56 research outputs found
Theoretical Atomic Spectroscopy of Earthbound and Stellar Plasma
Motivated by spectroscopic analysis of astrophysical and laboratory plasma, this thesis concerns the fundamental structure and spectral properties of atoms and their ions. The multiconfiguration Dirac-Hartree-Fock (MCDHF) method is used to predict the emission or absorption of radiation, by atomic systems in general, and of heavy and highly charged ions in particular.The first set of publications, paper AI to AVII, concerns ab-initio predictions of atomic structure and radiative transition rates, with a particular focus on relativistic and electron correlation effects. Systematic and large-scale MCDHF calculations have been carried out, often in combination with electron-beam ion trap experiments.The second set, BI to BVIII, presents a rigorous treatment of effects from non-spherical interactions with certain nuclei - hyperfine interaction - and external magnetic fields - Zeeman interaction - on atomic spectra. A general methodology has been developed and implemented in computer codes to include these perturbations in the wavefunctions and to determine their impact on the resulting spectra. Of particular interest are spectral intensity redistributions and unexpected transitions, and their applications to stellar abundance analyses, magnetic-fields effects in storage-ring experiments, and coronal magnetic-field measurements
Carbon monoxide formation and cooling in supernovae
The inclusion of molecular physics is an important piece that tends to be
missing from the puzzle when modeling the spectra of supernovae (SNe).
Molecules have both a direct impact on the spectra, particularly in the
infrared, and an indirect one as a result of their influence on certain
physical conditions, such as temperature. In this paper, we aim to investigate
molecular formation and non-local thermodynamic equilibrium (NLTE) cooling,
with a particular focus on CO, the most commonly detected molecule in
supernovae. We also aim to determine the dependency of supernova chemistry on
physical parameters and the relative sensitivity to rate uncertainties. We
implemented a chemical kinetic description of the destruction and formation of
molecules into the SN spectral synthesis code SUMO. In addition, selected
molecules were coupled into the full NLTE level population framework and, thus,
we incorporated molecular NLTE cooling into the temperature equation. We
produced a test model of the CO formation in SN 1987A between 150 and 600 days
and investigated the sensitivity of the resulting molecular masses to the input
parameters. We find that there is a close inter-dependency between the thermal
evolution and the amount of CO formed, mainly through an important
temperature-sensitive CO destruction process with O+. After a few hundred days,
CO completely dominates the cooling of the oxygen-carbon zone of the supernova
which, therefore, contributes little optical emission. The uncertainty of the
calculated CO mass scales approximately linearly with the typical uncertainty
factor for individual rates. We demonstrate how molecular masses can
potentially be used to constrain various physical parameters of the supernova
Coronal lines and the importance of deep core-valence correlation in Ag-like ions
We report on large-scale and critically evaluated {\em ab initio} MCDHF
calculations of the wavelength of the "coronal", M1 transition $4f\
^2\mathrm{F}_{5/2}^o-^2\mathrm{F}_{7/2}^oZ
\ge 62^{23+}^{27+}Z = 5094n=3$ shell in the theoretical model is emphasized. The
results show close to spectroscopic accuracy for these forbidden lines.Comment: 10 pages, 5 figures, 3 table
NLTE Spectra of Kilonovae
The electromagnetic transient following a binary neutron star merger is known
as a kilonova (KN). Owing to rapid expansion velocities and small ejecta
masses, KNe rapidly transition into the Non-Local Thermodynamic Equilibrium
(NLTE) regime. In this study, we present synthetic NLTE spectra of KNe from 5
to 20 days after merger using the \texttt{SUMO} spectral synthesis code. We
study three homogeneous composition, 1D multi-zone models with characteristic
electron fractions of and . We find that emission
features in the spectra tend to emerge in windows of reduced line blocking, as
the ejecta are still only partially transparent even at 20 days. For the (lanthanide-free) ejecta, we find that the neutral and singly
ionised species of Rb, Sr, Y and Zr dominate the spectra, all with good
potential for identification. We directly test and confirm an impact of Sr on
the 10000 angstrom spectral region in lanthanide-free ejecta, but also see that
its signatures may be complex. We suggest the Rb I -
7900 angstrom transition as a candidate for the 7500--7900
angstrom P-Cygni feature in AT2017gfo. For the and
compositions, lanthanides are dominant in the spectral formation, in particular
Nd, Sm, and Dy. We identify key processes in KN spectral formation, notably
that scattering and fluorescence play important roles even up to 20 days after
merger, implying that the KN ejecta are not yet optically thin at this time.Comment: 20 pages (29 with appendices), 17 figures, resubmitted to MNRAS after
referee repor
The effect of an external magnetic field on the determination of E1M1 two-photon decay rates in Be-like ions
In this work we report on ab initio theoretical results for the magnetic
field induced 2s2p ^3P_0 - 2s^2 ^1S_0 E1 transition for ions in the beryllium
isoelectronic sequence between Z=5 and 92. It has been proposed that the rate
of the E1M1 two-photon transition 2s2p ^3P_0 - 2s^2 ^1S_0 can be extracted from
the lifetime of the ^3P_0 state in Be-like ions with zero nuclear spin by
