures, isotope shifts and transition probabilities
This work is an ab initio study of the 2p3 4So_3/2, and 2Do_{3/2,5/2} states
of C- and 2p2 3P_{0,1,2}, 1D_2, and 1S_0 states of neutral carbon. We use the
multi-configuration Hartree-Fock approach, focusing on the accuracy of the wave
function itself. We obtain all C- detachment thresholds, including correlation
effects to about 0.5%. Isotope shifts and hyperfine structures are calculated.
The achieved accuracy of the latter is of the order of 0.1 MHz.
Intra-configuration transition probabilities are also estimated.Comment: 15 pages, 2 figures, 12 table
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