5,496 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
A Survey of Soil Diatoms
This investigation was undertaken to determine qualitatively the complement of diatom species in six different soils in the vicinity of Iowa City, Iowa. Separating diatoms from the various soils and cleaning them presented a problem requiring the trial of several existing techniques
Framing the neoliberal canon: resisting the market myth via literary enquiry
here is widespread recognition that neoliberal rhetoric about ‘free markets’ stands in considerable tension with ‘really existing’ neoliberalizing processes. However, the oft-utilized analytical distinction between ‘pure’ economic and political theory and ‘messy’ empirical developments takes for granted that neoliberalism, at its core, valorizes free markets. In contrast, the paper explores whether neoliberal intellectuals ever made such an argument. Using Friedrich Hayek and Milton Friedman as exemplars, our reading of canonical neoliberal texts focuses on author framing gestures, particular understandings of the term ‘science’, techniques of characterization, and constructions of epistemological legitimacy. This enables us to avoid the trap of assuming that these texts are about free markets and instead enquires into their constitution as literary artefacts. As such, we argue that the remaking of states and households rather than the promotion of free markets is at the core of neoliberalism. Our analysis has significant implications. For example, it means that authoritarian neoliberalism is not a departure from but actually more in line with the ‘pure’ neoliberal canon than in the past. Therefore, neoliberalism ought to be critiqued not for its rhetorical promotion of free markets but instead for seeking to reorganize societies in coercive, non-democratic and unequal ways. This also enables us to acknowledge that households are central to resistance to neoliberalism as well as to the neoliberal worldview itself
Commensurability and beyond: from Mises and Neurath to the future of the socialist calculation debate
Mises' 'calculation argument' against socialism argues that monetary calculation is indispensable as a commensurable unit for evaluating factors of production. This is not due to his conception of rationality being purely 'algorithmic,' for it accommodates non-monetary, incommensurable values. Commensurability is needed, rather, as an aid in the face of economic complexity. The socialist Neurath's response to Mises is unsatisfactory in rejecting the need to explore possible non-market techniques for achieving a certain degree of commensurability. Yet Neurath's contribution is valuable in emphasizing the need for a balanced, comparative approach to the question of market versus non-market that puts the commensurability question in context. These central issues raised by adversaries in the early socialist calculation debate have continued relevance for the contemporary discussion
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
Hubble Space Telescope Transmission Spectroscopy of the Exoplanet HD 189733b: High-altitude atmospheric haze in the optical and near-UV with STIS
We present Hubble Space Telescope optical and near-ultraviolet transmission
spectra of the transiting hot-Jupiter HD189733b, taken with the repaired Space
Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover
the range 2900-5700 Ang and reach per-exposure signal-to-noise levels greater
than 11,000 within a 500 Ang bandwidth. We used time series spectra obtained
during two transit events to determine the wavelength dependance of the
planetary radius and measure the exoplanet's atmospheric transmission spectrum
for the first time over this wavelength range. Our measurements, in conjunction
with existing HST spectra, now provide a broadband transmission spectrum
covering the full optical regime. The STIS data also shows unambiguous evidence
of a large occulted stellar spot during one of our transit events, which we use
to place constraints on the characteristics of the K dwarf's stellar spots,
estimating spot temperatures around Teff~4250 K. With contemporaneous
ground-based photometric monitoring of the stellar variability, we also measure
the correlation between the stellar activity level and transit-measured
planet-to-star radius contrast, which is in good agreement with predictions. We
find a planetary transmission spectrum in good agreement with that of Rayleigh
scattering from a high-altitude atmospheric haze as previously found from HST
ACS camera. The high-altitude haze is now found to cover the entire optical
regime and is well characterised by Rayleigh scattering. These findings suggest
that haze may be a globally dominant atmospheric feature of the planet which
would result in a high optical albedo at shorter optical wavelengths.Comment: 14 pages, 14 figures, 4 tables, accepted to MNRAS, revised version
has minor change
3D LTE spectral line formation with scattering in red giant stars
We investigate the effects of coherent isotropic continuum scattering on the
formation of spectral lines in local thermodynamic equilibrium (LTE) using 3D
hydrodynamical and 1D hydrostatic model atmospheres of red giant stars.
Continuum flux levels, spectral line profiles and curves of growth for
different species are compared with calculations that treat scattering as
absorption. Photons may escape from deeper, hotter layers through scattering,
resulting in significantly higher continuum flux levels beneath a wavelength of
5000 A. The magnitude of the effect is determined by the importance of
scattering opacity with respect to absorption opacity; we observe the largest
changes in continuum flux at the shortest wavelengths and lowest metallicities;
intergranular lanes of 3D models are more strongly affected than granules.
Continuum scattering acts to increase the profile depth of LTE lines: continua
gain more brightness than line cores due to their larger thermalization depth
in hotter layers. We thus observe the strongest changes in line depth for
high-excitation species and ionized species, which contribute significantly to
photon thermalization through their absorption opacity near the continuum
optical surface. Scattering desaturates the line profiles, leading to larger
abundance corrections for stronger lines, which reach -0.5 dex at 3000 A for Fe
II lines in 3D with excitation potential 2 eV at [Fe/H]=-3.0. The corrections
are less severe for low-excitation lines, longer wavelengths, and higher
metallicity. Velocity fields increase the effects of scattering by separating
emission from granules and intergranular lanes in wavelength. 1D calculations
exhibit similar scattering abundance corrections for weak lines, but those for
strong lines are generally smaller compared to 3D models and depend on the
choice of microturbulence.Comment: Astronomy & Astrophysics, Volume 529, 05/201
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