86 research outputs found
Fast and Simple Relational Processing of Uncertain Data
This paper introduces U-relations, a succinct and purely relational
representation system for uncertain databases. U-relations support
attribute-level uncertainty using vertical partitioning. If we consider
positive relational algebra extended by an operation for computing possible
answers, a query on the logical level can be translated into, and evaluated as,
a single relational algebra query on the U-relation representation. The
translation scheme essentially preserves the size of the query in terms of
number of operations and, in particular, number of joins. Standard techniques
employed in off-the-shelf relational database management systems are effective
for optimizing and processing queries on U-relations. In our experiments we
show that query evaluation on U-relations scales to large amounts of data with
high degrees of uncertainty.Comment: 12 pages, 14 figure
Magnetic field structure in single late-type giants: Beta Ceti in 2010 - 2012
The data were obtained using two spectropolarimeters - Narval at the Bernard
Lyot Telescope, Pic du Midi, France, and ESPaDOnS at CFHT, Hawaii. Thirty-eight
circularly-polarized spectra have been collected in the period June 2010 -
January 2012. The Least Square Deconvolution method was applied for extracting
high signal-to-noise ratio line profiles, from which we measure the
surface-averaged longitudinal magnetic field Bl. Chromospheric activity
indicators CaII K, H_alpha, CaII IR (854.2 nm) and radial velocity were
simultaneously measured and their variability was analysed together with the
behavior of Bl. The Zeeman Doppler Imaging (ZDI) inversion technique was
employed for reconstruction of the large-scale magnetic field and two magnetic
maps of Beta Ceti are presented for two periods (June 2010 - December 2010 and
June 2011 - January 2012). Bl remains of positive polarity for the whole
observational period. The behavior of the line activity indicators is in good
agreement with the Bl variations. The two ZDI maps show a mainly axisymmetric
and poloidal magnetic topology and a simple surface magnetic field
configuration dominated by a dipole. Little evolution is observed between the
two maps, in spite of a 1 yr interval between both subsets. We also use
state-of-the-art stellar evolution models to constrain the evolutionary status
of Beta Ceti. We derive a mass of 3.5 M_sun and propose that this star is
already in the central-helium burning phase. Taking into account all our
results and the evolutionary status of the star, we suggest that dynamo action
alone may not be eficient enough to account for the high magnetic activity of
Beta Ceti. As an alternate option, we propose that it may be an Ap star
descendant presently undergoing central helium-burning and still exhibiting a
remnant of the Ap star magnetic field.Comment: 10 pages; 5 figures; 3 table
A dominant magnetic dipole for the evolved Ap star candidate EK Eridani
EK Eri is one of the most slowly rotating active giants known, and has been
proposed to be the descendant of a strongly magnetic Ap star. We have performed
a spectropolarimetric study of EK Eri over 4 photometric periods with the aim
of inferring the topology of its magnetic field. We used the NARVAL
spectropolarimeter at the Bernard Lyot telescope at the Pic du Midi
Observatory, along with the least-squares deconvolution method, to extract high
signal-to-noise ratio Stokes V profiles from a timeseries of 28 polarisation
spectra. We have derived the surface-averaged longitudinal magnetic field Bl.
We fit the Stokes V profiles with a model of the large-scale magnetic field and
obtained Zeeman Doppler images of the surface magnetic strength and geometry.
Bl variations of up to about 80 G are observed without any reversal of its
sign, and which are in phase with photometric ephemeris. The activity
indicators are shown to vary smoothly on a timescale compatible with the
rotational period inferred from photometry (308.8 d.), however large deviations
can occur from one rotation to another. The surface magnetic field variations
of EK Eri appear to be dominated by a strong magnetic spot (of negative
polarity) which is phased with the dark (cool) photometric spot. Our modeling
shows that the large-scale magnetic field of EK Eri is strongly poloidal. For a
rotational axis inclination of i = 60{\deg}, we obtain a model that is almost
purely dipolar. In the dipolar model, the strong magnetic/photometric spot
corresponds to the negative pole of the dipole, which could be the remnant of
that of an Ap star progenitor of EK Eri. Our observations and modeling
conceptually support this hypothesis, suggesting an explanation of the
outstanding magnetic properties of EK Eri as the result of interaction between
deep convection and the remnant of an Ap star magnetic dipole.Comment: 8 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Supporting User-Defined Functions on Uncertain Data
Uncertain data management has become crucial in many sensing and scientific applications. As user-defined functions (UDFs) become widely used in these applications, an important task is to capture result uncertainty for queries that evaluate UDFs on uncertain data. In this work, we provide a general framework for supporting UDFs on uncertain data. Specifically, we propose a learning approach based on Gaussian processes (GPs) to compute approximate output distributions of a UDF when evaluated on uncertain input, with guaranteed error bounds. We also devise an online algorithm to compute such output distributions, which employs a suite of optimizations to improve accuracy and performance. Our evaluation using both real-world and synthetic functions shows that our proposed GP approach can outperform the state-of-the-art sampling approach with up to two orders of magnitude improvement for a variety of UDFs. 1
The Magnetic Fields at the Surface of Active Single G-K Giants
We investigate the magnetic field at the surface of 48 red giants selected as
promising for detection of Stokes V Zeeman signatures in their spectral lines.
