70 research outputs found
Determining Eccentricities of Transiting Planets: A Divide in the Mass-Period Plane
The two dominant features in the distribution of orbital parameters for
close-in exoplanets are the prevalence of circular orbits for very short
periods, and the observation that planets on closer orbits tend to be heavier.
The first feature is interpreted as a signature of tidal evolution, while the
origin of the second, a "mass-period relation" for hot Jupiters, is not
understood. In this paper we re-consider the ensemble properties of transiting
exoplanets with well-measured parameters, focussing on orbital eccentricity and
the mass-period relation. We recalculate the constraints on eccentricity in a
homogeneous way, using new radial-velocity data, with particular attention to
statistical biases. We find that planets on circular orbits gather in a
well-defined region of the mass-period plane, close to the minimum period for
any given mass. Exceptions to this pattern reported in the Literature can be
attributed to statistical biases. The ensemble data is compatible with
classical tide theory with orbital circularisation caused by tides raised on
the planet, and suggest that tidal circularisation and the stopping mechanisms
for close-in planets are closely related to each other. The position
mass-period relation is compatible with a relation between a planet's Hill
radius and its present orbit.Comment: 8 pages, to be published in MNRA
The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations
The hot Jupiter HD189733b is the most extensively observed exoplanet. Its
atmosphere has been detected and characterised in transmission and eclipse
spectroscopy, and its phase curve measured at several wavelengths. This paper
brings together results of our campaign to obtain the complete transmission
spectrum of the atmosphere of this planet from UV to IR with HST, using STIS,
ACS and WFC3. We provide a new tabulation of the transmission spectrum across
the entire visible and IR range. The radius ratio in each wavelength band was
rederived to ensure a consistent treatment of the bulk transit parameters and
stellar limb-darkening. Special care was taken to correct for, and derive
realistic estimates of the uncertainties due to, both occulted and unocculted
star spots. The combined spectrum is very different from the predictions of
cloud-free models: it is dominated by Rayleigh scattering over the whole
visible and near infrared range, the only detected features being narrow Na and
K lines. We interpret this as the signature of a haze of condensate grains
extending over at least 5 scale heights. We show that a dust-dominated
atmosphere could also explain several puzzling features of the emission
spectrum and phase curves, including the large amplitude of the phase curve at
3.6um, the small hot-spot longitude shift and the hot mid-infrared emission
spectrum. We discuss possible compositions and derive some first-order
estimates for the properties of the putative condensate haze/clouds. We finish
by speculating that the dichotomy between the two observationally defined
classes of hot Jupiter atmospheres, of which HD189733b and HD209458b are the
prototypes, might not be whether they possess a temperature inversion, but
whether they are clear or dusty. We also consider the possibility of a
continuum of cloud properties between hot Jupiters, young Jupiters and L-type
brown dwarfs.Comment: Accepted for publication in MNRAS. 31 pages, 19 figures, 8 table
Observational constraints on tidal effects using orbital eccentricities
We have analysed radial velocity measurements for known transiting exoplanets
to study the empirical signature of tidal orbital evolution for close-in
planets. Compared to standard eccentricity determination, our approach is
modified to focus on the rejection of the null hypothesis of a circular orbit.
We are using a MCMC analysis of radial velocity measurements and photometric
constraints, including a component of correlated noise, as well as Bayesian
model selection to check if the data justifies the additional complexity of an
eccentric orbit. We find that among planets with non-zero eccentricity values
quoted in the literature, there is no evidence for an eccentricity detection
for the 7 planets CoRoT-5b, WASP-5b, WASP-6b, WASP-10b, WASP-12b, WASP-17b, and
WASP-18b. In contrast, we confirm the eccentricity of HAT-P-16b,
e=0.034\pm0.003, the smallest eccentricity that is reliably measured so far for
an exoplanet as well as that of WASP-14b, which is the planet at the shortest
period (P=2.24 d), with a confirmed eccentricity, e= 0.088\pm0.003. As part of
the study, we present new radial velocity data using the HARPS spectrograph for
CoRoT-1, CoRoT-3, WASP-2, WASP-4, WASP-5 and WASP-7 as well as the SOPHIE
spectrograph for HAT-P-4, HAT-P-7, TrES-2 and XO-2.
We show that the dissipative effect of tides raised in the planet by the star
and vice-versa explain all the eccentricity and spin-orbit alignment
measurements available for transiting planets. We revisit the mass-period
relation (Mazeh et al. 2005, Pont 2011) and consider its relation to the
stopping mechanism of orbital migration for hot Jupiters. In addition to
CoRoT-2 and HD 189733 (Pont 2009), we find evidence for excess rotation of the
star in the systems CoRoT-18, HAT-P-20, WASP-19 and WASP-43.Comment: 31 pages, 17 figures. Accepted by MNRAS (Feb 2012
CAN KIDS LEARN BY THEMSELVES USING TECHNOLOGY AND WHAT DO THEY LEARN?
