652 research outputs found
Pitfall trap sampling bias depends on body mass, temperature, and trap number: insights from an individual-based model
The diversity and community composition of ground arthropods is routinely analyzed by pitfall trap sampling, which is a cost- and time-effective method to gather large numbers of replicates but also known to generate data that are biased by species-specific differences in locomotory activity. Previous studies have looked at factors that influence the sampling bias. These studies, however, were limited to one or few species and did rarely quantify how the species-specific sampling bias shapes community-level diversity metrics. In this study, we systematically quantify the species-specific and community-level sampling bias with an allometric individual-based model that simulates movement and pitfall sampling of 10 generic ground arthropod species differing in body mass. We perform multiple simulation experiments covering different scenarios of pitfall trap number, spatial trap arrangement, temperature, and population density. We show that the sampling bias decreased strongly with increasing body mass, temperature, and pitfall trap number, while population density had no effect and trap arrangement only had little effect. The average movement speed of a species in the field integrates body mass and temperature effects and could be used to derive reliable estimates of absolute species abundance. We demonstrate how unbiased relative species abundance can be derived using correction factors that need only information on species body mass. We find that community-level diversity metrics are sensitive to the particular community structure, namely the relation between body mass and relative abundance across species. Generally, pitfall trap sampling flattens the rank-abundance distribution and leads to overestimations of ground arthropod Shannon diversity. We conclude that the correction of the species-specific pitfall trap sampling bias is necessary for the reliability of conclusions drawn from ground arthropod field studies. We propose bias correction is a manageable task using either body mass to derive unbiased relative abundance or the average speed to derive reliable estimates of absolute abundance from pitfall trap sampling
Role of tyrosine M210 in the initial charge separation of reaction centers of Rhodobacter sphaeroides
Femtosecond spectroscopy was used in combination with site-directed mutagenesis to study the
influence of tyrosine M210 (YM210) on the primary electron transfer in the reaction center of Rhodobacter
sphaeroides. The exchange of YM210 to phenylalanine caused the time constant of primary electron transfer
to increase from 3.5 f 0.4 ps to 16 f 6 ps while the exchange to leucine increased the time constant even
more to 22 f 8 ps. The results suggest that tyrosine M210 is important for the fast rate of the primary
electron transfer
Finding rare objects and building pure samples: Probabilistic quasar classification from low resolution Gaia spectra
We develop and demonstrate a probabilistic method for classifying rare
objects in surveys with the particular goal of building very pure samples. It
works by modifying the output probabilities from a classifier so as to
accommodate our expectation (priors) concerning the relative frequencies of
different classes of objects. We demonstrate our method using the Discrete
Source Classifier, a supervised classifier currently based on Support Vector
Machines, which we are developing in preparation for the Gaia data analysis.
DSC classifies objects using their very low resolution optical spectra. We look
in detail at the problem of quasar classification, because identification of a
pure quasar sample is necessary to define the Gaia astrometric reference frame.
By varying a posterior probability threshold in DSC we can trade off sample
completeness and contamination. We show, using our simulated data, that it is
possible to achieve a pure sample of quasars (upper limit on contamination of 1
in 40,000) with a completeness of 65% at magnitudes of G=18.5, and 50% at
G=20.0, even when quasars have a frequency of only 1 in every 2000 objects. The
star sample completeness is simultaneously 99% with a contamination of 0.7%.
Including parallax and proper motion in the classifier barely changes the
results. We further show that not accounting for class priors in the target
population leads to serious misclassifications and poor predictions for sample
completeness and contamination. (Truncated)Comment: MNRAS accepte
The Stellar Populations in the Outer Regions of M33. I. Metallicity Distribution Function
We present deep CCD photometry in the VI passbands using the WIYN 3.5m
telescope of a field located approximately 20' southeast of the center of M33;
this field includes the region studied by Mould & Kristian in their 1986 paper.
The color-magnitude diagram (CMD) extends to I~25 and shows a prominent red
giant branch (RGB), along with significant numbers of asymptotic giant branch
and young main sequence stars. The red clump of core helium burning stars is
also discernable near the limit of our CMD. The I-band apparent magnitude of
the red giant branch tip implies a distance modulus of (m-M)_I = 24.77 +/-
0.06, which combined with an adopted reddening of E(V-I)=0.06 +/- 0.02 yields
an absolute modulus of (m-M)_0 = 24.69 +/- 0.07 (867 +/- 28 kpc) for M33. Over
the range of deprojected radii covered by our field (~8.5 to ~12.5 kpc), we
find a significant age gradient with an upper limit of ~1 Gyr (~0.25 Gyr/kpc).
