678 research outputs found
Characterising exo-ringsystems around fast-rotating stars using the Rossiter-McLaughlin effect
Planetary rings produce a distinct shape distortion in transit lightcurves.
However, to accurately model such lightcurves the observations need to cover
the entire transit, especially ingress and egress, as well as an out-of-transit
baseline. Such observations can be challenging for long period planets, where
the transits may last for over a day. Planetary rings will also impact the
shape of absorption lines in the stellar spectrum, as the planet and rings
cover different parts of the rotating star (the Rossiter-McLaughlin effect).
These line-profile distortions depend on the size, structure, opacity,
obliquity and sky projected angle of the ring system. For slow rotating stars,
this mainly impacts the amplitude of the induced velocity shift, however, for
fast rotating stars the large velocity gradient across the star allows the line
distortion to be resolved, enabling direct determination of the ring
parameters. We demonstrate that by modeling these distortions we can recover
ring system parameters (sky-projected angle, obliquity and size) using only a
small part of the transit. Substructure in the rings, e.g. gaps, can be
recovered if the width of the features () relative to the size of the
star is similar to the intrinsic velocity resolution (set by the width of the
local stellar profile, ) relative to the stellar rotation velocity (
sin, i.e. sin/). This opens up a new
way to study the ring systems around planets with long orbital periods, where
observations of the full transit, covering the ingress and egress, are not
always feasible.Comment: Accepted for publication in MNRA
Josephson squelch filter for quantum nanocircuits
We fabricated and tested a squelch circuit consisting of a copper powder
filter with an embedded Josephson junction connected to ground. For small
signals (squelch-ON), the small junction inductance attenuates strongly from DC
to at least 1 GHz, while for higher frequencies dissipation in the copper
powder increases the attenuation exponentially with frequency. For large
signals (squelch-OFF) the circuit behaves as a regular metal powder filter. The
measured ON/OFF ratio is larger than 50dB up to 50 MHz. This squelch can be
applied in low temperature measurement and control circuitry for quantum
nanostructures such as superconducting qubits and quantum dots.Comment: Corrected and completed references 6,7,8. Updated some minor details
in figure
Detection of water absorption in the day side atmosphere of HD 189733 b using ground-based high-resolution spectroscopy at 3.2 microns
We report a 4.8 sigma detection of water absorption features in the day side
spectrum of the hot Jupiter HD 189733 b. We used high-resolution (R~100,000)
spectra taken at 3.2 microns with CRIRES on the VLT to trace the
radial-velocity shift of the water features in the planet's day side atmosphere
during 5 h of its 2.2 d orbit as it approached secondary eclipse. Despite
considerable telluric contamination in this wavelength regime, we detect the
signal within our uncertainties at the expected combination of systemic
velocity (Vsys=-3 +5-6 km/s) and planet orbital velocity (Kp=154 +14-10 km/s),
and determine a H2O line contrast ratio of (1.3+/-0.2)x10^-3 with respect to
the stellar continuum. We find no evidence of significant absorption or
emission from other carbon-bearing molecules, such as methane, although we do
note a marginal increase in the significance of our detection to 5.1 sigma with
the inclusion of carbon dioxide in our template spectrum. This result
demonstrates that ground-based, high-resolution spectroscopy is suited to
finding not just simple molecules like CO, but also to more complex molecules
like H2O even in highly telluric contaminated regions of the Earth's
transmission spectrum. It is a powerful tool that can be used for conducting an
immediate census of the carbon- and oxygen-bearing molecules in the atmospheres
of giant planets, and will potentially allow the formation and migration
history of these planets to be constrained by the measurement of their
atmospheric C/O ratios.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
The GROUSE project III: Ks-band observations of the thermal emission from WASP-33b
In recent years, day-side emission from about a dozen hot Jupiters has been
detected through ground-based secondary eclipse observations in the
near-infrared. These near-infrared observations are vital for determining the
energy budgets of hot Jupiters, since they probe the planet's spectral energy
distribution near its peak. The aim of this work is to measure the Ks-band
secondary eclipse depth of WASP-33b, the first planet discovered to transit an
A-type star. This planet receives the highest level of irradiation of all
transiting planets discovered to date. Furthermore, its host-star shows
pulsations and is classified as a low-amplitude delta-Scuti. As part of our
