1,696 research outputs found
Inducing Probabilistic Grammars by Bayesian Model Merging
We describe a framework for inducing probabilistic grammars from corpora of
positive samples. First, samples are {\em incorporated} by adding ad-hoc rules
to a working grammar; subsequently, elements of the model (such as states or
nonterminals) are {\em merged} to achieve generalization and a more compact
representation. The choice of what to merge and when to stop is governed by the
Bayesian posterior probability of the grammar given the data, which formalizes
a trade-off between a close fit to the data and a default preference for
simpler models (`Occam's Razor'). The general scheme is illustrated using three
types of probabilistic grammars: Hidden Markov models, class-based -grams,
and stochastic context-free grammars.Comment: To appear in Grammatical Inference and Applications, Second
International Colloquium on Grammatical Inference; Springer Verlag, 1994. 13
page
On the Link Between Central Black Holes, Bar Dynamics, and Dark Matter Halos in Spiral Galaxies
The discovery of a relationship between supermassive black hole (SMBH) mass
and spiral arm pitch angle (P) is evidence that SMBHs are tied to the overall
secular evolution of a galaxy. The discovery of SMBHs in late-type galaxies
with little or no bulge suggests that an underlying correlation between the
dark matter halo concentration and SMBH mass (MBH) exists, rather than between
the bulge mass and MBH. In this paper we measure P using a two-dimensional fast
fourier transform and estimate the bar pattern speeds of 40 barred spiral
galaxies from the Carnegie-Irvine Galaxy Survey. The pattern speeds were
derived by estimating the gravitational potentials of our galaxies from Ks-band
images and using them to produce dynamical simulation models. The pattern
speeds allow us to identify those galaxies with low central dark halo
densities, or fast rotating bars, while P provides an estimate of MBH. We find
that a wide range of MBH exists in galaxies with low central dark matter halo
densities, which appears to support other theoretical results. We also find
that galaxies with low central dark halo densities appear to follow more
predictable trends in P versus de Vaucouleurs morphological type (T) and bar
strength versus T than barred galaxies in general. The empirical relationship
between MBH and total gravitational mass of a galaxy (Mtot) allows us to
predict the minimum Mtot that will be observationally measured of our fast bar
galaxies. These predictions will be investigated in a subsequent paper.Comment: 17 pages, 1 table, 11 figures, accepted for publication in MNRA
Milliarcsecond N-Band Observations of the Nova RS Ophiuchi: First Science with the Keck Interferometer Nuller
We report observations of the nova RS Ophiuchi (RS Oph) using the Keck
Interferometer Nuller (KIN), approximately 3.8 days following the most recent
outburst that occurred on 2006 February 12. These observations represent the
first scientific results from the KIN, which operates in N-band from 8 to 12.5
microns in a nulling mode. By fitting the unique KIN data, we have obtained an
angular size of the mid-infrared continuum of 6.2, 4.0, or 5.4 mas for a disk
profile, gaussian profile (FWHM), and shell profile respectively. The data show
evidence of enhanced neutral atomic hydrogen emission and atomic metals
including silicon located in the inner spatial regime near the white dwarf (WD)
relative to the outer regime. There are also nebular emission lines and
evidence of hot silicate dust in the outer spatial region, centered at ! 17 AU
from the WD, that are not found in the inner regime. Our evidence suggests that
these features have been excited by the nova flash in the outer spatial regime
before the blast wave reached these regions. These identifications support a
model in which the dust appears to be present between outbursts and is not
created during the outburst event. We further discuss the present results in
terms of a unifying model of the system that includes an increase in density in
the plane of the orbit of the two stars created by a spiral shock wave caused
by the motion of the stars through the cool wind of the red giant star. These
data show the power and potential of the nulling technique which has been
developed for the detection of Earth-like planets around nearby stars for the
Terrestrial Planet Finder Mission and Darwin missions.Comment: 41 pages, 10 figure
The near-infrared size-luminosity relations for Herbig Ae/Be disks
We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes
using the 85-m baseline Keck Interferometer. Targets were chosen to span the
maximum range of stellar properties to probe the disk size dependence on
luminosity and effective temperature. For most targets, the measured
near-infrared sizes (ranging from 0.2 to 4 AU) support a simple disk model
possessing a central optically-thin (dust-free) cavity, ringed by hot dust
emitting at the expected sublimation temperatures (T_sub~1000-1500K).
