2,512 research outputs found
Data Warehousing Failures: Case Studies and Findings
Eight studies of data warehousing failures are presented. They were written based on interviews with people who were associated with the projects. The extent of the failure varies with the organization, but in all cases, the project was at least a disappointment. Read the cases and prepare a one or two page discussion of the following: 1. Whatâs the scope of what can be considered a data warehousing failure? Discuss. 2. What generalizations apply across the cases? 3. What do you find most interesting in the failure stories? 4. Do they provide any insights about how a failure might be avoided
Stellar surface magneto-convection as a source of astrophysical noise II. Center-to-limb parameterisation of absorption line profiles and comparison to observations
Manifestations of stellar activity (such as star-spots, plage/faculae, and
convective flows) are well known to induce spectroscopic signals often referred
to as astrophysical noise by exoplanet hunters. For example, setting an
ultimate goal of detecting true Earth-analogs demands reaching radial velocity
(RV) precisions of ~9 cm/s. While this is becoming technically feasible with
the latest generation of highly stabilised spectrographs, it is astrophysical
noise that sets the true fundamental barrier on attainable RV precisions. In
this paper we parameterise the impact of solar surface magneto-convection on
absorption line profiles, and extend the analysis from the solar disc centre
(Paper I) to the solar limb. Off disc-centre, the plasma flows orthogonal to
the granule tops begin to lie along the line-of-sight and those parallel to the
granule tops are no longer completely aligned with the observer. Moreover, the
granulation is corrugated and the granules can block other granules, as well as
the intergranular lane components. Overall, the visible plasma flows and
geometry of the corrugated surface significantly impact the resultant line
profiles and induce centre-to-limb variations in shape and net position. We
detail these herein, and compare to various solar observations. We find our
granulation parameterisation can recreate realistic line profiles and induced
radial velocity shifts, across the stellar disc, indicative of both those found
in computationally heavy radiative 3D magnetohydrodynamical simulations and
empirical solar observations.Comment: 17 pages, 14 figures, accepted to Ap
Breed, age and gender distribution of dogs with chronic hepatitis in the United Kingdom
AbstractStandardised histological criteria are now available for the diagnosis of canine chronic hepatitis (CH). CH is common in dogs, but no studies have reported breed, age and gender distributions in the United Kingdom (UK). The objective of this study was to determine which breeds had an increased risk for developing CH in the UK and to report the age and gender distribution for those breeds. The databases of six veterinary histopathology laboratories were searched for cases with a histological diagnosis of CH according to standardised criteria. The breed, age and gender of dogs was recorded and compared to a control population to calculate the odds ratio and 95% confidence intervals for developing CH.A total of 551 cases of CH were identified, consisting of 61 breeds. Nineteen breeds were represented by five or more cases. Breeds with an increased risk for developing CH included the American cocker spaniel, Cairn terrier, Dalmatian, Dobermann pinscher, English cocker spaniel, English springer spaniel, Great Dane, Labrador retriever and Samoyed. The median age at diagnosis for all breeds with CH was 8years (range 7months to 16years). Dalmatians, Dobermann pinschers and English springer spaniels with CH were significantly younger than Cairn terriers, English cocker spaniels and Labrador retrievers with CH. Females were over-represented when all cases were examined together. In conclusion, several breeds in the UK have an increased risk of CH, some of which have not been previously reported
How well are aerosolâcloud interactions represented in climate models? â Part 1: Understanding the sulfate aerosol production from the 2014â15 Holuhraun eruption