We use the spectropolarimeters Narval and ESPaDOnS to detect circular
polarization within the photospheric absorption lines of our targets and use
the least-squares deconvolution (LSD) method. We also measure the classical
S-index activity indicator, and the stellar radial velocity. To infer the
evolutionary status of our giants and to interpret our results, we use
state-of-the-art stellar evolutionary models with predictions of convective
turnover times. We unambiguously detect magnetic fields via Zeeman signatures
in 29 of the 48 red giants in our sample. Zeeman signatures are found in all
but one of the 24 red giants exhibiting signs of activity, as well as 6 out of
17 bright giant stars.The majority of the magnetically detected giants are
either in the first dredge up phase or at the beginning of core He burning,
i.e. phases when the convective turnover time is at a maximum: this corresponds
to a 'magnetic strip' for red giants in the Hertzsprung-Russell diagram. A
close study of the 16 giants with known rotational periods shows that the
measured magnetic field strength is tightly correlated with the rotational
properties, namely to the rotational period and to the Rossby number Ro. Our
results show that the magnetic fields of these giants are produced by a dynamo.
Four stars for which the magnetic field is measured to be outstandingly strong
with respect to that expected from the rotational period/magnetic field
relation or their evolutionary status are interpreted as being probable
descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4
bright giants (Aldebaran, Alphard, Arcturus, eta Psc) are detected with
magnetic field strength at the sub-gauss level.Comment: 34 pages, 22 Figures, accepted for publication in Astronomy &
Astrophysic
EK Eridani: the tip of the iceberg of giants which have evolved from magnetic Ap stars
We observe the slowly-rotating, active, single giant, EK Eri, to study and
infer the nature of its magnetic field directly. We used the spectropolarimeter
NARVAL at the Telescope Bernard Lyot, Pic du Midi Observatory, and the Least
Square Deconvolution method to create high signal-to-noise ratio Stokes V
profiles. We fitted the Stokes V profiles with a model of the large-scale
magnetic field. We studied the classical activity indicators, the CaII H and K
lines, the CaII infrared triplet, and H\alpha line. We detected the Stokes V
signal of EK Eri securely and measured the longitudinal magnetic field Bl for
seven individual dates spanning 60% of the rotational period. The measured
longitudinal magnetic field of EK Eri reached about 100 G and was as strong as
fields observed in RSCVn or FK Com type stars: this was found to be
extraordinary when compared with the weak fields observed at the surfaces of
slowly-rotating MS stars or any single red giant previously observed with
NARVAL. From our modeling, we infer that the mean surface magnetic field is
about 270 G, and that the large scale magnetic field is dominated by a poloidal
component. This is compatible with expectations for the descendant of a
strongly magnetic Ap star.Comment: 8 pages, 6 figures. Accepted for publication in A&
Recommended from our members
Health âBrexternalitiesâ: The Brexit effect on health and health care outside the UK
The principal effects of Brexit on health and health care will fall within the United Kingdom, and all forms of Brexit have overwhelmingly negative implications for health care and health within the UK. This article focuses on the external effects of Brexit (âBrexternalitiesâ) for health and health care. The EU is a particularly powerful institutional and legal arrangement for managing economic and political externalities, in health policy as in any other policy. Equally, when a state leaves the EU, the manner of leaving will result in better or worse management of relevant externalities. âBrexternalitiesâ thus involve questions about policy legitimacy and accountability. Health âBrexternalitiesâ do not fall equally in all EU countries. They are felt more distinctly in the context of those elements of health policy that are most closely entwined with the UKâs health policy: for instance, on the island of Ireland; certain areas of Spain and other parts of southern Europe. Some health âBrexternalitiesâ, such as in medicines safety, will be imposed on the whole population of the EU. And some health âBrexternalitiesâ, such as communicable disease control, will be felt globally
Discovery of a weak magnetic field in the photosphere of the single giant Pollux
Aims: We observe the nearby, weakly-active single giant, Pollux, in order to
directly study and infer the nature of its magnetic field. Methods: We used the
new generation spectropolarimeters ESPaDOnS and NARVAL to observe and detect
circular polarization within the photospheric absorption lines of Pollux. Our
observations span 18 months from 2007-2009. We treated the spectropolarimetric
data using the Least-Squares Deconvolution method to create high
signal-to-noise ratio mean Stokes V profiles. We also measured the classical
activity indicator S-index for the Ca H&K lines, and the stellar radial
velocity (RV). Results: We have unambiguously detected a weak Stokes V signal
in the spectral lines of Pollux, and measured the related surface-averaged
longitudinal magnetic field Bl. The longitudinal field averaged over the span
of the observations is below one gauss. Our data suggest variations of the
longitudinal magnetic field, but no significant variation of the S-index. We
observe variations of RV which are qualitatively consistent with the published
ephemeris for a proposed exoplanet orbiting Pollux. The observed variations of
Bl appear to mimic those of RV, but additional data for this relationship to be
established. Using evolutionary models including the effects of rotation, we
derive the mass of Pollux and we discuss its evolutionary status and the origin
of its magnetic field. Conclusions: This work presents the first direct
detection of the magnetic field of Pollux, and demonstrates that ESPaDOnS and
NARVAL are capable of obtaining sub-G measurements of the surface-averaged
longitudinal magnetic field of giant stars, and of directly studying the
relationships between magnetic activity, stellar evolution and planet hosting
of these stars.Comment: 8 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
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