Furthermore, we find a tight correlation of disk size with source luminosity R
propto L^(1/2) for Ae and late Be systems (valid over more than 2 decades in
luminosity), confirming earlier suggestions based on lower-quality data.
Interestingly, the inferred dust-free inner cavities of the highest luminosity
sources (Herbig B0-B3 stars) are under-sized compared to predictions of the
optically-thin cavity model, likely due to optically-thick gas within the inner
AU.Comment: Accepted by Astrophysical Journal; 24 pages, 4 figures, 4 table
The dusty AGB star RS CrB: first mid-infrared interferometric observations with the Keck Telescopes
We report interferometric observations of the semi-regular variable star RS
CrB, a red giant with strong silicate emission features. The data were among
the first long baseline mid-infrared stellar fringes obtained between the Keck
telescopes, using parts of the new nulling beam combiner. The light was
dispersed by a low-resolution spectrometer, allowing simultaneous measurement
of the source visibility and intensity spectra from 8 to 12 microns. The
interferometric observations allow a non-ambiguous determination of the dust
shell spatial scale and relative flux contribution. Using a simple
spherically-symmetric model, in which a geometrically thin shell surrounds the
stellar photosphere, we find that ~30% to ~70% of the overall mid-infrared flux
- depending on the wavelength - originates from 7-8 stellar radii. The derived
shell opacity profile shows a broad peak around 11 microns (tau ~ 0.06),
characteristic of Mg-rich silicate dust particles.Comment: Accepted for publication in ApJ Letter
Safer Sex as the Bolder Choice: Testosterone Is Positively Correlated with Safer Sex Behaviorally Relevant Attitudes in Young Men
Introduction.â Higher testosterone (T) is tied to riskâtaking, especially in financial domains but also in health domains relevant to acquiring sexually transmitted infections (STIs). However, safer sex constructs could themselves carry the possibility of âsocial riskâ due to sexual stigma or embarrassment, or could involve boldness or confidence because they could represent status displays of frequent sexual activity. Aim.â To determine how T and behaviorally relevant attitudes about sexual riskâtaking are linked, to better understand biopsychosocial aspects of sexual health related to STIs. Methods.â In 78 firstâyear male college students, we examined correlations between salivary T and behaviorally relevant safer sex attitudes assessed via questionnaires. Main Outcome Measures.â T, via saliva; safer sex attitudes, via a composite and the University of California, Los Angeles Multidimensional Condom Attitudes Scale (MCAS). Results.â Higher T was significantly correlated with higher scores on the following: safer sex likelihood composite, r (73)â=â0.33, P â=â0.003; the MCAS safer sex resilience, r (32)â=â0.36, P â=â0.037; and the MCAS condom purchase comfort, r (32)â=â0.37, P â=â0.031. Associations between T and safer sex likelihood and resilience were still robust after controlling for potential confounds, though the association between T and purchase comfort diminished to a trend. Conclusions.â Higher T was positively linked with safer sex attitudes, especially those most closely tied to STI risk avoidance. Thus, future research and interventions for STI prevention should address the possibility that safer sex may be paradoxically perceived as a âboldâ or âriskyâ choice even as it decreases STI risk. van Anders SM, Goldey KL, Conley TD, Snipes DJ, and Patel DA. Safer sex as the bolder choice: Testosterone is positively correlated with safer sex behaviorally relevant attitudes in young men. J Sex Med 2012;9:727â734.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90413/1/j.1743-6109.2011.02544.x.pd
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Risk measures for direct real estate investments with non-normal or unknown return distributions
The volatility of returns is probably the most widely used risk measure for real estate. This is rather surprising since a number of studies have cast doubts on the view that volatility can capture the manifold risks attached to properties and corresponds to the risk attitude of investors. A central issue in this discussion is the statistical properties of real estate returnsâin contrast to neoclassical capital market theory they are mostly non-normal and often unknown, which render many statistical measures useless. Based on a literature review and an analysis of data from Germany we provide evidence that volatility alone is inappropriate for measuring the risk of direct real estate.
