2,663 research outputs found
Comment on ``Force Balance at the Transition from Selective Withdrawal to Viscous Entrainment
Comment on paper by Blanchette and Zhang, Phys. Rev. Lett. 102, 144501
(2009)
Controlled energy-selected electron capture and release in double quantum dots
Highly accurate quantum electron dynamics calculations demonstrate that
energy can be efficiently transferred between quantum dots. Specifically, in a
double quantum dot an incoming electron is captured by one dot and the excess
energy is transferred to the neighboring dot and used to remove an electron
from this dot. This process is due to long-range electron correlation and shown
to be operative at rather large distances between the dots. The efficiency of
the process is greatly enhanced by preparing the double quantum dot such that
the incoming electron is initially captured by a two-electron resonance state
of the system. In contrast to atoms and molecules in nature, double quantum
dots can be manipulated to achieve this enhancement. This mechanism leads to a
surprisingly narrow distribution of the energy of the electron removed in the
process which is explained by resonance theory. We argue that the process could
be exploited in practice.Comment: Lette
Electron-correlation driven capture and release in double quantum dots
We recently predicted that the interatomic Coulombic electron capture (ICEC)
process, a long-range electron correlation driven capture process, is
achievable in gated double quantum dots (DQDs). In ICEC an incoming electron is
captured by one QD and the excess energy is used to remove an electron from the
neighboring QD. In this work we present systematic full three-dimensional
electron dynamics calculations in quasi-one dimensional model potentials that
allow for a detailed understanding of the connection between the DQD geometry
and the reaction probability for the ICEC process. We derive an effective
one-dimensional approach and show that its results compare very well with those
obtained using the full three-dimensional calculations. This approach
substantially reduces the computation times. The investigation of the
electronic structure for various DQD geometries for which the ICEC process can
take place clarify the origin of its remarkably high probability in the
presence of two-electron resonances
Accuracy of stellar parameters of exoplanet-host stars determined from asteroseismology
In the context of the space-based mission CoRoT, devoted to asteroseismology
and search for planet transits, we analyse the accuracy of fundamental stellar
parameters (mass, radius, luminosity) that can be obtained from
asteroseismological data.} Our work is motivated by the large uncertainties on
planetary radius determination of transiting planets which are mainly due to
uncertainties on the stellar parameters. Our goal is to analyse uncertainties
of fundamental stellar parameters for a given accuracy of oscillation frequency
determination. We generate grids of equilibrium models of stars and compute
their pulsation spectra based on a linear nonadiabatic stability analysis.
Using differents methods of comparison of oscillation mode spectra, we derive
uncertainties on fundamental stellar parameters and analyse the effect of
varying the number of considered modes.} The limits obtained depend strongly on
the adapted method to compare spectra. We find a degeneracy in the stellar
parameter solutions, up to a few % in mass (from less than 1% to more than 7%
depending on the method used and the number of considered modes), luminosity
(from 2% to more than 10%) or radius (from less than 1% to 3%), for a given
pulsation spectrum.Comment: 7 pages, 9 figure
A transiting planet among 23 new near-threshold candidates from the OGLE survey - OGLE-TR-182
By re-processing the data of the second season of the OGLE survey for
planetary transits and adding new mesurements on the same fields gathered in
subsequent years with the OGLE telescope, we have identified 23 new transit
candidates, recorded as OGLE-TR-178 to OGLE-TR-200. We studied the nature of
these objects with the FLAMES/UVES multi-fiber spectrograph on the VLT. One of
the candidates, OGLE-TR-182, was confirmed as a transiting gas giant planet on
a 4-day orbit. We characterised it with further observations using the FORS1
camera and UVES spectrograph on the VLT. OGLE-TR-182b is a typical ``hot
Jupiter'' with an orbital period of 3.98 days, a mass of 1.01 +- 0.15 MJup and
a radius of 1.13 (+0.24-0.08) RJup. Confirming this transiting planet required
a large investment in telescope time with the best instruments available, and
we comment on the difficulty of the confirmation process for transiting planets
in the OGLE survey. We delienate the zone were confirmation is difficult or
impossible, and discuss the implications for the Corot space mission in its
quest for transiting telluric planets.Comment: 7 pages, submitted to Astronomy and Astrophysic
A Gaussian process framework for modelling instrumental systematics: application to transmission spectroscopy
Transmission spectroscopy, which consists of measuring the
wavelength-dependent absorption of starlight by a planet's atmosphere during a
transit, is a powerful probe of atmospheric composition. However, the expected
signal is typically orders of magnitude smaller than instrumental systematics,
and the results are crucially dependent on the treatment of the latter. In this
paper, we propose a new method to infer transit parameters in the presence of
systematic noise using Gaussian processes, a technique widely used in the
machine learning community for Bayesian regression and classification problems.
Our method makes use of auxiliary information about the state of the
instrument, but does so in a non-parametric manner, without imposing a specific
dependence of the systematics on the instrumental parameters, and naturally
allows for the correlated nature of the noise. We give an example application
of the method to archival NICMOS transmission spectroscopy of the hot Jupiter
HD 189733, which goes some way towards reconciling the controversy surrounding
this dataset in the literature. Finally, we provide an appendix giving a
general introduction to Gaussian processes for regression, in order to
encourage their application to a wider range of problems.Comment: 6 figures, 1 table, accepted for publication in MNRA
A new look at NICMOS transmission spectroscopy of HD189733, GJ-436 and XO-1: no conclusive evidence for molecular features
We present a re-analysis of archival HST/NICMOS transmission spectroscopy of
three exoplanet systems; HD 189733, GJ-436 and XO-1. Detections of several
molecules, including H20, CH4 and CO2, have been claimed for HD 189733 and
XO-1, but similarly sized features are attributed to systematic noise for
GJ-436. The data consist of time-series grism spectra covering a planetary
transit. After extracting light curves in independent wavelength channels, we
use a linear decorrelation technique account for instrumental systematics
(which is becoming standard in the field), and measure the planet-to-star
radius ratio as a function of wavelength. For HD 189733, the uncertainties in
the transmission spectrum are significantly larger than those previously
reported. We also find the transmission spectrum is considerably altered when
using different out-of-transit orbits to remove the systematics, when some
parameters are left out of the decorrelation procedure, or when we perform the
decorrelation with quadratic functions rather than linear functions. Given that
there is no physical reason to believe the baseline flux should be modelled as
a linear function of any particular set of parameters, we interpret this as
evidence that the linear decorrelation technique is not a robust method to
remove systematic effects from the light curves for each wavelength channel.
For XO-1, the parameters measured to decorrelate the light curves would require
extrapolation to the in-transit orbit to remove the systematics, and we cannot
reproduce the previously reported results. We conclude that the resulting
NICMOS transmission spectra are too dependent on the method used to remove
systematics to be considered robust detections of molecular species in
planetary atmospheres, although the presence of these molecules is not ruled
out.Comment: 17 pages, 28 figures, accepted in MNRA
Electrically induced tunable cohesion in granular systems
Experimental observations of confined granular materials in the presence of
an electric field that induces cohesive forces are reported. The angle of
repose is found to increase with the cohesive force. A theoretical model for
the stability of a granular heap, including both the effect of the sidewalls
and cohesion is proposed. A good agreement between this model and the
experimental results is found. The steady-state flow angle is practically
unaffected by the electric field except for high field strengths and low flow
rates.Comment: accepted for publication in "Journal of Statistical Mechanics: Theory
and Experiment
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