10,200 research outputs found
Magnification relations in gravitational lensing via multidimensional residue integrals
We investigate the so-called magnification relations of gravitational lensing
models. We show that multidimensional residue integrals provide a simple
explanation for the existence of these relations, and an effective method of
computation. We illustrate the method with several examples, thereby deriving
new magnification relations for galaxy lens models and microlensing (point mass
lensing).Comment: 16 pages, uses revtex4, submitted to Journal of Mathematical Physic
Star-dust geometries in galaxies: The effect of interstellar matter distributions on optical and infrared properties of late-type galaxies
The presence of substantial amounts of interstellar dust in late-type galaxies affects observable parameters such as the optical surface brightness, the color, and the ratio of far-infrared to optical luminosity of these galaxies. We conducted radiative transfer calculations for late-type galaxy environments to examine two different scenarios: (1) the effects of increasing amounts of dust in two fixed geometries with different star distributions; and (2) the effects of an evolving dust-star geometry in which the total amount of dust is held constant, for three different star distributions. The calculations were done for ten photometric bands, ranging from the far-ultraviolet to the near-infrared (K), and scattered light was included in the galactic surface brightness at each wavelength. The energy absorbed throughout these ten photometric bands was assumed to re-emerge in the far-infrared as thermal dust emission. We also considered the evolutionary contraction of a constant amount of dust relative to pre-existing star distributions
Expansion of the Planet Detection Channels in Next-Generation Microlensing Surveys
We classify various types of planetary lensing signals and the channels of
detecting them. We estimate the relative frequencies of planet detections
through the individual channels with special emphasis on the new channels to be
additionally provided by future lensing experiments that will survey wide
fields continuously at high cadence by using very large-format imaging cameras.
From this investigation, we find that the fraction of wide-separation planets
that would be discovered through the new channels of detecting planetary
signals as independent and repeating events would be substantial. We estimate
that the fraction of planets detectable through the new channels would comprise
~15 -- 30% of all planets depending on the models of the planetary separation
distribution and mass ratios of planets. Considering that a significant
fraction of planets might exist in the form of free-floating planets, the
frequency of planets to be detected through the new channel would be even
higher. With the expansion of the channels of detecting planet, future lensing
surveys will greatly expand the range of planets to be probed.Comment: 6 pages, 3 figures, one tabl
The value of continuous professional development: Teachers' perceptions
The central argument is that in the presentation of workshops for teachers, presenters should focus on the principles underlying continuous professional development (CPD), since teachers are likely to be more willing to attend workshops if they are worth the time spent and the sacrifices made. In a workshop, on supporting learners with learning difficulties, such principles were therefore applied. A hands-on presentation was used to ensure that the teachers fully understood the use of the suggested teaching methods. The teachers were then asked to complete a questionnaire to determine their perceptions of how they had experienced the workshop. The questionnaire focused on the various CPD principles applied in the workshop, as well as the personal value and expected effect of the workshop on their teaching approach. In general, the teachers reported that regardless of their teaching position, qualifications, gender, or age they had experienced the CPD workshop positively. Keywords: continuous professional development; in-service training; learning difficulties; teaching methods; workshop South African Journal of Education Vol. 27 (1) 2007: pp. 53-6
Gravitational microlensing as a test of stellar model atmospheres
We present calculations illustrating the potential of gravitational
microlensing to discriminate between classical models of stellar surface
brightness profiles and the recently computed ``Next Generation'' models of
Hauschildt et al. These spherically-symmetric models include a much improved
treatment of molecular lines in the outer atmospheres of cool giants -- stars
which are very typical sources in Galactic bulge microlensing events. We show
that the microlensing signatures of intensively monitored point and fold
caustic crossing events are readily able to distinguish between NextGen and the
classical models, provided a photometric accuracy of 0.01 magnitudes is
reached. This accuracy is now routinely achieved by alert networks, and hence
current observations can discriminate between such model atmospheres, providing
a unique insight on stellar photospheres.Comment: 4 pages, 4 figures, Astronomy & Astrophysics (Letters), vol. 388, L1
(2002
Double precision trajectory program /DPTRAJ 2.2C/
Four part program computes trajectory of space probe moving in solar system and subject to variety of forces
The Flux Ratio Method for Determining the Dust Attenuation of Starburst Galaxies
The presence of dust in starburst galaxies complicates the study of their
stellar populations as the dust's effects are similar to those associated with
changes in the galaxies' stellar age and metallicity. This degeneracy can be
overcome for starburst galaxies if UV/optical/near-infrared observations are
combined with far-infrared observations. We present the calibration of the flux
ratio method for calculating the dust attenuation at a particular wavelength,
Att(\lambda), based on the measurement of F(IR)/F(\lambda) flux ratio. Our
calibration is based on spectral energy distributions (SEDs) from the PEGASE
stellar evolutionary synthesis model and the effects of dust (absorption and
scattering) as calculated from our Monte Carlo radiative transfer model. We
tested the attenuations predicted from this method for the Balmer emission
lines of a sample starburst galaxies against those calculated using radio
observations and found good agreement. The UV attenuation curves for a handful
of starburst galaxies were calculated using the flux ratio method, and they
compare favorably with past work. The relationship between Att(\lambda) and
F(IR)/F(\lambda) is almost completely independent of the assumed dust
properties (grain type, distribution, and clumpiness). For the UV, the
relationship is also independent of the assumed stellar properties (age,
metallicity, etc) accept for the case of very old burst populations. However at
longer wavelengths, the relationship is dependent on the assumed stellar
properties.Comment: accepted by the ApJ, 18 pages, color figures, b/w version at
http://mips.as.arizona.edu/~kgordon/papers/fr_method.htm
Astrometric Microlensing as a Method of Discovering and Characterizing Extra-Solar Planets
We introduce a new method of searching for and characterizing extra-solar
planets. We show that by monitoring the center-of-light motion of microlensing
alerts using the next generation of high precision astrometric instruments the
probability of detecting a planet orbiting the lens is high. We show that
adding astrometric information to the photometric microlensing lightcurve
greatly helps in determining the planetary mass and semi-major axis. We
introduce astrometric maps as a new way for calculating astrometric motion and
planet detection probabilities. Finite source effects are important for low
mass planets, but even Earth mass planets can give detectable signals.Comment: 9 pages includes 8 postscript figures, AAS Latex, submitted to Ap
The Ins and Outs of CCR7 in the Thymus
Although it is widely supposed that chemokines play a role in the thymus, most existing evidence is circumstantial. In this issue, two groups provide direct evidence that the chemokine receptor CCR7 is required for normal thymocyte migration (Ueno, T., F. Saito, D. Gray, S. Kuse, K. Hieshima, H. Nakano, T. Kakiuchi, M. Lipp, R. Boyd, and Y. Takahama. 2004. J. Exp. Med. 200:493–505; Misslitz, A., O. Pabst, G. Hintzen, L. Ohl, E. Kremmer, H. T. Petrie, and R. Forster. 2004. J. Exp. Med. 200:481–491). The two papers focus on distinct and opposite migration events, an early outward migration and a later inward migration. Together these papers provide a fascinating picture of the complex role of CCR7 in orchestrating thymocyte migration
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