Comparison of the RGB photometry to empirical giant branch sequences for
Galactic globulars allows us to use the dereddened color of these stars to
construct a metallicity distribution function (MDF). The primary peak in the
MDF is at a metallicity of [Fe/H] ~ -1.0 with a tail to lower abundances. The
peak does show radial variation with a slope of d[Fe/H]/dR_{deproj} = -0.06 +/-
0.01 dex/kpc. This gradient is consistent with the variation seen in the inner
disk regions of M33. As such, we conclude that the vast majority of stars in
this field belong to the disk of M33, not the halo as previously thought.Comment: 34 pages, 13 figures, accepted to The Astronomical Journal, July
2004, high resolution version available at
ftp://www.astro.ufl.edu/pub/ata/sarajedini_m33.pd
Bayesian Accretion Modeling: Axisymmetric Equatorial Emission in the Kerr Spacetime
The Event Horizon Telescope (EHT) has produced images of two supermassive
black holes, Messier~87* (M 87*) and Sagittarius~A* (Sgr A*). The EHT
collaboration used these images to indirectly constrain black hole parameters
by calibrating measurements of the sky-plane emission morphology to images of
general relativistic magnetohydrodynamic (GRMHD) simulations. Here, we develop
a model for directly constraining the black hole mass, spin, and inclination
through signatures of lensing, redshift, and frame dragging, while
simultaneously marginalizing over the unknown accretion and emission
properties. By assuming optically thin, axisymmetric, equatorial emission near
the black hole, our model gains orders of magnitude in speed over similar
approaches that require radiative transfer. Using 2017 EHT M 87* baseline
coverage, we use fits of the model to itself to show that the data are
insufficient to demonstrate existence of the photon ring. We then survey
time-averaged GRMHD simulations fitting EHT-like data, and find that our model
is best-suited to fitting magnetically arrested disks, which are the favored
class of simulations for both M 87* and Sgr A*. For these simulations, the
best-fit model parameters are within of the true mass and within
for inclination. With 2017 EHT coverage and 1\% fractional
uncertainty on amplitudes, spin is unconstrained. Accurate inference of spin
axis position angle depends strongly on spin and electron temperature. Our
results show the promise of directly constraining black hole spacetimes with
interferometric data, but they also show that nearly identical images permit
large differences in black hole properties, highlighting degeneracies between
the plasma properties, spacetime, and most crucially, the unknown emission
geometry when studying lensed accretion flow images at a single frequency.Comment: Accepted to ApJ, 16 pages, 10 figure
Change in hematologic indices over time in pediatric inflammatory bowel disease treated with azathioprine
Azathioprine leads to changes in mean corpuscular volume (MCV) and white blood cell (WBC) indices reflecting efficacy or toxicity. Understanding the interactions between bone marrow stem cells and azathioprine could highlight abnormal response patterns as forerunners for hematologic malig-nancies. This study gives a statistical description of factors influencing the relationship between MCV and WBC in children with inflammatory bowel disease treated with azathioprine. We found that leukopenia preceded macro¬cytosis. Macrocytosis is therefore not a good predictor of leukopenia. Further studies will be necessary to determine the subgroup of patients at increased risk of malignancies based on bone marrow response
A real-time articulatory visual feedback approach with target presentation for second language pronunciation learning
International audienceArticulatory information can support learning or remediating pronunciation of a second language (L2). This paper describes an electromagnetic articulometer-based visual-feedback approach using an articulatory target presented in real-time to facilitate L2 pronunciation learning. This approach trains learners to adjust articulatory positions to match targets for a L2 vowel estimated from productions of vowels that overlap in both L1 and L2. Training of Japanese learners for the American English vowel /ae/ that included visual training improved its pronunciation regardless of whether audio training was also included. Articulatory visual feedback is shown to be an effective method for facilitating L2 pronunciation learning
The IRAC Shallow Survey
The IRAC shallow survey covers 8.5 square degrees in the NOAO Deep Wide-Field
Survey in Bootes with 3 or more 30 second exposures per position. An overview
of the survey design, reduction, calibration, star-galaxy separation, and
initial results is provided. The survey includes approximately 370,000,
280,000, 38,000, and 34,000 sources brighter than the 5 sigma limits of 6.4,
8.8, 51, and 50 microJy at 3.6, 4.5, 5.8, and 8 microns respectively, including
some with unusual spectral energy distributions.Comment: To appear in ApJS, Spitzer special issue. For full resolution see
http://cfa-www.harvard.edu/irac/publication
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