GROUnd-based Secondary Eclipse (GROUSE) project we have obtained observations
of two separate secondary eclipses of WASP-33b in the Ks-band using the LIRIS
instrument on the William Herschel Telescope (WHT). The telescope was
significantly defocused to avoid saturation of the detector for this bright
star (K~7.5). To increase the stability and the cadence of the observations,
they were performed in staring mode. We collected a total of 5100 and 6900
frames for the first and the second night respectively, both with an average
cadence of 3.3 seconds. On the second night the eclipse is detected at the
12-sigma level, with a measured eclipse depth of 0.244+0.027-0.020 %. This
eclipse depth corresponds to a brightness temperature of 3270+115-160 K. The
measured brightness temperature on the second night is consistent with the
expected equilibrium temperature for a planet with a very low albedo and a
rapid re-radiation of the absorbed stellar light. For the other night the short
out-of-eclipse baseline prevents good corrections for the stellar pulsations
and systematic effects, which makes this dataset unreliable for eclipse depth
measurements. This demonstrates the need of getting a sufficient out-of-eclipse
baseline.Comment: 12 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
Geometrical Defects in Josephson Junction Arrays
Dislocations and disclinations in a lattice of Josephson junctions will
affect the dynamics of vortex excitations within the array. These defects
effectively distort the space in which the excitations move and interact. The
interaction energy between such defects and excitations are determined and
vortex trajectories in twisted lattices are calculated. Finally, possible
experiments observing these effects are presented.Comment: 26 pages including 5 figure
The Changing Face of α Centauri B: Probing plage and stellar activity in K-dwarfs
A detailed knowledge of stellar activity is crucial for understanding stellar
dynamos, as well as pushing exoplanet radial-velocity detection limits towards
Earth analogue confirmation. We directly compare archival HARPS spectra taken
at the minimum in Cen B's activity cycle to a high-activity state when
clear rotational modulation of is visible. Relative to the
inactive spectra, we find a large number of narrow pseudo-emission features in
the active spectra with strengths that are rotationally modulated. These
features most likely originate from plage, spots, or a combination of both.
They also display radial velocity variations of 300 m s -
consistent with an active region rotating across the stellar surface.
Furthermore, we see evidence that some of the lines originating from the
`active immaculate' photosphere appear broader relative to the `inactive
immaculate' case. This may be due to enhanced contributions of e.g. magnetic
bright points to these lines, which then causes additional line broadening.
More detailed analysis may enable measurements of plage and spot coverage using
single spectra in the future.Comment: 6 pages, 4 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society (MNRAS) Letter
Conductance Fluctuations in a Metallic Wire Interrupted by a Tunnel Junction
The conductance fluctuations of a metallic wire which is interrupted by a
small tunnel junction has been explored experimentally. In this system, the
bias voltage V, which drops almost completely inside the tunnel barrier, is
used to probe the energy dependence of conductance fluctuations due to disorder
in the wire. We find that the variance of the fluctuations is directly
proportional to V. The experimental data are consistently described by a
theoretical model with two phenomenological parameters: the phase breaking time
at low temperatures and the diffusion coefficient.Comment: 9 pages RevTeX and 4 PS figures (accepted for publication in Physical
Review Letters
Electronic structure of superposition states in flux qubits
Flux qubits, small superconducting loops interrupted by Josephson junctions,
are successful realizations of quantum coherence for macroscopic variables.
Superconductivity in these loops is carried by --
electrons, which has been interpreted as suggesting that coherent
superpositions of such current states are macroscopic superpositions analogous
to Schr\"odinger's cat. We provide a full microscopic analysis of such qubits,
from which the macroscopic quantum description can be derived. This reveals
that the number of microscopic constituents participating in superposition
states for experimentally accessible flux qubits is surprisingly but not
trivially small. The combination of this relatively small size with large
differences between macroscopic observables in the two branches is seen to
result from the Fermi statistics of the electrons and the large disparity
between the values of superfluid and Fermi velocity in these systems.Comment: Minor cosmetic changes. Published version
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