6,437 research outputs found
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
Highly Excited Core Resonances in Photoionization of Fe XVII : Implications for Plasma Opacities
A comprehensive study of high-accuracy photoionization cross sections is
carried out using the relativistic Breit-Pauli R-matrix (BPRM) method for (hnu
+ Fe XVII --> Fe XVIII + e). Owing to its importance in high-temperature
plasmas the calculations cover a large energy range, particularly the myriad
photoexciation-of-core (PEC) resonances including the n = 3 levels not
heretofore considered. The calculations employ a close coupling wave function
expansion of 60 levels of the core ion Fe XVIII ranging over a wide energy
range of nearly 900 eV between the n = 2 and n = 3 levels. Strong coupling
effects due to dipole transition arrays 2p^5 --> 2p^4 (3s,3d) manifest
themselves as large PEC resonances throughout this range, and enhance the
effective photoionization cross sections orders of magnitude above the
background. Comparisons with the erstwhile Opacity Project (OP) and other
previous calculations shows that the currently available cross sections
considerably underestimate the bound-free cross sections. A
level-identification scheme is used for spectroscopic designation of the 454
bound fine structure levels of Fe XVII. Level-specific photoionization cross
sections are computed for all levels. In addition, partial cross sections for
leaving the core ion Fe XVII in the ground state are also obtained. These
results should be relevant to modeling of astrophysical and laboratory plasma
sources requiring (i) photoionization rates, (ii) extensive
non-local-thermodynamic-equilibrium models, (iii) total unified electron-ion
recombination rates including radiative and dielectronic recombination, and
(iv) plasma opacities. We particularly examine PEC and non-PEC resonance
strengths and emphasize their expanded role to incorporate inner-shell
excitations for improved opacities, as shown by the computed monochromatic
opacity of Fe XVII.Comment: 12 pages, 5 figures, Physical Review A (in press
R-matrix calculations for opacities: III. Plasma broadening of autoionizing resonances
A general formulation is employed to study and quantitatively ascertain the
effect of plasma broadening of {\it intrinsic} autoionizing (AI) resonances in
photoionization cross sections. In particular, R-matrix data for iron ions
described in the previous paper in the RMOP series (RMOP-II, hereafter RMOP2)
are used to demonstrate underlying physical mechanisms due to electron
collisions, ion microfields (Stark), thermal Doppler effects, core excitations,
and free-free transitions. Breit-Pauli R-matrix (BPRM) cross section for the
large number of bound levels of Fe ions are considered, 454 levels of Fe~XVII,
1,184 levels of Fe~XVIII and 508 levels of Fe~XIX. Following a description of
theoretical and computational methods, a sample of results is presented to show
significant broadening and shifting of AI resonances due to {\it Extrinsic}
plasma broadening as a function of temperature and density. Redistribution of
AI resonance strengths broadly preserves their integrated strengths as well as
the naturally {\it intrinsic} asymmetric shapes of resonance complexes which
are broadened, smeared and flattened, eventually dissolving into the bound-free
continua.Comment: 20 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:2301.0773
K-shell dielectronic resonances in photoabsorption: differential oscillator strengths for Li-like C IV, O VI, and Fe XXIV
Recently X-ray photoabsorption in KLL resonances of O VI was predicted
[Pradhan, Astrophys.J. Lett. 545, L165 (2000)], and detected by the Chandra
X-ray Observatory [Lee et al, Astrophys. J. {\it Lett.}, submitted].
The required resonance oscillator strengths f_r, are evaluated in terms of
the differential oscillator strength df/de that relates bound and continuum
absorption. We present the f_r values from radiatively damped and undamped
photoionization cross sections for Li-like C,O, and Fe calculated using
relativistic close coupling Breit-Pauli R-matrix method. The KLL resonances of
interest here are: 1s2p (^3P^o) 2s [^4P^o_{1/2,3/2}, ^2P^o_{1/2,3/2}] and 1s2p
(^1P^o) 2s [^2P^o_{1/2,3/2}]. The KLL photoabsorption resonances in Fe XXIV are
fully resolved up to natural autoionization profiles for the first time. It is
demonstrated that the undamped f_r independently yield the resonance radiative
decay rates, and thereby provide a precise check on the resolution of
photoionization calculations in general. The predicted photoabsorption features
should be detectable by the X-ray space observatories and enable column
densities in highly ionized astrophysical plasmas to be determined from the
calculated f_r. The dielectronic satellites may appear as redward broadening of
resonances lines in emission and absorption.Comment: 9 pages, 2 figurs, Phys. Rev. A, Rapid Communication (submitted
Magnetic Order Beyond RKKY in the Classical Kondo Lattice
We study the Kondo lattice model of band electrons coupled to classical
spins, in three dimensions, using a combination of variational calculation and
Monte Carlo. We use the weak coupling `RKKY' window and the strong coupling
regime as benchmarks, but focus on the physically relevant intermediate
coupling regime. Even for modest electron-spin coupling the phase boundaries
move away from the RKKY results, the non interacting Fermi surface no longer
dictates magnetic order, and weak coupling `spiral' phases give way to
collinear order. We use these results to revisit the classic problem of 4f
magnetism and demonstrate how both electronic structure and coupling effects
beyond RKKY control the magnetism in these materials.Comment: 6 pages, 4 figs. Improved figures, expanded captions. To appear in
Europhys. Let
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