2,497 research outputs found
Metallicities of M Dwarf Planet Hosts from Spectral Synthesis
We present the first spectroscopic metallicities of three M dwarfs with known
or candidate planetary mass companions. We have analyzed high resolution, high
signal-to-noise spectra of these stars which we obtained at McDonald
Observatory. Our analysis technique is based on spectral synthesis of atomic
and molecular features using recently revised cool-star model atmospheres and
spectrum synthesis code. The technique has been shown to yield results
consistent with the analyses of solar-type stars and allows measurements of M
dwarf [M/H] values to 0.12 dex precision. From our analysis, we find [M/H] =
-0.12, -0.32, and -0.33 for GJ 876, GJ 436, and GJ 581 respectively. These
three M dwarf planet hosts have sub-solar metallicities, a surprising departure
from the trend observed in FGK-type stars. This study is the first part of our
ongoing work to determine the metallicities of the M dwarfs included in the
McDonald Observatory planet search program.Comment: 13 pages, 2 figures, accepted for publication in ApJ
Learning Design: reflections on a snapshot of the current landscape
The mounting wealth of open and readily available information and the swift evolution of social, mobile and creative technologies warrant a re-conceptualisation of the role of educators: from providers of knowledge to designers of learning. This need is being addressed by a growing trend of research in Learning Design. Responding to this trend, the Art and Science of Learning Design workshop brought together leading voices in the field and provided a forum for discussing its key issues. It focused on three thematic axes: practices and methods, tools and resources, and theoretical frameworks. This paper reviews some definitions of Learning Design and then summarises the main contributions to the workshop. Drawing upon these, we identify three key challenges for Learning Design that suggest directions for future research
Nanoscale tunnel field effect transistor based on a complex oxide lateral heterostructure
We demonstrate a tunnel field effect transistor based on a lateral
heterostructure patterned from an electron gas.
Charge is injected by tunneling from the /
contacts and the current through a narrow channel of insulating
is controlled via an electrostatic side gate. Drain-source
I/V-curves have been measured at low and elevated temperatures. The transistor
shows strong electric-field and temperature-dependent behaviour with a steep
sub-threshold slope %of up to as small as and a
transconductance as high as . A fully
consistent transport model for the drain-source tunneling reproduces the
measured steep sub-threshold slope.Comment: 20 pages, 6 figures, Supplementary material: 4 pages, 2 figure
Sodium abundances in nearby disk stars
We present sodium abundances for a sample of nearby stars. All results have
been derived from NLTE statistical equilibrium calculations. The influence of
collisional interactions with electrons and hydrogen atoms is evaluated by
comparison of the solar spectrum with very precise fits to the Na I line cores.
The NLTE effects are more pronounced in metal-poor stars since the statistical
equilibrium is dominated by collisions of which at least the electronic
component is substantially reduced. The resulting influence on the
determination of sodium abundances is in a direction opposite to that found
previously for Mg and Al. The NLTE corrections are about -0.1 in thick-disk
stars with [Fe/H] about -0.6. Our [Na/Fe] abundance ratios are about solar for
thick- and thin-disk stars. The increase in [Na/Fe] as a function of [Fe/H] for
metal-rich stars found by Edvardsson et al. (1993) is confirmed. Our results
suggest that sodium yields increase with the metallicity, and quite large
amounts of sodium may be produced by AGB stars. We find that [Na/Fe]ratios,
together with either [Mg/Fe] ratio, kinematic data or stellar evolutionary
ages, make possible the individual discrimination between thin- and thick-disk
membership.Comment: 11pages, 11 figures. A&A accepte
Momentum conservation and local field corrections for the response of interacting Fermi gases
We reanalyze the recently derived response function for interacting systems
in relaxation time approximation respecting density, momentum and energy
conservation. We find that momentum conservation leads exactly to the local
field corrections for both cases respecting only density conservation and
respecting density and energy conservation. This rewriting simplifies the
former formulae dramatically. We discuss the small wave vector expansion and
find that the response function shows a high frequency dependence of
which allows to fulfill higher order sum rules. The momentum
conservation also resolves a puzzle about the conductivity which should only be
finite in multicomponent systems
Signal processing in local neuronal circuits based on activity-dependent noise and competition
We study the characteristics of weak signal detection by a recurrent neuronal
network with plastic synaptic coupling. It is shown that in the presence of an
asynchronous component in synaptic transmission, the network acquires
selectivity with respect to the frequency of weak periodic stimuli. For
non-periodic frequency-modulated stimuli, the response is quantified by the
mutual information between input (signal) and output (network's activity), and
is optimized by synaptic depression. Introducing correlations in signal
structure resulted in the decrease of input-output mutual information. Our
results suggest that in neural systems with plastic connectivity, information
is not merely carried passively by the signal; rather, the information content
of the signal itself might determine the mode of its processing by a local
neuronal circuit.Comment: 15 pages, 4 pages, in press for "Chaos
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out following the
classical recipe: LTE line formation and homogeneous, plane-parallel,
flux-constant, and LTE model atmospheres. We review different results in the
literature that have suggested significant inconsistencies in the spectroscopic
analyses, pointing out the difficulties in deriving independent estimates of
the stellar fundamental parameters and hence,detecting systematic errors.
The trigonometric parallaxes measured by the HIPPARCOS mission provide
accurate appraisals of the stellar surface gravity for nearby stars, which are
used here to check the gravities obtained from the photospheric iron ionization
balance. We find an approximate agreement for stars in the metallicity range -1
<= [Fe/H] <= 0, but the comparison shows that the differences between the
spectroscopic and trigonometric gravities decrease towards lower metallicities
for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow
upon the abundance analyses for extreme metal-poor stars that make use of the
ionization equilibrium to constrain the gravity. The comparison with the
strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)
confirms that this method provides systematically larger gravities than the
ionization balance. The strong-line gravities get closer to the physical ones
for the stars analyzed by Fuhrmann, but they are even further away than the
iron ionization gravities for the stars of lower gravities in Edvardsson's
sample. The confrontation of the deviations of the iron ionization gravities in
metal-poor stars reported here with departures from the excitation balance
found in the literature, show that they are likely to be induced by the same
physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The
Astrophysical Journa
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