248 research outputs found
Excitation and charge transfer in low-energy hydrogen atom collisions with neutral oxygen
Excitation and charge transfer in low-energy O+H collisions is studied; it is
a problem of importance for modelling stellar spectra and obtaining accurate
oxygen abundances in late-type stars including the Sun. The collisions have
been studied theoretically using a previously presented method based on an
asymptotic two-electron linear combination of atomic orbitals (LCAO) model of
ionic-covalent interactions in the neutral atom-hydrogen-atom system, together
with the multichannel Landau-Zener model. The method has been extended to
include configurations involving excited states of hydrogen using an estimate
for the two-electron transition coupling, but this extension was found to not
lead to any remarkably high rates. Rate coefficients are calculated for
temperatures in the range 1000 - 20000 K, and charge transfer and
(de)excitation processes involving the first excited S-states, 4s.5So and
4s.3So, are found to have the highest rates.Comment: Accepted for A&A. Data will be made available at CDS. Is available
here: https://github.com/barklem/public-data. Replaced version corrects url
and adds it to the pape
Excitation and charge transfer in low-energy hydrogen atom collisions with neutral iron
Data for inelastic processes due to hydrogen atom collisions with iron are
needed for accurate modelling of the iron spectrum in late-type stars.
Excitation and charge transfer in low-energy Fe+H collisions is studied
theoretically using a previously presented method based on an asymptotic
two-electron linear combination of atomic orbitals (LCAO) model of
ionic-covalent interactions in the neutral atom-hydrogen-atom system, together
with the multi-channel Landau-Zener model. An extensive calculation including
166 covalent states and 25 ionic states is presented and rate coefficients are
calculated for temperatures in the range 1000 - 20000 K. The largest rates are
found for charge transfer processes to and from two clusters of states around
6.3 and 6.6 eV excitation, corresponding in both cases to active 4d and 5p
electrons undergoing transfer. Excitation and de-excitation processes among
these two sets of states are also significant.Comment: Accepted by A&
The broadening of Fe II lines by neutral hydrogen collisions
Data for the broadening of 24188 Fe II lines by collisions with neutral
hydrogen atoms have been computed using the theory of Anstee & O'Mara as
extended to singly ionised species and higher orbital angular momentum states
by Barklem & O'Mara. Data have been computed for all Fe II lines between
observed energy levels in the line lists of Kurucz with log gf > -5 for which
the theory is applicable. The variable energy debt parameter Ep used in
computing the second order perturbation theory potential is chosen to be
consistent with the long range dispersion interaction constant C6 computed
using the f-values from Kurucz.Comment: Accepted for A&A. 5 pages, 5 figures, 2 electronic tables. Tables
will be available via CDS; presently also at
http://www.astro.uu.se/~barklem/papers/fe2_data.tar.g
A unified numerical model of collisional depolarization and broadening rates due to hydrogen atom collisions
Interpretation of solar polarization spectra accounting for partial or
complete frequency redistribution requires data on various collisional
processes. Data for depolarization and polarization transfer are needed but
often missing, while data for collisional broadening are usually more readily
available. Recent work by Sahal-Br\'echot and Bommier concluded that despite
underlying similarities in the physics of collisional broadening and
depolarization processes, relationships between them are not possible to derive
purely analytically.
We aim to derive accurate numerical relationships between the collisional
broadening rates and the collisional depolarization and polarization transfer
rates due to hydrogen atom collisions. Such relationships would enable accurate
and efficient estimation of collisional data for solar applications.
Using earlier results for broadening and depolarization processes based on
general (i.e. not specific to a given atom), semi-classical calculations
employing interaction potentials from perturbation theory, genetic programming
(GP) has been used to fit the available data and generate analytical functions
describing the relationships between them. The predicted relationships from the
GP-based model are compared with the original data to estimate the accuracy of
the method.Comment: 10 pages, 7 figures, Accepted for publication in Astronomy &
Astrophysic
Non-LTE Balmer line formation in late-type spectra: Effects of atomic processes involving hydrogen atoms
(*** abridged ***) Context: The wings of Balmer lines are often used as
effective temperature diagnostics for late-type stars under the assumption they
form in local thermodynamic equilibrium.
Aims: Our goal is to investigate the non-LTE formation of Balmer lines in
late-type stellar atmospheres, to establish if the assumption of LTE is
justified. Furthermore, we aim to determine which collision processes are
important for the problem; in particular, the role of collision processes with
hydrogen atoms is investigated.
Method: A model hydrogen atom for non-LTE calculations has been constructed
accounting for various collision processes using the best available data from
the literature. The processes included are inelastic collisions with electrons
and hydrogen atoms, mutual neutralisation and Penning ionisation. Non-LTE
calculations are performed, and the relative importance of the collision
processes is investigated.
Results: Our calculations show electron collisions alone are not sufficient
to establish LTE for the formation of Balmer line wings. The role of inelastic
collisions with neutral hydrogen is unclear. The available data for these
processes is of questionable quality, and different prescriptions for the rate
coefficents give significantly different results for the Balmer line wings.
