4,160 research outputs found
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
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
Gender Differences in Student Engagement Among African American Undergraduates at Historically Black Colleges and Universities
Differences in student engagement between women and men at historically Black colleges and universities (HBCUs) are examined in this study. Data were collected from 1,167 African American undergraduate students at 12 four-year HBCUs that participated in the National Survey of Student Engagement. Controlling for several factors that might obscure gender differences, the results counter previous research regarding gender gaps on HBCU campuses by illustrating that African American women enjoy an equally engaging experience as their same-race male counterparts
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