For over 6 months, the 2014â2015 effusive eruption at Holuhraun, Iceland, injected considerable amounts of sulfur dioxide (SO2) into the lower troposphere with a daily rate of up to one-third of the global emission rate, causing extensive air pollution across Europe. The large injection of SO2, which oxidises to form sulfate aerosol (SO42-), provides a natural experiment offering an ideal opportunity to scrutinise state-of-the-art general circulation models' (GCMs) representation of aerosolâcloud interactions (ACIs). Here we present Part 1 of a two-part model inter-comparison using the Holuhraun eruption as a framework to analyse ACIs. We use SO2 retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) instrument and ground-based measurements of SO2 and SO42- mass concentrations across Europe, in conjunction with a trajectory analysis using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, to assess the spatial and chemical evolution of the volcanic plume as simulated by five GCMs and a chemical transport model (CTM). IASI retrievals of plume altitude and SO2 column load reveal that the volcanic perturbation is largely contained within the lower troposphere. Compared to the satellite observations, the models capture the spatial evolution and vertical variability of the plume reasonably well, although the models often overestimate the plume altitude. HYSPLIT trajectories are used to attribute to Holuhraun emissions 111 instances of elevated sulfurous surface mass concentrations recorded at European Monitoring and Evaluation Programme (EMEP) stations during September and October 2014. Comparisons with the simulated concentrations show that the modelled ratio of SO2 to SO42- during these pollution episodes is often underestimated and overestimated for the young and mature plume, respectively. Models with finer vertical resolutions near the surface are found to better capture these elevated sulfurous ground-level concentrations. Using an exponential function to describe the decay of observed surface mass concentration ratios of SO2 to SO42- with plume age, the in-plume oxidation rate constant is estimated as 0.032â±â0.002âhâ1 (1.30â±â0.08âd e-folding time), with a near-vent ratio of 25â±â5 (”gâmâ3 of SO2â/â”gâmâ3 of SO42-). The majority of the corresponding derived modelled oxidation rate constants are lower than the observed estimate. This suggests that the representation of the oxidation pathway/s in the simulated plumes is too slow. Overall, despite their coarse spatial resolutions, the six models show reasonable skill in capturing the spatial and chemical evolution of the Holuhraun plume. This capable representation of the underlying aerosol perturbation is essential to enable the investigation of the eruption's impact on ACIs in the second part of this study.</p
The Kepler-10 planetary system revisited by HARPS-N: A hot rocky world and a solid Neptune-mass planet
Kepler-10b was the first rocky planet detected by the Kepler satellite and
con- firmed with radial velocity follow-up observations from Keck-HIRES. The
mass of the planet was measured with a precision of around 30%, which was
insufficient to constrain models of its internal structure and composition in
detail. In addition to Kepler-10b, a second planet transiting the same star
with a period of 45 days was sta- tistically validated, but the radial
velocities were only good enough to set an upper limit of 20 Mearth for the
mass of Kepler-10c. To improve the precision on the mass for planet b, the
HARPS-N Collaboration decided to observe Kepler-10 intensively with the HARPS-N
spectrograph on the Telescopio Nazionale Galileo on La Palma. In to- tal, 148
high-quality radial-velocity measurements were obtained over two observing
seasons. These new data allow us to improve the precision of the mass
determina- tion for Kepler-10b to 15%. With a mass of 3.33 +/- 0.49 Mearth and
an updated radius of 1.47 +0.03 -0.02 Rearth, Kepler-10b has a density of 5.8
+/- 0.8 g cm-3, very close to the value -0.02 predicted by models with the same
internal structure and composition as the Earth. We were also able to determine
a mass for the 45-day period planet Kepler-10c, with an even better precision
of 11%. With a mass of 17.2 +/- 1.9 Mearth and radius of 2.35 +0.09 -0.04
Rearth, -0.04 Kepler-10c has a density of 7.1 +/- 1.0 g cm-3. Kepler-10c
appears to be the first strong evidence of a class of more massive solid
planets with longer orbital periods.Comment: 44 pages, 8 figures, accepted for publication in Ap
Estimating magnetic filling factors from simultaneous spectroscopy and photometry : disentangling spots, plage, and network
A.C.C. acknowledges support from the Science and Technology Facilities Council (STFC) consolidated grant number ST/R000824/1.State-of-the-art radial velocity (RV) exoplanet searches are limited by the effects of stellar magnetic activity. Magnetically active spots, plage, and network regions each have different impacts on the observed spectral lines and therefore on the apparent stellar RV. Differentiating the relative coverage, or filling factors, of these active regions is thus necessary to differentiate between activity-driven RV signatures and Doppler shifts due to planetary orbits. In this work, we develop a technique to estimate feature-specific magnetic filling factors on stellar targets using only spectroscopic and photometric observations. We demonstrate linear and neural network implementations of our technique using observations from the solar telescope at HARPS-N, the HK Project at the Mt. Wilson Observatory, and