753 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
Stellar laboratories III. New Ba V, Ba VI, and Ba VII oscillator strengths and the barium abundance in the hot white dwarfs G191-B2B and RE0503-289
For the spectral analysis of high-resolution and high-signal-to-noise (S/N)
spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium
(NLTE) model atmospheres are mandatory. These are strongly dependent on the
reliability of the atomic data that is used for their calculation. Reliable Ba
V - VII oscillator strengths are used to identify Ba lines in the spectra of
the DA-type white dwarf G191-B2B and the DO-type white dwarf RE0503-289 and to
determine their photospheric Ba abundances. We newly calculated Ba V - VII
oscillator strengths to consider their radiative and collisional bound-bound
transitions in detail in our NLTE stellar-atmosphere models for the analysis of
Ba lines exhibited in high-resolution and high-S/N UV observations of G191-B2B
and RE0503-289. For the first time, we identified highly ionized Ba in the
spectra of hot white dwarfs. We detected Ba VI and Ba VII lines in the Far
Ultraviolet Spectroscopic Explorer (FUSE) spectrum of RE0503-289. The Ba VI /
Ba VII ionization equilibrium is well reproduced with the previously determined
effective temperature of 70000 K and surface gravity of . The Ba
abundance is (mass fraction, about 23000 times the
solar value). In the FUSE spectrum of G191-B2B, we identified the strongest Ba
VII line (at 993.41 \AA) only, and determined a Ba abundance of (about 265 times solar). Reliable measurements and calculations
of atomic data are a pre-requisite for stellar-atmosphere modeling. Observed Ba
VI - VII line profiles in two white dwarfs' (G191-B2B and RE0503-289)
far-ultraviolet spectra were well reproduced with our newly calculated
oscillator strengths. This allowed to determine the photospheric Ba abundance
of these two stars precisely.Comment: 36 pages, 8 figure
Stellar laboratories. V. The Xe VI ultraviolet spectrum and the xenon abundance in the hot DO-type white dwarf RE0503-289
For the spectral analysis of spectra of hot stars with a high resolution and
high signal-to-noise ratio (S/N), advanced non-local thermodynamic equilibrium
(NLTE) model atmospheres are mandatory. These are strongly dependent on the
reliability of the atomic data that are used for their calculation.
Reliable Xe VI oscillator strengths are used to identify Xe lines in the
ultraviolet spectrum of the DO-type white dwarf RE0503-289 and to determine its
photospheric Xe abundance.
We publish newly calculated oscillator strengths that are based on a recently
measured Xe VI laboratory line spectrum. These strengths were used to consider
their radiative and collisional bound-bound transitions in detail in our NLTE
stellar-atmosphere models to analyze Xe VI lines exhibited in high-resolution
and high S/N UV observations of RE0503-289.
We identify three hitherto unknown Xe VI lines in the ultraviolet spectrum of
RE0503-289 and confirm the previously measured photospheric Xe abundance of
this white dwarf (log Xe = -4.2 +/- 0.6).
Reliable measurements and calculations of atomic data are prerequisite for
stellar-atmosphere modeling. Observed Xe VI line profiles in the ultraviolet
spectrum of the white dwarf RE0503-289 were well reproduced with the newly
calculated Xe VI oscillator strengths.Comment: 3 pages, 4 figure
Stellar laboratories: new Ge V and Ge VI oscillator strengths and their validation in the hot white dwarf RE 0503-289
State-of-the-art spectral analysis of hot stars by means of non-LTE
model-atmosphere techniques has arrived at a high level of sophistication. The
analysis of high-resolution and high-S/N spectra, however, is strongly
restricted by the lack of reliable atomic data for highly ionized species from
intermediate-mass metals to trans-iron elements. Especially data for the latter
has only been sparsely calculated. Many of their lines are identified in
spectra of extremely hot, hydrogen-deficient post-AGB stars. A reliable
determination of their abundances establishes crucial constraints for AGB
nucleosynthesis simulations and, thus, for stellar evolutionary theory.
In a previous analysis of the UV spectrum of RE 0503-289, spectral lines of
highly ionized Ga, Ge, As, Se, Kr, Mo, Sn, Te, I, and Xe were identified.
Individual abundance determinations are hampered by the lack of reliable
oscillator strengths. Most of these identified lines stem from Ge V. In
addition, we identified Ge VI lines for the first time. We calculated Ge V and
Ge VI oscillator strengths to consider their radiative and collisional
bound-bound transitions in detail in our non-LTE stellar-atmosphere models for
the analysis of the Ge IV - VI spectrum exhibited in high-resolution and
high-S/N UV spectra of RE 0503-289. We identify four Ge IV, 37 Ge V, and seven
Ge VI lines. Most of these are identified for the first time in any star. We
reproduce almost all Ge IV, Ge VI, and Ge VI lines in the observed spectrum of
RE 0503-289 (Teff = 70 kK, log g = 7.5) at log Ge = -3.8 +/- 0.3 (mass
fraction, about 650 times solar).
