86 research outputs found
Multi-level 3D non-LTE computations of lithium lines in the metal-poor halo stars HD140283 and HD84937
The lithium abundances in metal-poor halo stars are of importance for
cosmology, galaxy evolution and stellar structure. In an attempt to study
possible systematic errors in the derived Li abundances, the line formation of
LiI lines has been investigated by means of realistic 3D hydrodynamical model
atmospheres of halo stars and 3D non-LTE radiative transfer calculations. These
are the first detailed 3D non-LTE computations reported employing a multi-level
atomic model showing that such problems are now computationally tractable. The
detailed computations reveal that the LiI population has a strong influence
from the radiation field rather than the local gas temperature, indicating that
the low derived Li abundances found by Asplund et al. (1999) are an artifact of
their assumption of LTE. Relative to 3D LTE, the detailed calculations show
pronounced over-ionization. In terms of abundances the 3D non-LTE values are
within 0.05 dex of the 1D non-LTE results for the particular cases of HD140283
and HD84937, which is a consequence of the dominance of the radiation in
determining the population density of LiI. Although 3D non-LTE can be expected
to give results rather close (+/-0.1 dex) to 1D non-LTE for this reason, there
may be systematic trends with metallicity and effective temperature.Comment: A&A, in press (Letters to the editors), 5 page
Three-dimensional non-LTE radiative transfer computation of the Ca 8542 infrared line from a radiation-MHD simulation
Interpretation of imagery of the solar chromosphere in the widely used
\CaIIIR infrared line is hampered by its complex, three-dimensional and non-LTE
formation. Forward modelling is required to aid understanding. We use a 3D
non-LTE radiative transfer code to compute synthetic \CaIIIR images from a
radiation-MHD simulation of the solar atmosphere spanning from the convection
zone to the corona. We compare the simulation with observations obtained with
the CRISP filter at the Swedish 1--m Solar Telescope. We find that the
simulation reproduces dark patches in the blue line wing caused by Doppler
shifts, brightenings in the line core caused by upward-propagating shocks and
thin dark elongated structures in the line core that form the interface between
upward and downward gas motion in the chromosphere. The synthetic line core is
narrower than the observed one, indicating that the sun exhibits both more
vigorous large-scale dynamics as well as small scale motions that are not
resolved within the simulation, presumably owing to a lack of spatial
resolution.Comment: accepted as ApJ lette
Observational manifestations of solar magneto-convection -- center-to-limb variation
We present the first center-to-limb G-band images synthesized from high
resolution simulations of solar magneto-convection. Towards the limb the
simulations show "hilly" granulation with dark bands on the far side, bright
granulation walls and striated faculae, similar to observations. At disk center
G-band bright points are flanked by dark lanes. The increased brightness in
magnetic elements is due to their lower density compared with the surrounding
intergranular medium. One thus sees deeper layers where the temperature is
higher. At a given geometric height, the magnetic elements are cooler than the
surrounding medium. In the G-band, the contrast is further increased by the
destruction of CH in the low density magnetic elements. The optical depth unity
surface is very corrugated. Bright granules have their continuum optical depth
unity 80 km above the mean surface, the magnetic elements 200-300 km below. The
horizontal temperature gradient is especially large next to flux
concentrations. When viewed at an angle, the deep magnetic elements optical
surface is hidden by the granules and the bright points are no longer visible,
except where the "magnetic valleys" are aligned with the line of sight. Towards
the limb, the low density in the strong magnetic elements causes unit
line-of-sight optical depth to occur deeper in the granule walls behind than
for rays not going through magnetic elements and variations in the field
strength produce a striated appearance in the bright granule walls.Comment: To appear in ApJL. 6 pages 4 figure
The non-LTE line formation of neutral carbon in late-type stars
We investigate the non-Local Thermodynamic Equilibrium (non-LTE) line
formation of neutral carbon in late-type stars in order to remove some of the
potential systematic errors in stellar abundance analyses employing C I
features. The statistical equilibrium code MULTI was used on a grid of
plane-parallel 1D MARCS atmospheric models. Within the parameter space
explored, the high-excitation C I lines are stronger in non-LTE due to the
combined effect of line source function drop and increased lower level
population; the relative importance of the two effects depends on the
particular combination of Teff, log g, [Fe/H] and [C/Fe] and on the analysed C
I line. As a consequence, the non-LTE abundance corrections are negative and
can be substantially so, for example ~-0.4 dex in halo turn-off stars at
[Fe/H]~-3. The magnitude of the non-LTE corrections is rather insensitive to
whether inelastic H collisions are included or not. Our results have
implications on studies of nucleosynthetic processes and on Galactic chemical
evolution models. When applying our calculated corrections to recent
observational data, the upturn in [C/O] at low metallicity might still be
present (thus apparently still necessitating contributions from massive Pop.