employing resonant recombination in a storage-ring. This experimental approach
involves a perturbing external magnetic field. The effect of this field needs
to be evaluated in order to properly extract the two-photon rate from the
measured decay curves. The magnetic field induced transition rates are
carefully evaluated and it is shown that, with a typical storage-ring field
strength, it is dominant or of the same order as the E1M1 rate for low- and
mid-Z ions. Results for several field strengths and ions are presented and we
also give a simple Z-dependent formula for the rate. We estimate the
uncertainties of our model to be within 5% for low- and mid-Z ions, and
slightly larger for more highly charged ions. Furthermore we evaluate the
importance of including both perturber states, ^3P_1 and ^1P_1, and it is shown
that excluding the influence of the ^1P_1 perturber overestimates the rate by
up to 26% for the mid-Z ions.Comment: 21 pages, 5 figure
Extended MCDHF calculations of energy levels and transition data for N I
Accurate and extensive atomic data are essential for spectroscopic analyses
of stellar atmospheres and other astronomical objects. We present energy
levels, lifetimes, and transition probabilities for neutral nitrogen, the sixth
most abundant element in the cosmos. The calculations employ the fully
relativistic multiconfiguration Dirac-Hartree-Fock and relativistic
configuration interaction methods, and span the 103 lowest states up to and
including 2s2p5s. Our theoretical energies are in excellent agreement
with the experimental data, with an average relative difference of 0.07%. In
addition, our transition probabilities are in good agreement with available
experimental and theoretical data. We further verify the agreement of our data
with experimental results via a re-analysis of the solar nitrogen abundance,
with the results from the Babushkin and Coulomb gauges consistent to 2% or 0.01
dex. We estimated the uncertainties of the computed transition data based on a
statistical analysis of the differences between the transition rates in
Babushkin and Coulomb gauges. Out of the 1701 computed electric dipole
transitions in this work, 83 (536) are associated with uncertainties less than
5% (10%).Comment: 17 pages, 7 figures; Accepted for publication in The Astrophysical
Journal Supplement Serie
A first spectroscopic measurement of the magnetic field strength for an active region of the solar corona
For all involved in astronomy, the importance of monitoring and determining
astrophysical magnetic field strengths is clear. It is also a well-known fact
that the corona magnetic fields play an important part in the origin of solar
flares and the variations of space weather. However, after many years of solar
corona studies, there is still no direct and continuous way to measure and
monitor the solar magnetic field strength. We will here present a scheme which
allows such a measurement, based on a careful study of an exotic class of
atomic transitions known as magnetic induced transitions in Fe. In this
contribution we present a first application of this methodology and determine a
value of the coronal field strength using the spectroscopic data from HINODE
Resolving a discrepancy between experimental and theoretical lifetimes in atomic negative ions
A recent measurement of the lifetime of the excited 3p5 state in the S- negative ion, which is dominated by a forbidden magnetic dipole transition to the 2 P3/ 2 ground state, reveals a discrepancy with earlier theoretical predictions. To investigate this we have performed systematic and large-scale multiconfiguration Dirac-Hartree-Fock calculations for this system. After including a careful treatment of correlation and relativistic effects, we predict a well-converged value for this lifetime, with an uncertainty considerably less than 1%, thereby removing the apparent conflict between theory and experiment. We also show that this result corresponds to the non-relativistic limit in the LS coupling approximation for the magnetic dipole transition within this 2 P term. In addition we demonstrate the usefulness of the latter approach for 2 P transitions in O-, Se- and Te-, as well as for analogous M1 transitions within 2 D terms in Ni- and Pt- ions
Targeted optimization in small-scale atomic structure calculations : application to Au I
The lack of reliable atomic data can be a severe limitation in astrophysical modelling, in particular of events such as kilonovae that require information on all neutron-capture elements across a wide range of ionization stages. Notably, the presence of non-orthonormalities between electron orbitals representing configurations that are close in energy can introduce significant inaccuracies in computed energies and transition probabilities. Here, we propose an explicit targeted optimization (TO) method that can effectively circumvent this concern while retaining an orthonormal orbital basis set. We illustrate this method within the framework of small-scale atomic structure models of Au I, using the Grasp2018 multiconfigurational Dirac-Hartree-Fock atomic structure code. By comparing to conventional optimization schemes we show how a TO approach improves the energy level positioning and ordering. TO also leads to better agreement with experimental data for the strongest E1 transitions. This illustrates how small-scale models can be significantly improved with minor computational costs if orbital non-orthonormalities are considered carefully. These results should prove useful to multi-element atomic structure calculations in, for example, astrophysical opacity applications involving neutron-capture elements
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