We use a unique data sample by IPD, which includes the total returns of 939 properties across different usage types (56% office, 20% retail, 8% others and 16% residential properties) from 1996 to 2009, the German IPD Index, and the German Property Index. The analysis of the distributional characteristics shows that German real estate returns in this period were not normally distributed and that a logistic distribution would have been a better fit. This is in line with most of the current literature on this subject and leads to the question which indicators are more appropriate to measure real estate risks. We suggest that a combination of quantitative and qualitative risk measures more adequately captures real estate risks and conforms better with investor attitudes to risk. Furthermore, we present criteria for the purpose of risk classification
A Revised View of the Transient Radio Sky
We report on a re-analysis of archival data from the Very Large Array for a
sample of ten long duration radio transients reported by Bower and others.
These transients have an implied all-sky rate that would make them the most
common radio transient in the sky and yet most have no quiescent counterparts
at other wavelengths and therefore no known progenitor (other than Galactic
neutron stars). We find that more than half of these transients are due to rare
data artifacts. The remaining sources have lower signal-to-noise ratio (SNR)
than initially reported by 1 to 1.5-sigma. This lowering of SNR matters greatly
since the sources are at the threshold. We are unable to decisively account for
the differences. By two orthogonal criteria one source appears to be a good
detection. Thus the rate of long duration radio transients without optical
counterparts is, at best, comparable to that of the class of recently
discovered Swift J1644+57 nuclear radio transients. We revisit the known and
expected classes of long duration radio transients and conclude that the
dynamic radio sky remains a rich area for further exploration. Informed by the
experience of past searches for radio transients, we suggest that future
surveys pay closer attention to rare data errors and ensure that a wealth of
sensitive multi-wavelength data be available in advance of the radio
observations and that the radio searches should have assured follow-up
resources.Comment: ApJ, submitte
Herd-level animal management factors associated with the occurrence of bovine neonatal pancytopenia in calves in a multicountry study
Since 2007, mortality associated with a previously unreported haemorrhagic disease has been observed in young calves in several European countries. The syndrome, which has been named âbovine neonatal pancytopeniaâ (BNP), is characterised by thrombocytopenia, leukocytopenia and a panmyelophthisis. A herd-level case-control study was conducted in four BNP affected countries (Belgium, France, Germany and the Netherlands) to identify herd management risk factors for BNP occurrence. Data were collected using structured face-to-face and telephone interviews of farm managers and their local veterinarians. In total, 363 case farms and 887 control farms were included in a matched multivariable conditional logistic regression analysis. Case-control status was strongly associated with the odds of herd level use of the vaccine PregSureÂź BVD (PregSure, Pfizer Animal Health) (matched adjusted odds ratio (OR) 107.2; 95% CI: 41.0â280.1). This was also the case for the practices of feeding calves colostrum from the calfâs own dam (OR 2.0; 95% CI: 1.1â3.4) or feeding pooled colostrum (OR 4.1; 95% CI: 1.9â8.8). Given that the study had relatively high statistical power and represented a variety of cattle production and husbandry systems, it can be concluded with some confidence that no other herd level management factors are competent causes for a sufficient cause of BNP occurrence on herd level. It is suggested that genetic characteristics of the dams and BNP calves should be the focus of further investigations aimed at identifying the currently missing component causes that together with PregSure vaccination and colostrum feeding represent a sufficient cause for occurrence of BNP in calves
Quenching Massive Galaxies with On-the-fly Feedback in Cosmological Hydrodynamic Simulations
Massive galaxies today typically are not forming stars despite being
surrounded by hot gaseous halos with short central cooling times. This likely
owes to some form of "quenching feedback" such as merger-driven quasar activity
or radio jets emerging from central black holes. Here we implement heuristic
prescriptions for these phenomena on-the-fly within cosmological hydrodynamic
simulations. We constrain them by comparing to observed luminosity functions
and color-magnitude diagrams from SDSS. We find that quenching from mergers
alone does not produce a realistic red sequence, because 1 - 2 Gyr after a
merger the remnant accretes new fuel and star formation reignites. In contrast,
quenching by continuously adding thermal energy to hot gaseous halos
quantitatively matches the red galaxy luminosity function and produces a
reasonable red sequence. Small discrepancies remain - a shallow red sequence
slope suggests that our models underestimate metal production or retention in
massive red galaxies, while a deficit of massive blue galaxies may reflect the
fact that observed heating is intermittent rather than continuous. Overall,
injection of energy into hot halo gas appears to be a necessary and sufficient
condition to broadly produce red and dead massive galaxies as observed.Comment: 23 pages, 14 figures. MNRAS accepted. Added Sec. 4.4 and
significantly modified the Discussion at the suggestion of the refere
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