739 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
Stemmen voor duurzaamheid
In de verkiezingsprogramma's van de vijf grote partijen schuilt een elementair verschil in opvatting over de relatie tussen milieu en
economie
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
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
Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates
We present a theoretical analysis of the selective darkening method for
implementing quantum controlled-NOT (CNOT) gates. This method, which we
recently proposed and demonstrated, consists of driving two
transversely-coupled quantum bits (qubits) with a driving field that is
resonant with one of the two qubits. For specific relative amplitudes and
phases of the driving field felt by the two qubits, one of the two transitions
in the degenerate pair is darkened, or in other words, becomes forbidden by
effective selection rules. At these driving conditions, the evolution of the
two-qubit state realizes a CNOT gate. The gate speed is found to be limited
only by the coupling energy J, which is the fundamental speed limit for any
entangling gate. Numerical simulations show that at gate speeds corresponding
to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and
increases further for lower gate speeds. In addition, the effect of
higher-lying energy levels and weak anharmonicity is studied, as well as the
scalability of the method to systems of multiple qubits. We conclude that in
all these respects this method is competitive with existing schemes for
creating entanglement, with the added advantages of being applicable for qubits
operating at fixed frequencies (either by design or for exploitation of
coherence sweet-spots) and having the simplicity of microwave-only operation.Comment: 25 pages, 5 figure
Low-crosstalk bifurcation detectors for coupled flux qubits
We present experimental results on the crosstalk between two AC-operated
dispersive bifurcation detectors, implemented in a circuit for high-fidelity
readout of two strongly coupled flux qubits. Both phase-dependent and
phase-independent contributions to the crosstalk are analyzed. For proper
tuning of the phase the measured crosstalk is 0.1 % and the correlation between
the measurement outcomes is less than 0.05 %. These results show that
bifurcative readout provides a reliable and generic approach for multi-partite
correlation experiments.Comment: Copyright 2010 American Institute of Physics. This article may be
downloaded for personal use only. Any other use requires prior permission of
the author and the American Institute of Physics. The following article
appeared in Applied Physics Letters and may be found at
http://link.aip.org/link/?apl/96/12350
The GROUSE project II: Detection of the Ks-band secondary eclipse of exoplanet HAT-P-1b
Context: Only recently it has become possible to measure the thermal emission
from hot-Jupiters at near-Infrared wavelengths using ground-based telescopes,
by secondary eclipse observations. This allows the planet flux to be probed
around the peak of its spectral energy distribution, which is vital for the
understanding of its energy budget. Aims: The aim of the reported work is to
measure the eclipse depth of the planet HAT-P-1b at 2.2micron. This planet is
an interesting case, since the amount of stellar irradiation it receives falls
in between that of the two best studied systems (HD209458 and HD189733), and it
has been suggested to have a weak thermal inversion layer. Methods: We have
used the LIRIS instrument on the William Herschel Telescope (WHT) to observe
the secondary eclipse of HATP-1b in the Ks-band, as part of our Ground-based
secondary eclipse (GROUSE) project. The observations were done in staring mode,
while significantly defocusing the telescope to avoid saturation on the K=8.4
star. With an average cadence of 2.5 seconds, we collected 6520 frames during
one night. Results: The eclipse is detected at the 4sigma level, the measured
depth being 0.109+/-0.025%. The uncertainties are dominated by residual
systematic effects, as estimated from different reduction/analysis procedures.
The measured depth corresponds to a brightness temperature of 2136+150-170K.
This brightness temperature is significantly higher than those derived from
longer wavelengths, making it difficult to fit all available data points with a
plausible atmospheric model. However, it may be that we underestimate the true
uncertainties of our measurements, since it is notoriously difficult to assign
precise statistical significance to a result when systematic effects are
important.Comment: 7 pages, 10 figures, Accepted for publication in A&
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
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