81 research outputs found
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
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
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
3D non-LTE iron abundances in FG-type dwarfs
Spectroscopic measurements of iron abundances are prone to systematic
modelling errors. We present 3D non-LTE calculations across 32 STAGGER-grid
models with effective temperatures from 5000 K to 6500 K, surface gravities of
4.0 dex and 4.5 dex, and metallicities from 3 dex to 0 dex, and study the
effects on 171 Fe I and 12 Fe II optical lines. In warm metal-poor stars, the
3D non-LTE abundances are up to 0.5 dex larger than 1D LTE abundances inferred
from Fe I lines of intermediate excitation potential. In contrast, the 3D
non-LTE abundances can be 0.2 dex smaller in cool metal-poor stars when using
Fe I lines of low excitation potential. The corresponding abundance differences
between 3D non-LTE and 1D non-LTE are generally less severe but can still reach
0.2 dex. For Fe II lines the 3D abundances range from up to 0.15 dex
larger, to 0.10 dex smaller, than 1D abundances, with negligible departures
from 3D LTE except for the warmest stars at the lowest metallicities. The
results were used to correct 1D LTE abundances of the Sun and Procyon (HD
61421), and of the metal-poor stars HD 84937 and HD 140283, using an
interpolation routine based on neural networks. The 3D non-LTE models achieve
an improved ionisation balance in all four stars. In the two metal-poor stars,
they remove excitation imbalances that amount to 250 K to 300 K errors in
effective temperature. For Procyon, the 3D non-LTE models suggest [Fe/H] = 0.11
0.03, which is significantly larger than literature values based on
simpler models. We make the 3D non-LTE interpolation routine for FG-type dwarfs
publicly available, in addition to 1D non-LTE departure coefficients for
standard MARCS models of FGKM-type dwarfs and giants. These tools, together
with an extended 3D LTE grid for Fe II from 2019, can help improve the accuracy
of stellar parameter and iron abundance determinations for late-type stars.Comment: 17 pages, 11 figures, 5 tables; arXiv abstract abridged; accepted for
publication in Astronomy & Astrophysic
Two-dimensional non-LTE \ion{O}{I} 777\,nm line formation in radiation hydrodynamics simulations of Cepheid atmospheres
Oxygen abundance measurements are important for understanding stellar
structure and evolution. Measured in Cepheids, they further provide clues on
the metallicity gradient and chemo-dynamical evolution in the Galaxy. However,
most of the abundance analyses of Cepheids to date have been based on
one-dimensional (1D) hydrostatic model atmospheres. Here, we test the validity
of this approach for the key oxygen abundance diagnostic, the \ion{O}{I}
~triplet lines. We carry out 2D non-LTE radiative transfer
clculations across two different 2D radiation hydrodynamics simulations of
Cepheid atmospheres, having stellar parameters of K,
solar chemical compositions, and and , corresponding to
pulsation periods of 9 and 3 days, respectively. We find that the 2D non-LTE
versus 1D LTE abundance differences range from ~dex to ~dex
depending on pulsational phase. The 2D non-LTE versus 1D non-LTE abundance
differences range from ~dex to ~dex. The abundance differences are
smallest when the Cepheid atmospheres are closest to hydrostatic equilibrium,
corresponding to phases of around to , and we recommend these phases
for observers deriving the oxygen abundance from \ion{O}{I}
triplet with 1D hydrostatic models.Comment: 9 pages, 10 figures; Published in Astronomy and Astrophysic
Extended atomic data for oxygen abundance analyses
As the most abundant element in the universe after hydrogen and helium,
oxygen plays a key role in planetary, stellar, and galactic astrophysics. Its
abundance is especially influential on stellar structure and evolution, and as
the dominant opacity contributor at the base of the Sun's convection zone it is
central to the discussion around the solar modelling problem. However,
abundance analyses require complete and reliable sets of atomic data. We
present extensive atomic data for O I, by using the multiconfiguration
Dirac-Hartree-Fock and relativistic configuration interaction methods.
Lifetimes and transition probabilities for radiative electric dipole
transitions are given and compared with results from previous calculations and
available measurements. The accuracy of the computed transition rates is
evaluated by the differences between the transition rates in Babushkin and
Coulomb gauges, as well as by a cancellation factor analysis. Out of the 989
computed transitions in this work, 205 are assigned to the accuracy classes
AA-B, that is, with uncertainties less than 10%, following the criteria defined
by the National Institute of Standards and Technology Atomic Spectra Database.
We discuss the influence of the new log(gf) values on the solar oxygen
abundance and ultimately advocate .Comment: 13 pages, 5 figures; Accepted for publication in Astronomy &
Astrophysic
3D NLTE spectral line formation of lithium in late-type stars
Accurately known stellar lithium abundances may be used to shed light on a
variety of astrophysical phenomena such as Big Bang nucleosynthesis, radial
migration, ages of stars and stellar clusters, and planet engulfment events. We
present a grid of synthetic lithium spectra that are computed in non-local
thermodynamic equilibrium (NLTE) across the STAGGER grid of three-dimensional
(3D) hydrodynamic stellar atmosphere models. This grid covers three Li lines at
610.4 nm, 670.8 nm, and 812.6 nm for stellar parameters representative of
FGK-type dwarfs and giants, spanning -7000 K, -5.0, -0.5, and -4.0. We
find that our abundance corrections are up to 0.15 dex more negative than in
previous work, due to a previously overlooked NLTE effect of blocking of UV
lithium lines by background opacities, which has important implications for a
wide range of science cases. We derive a new 3D NLTE solar abundance of
, which is 0.09 dex lower than the commonly
used value. We make our grids of synthetic spectra and abundance corrections
publicly available through the Breidablik package. This package includes
methods for accurately interpolating our grid to arbitrary stellar parameters
through methods based on Kriging (Gaussian process regression) for line
profiles, and MLP (Multi-Layer Perceptrons, a class of fully connected
feedforward neural networks) for NLTE corrections and 3D NLTE abundances from
equivalent widths, achieving interpolation errors of the order 0.01 dex.Comment: 20 pages, 12 figures, accepted for publication in MNRA
Higher metal abundances do not solve the solar problem
Context. The Sun acts as a cornerstone of stellar physics. Thanks to
spectroscopic, helioseismic and neutrino flux observations, we can use the Sun
as a laboratory of fundamental physics in extreme conditions. The conclusions
we draw are then used to inform and calibrate evolutionary models of all other
stars in the Universe. However, solar models are in tension with helioseismic
constraints. The debate on the ``solar problem'' has hitherto led to numerous
publications discussing potential issues with solar models and abundances.
Aims. Using the recently suggested high-metallicity abundances for the Sun, we
investigate whether standard solar models, as well as models with macroscopic
transport reproducing the solar surface lithium abundances and analyze their
properties in terms of helioseismic and neutrino flux observations. Methods. We
compute solar evolutionary models and combine spectroscopic and helioseismic
constraints as well as neutrino fluxes to investigate the impact of macroscopic
transport on these measurements. Results. When high-metallicity solar models
are calibrated to reproduce the measured solar lithium depletion, tensions
arise with respect to helioseismology and neutrino fluxes. This is yet another
demonstration that the solar problem is also linked to the physical
prescriptions of solar evolutionary models and not to chemical composition
alone. Conclusions. A revision of the physical ingredients of solar models is
needed in order to improve our understanding of stellar structure and
evolution. The solar problem is not limited to the photospheric abundances if
the depletion of light elements is considered. In addition, tighter constraints
on the solar beryllium abundance will play a key role in the improvement of
solar models.Comment: Accepted for publication in Astronomy and Astrophysic
- …