the Total Irradiance Monitor onboard SORCE. We then compare the results of each technique to direct observations by the Solar Dynamics Observatory. Both implementations yield filling factor estimates that are highly correlated with the observed values. Modeling the solar RVs using these filling factors reproduces the expected contributions of the suppression of convective blueshift and rotational imbalance due to brightness inhomogeneities. Both implementations of this technique reduce the overall activity-driven rms RVs from 1.64 to 1.02 m s(-1), corresponding to a 1.28 m s(-1) reduction in the rms variation. The technique provides an additional 0.41 m s(-1) reduction in the rms variation compared to traditional activity indicators.PostprintPeer reviewe
Bounding global aerosol radiative forcing of climate change
Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol-radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol-driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed-phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of -1.6 to -0.6 W mâ2, or -2.0 to -0.4 W mâ2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial-era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds
Detection Limits of Low-mass, Long-period Exoplanets Using Gaussian Processes Applied to HARPS-N Solar Radial Velocities
Radial velocity (RV) searches for Earth-mass exoplanets in the habitable zone
around Sun-like stars are limited by the effects of stellar variability on the
host star. In particular, suppression of convective blueshift and brightness
inhomogeneities due to photospheric faculae/plage and starspots are the
dominant contribution to the variability of such stellar RVs. Gaussian process
(GP) regression is a powerful tool for statistically modeling these
quasi-periodic variations. We investigate the limits of this technique using
800 days of RVs from the solar telescope on the High Accuracy Radial velocity
Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph. These data
provide a well-sampled time series of stellar RV variations. Into this data
set, we inject Keplerian signals with periods between 100 and 500 days and
amplitudes between 0.6 and 2.4 m s. We use GP regression to fit the
resulting RVs and determine the statistical significance of recovered periods
and amplitudes. We then generate synthetic RVs with the same covariance
properties as the solar data to determine a lower bound on the observational
baseline necessary to detect low-mass planets in Venus-like orbits around a
Sun-like star. Our simulations show that discovering planets with a larger mass
( 0.5 m s) using current-generation spectrographs and GP
regression will require more than 12 yr of densely sampled RV observations.
Furthermore, even with a perfect model of stellar variability, discovering a
true exo-Venus ( 0.1 m s) with current instruments would take over
15 yr. Therefore, next-generation spectrographs and better models of stellar
variability are required for detection of such planets
Separating planetary reflex Doppler shifts from stellar variability in the wavelength domain
Stellar magnetic activity produces time-varying distortions in the
photospheric line profiles of solar-type stars. These lead to systematic errors
in high-precision radial-velocity measurements, which limit efforts to discover
and measure the masses of low-mass exoplanets with orbital periods of more than
a few tens of days. We present a new data-driven method for separating Doppler
shifts of dynamical origin from apparent velocity variations arising from
variability-induced changes in the stellar spectrum. We show that the
autocorrelation function (ACF) of the cross-correlation function used to
measure radial velocities is effectively invariant to translation. By
projecting the radial velocities on to a subspace labelled by the observation
identifiers and spanned by the amplitude coefficients of the ACF's principal
components, we can isolate and subtract velocity perturbations caused by
stellar magnetic activity. We test the method on a 5-year time sequence of 853
daily 15-minute observations of the solar spectrum from the HARPS-N instrument
and solar-telescope feed on the 3.58-m Telescopio Nazionale Galileo. After
removal of the activity signals, the heliocentric solar velocity residuals are
found to be Gaussian and nearly uncorrelated. We inject synthetic low-mass
planet signals with amplitude cm s into the solar observations at
a wide range of orbital periods. Projection into the orthogonal complement of
the ACF subspace isolates these signals effectively from solar activity
signals. Their semi-amplitudes are recovered with a precision of cm
s, opening the door to Doppler detection and characterization of
terrestrial-mass planets around well-observed, bright main-sequence stars
across a wide range of orbital periods
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