This study presents the method and results of an experiment conducted to investigate whether kids (5-8 years) can use technology to learn in such unsupervised learning conditions. This study also assesses the skills and understanding that children develop through unsupervised technology exposure. The study, carried over 6 months, focused on two primary schools (School âAâ and School âBâ) both found in Mauritius. Both quantitative and qualitative approaches were adopted to collect data for this investigation. The qualitative data focused on interviews with kids, examination of the studentsâ work on PCs, observation in their ability to explore an interface, use a mouse, keyboard and the Internet. The quantitative data provide information which is easy to analyze statistically and fairly reliable. Descriptive statistics and charts have been used to analyze the quantitative data. It has been found that kids who had access to computers and Internet-based resources both at school and at home can self â educate themselves, but only to some extent because there will come such a moment when they will need a facilitator for guidanc
The migration of single hot Jupiters: Balance of evidence tips towards dynamical and tidal evolution
In this thesis, we revisit the seventeen year old question of how hot Jupiters got to their short period orbits, given that gas-giant planet building is supposed to take place beyond the ice-line at about 3 AU. Two major theories are generally used to explain this mystery. Firstly, exchange of energy and angular momentum between the newly-built planet and the progenitor dust and gas disk could result in planetary migration to a short period. This is generally believed to result in planets on circular orbits, with orbital angular momenta that are aligned with the host star spin. The competing theory which has gained more support in recent years, is that gravitational interactions leading to planet-planet scattering and/or Kozai interactions with massive and distant objects caused the planets to migrate violently (scattering) or slowly (Kozai) to short period, eccentric and misaligned orbits. These orbits are then expected to circularise and align under tidal interactions with the host star. In addition, the host star is expected to show evidence of spin-up if the tide on the star is strong enough.
Our contribution to this field is to provide additional support for the scenario involving dynamical interaction and tidal damping. We present observational evidence in the form of 158 new radial velocity measurements for 12 planets and a reanalysis of existing radial velocity data and photometric constraints from the literature for a total of 64 planetary systems. We also critically consider a further 30 newly announced planets from the literature. We show that there is no evidence for a finite eccentricity in several cases that were previously claimed to be âexceptionsâ to the observed trend that close-in planets are on circular orbits and the generally accepted reason that they underwent strong tidal interactions. We also show that the dissipative effect of tides raised in the planet by the star and vice versa explain all the eccentricity and spin-orbit alignment measurements available for transiting planets. We find evidence for excess rotation of the star in 6 systems, showing that heavy and close-in objects can exert strong tidal effects on the star. Hot Jupiters on circular orbits clump on the mass-period relation, which thus appears to be related to the stopping mechanism of orbital migration for hot Jupiters
Duodenal biopsies for the diagnosis of coeliac disease: are we adhering to current guidance?
Background: The British Society of Gastroenterology guidelines recommend taking at least four duodenal biopsy specimens at the time of upper gastrointestinal (UGI) endoscopy if coeliac disease (CD) is suspected and it has been shown to increase the diagnostic yield of CD. We assessed the compliance to these guidelines within our institution. We then applied measures to improve our compliance rate and assessed the resulting impact on our diagnostic rate of CD.
Methods: We performed a retrospective audit of electronic records for all patients, with no prior diagnosis of CD, who underwent UGI endoscopy with duodenal biopsies between August 2014 and May 2015. We implemented measures to raise awareness among endoscopy users at our institution and carried out a reaudit between February and May 2016.
Results: 924 patients were found to be eligible in the first part of the study and 278 in the second part. The proportion of patients who had â„4 biopsy specimens submitted increased from 21.9% to 60.8% (p<0.001). The diagnostic rate of CD increased from 3.5% in the audit group to 7.6% in the reaudit group (p=0.004). A positive serology result and suspected CD as an indication for biopsy were found to be independent predictors of the likelihood of complying with guidelines.
Conclusions: Our study suggests that taking <4 duodenal biopsy specimens to assess for the presence of CD may lead to the diagnosis of CD being missed. Simple measures can improve the local compliance rate to current guidelines
The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.The hot Jupiter HD 189733b is the most extensively observed exoplanet. Its atmosphere has been detected and characterized in transmission and eclipse spectroscopy, and its phase curve measured at several wavelengths. This paper brings together the results of our campaign to obtain the complete transmission spectrum of the atmosphere of this planet from UV to infrared with the Hubble Space Telescope, using the STIS, ACS and WFC3 instruments. We provide a new tabulation of the transmission spectrum across the entire visible and infrared range. The radius ratio in each wavelength band was re-derived, where necessary, to ensure a consistent treatment of the bulk transit parameters and stellar limb darkening. Special care was taken to correct for, and derive realistic estimates of the uncertainties due to, both occulted and unocculted star spots.