Conclusions: Improved calculations or experimental data are needed for
excitation and, particularly, ionisation of hydrogen atoms in low-lying states
by hydrogen atom impact at near threshold energies. Until such data are
available, the assumption of LTE for the formation of Balmer line wings in
late-type stars is questionable.Comment: Accepted for A&A; 12 pages, 11 figure
Electron-impact excitation of neutral oxygen
Aims: To calculate transition rates from ground and excited states in neutral
oxygen atoms due to electron collisions for non-LTE modelling of oxygen in
late-type stellar atmospheres, thus enabling reliable interpretation of oxygen
lines in stellar spectra.
Methods: A 38-state R-matrix calculation in LS-coupling has been performed.
Basis orbitals from the literature (Thomas et al.) are adopted, and a large set
of configurations are included to obtain good representations of the target
wavefunctions. Rate coefficients are calculated by averaging over a Maxwellian
velocity distribution.
Results: Estimates for the cross sections and rate coefficients are presented
for transitions between the seven lowest LS states of neutral oxygen. The cross
sections for excitation from the ground state compare well with existing
experimental and recent theoretical results.Comment: Accepted for A&A; 9 pages, 2 figures, 6 table
Carbon and oxygen in metal-poor halo stars
Carbon and oxygen are key tracers of the Galactic chemical evolution; in
particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor
halo stars could be a signature of nucleosynthesis by massive Population III
stars. We reanalyse carbon, oxygen, and iron abundances in thirty-nine
metal-poor turn-off stars. For the first time, we take into account
three-dimensional (3D) hydrodynamic effects together with departures from local
thermodynamic equilibrium (LTE) when determining both the stellar parameters
and the elemental abundances, by deriving effective temperatures from 3D
non-LTE H profiles, surface gravities from Gaia parallaxes, iron
abundances from 3D LTE Feii equivalent widths, and carbon and oxygen abundances
from 3D non-LTE Ci and Oi equivalent widths. We find that [C/Fe] stays flat
with [Fe/H], whereas [O/Fe] increases linearly up to dex with decreasing
[Fe/H] down to dex. As such [C/O] monotonically decreases towards
decreasing [O/H], in contrast to previous findings, mainly by virtue of less
severe non-LTE effects for Oi at low [Fe/H] with our improved calculations.Comment: 5 pages, 2 figures; published in A&A Letter
Inelastic O+H collisions and the OI 777nm solar centre-to-limb variation
The OI 777 nm triplet is a key diagnostic of oxygen abundances in the
atmospheres of FGK-type stars; however it is sensitive to departures from local
thermodynamic equilibrium (LTE). The accuracy of non-LTE line formation
calculations has hitherto been limited by errors in the inelastic O+H
collisional rate coefficients: several recent studies have used the so-called
Drawin recipe, albeit with a correction factor that is
calibrated to the solar centre-to-limb variation of the triplet. We present a
new model oxygen atom that incorporates inelastic O+H collisional rate
coefficients using an asymptotic two-electron model based on linear
combinations of atomic orbitals, combined with a free electron model, based on
the impulse approximation. Using a 3D hydrodynamic stagger model solar
atmosphere and 3D non-LTE line formation calculations, we demonstrate that this
physically-motivated approach is able to reproduce the solar centre-to-limb
variation of the triplet to 0.02 dex, without any calibration of the inelastic
collisional rate coefficients or other free parameters. We infer
from the triplet alone, strengthening
the case for a low solar oxygen abundance.Comment: 13 pages, 8 figures; published in Astronomy & Astrophysic
Effective temperature determinations of late-type stars based on 3D non-LTE Balmer line formation
Hydrogen Balmer lines are commonly used as spectroscopic effective
temperature diagnostics of late-type stars. However, the absolute accuracy of
classical methods that are based on one-dimensional (1D) hydrostatic model
atmospheres and local thermodynamic equilibrium (LTE) is still unclear. To
investigate this, we carry out 3D non-LTE calculations for the Balmer lines,
performed, for the first time, over an extensive grid of 3D hydrodynamic
STAGGER model atmospheres. For H, H, and H, we find
significant 1D non-LTE versus 3D non-LTE differences (3D effects): the outer
wings tend to be stronger in 3D models, particularly for H, while the
inner wings can be weaker in 3D models, particularly for H. For
H, we also find significant 3D LTE versus 3D non-LTE differences
(non-LTE effects): in warmer stars (K) the inner
wings tend to be weaker in non-LTE models, while at lower effective
temperatures (K) the inner wings can be stronger in
non-LTE models; the non-LTE effects are more severe at lower metallicities. We
test our 3D non-LTE models against observations of well-studied benchmark
stars. For the Sun, we infer concordant effective temperatures from H,
H, and H; however the value is too low by around 50K which could
signal residual modelling shortcomings. For other benchmark stars, our 3D
non-LTE models generally reproduce the effective temperatures to within
uncertainties. For H, the absolute 3D effects and non-LTE
effects can separately reach around 100K, in terms of inferred effective
temperatures. For metal-poor turn-off stars, 1D LTE models of H can
underestimate effective temperatures by around 150K. Our 3D non-LTE model
spectra are publicly available, and can be used for more reliable spectroscopic
effective temperature determinations.Comment: 19 pages, 10 figures, abstract abridged; accepted for publication in
Astronomy & Astrophysic
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