350 research outputs found
Spectrophotometric Resolution of Stellar Atmospheres with Microlensing
Microlensing is a powerful tool for studying stellar atmospheres because as
the source crosses regions of formally infinite magnification (caustics) the
surfaceof the star is resolved, thereby allowing one to measure the radial
intensity profile, both photometrically and spectroscopically. However, caustic
crossing events are relatively rare, and monitoring them requires intensive
application of telescope resources. It is therefore essential that the
observational parameters needed to accurately measure the intensity profile are
quantified. We calculate the expected errors in the recovered radial intensity
profile as a function of the unlensed flux, source radius, spatial resolution
the recovered intensity profile, and caustic crossing time for the two
principle types of caustics: point-mass and binary lenses. We demonstrate that
for both cases there exist simple scaling relations between these parameters
and the resultant errors. We find that the error as a function of the spatial
resolution of the recovered profile, parameterized by the number of radial
bins, increases as , considerably faster than the naive
expectation. Finally, we discuss the relative advantages of binary
caustic-crossing events and point-lens events. Binary events are more common,
easier to plan for, and provide more homogeneous information about the stellar
atmosphere. However, a sub-class of point-mass events with low impact
parameters can provide dramatically more information provided that they can be
recognized in time to initiate observations.Comment: 20 pages, 5 figures, submitted to the Astrophysical Journa
Seleção preliminar de estirpes de rizóbios para inoculação em feijoeiro comum (Phaseolus vulgaris L.)
bitstream/item/66227/1/31297.pdfOrganizado por: Alberto Feiden, Milton Parron Padovan, Adalgiza Inês Campolim, Aurélio Vinícius Borsato, Ivo de Sá Motta, João Batista Catto, Tércio Jacques Fehlauer
Theoretical Limb Darkening for Classical Cepheids: II. Corrections for the Geometric Baade-Wesselink Method
The geometric Baade-Wesselink method is one of the most promising techniques
for obtaining a better calibration of the Cepheid period-luminosity relation by
means of interferometric measurements of accurate diameters. In this paper we
present new wavelength- and phase-dependent limb darkening corrections based on
our time-dependent hydrodynamic models of the classical Cepheid zeta Gem. We
show that a model simulation of a Cepheid atmosphere, taking into account the
hydrodynamic effects associated with the pulsation, shows strong departures
from the limb darkening otherwise predicted by a static model. For most of its
pulsational cycle the hydrodynamic model predicts a larger limb darkening then
the equivalent static model. The hydrodynamics affects the limb darkening
mainly at UV and optical wavelengths. Most of these effects evolve slowly as
the star pulsates, but there are phases, associated with shocks propagating
into the photosphere, in which significant changes in the limb darkening take
place on time-scales of the order of less than a day. We assess the implication
of our model LD corrections fitting the geometric Baade-Wesselink distance of
zeta Gem for the available near-IR PTI data. We discuss the effects of our
model limb darkening on the best fit result, and analyze the requirements
needed to test the time-dependence of the limb darkening with future
interferometric measurements.Comment: 22 pages, 5 figures, to be published on the Astrophysical Journal,
June 1 2003 issu
Theoretical Limb Darkening for Pulsating Cepheids
This work presents a new method to compute time and wavelength dependent
center-to-limb brightness distributions for Classical Cepheids. Our model
atmospheres are based on second-order accurate 1-D hydrodynamic calculations,
performed in spherical geometry. The brightness intensity distributions, and
the resulting limb darkening, are computed through the dynamic atmospheres by
using a full set of atomic and molecular opacities. Our results confirm
important differences with respect to equivalent hydrostatic models. The amount
of limb darkening, and the shape of the limb profiles, show a strong dependence
on the pulsational phase of the Cepheid, which cannot be reproduced by static
models. Non-linear effects in our hydrodynamic equations add a new level of
complexity in the wavelength dependence of our limb profiles, which are
affected by the presence of shock-waves traveling through the atmosphere. These
effects, already detectable by present-day interferometers, should be taken
into consideration when deriving limb darkened diameters for nearby Cepheids
with the accuracy required to measure their radial pulsations.Comment: Accepted for publication in the Astrophysical Journa
Wavelet Analysis in Virtual Colonoscopy