The combined spectrum is very different from the predictions of cloud-free models for hot Jupiters: it is dominated by Rayleigh scattering over the whole visible and near-infrared range, the only detected features being narrow sodium and potassium lines. We interpret this as the signature of a haze of condensate grains extending over at least five scaleheights. We show that a dust-dominated atmosphere could also explain several puzzling features of the emission spectrum and phase curves, including the large amplitude of the phase curve at 3.6âÎŒm, the small hotspot longitude shift and the hot mid-infrared emission spectrum. We discuss possible compositions and derive some first-order estimates for the properties of the putative condensate haze/clouds. We finish by speculating that the dichotomy between the two observationally defined classes of hot Jupiter atmospheres, of which HD 189733b and HD 209458b are the prototypes, might not be whether they possess a temperature inversion, but whether they are clear or dusty. We also consider the possibility of a continuum of cloud properties between hot Jupiters, young Jupiters and L-type brown dwarfs.Science and Technology Facilities Council (STFC)NASANSFTennessee State UniversityState of Tennesse
A Secure Federated Learning Framework for Residential Short Term Load Forecasting
Smart meter measurements, though critical for accurate demand forecasting,
face several drawbacks including consumers' privacy, data breach issues, to
name a few. Recent literature has explored Federated Learning (FL) as a
promising privacy-preserving machine learning alternative which enables
collaborative learning of a model without exposing private raw data for short
term load forecasting. Despite its virtue, standard FL is still vulnerable to
an intractable cyber threat known as Byzantine attack carried out by faulty
and/or malicious clients. Therefore, to improve the robustness of federated
short-term load forecasting against Byzantine threats, we develop a
state-of-the-art differentially private secured FL-based framework that ensures
the privacy of the individual smart meter's data while protect the security of
FL models and architecture. Our proposed framework leverages the idea of
gradient quantization through the Sign Stochastic Gradient Descent (SignSGD)
algorithm, where the clients only transmit the `sign' of the gradient to the
control centre after local model training. As we highlight through our
experiments involving benchmark neural networks with a set of Byzantine attack
models, our proposed approach mitigates such threats quite effectively and thus
outperforms conventional Fed-SGD models
Mass Transfer, Transiting Stream and Magnetopause in Close-in Exoplanetary Systems with Applications to WASP-12
We study mass transfer by Roche lobe overflow in close-in exoplanetary
systems. The planet's atmospheric gas passes through the inner Lagrangian point
and flows along a narrow stream, accelerating to 100-200\kms velocity before
forming an accretion disk. We show that the cylinder-shaped accretion stream
can have an area (projected in the plane of the sky) comparable to that of the
planet and a significant optical depth to spectral line absorption. Such a
"transiting cylinder" may produce an earlier ingress of the planet transit, as
suggested by recent HST observations of the WASP-12 system. The asymmetric disk
produced by the accretion stream may also lead to time-dependent obscuration of
the star light and apparent earlier ingress. We also consider the interaction
of the stellar wind with the planetary magnetosphere. Since the wind speed is
subsonic/sub-Alfvenic and comparable to the orbital velocity of the planet, the
head of the magnetopause lies eastward relative to the substellar line (the
line joining the planet and the star). The gas around the magnetopause may, if
sufficiently compressed, give rise to asymmetric ingress/egress during the
planet transit, although more works are needed to evaluate this possibility.Comment: 6 pages with 2 figures. Accepted in ApJ. Small changes (add
discussion on asymmetric disks
Factors Affecting the Radii of Close-in Transiting Exoplanets
The radius of an exoplanet may be affected by various factors, including
irradiation, planet mass and heavy element content. A significant number of
transiting exoplanets have now been discovered for which the mass, radius,
semi-major axis, host star metallicity and stellar effective temperature are
known. We use multivariate regression models to determine the dependence of
planetary radius on planetary equilibrium temperature T_eq, planetary mass M_p,
stellar metallicity [Fe/H], orbital semi-major axis a, and tidal heating rate
H_tidal, for 119 transiting planets in three distinct mass regimes. We
determine that heating leads to larger planet radii, as expected, increasing
mass leads to increased or decreased radii of low-mass (<0.5R_J) and high-mass
(>2.0R_J) planets, respectively (with no mass effect on Jupiter-mass planets),
and increased host-star metallicity leads to smaller planetary radii,
indicating a relationship between host-star metallicity and planet heavy
element content. For Saturn-mass planets, a good fit to the radii may be
obtained from log(R_p/R_J)=-0.077+0.450 log(M_p/M_J)-0.314[Fe/H]+0.671
log(a/AU)+0.398 log(T_eq/K). The radii of Jupiter-mass planets may be fit by
log(R_p/R_J)=-2.217+0.856 log(T_eq/K)+0.291 log(a/AU). High-mass planets' radii
are best fit by log(R_p/R_J)=-1.067+0.380 log(T_eq/K)-0.093
log(M_p/M_J)-0.057[Fe/H]+0.019 log(H_tidal/1x10^{20}). These equations produce
a very good fit to the observed radii, with a mean absolute difference between
fitted and observed radius of 0.11R_J. A clear distinction is seen between the
core-dominated Saturn-mass (0.1-0.5M_J) planets, whose radii are determined
almost exclusively by their mass and heavy element content, and the gaseous
envelope-dominated Jupiter-mass (0.5-2.0M_J) planets, whose radii increase
strongly with irradiating flux, partially offset by a power-law dependence on
orbital separation.Comment: 14 pages, 14 figures, accepted in A&
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