173 research outputs found
Line formation in solar granulation: II. The photospheric Fe abundance
The solar photospheric Fe abundance has been determined using realistic ab
initio 3D, time-dependent, hydrodynamical model atmospheres. The study is based
on the excellent agreement between the predicted and observed line profiles
directly rather than equivalent width, since the intrinsic Doppler broadening
from the convective motions and oscillations provide the necessary non-thermal
broadening. Thus, three of the four hotly debated parameters (equivalent
widths, microturbulence and damping enhancement factors) in the center of the
recent solar Fe abundance dispute regarding FeI lines no longer enter the
analysis, leaving the transition probabilities as the main uncertainty. Both
FeI (using the samples of lines of both the Oxford and Kiel studies) and FeII
lines have been investigated, which give consistent results: log FeI = 7.44 +-
0.05 and log FeII = 7.45 +- 0.10. Also the wings of strong FeI lines return
consistent abundances, log FeII = 7.42 +- 0.03, but due to the uncertainties
inherent in analyses of strong lines we give this determination lower weight
than the results from weak and intermediate strong lines. In view of the recent
slight downward revision of the meteoritic Fe abundance log Fe = 7.46 +- 0.01,
the agreement between the meteoritic and photospheric values is very good, thus
appearingly settling the debate over the photospheric Fe abundance from FeI
lines.Comment: Accepted for A&
Abundance Analysis of the Halo Giant HD122563 with Three-Dimensional Model Stellar Atmospheres
We present a preliminary local thermodynamic equilibrium (LTE) abundance
analysis of the template halo red giant HD122563 based on a realistic,
three-dimensional (3D), time-dependent, hydrodynamical model atmosphere of the
very metal-poor star. We compare the results of the 3D analysis with the
abundances derived by means of a standard LTE analysis based on a classical,
1D, hydrostatic model atmosphere of the star. Due to the different upper
photospheric temperature stratifications predicted by 1D and 3D models, we find
large, negative, 3D-1D LTE abundance differences for low-excitation OH and Fe I
lines. We also find trends with lower excitation potential in the derived Fe
LTE abundances from Fe I lines, in both the 1D and 3D analyses. Such trends may
be attributed to the neglected departures from LTE in the spectral line
formation calculations.Comment: 4 pages, 4 figures, contribution to proceedings for Joint Discussion
10 at the IAU General Assembly, Rio de Janeiro, Brazil, August 200
Line formation in solar granulation: I. Fe line shapes, shifts and asymmetries
Realistic ab-initio 3D, radiative-hydrodynamical convection simulations of
the solar granulation have been applied to FeI and FeII line formation. In
contrast to classical analyses based on hydrostatic 1D model atmospheres the
procedure contains no adjustable free parameters but the treatment of the
numerical viscosity in the construction of the 3D, time-dependent,
inhomogeneous model atmosphere and the elemental abundance in the 3D spectral
synthesis. However, the numerical viscosity is introduced purely for numerical
stability purposes and is determined from standard hydrodynamical test cases
with no adjustments allowed to improve the agreement with the observational
constraints from the solar granulation. The non-thermal line broadening is
mainly provided by the Doppler shifts arising from the convective flows in the
solar photosphere and the solar oscillations. The almost perfect agreement
between the predicted temporally and spatially averaged line profiles for weak
Fe lines with the observed profiles and the absence of trends in derived
abundances with line strengths, seem to imply that the micro- and
macroturbulence concepts are obsolete in these 3D analyses. Furthermore, the
theoretical line asymmetries and shifts show a very satisfactory agreement with
observations with an accuracy of typically 50-100 m/s on an absolute velocity
scale. The remaining minor discrepancies point to how the convection
simulations can be refined further.Comment: Accepted for A&
Damping rates and frequency corrections of Kepler LEGACY stars
Linear damping rates and modal frequency corrections of radial oscillation
modes in selected LEGACY main-sequence stars are estimated by means of a
nonadiabatic stability analysis. The selected stellar sample covers stars
observed by Kepler with a large range of surface temperatures and surface
gravities. A nonlocal, time-dependent convection model is perturbed to assess
stability against pulsation modes. The mixing-length parameter is calibrated to