III stars for the carbon production), but at a lower level and possibly with a
rather shallow trend of ~-0.2 dex/dex below [O/H]~-1Comment: Accepted for publication in A&A, 19 pages, 2 tables, 11 figures,
aa.cls v6.0 include
Approximations for radiative cooling and heating in the solar chromosphere
Context. The radiative energy balance in the solar chromosphere is dominated
by strong spectral lines that are formed out of LTE. It is computationally
prohibitive to solve the full equations of radiative transfer and statistical
equilibrium in 3D time dependent MHD simulations.
Aims. To find simple recipes to compute the radiative energy balance in the
dominant lines under solar chromospheric conditions.
Methods. We use detailed calculations in time-dependent and 2D MHD snapshots
to derive empirical formulae for the radiative cooling and heating.
Results. The radiative cooling in neutral hydrogen lines and the Lyman
continuum, the H and K and intrared triplet lines of singly ionized calcium and
the h and k lines of singly ionized magnesium can be written as a product of an
optically thin emission (dependent on temperature), an escape probability
(dependent on column mass) and an ionization fraction (dependent on
temperature). In the cool pockets of the chromosphere the same transitions
contribute to the heating of the gas and similar formulae can be derived for
these processes. We finally derive a simple recipe for the radiative heating of
the chromosphere from incoming coronal radiation. We compare our recipes with
the detailed results and comment on the accuracy and applicability of the
recipes.Comment: accepted for publication in Astronomy & Astrophysic
3D Hydrodynamical Simulations of Surface Convection in Red Giant Stars. Impact on spectral line formation and abundance analysis
We investigate the impact of 3D hydrodynamical model atmospheres of red giant
stars at different metallicities on the formation of spectral lines of a number
of ions and molecules. We carry out realistic 3D simulations of surface
convection in red giant stars with varying stellar parameters. We use the
simulations as time-dependent hydrodynamical model stellar atmospheres to
compute atomic (Li, O, Na, Mg, Ca, Fe) and molecular (CH, NH, OH) spectral
lines under the assumption of local thermodynamic equilibrium (LTE). We compare
the line strengths computed in 3D with the results of analogous line formation
calculations for 1D, hydrostatic, plane-parallel MARCS model atmospheres in
order to estimate the impact of 3D models on the derivation of elemental
abundances. The temperature and density inhomogeneities and correlated
velocities in 3D models, as well as the differences between the 1D and mean 3D
structures significantly affect the predicted line strengths. Under the
assumption of LTE, the low atmospheric temperatures of very metal-poor 3D model
atmospheres cause the lines from neutral species and molecules to appear
stronger than in 1D. Therefore, elemental abundances derived from these lines
using 3D models are significantly lower than according to 1D analyses.