The computed tomographic colonography (CTC) computer aided detection (CAD) program is a new method in development to detect colon polyps in virtual colonoscopy. While high sensitivity is consistently achieved, additional features are desired to increase specificity. In this paper, a wavelet analysis was applied to CTCCAD outputs in an attempt to filter out false positive detections. 52 CTCCAD detection images were obtained using a screen capture application. 26 of these images were real polyps, confirmed by optical colonoscopy and 26 were false positive detections. A discrete wavelet transform of each image was computed with the MATLAB wavelet toolbox using the Haar wavelet at levels 1-5 in the horizontal, vertical and diagonal directions. From the resulting wavelet coefficients at levels 1-3 for all directions, a 72 feature vector was obtained for each image, consisting of descriptive statistics such as mean, variance, skew, and kurtosis at each level and orientation, as well as error statistics based on a linear predictor of neighboring wavelet coefficients. The vectors for each of the 52 images were then run through a support vector machine (SVM) classifier using ten-fold cross-validation training to determine its efficiency in distinguishing polyps from false positives. The SVM results showed 100% sensitivity and 51% specificity in correctly identifying the status of detections. If this technique were added to the filtering process of the CTCCAD polyp detection scheme, the number of false positive results could be reduced significantly
Improving undergraduate STEM education: The efficacy of discipline-based professional development
We sought to determine whether instructional practices used by undergraduate faculty in the geosciences have shifted from traditional teacher-centered lecture toward student-engaged teaching practices and to evaluate whether the national professional development program On the Cutting Edge (hereinafter Cutting Edge) has been a contributing factor in this change. We surveyed geoscience faculty across the United States in 2004, 2009, and 2012 and asked about teaching practices as well as levels of engagement in education research, scientific research, and professional development related to teaching. We tested these self-reported survey results with direct observations of teaching using the Reformed Teaching Observation Protocol, and we conducted interviews to understand what aspects of Cutting Edge have supported change. Survey data show that teaching strategies involving active learning have become more common, that these practices are concentrated in faculty who invest in learning about teaching, and that faculty investment in learning about teaching has increased. Regression analysis shows that, after controlling for other key influences, faculty who have participated in Cutting Edge programs and who regularly use resources on the Cutting Edge website are statistically more likely to use active learning teaching strategies. Cutting Edge participants also report that learning about teaching, the availability of teaching resources, and interactions with peers have supported changes in their teaching practice. Our data suggest that even one-time participation in a workshop with peers can lead to improved teaching by supporting a combination of affective and cognitive learning outcomes
Principles And Practices Fostering Inclusive Excellence: Lessons From The Howard Hughes Medical Institute’s Capstone Institutions
Best-practices pedagogy in science, technology, engineering, and mathematics (STEM) aims for inclusive excellence that fosters student persistence. This paper describes principles of inclusivity across 11 primarily undergraduate institutions designated as Capstone Awardees in Howard Hughes Medical Institute’s (HHMI) 2012 competition. The Capstones represent a range of institutional missions, student profiles, and geographical locations. Each successfully directed activities toward persistence of STEM students, especially those from traditionally underrepresented groups, through a set of common elements: mentoring programs to build community; research experiences to strengthen scientific skill/identity; attention to quantitative skills; and outreach/bridge programs to broaden the student pool. This paper grounds these program elements in learning theory, emphasizing their essential principles with examples of how they were implemented within institutional contexts. We also describe common assessment approaches that in many cases informed programming and created traction for stakeholder buy-in. The lessons learned from our shared experiences in pursuit of inclusive excellence, including the resources housed on our companion website, can inform others’ efforts to increase access to and persistence in STEM in higher education
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