the surface-convection-zone depth of a stellar model obtained from fitting
adiabatic frequencies to the LEGACY observations, and two of the nonlocal
convection parameters are calibrated to the corresponding LEGACY linewidth
measurements. The remaining nonlocal convection parameters in the 1D
calculations are calibrated so as to reproduce profiles of turbulent pressure
and of the anisotropy of the turbulent velocity field of corresponding 3D
hydrodynamical simulations. The atmospheric structure in the 1D stability
analysis adopts a temperature-optical-depth relation derived from 3D
hydrodynamical simulations. Despite the small number of parameters to adjust,
we find good agreement with detailed shapes of both turbulent pressure profiles
and anisotropy profiles with depth, and with damping rates as a function of
frequency. Furthermore, we find the absolute modal frequency corrections,
relative to a standard adiabatic pulsation calculation, to increase with
surface temperature and surface gravity.Comment: accepted for publication in Monthly Notices of the Royal Astronomical
Society (MNRAS); 15 pages, 8 figure
The Stagger-grid: A Grid of 3D Stellar Atmosphere Models - I. Methods and General Properties
We present the Stagger-grid, a comprehensive grid of time-dependent, 3D
hydrodynamic model atmospheres for late-type stars with realistic treatment of
radiative transfer, covering a wide range in stellar parameters. This grid of
3D models is intended for various applications like stellar spectroscopy,
asteroseismology and the study of stellar convection. In this introductory
paper, we describe the methods used for the computation of the grid and discuss
the general properties of the 3D models as well as their temporal and spatial
averages (). All our models were generated with the Stagger-code, using
realistic input physics for the equation of state (EOS) and for continuous and
line opacities. Our ~220 grid models range in Teff from 4000 to 7000K in steps
of 500K, in log g from 1.5 to 5.0 in steps of 0.5 dex, and [Fe/H] from -4.0 to
+0.5 in steps of 0.5 and 1.0 dex. We find a tight scaling relation between the
vertical velocity and the surface entropy jump, which itself correlates with
the constant entropy value of the adiabatic convection zone. The range in
intensity contrast is enhanced at lower metallicity. The granule size
correlates closely with the pressure scale height sampled at the depth of
maximum velocity. We compare the models with widely applied 1D models, as
well as with theoretical 1D hydrostatic models generated with the same EOS and
opacity tables as the 3D models, in order to isolate the effects of using
self-consistent and hydrodynamic modeling of convection, rather than the
classical mixing length theory approach. For the first time, we are able to
quantify systematically over a broad range of stellar parameters the
uncertainties of 1D models arising from the simplified treatment of physics, in
particular convective energy transport. In agreement with previous findings, we
find that the differences can be significant, especially for metal-poor stars.Comment: Accepted for publication in A&A, 31 pages, 29 figure
Line formation in solar granulation: II. The photospheric Fe abundance
The solar photospheric Fe abundance has been determined using realistic ab initio 3D, time-dependent, hydrodynamical model atmospheres. The study is based on the excellent agreement between the predicted and observed line profiles directly rather than equivalent widths, since the intrinsic Doppler broadening from the convective motions and oscillations provide the necessary non-thermal broadening. Thus, three of the four hotly debated parameters (equivalent widths, microturbulence and damping enhancement factors) in the center of the recent solar Fe abundance dispute regarding Fe i lines no longer enter the analysis, leaving the transition probabilities as the main uncertainty. Both Fe i (using the samples of lines of both the Oxford and Kiel studies) and Fe ii lines have been investigated, which give consistent results: log epsilon_FeI = 7.44 +/- 0.05 and log epsilon_FeII = 7.45 +/- 0.10. Also the wings of strong Fe i lines return consistent abundances, log epsilon_FeII = 7.42 +/- 0.03, but due to the uncertainties inherent in analyses of strong lines we give this determination lower weight than the results from weak and intermediate strong lines. In view of the recent slight downward revision of the meteoritic Fe abundance log epsilon_Fe = 7.46 +/- 0.01, the agreement between the meteoritic and photospheric values is very good, thus appearingly settling the debate over the photospheric Fe abundance from Fe i lines
- …