2,498 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
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
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
Intra-day variability observations and the VLBI structure analysis of quasar S4 0917+624
The IDV observations of S4 0917+624 were carried out monthly, from August
2005 to January 2010, with the Urumqi 25m radio telescope at 4.8 GHz. The
quasar S4 0917+624 exhibits only very weak or no IDV during our 4.5 year
observing interval. Prior to the year 2000, the source S4 0917+624 was one of
the most prominent IDV sources. Our new data indicate that the previous strong
IDV has ceased. We analyzed the long-term VLBI structural variability using
Gaussian model-fitting. From this we obtained the flux densities and the
deconvolved sizes of core and inner-jet components of the source. We studied
the properties such as core fraction, angular size, spectral index, and
brightness temperature of VLBI core for S4 0917+624, as well as the time delay
between 5 and 15 GHz variations, and compared them with the IDV properties of
S4 0917+624. The source shows ejection of several jet components that are
suspected to have partially reduced the IDV amplitude of S4 0917+624. However,
during 2005-2006, the VLBI core size was comparable to the size before the year
2000, but no strong IDV was detected in the period, suggesting that the
quenching effect due to source size changes may not be responsible for the lack
of strong IDV after the year 2000. The refractive scattering properties for the
strong IDV phase of S4 0917+624 before the year 2000 are discussed. The
disappearance of strong IDV in S4 0917+624 after the year 2000 is a mystery and
cannot be explained via the quenching effect by changes in the observable VLBI
structure. However, it may be caused by changes in the interstellar medium,
i.e. by interstellar weather, which induces changes in the scintillation
pattern on timescales of several years. Further coordinated multi-frequency
observations will be required to distinguish between the effect of
source-intrinsic variability and changing properties of the interstellar
medium.Comment: 8 pages, 6 figures, accepted for publication in A&
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