2,622 research outputs found
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. VII. Relativistic calculations for O VI and O VII for UV and X-ray modeling
Aimed at ionization balance and spectral analysis of UV and X-ray sources, we
present self-consistent sets of photoionization cross sections, recombination
cross sections, and rate coefficients for Li-like O VI and He-like O VII.
Relativistic fine structure is considered through the Breit-Pauli R-matrix
(BPRM) method in the close coupling approximation, implementing the unified
treatment for total electron-ion recombination subsuming both radiative and
di-electronic recombination processes. Self-consistency is ensured by using an
identical wavefunction expansion for the inverse processes of photoionization
and photo-recombination. Radiation damping of resonances, important for H-like
and He-like core ions, is included. Compared to previous LS coupling results
without radiative decay of low-n (<= 10) resonances, the presents results show
significant reduction in O VI recombination rates at high temperatures. In
addition to the total rates, level-specific photoionization cross sections and
recombination rates are presented for all fine structure levels n (lSLJ) up to
n <= 10, to enable accurate computation of recombination-cascade matrices and
spectral formation of prominent UV and X-ray lines such as the 1032,1038 A
doublet of O VI, and the `triplet' forbidden, intercombination, and resonance
X-ray lines of O VII at 22.1, 21.8, and 21.6 \ang respectively. Altogether,
atomic parameters for 98 levels of O VI and 116 fine structure levels of O VII
are theoretically computed. These data should provide a reasonably complete set
of photoionization and recombination rates in collisional or radiative
equilibrium.Comment: 33 pages, 8 figures, submitted to ApJ
Lived Experiences of a Rohingya Journalist
Edited by Kirandeep Kau
[O II] line ratios
Based on new calculations we reconfirm the low and high density limits on the
forbidden fine structure line ratio [O II] I(3729)/I(3726): lim_{N_ e} --> 0} =
1.5 and lim_{N_ e} --> \infty} = 0.35. Employing [O II] collision strengths
calculated using the Breit-Pauli R-matrix method we rule out any significant
deviation due to relativistic effects from these canonical values. The present
results are in substantial agreement with older calculations by Pradhan (1976)
and validate the extensive observational analysis of gaseous nebulae by Copetti
and Writzel (2002) and Wang et al (2004) that reach the same conclusions. The
present theoretical results and the recent observational analyses differ
significantly from the calculations by MacLaughlin and Bell (1998) and Keenan
et al (1999). The new maxwellian averaged effective collision strengths are
presented for the 10 transitions among the first 5 levels to enable
computations of [O II] line ratios.Comment: Submitted to MNRAS (Letters), 4 pages, 2 figures, 1 tabl
High Field (up to 140kOe) Angle Dependent Magneto Transport of Bi2Te3 Single Crystals
We report the angle dependent high field (up to 140kOe) magneto transport of
Bi2Te3 single crystals, a well-known topological insulator. The crystals were
grown from melt of constituent elements via solid state reaction route by
self-flux method. Details of crystal growth along with their brief
characterization up to 5 Tesla applied field was reported by some of us
recently [J. Magn. Mag. Mater. 428, 213 (2017)]. The angle dependence of the
magneto-resistance (MR) of Bi2Te3 follows the cos Theta function i.e., MR is
responsive, when the applied field is perpendicular (tilt angle Theta = o
and/or 180) to the transport current. The low field (10 kOe) MR showed the
signatures of weak anti localization (WAL) character with typical cusp near
origin at 5 K. Further, the MR is linear right up to highest applied field of
140 kOe. The large positive MR are observed up to high temperatures and are
above 250 and 150 percent at 140 kOe in perpendicular fields at 50 K and 100 K
respectively. Heat capacity CP(T) measurements revealed the value of Debye
temperature to be 135 K. ARPES (angle resolved photoemission spectroscopy) data
clearly showed that the bulk Bi2Te3 single crystal consists of a single Dirac
cone.Comment: 13 Pages text + Figs... Letter - Mat. Res. Ex
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
New Results for Photoionization and Recombination of Astrophysically Abundant Atoms and Ions: The Carbon Sequence
Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlPhotoionization cross sections and electron-ion recombination rate coefficients are presented for carbon-like ions obtained using methods developed for the Opacity Project. General features of the new data are discussed, and their astrophysical relevance is pointed out. The illustrative results for photoionization include
extensive autoionization resonance structures and partial photoionization into specific states of the residual ion. It is shown that the excited state photoionization cross sections are not, in general, described by hydrogenic behavior and often contain strong resonances due to photoexcitation corresponding to dipole transitions in the core ion; these are referred to as photoexcitation-of-core resonances that attenuate the background
cross section over much larger energy ranges than the Rydberg type of resonances. The effective photoionization cross sections are thus considerably enhanced, for excited bound states, relative to the hydrogenic form. A new method is described for the calculation of total recombination rate coefficients that accounts for both the
radiative and the dielectronic recombination processes in an ab initio manner. The recombination calculations are carried out in the close coupling approximation using the Milne relation with detailed photoionization cross sections for large numbers of excited states of each atom or ion, and employing a precise theory of
dielectronic recombination given by Bell & Seaton. The present recombination rate coefficients are compared with earlier works on radiative and dielectronic recombinations treated separately and significant differences are noted. Recombination rate coefficients are presented for C I, N II, 0 III, F IV, and Ne V at a wide range of
temperatures for astrophysical applications.This work was supported in part by a grant from the National Science Foundation (AST-8996215)
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