Differences between 3D and 1D abundances of C, N, and O derived from CH, NH,
and OH weak low-excitation lines are found to be in the range -0.5 dex to -1.0
dex for the the red giant stars at [Fe/H]=-3 considered here. At this
metallicity, large negative corrections (about -0.8 dex) are also found for
weak low-excitation Fe I lines. We caution, however, that departures from LTE
might be significant for these and other elements and comparable to the effects
due to stellar granulation.Comment: Accepted for publication on A&A, 23 pages, 15 figure
The Solar Heavy Element Abundances: II. Constraints from Stellar Atmospheres
Estimates of the bulk metal abundance of the Sun derived from the latest
generation of model atmospheres are significantly lower than the earlier
standard values. In Paper I we demonstrated that a low solar metallicity is
inconsistent with helioseismology if the quoted errors in the atmospheres
models (of order 0.05 dex) are correct. In this paper we undertake a critical
analysis of the solar metallicity and its uncertainty from a model atmospheres
perspective, focusing on CNO. We argue that the non-LTE corrections for
abundances derived from atomic features are overestimated in the recent
abundance studies, while systematic errors in the absolute abundances are
underestimated. If we adopt the internal consistency between different
indicators as a measure of goodness of fit, we obtain intermediate abundances
[C/H] = 8.44 +/- 0.06, [N/H] = 7.96 +/- 0.10 and [O/H] = 8.75 +/- 0.08. The
errors are too large to conclude that there is a solar abundance problem, and
permit both the high and low scales. However, the center-to-limb continuum flux
variations predicted in the simulations appear to be inconsistent with solar
data, which would favor the traditional thermal structure and lead to high CNO
abundances of (8.52, 7.96, 8.80) close to the seismic scale. We argue that
further empirical tests of non-LTE corrections and the thermal structure are
required for precise absolute abundances. The implications for beryllium
depletion and possible sources of error in the numerical simulations are
discussed.Comment: 36 pages, 4 figures, submitted Ap
Non-LTE calculations for neutral Na in late-type stars using improved atomic data
Neutral sodium is a minority species in the atmospheres of late-type stars,
and line formation in local thermodynamic equilibrium (LTE) is often a poor
assumption, in particular for strong lines. We present an extensive grid of
non-LTE calculations for several NaI lines in cool stellar atmospheres,
including metal-rich and metal-poor dwarfs and giants. For the first time, we
constructed a Na model atom that incorporates accurate quantum mechanical
calculations for collisional excitation and ionisation by electrons as well as
collisional excitation and charge exchange reactions with neutral hydrogen.
Similar to LiI, the new rates for hydrogen impact excitation do not affect the
statistical equilibrium calculations, while charge exchange reactions have a
small but non-negligible influence. The presented LTE and non-LTE
curves-of-growth can be interpolated to obtain non-LTE abundances and abundance
corrections for arbitrary stellar parameter combinations and line strengths.
The typical corrections for weak lines are -0.1...-0.2dex, whereas saturated
lines may overestimate the abundance in LTE by more than 0.5dex. The non-LTE Na
abundances appear very robust with respect to uncertainties in the input
collisional data.Comment: 9 pages, 8 figures, accepted for publication in A&
Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity
We present a detailed study of the non-Local Thermodynamic Equilibrium
(non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in
MARCS model atmospheres representative of late-type stars with spectral types F
to K. We carried out the calculations using the statistical equilibrium code
MULTI, including estimates of the impact on elemental abundance analysis. The
atomic model employed includes, in particular, recent quantum-mechanical
electron collision data. We confirm that the O I triplet lines form under
non-LTE conditions in late-type stars, suffering negative abundance corrections
with respect to LTE. At low metallicity, large line opacity stems from
triplet-quintet intersystem electron collisions, a form of coupling previously
not considered or seriously underestimated. The non-LTE effects become
generally severe for models (both giants and dwarfs) with higher T_eff.
Interestingly, in metal-poor turn-off stars, the negative non-LTE abundance
corrections tend to rapidly become more severe towards lower metallicity. When
neglecting H collisions, they amount to as much as ~ 0.9 dex and ~ 1.2 dex,
respectively at [Fe/H]=-3 and [Fe/H]=-3.5. Even when such collisions are
included, the LTE abundance remains a serious overestimate, correspondingly by
~ 0.5 dex and ~ 0.9 dex at such low metallicities. Although the poorly known
inelastic hydrogen collisions thus remain an important uncertainty, the large
metallicity-dependent non-LTE effects seem to point to a resulting "low"
(compared to LTE) [O/Fe] in metal-poor halo stars.Comment: 19 pages, 10 figures, aa.cls v6.1 included. Accepted for publication
in A&
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