Reliable measurements and calculations of atomic data are a prerequisite for
stellar-atmosphere modeling. Our oscillator-strength calculations have allowed,
for the first time, Ge V and Ge VI lines to be successfully reproduced in a
white dwarf's spectrum and to determine its photospheric Ge abundance.Comment: 54 pages, 8 figure
High-precision Atomic Physics Laboratories in Space: White Dwarfs and Subdwarfs
The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018Stellar atmospheres are prime laboratories to determine
atomic properties of highly ionized species.
Reliable opacities are crucial ingredients for the
calculation of stellar atmospheres of white dwarfs
and subdwarfs. A detailed investigation on the
precision of many iron-group oscillator strengths
is still outstanding. To make progress, we used
the Hubble Space Telescope Imaging Spectrograph
to measure high-resolution spectra of three hot
subdwarfs that exhibit extremely high iron-group
abundances. The predicted relative strengths of
the identified lines are compared with the observations
to judge the quality of Kurucz’s line data
and to determine correction factors for abundance
determinations of the respective elements.Astronom
Complete spectral energy distribution of the hot, helium-rich white dwarf RX J0503.9-2854
In the line-of-sight toward the DO-type white dwarf RX J0503.9-2854, the
density of the interstellar medium (ISM) is very low, and thus the
contamination of the stellar spectrum almost negligible. This allows us to
identify many metal lines in a wide wavelength range from the extreme
ultraviolet to the near infrared. In previous spectral analyses, many metal
lines in the ultraviolet spectrum of RX J0503.9-2854 have been identified. A
complete line list of observed and identified lines is presented here. We
compared synthetic spectra that had been calculated from model atmospheres in
non-local thermodynamical equilibrium, with observations. In total, we
identified 1272 lines (279 of them were newly assigned) in the wavelength range
from the extreme ultraviolet to the near infrared. 287 lines remain
unidentified. A close inspection of the EUV shows that still no good fit to the
observed shape of the stellar continuum flux can be achieved although He, C, N,
O, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Cr, Ni Zn, Ga, Ge, As, Kr, Zr, Mo,
Sn, Xe, and Ba are included in the stellar atmosphere models. There are two
possible reasons for the deviation between observed and synthetic flux in the
EUV. Opacities from hitherto unconsidered elements in the model-atmosphere
calculation may be missing and/or the effective temperature is slightly lower
than previously determined.Comment: 92 pages, 45 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
Stellar laboratories. IX. New Se V, Sr IV - VII, Te VI, and I VI oscillator strengths and the Se, Sr, Te, and I abundances in the hot white dwarfs G191-B2B and RE 0503-289
To analyze spectra of hot stars, advanced non-local thermodynamic equilibrium
(NLTE) model-atmosphere techniques are mandatory. Reliable atomic data is for
the calculation of such model atmospheres.
We aim to calculate new Sr IV - VII oscillator strengths to identify for the
first time Sr spectral lines in hot white dwarf (WD) stars and to determine the
photospheric Sr abundances. o measure the abundances of Se, Te, and I in hot
WDs, we aim to compute new Se V, Te VI, and I VI oscillator strengths.
To consider radiative and collisional bound-bound transitions of Se V, Sr IV
- VII, Te VI, and I VI in our NLTE atmosphere models, we calculated oscillator
strengths for these ions.
We newly identified four Se V, 23 Sr V, 1 Te VI, and three I VI lines in the
ultraviolet (UV) spectrum of RE0503-289. We measured a photospheric Sr
abundance of 6.5 +3.8/-2.4 x 10**-4 (mass fraction, 9500 - 23800 times solar).
We determined the abundances of Se (1.6 +0.9/-0.6 x 10**-3, 8000 - 20000), Te
(2.5 +1.5/-0.9 x 10**-4, 11000 - 28000), and I (1.4 +0.8/-0.5 x 10**-5, 2700 -
6700). No Se, Sr, Te, and I line was found in the UV spectra of G191-B2B and we
could determine only upper abundance limits of approximately 100 times solar.
All identified Se V, Sr V, Te VI, and I VI lines in the UV spectrum of
RE0503-289 were simultaneously well reproduced with our newly calculated
oscillator strengths.Comment: 26 pages, 5 figure
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