16,561 research outputs found
Identification and control of structures in space
The derivation of the equations of motion for the Spacecraft Control Laboratory Experiment (SCOLE) is reported and the equations of motion of a similar structure orbiting the earth are also derived. The structure is assumed to undergo large rigid-body maneuvers and small elastic deformations. A perturbation approach is proposed whereby the quantities defining the rigid-body maneuver are assumed to be relatively large, with the elastic deformations and deviations from the rigid-body maneuver being relatively small. The perturbation equations have the form of linear equations with time-dependent coefficients. An active control technique can then be formulated to permit maneuvering of the spacecraft and simultaneously suppressing the elastic vibration
Axiomatic approach to radiation reaction of scalar point particles in curved spacetime
Several different methods have recently been proposed for calculating the
motion of a point particle coupled to a linearized gravitational field on a
curved background. These proposals are motivated by the hope that the point
particle system will accurately model certain astrophysical systems which are
promising candidates for observation by the new generation of gravitational
wave detectors. Because of its mathematical simplicity, the analogous system
consisting of a point particle coupled to a scalar field provides a useful
context in which to investigate these proposed methods. In this paper, we
generalize the axiomatic approach of Quinn and Wald in order to produce a
general expression for the self force on a point particle coupled to a scalar
field following an arbitrary trajectory on a curved background. Our equation
includes the leading order effects of the particle's own fields, commonly
referred to as ``self force'' or ``radiation reaction'' effects. We then
explore the equations of motion which follow from this expression in the
absence of non-scalar forces.Comment: 17 pages, 1 figur
Zero-bias Anomaly of Tunneling into the Edge of a 2D Electron System
We investigate the electron tunneling into the edge of a clean weakly
interacting two-dimensional electron gas. It is shown that the corresponding
differential conductance has a cusp at zero bias, and is characterized
by a universal slope at . This singularity originates from the
electron scattering on the Friedel oscillation caused by the boundary of the
system.Comment: 10 pages, uuencoded compressed Postscript file, to appear in Phys.
Rev. B (Rapid Communications
Image guidance during breast radiotherapy: A phantom dosimetry and radiation-induced second cancer risk study
Imaging procedures utilised for patient position verification during breast radiotherapy can add a considerable dose to organs surrounding the target volume on top of therapeutic scatter dose. This study investigated the dose from a breast kilovoltage cone-beam CT (kV-CBCT), a breast megavoltage fan-beam CT (MV-FBCT), and a TomoDirectTM breast treatment. Thermoluminescent dosimeters placed within a female anthropomorphic phantom were utilised to measure the dose to various organs and tissues. The contralateral breast, lungs and heart received 0.40 cGy, 0.45 cGy and 0.40 cGy from the kV-CBCT and 1.74 cGy, 1.39 cGy and 1.73 cGy from the MV-FBCT. In comparison to treatment alone, daily imaging would increase the contralateral breast, contralateral lung and heart dose by a relative 12%, 24% and 13% for the kV-CBCT, and 52%, 101% and 58% for the MV-FBCT. The impact of the imaging dose relative to the treatment dose was assessed with linear and linear-quadratic radiation-induced secondary cancer risk models for the contralateral breast. The additional imaging dose and risk estimates presented in this study should be taken into account when considering an image modality and frequency for patient position verification protocols in breast radiotherapy
Untangling the Health Impacts of Mexico – U.S. Migration
Research has found that immigrant health has a tendency to decline with time spent in the United States. Using data from the Mexican Migration Project from 2007-2014, this paper is the first to test the impact of domestic and international migration on different types of health measures. Results find cumulative U.S. migration experience has a negative impact both on self-reported and objective health measures. By contrast, the number of trips to the United States and migrations made within Mexico impact individual’s self-assessment of their health but not objective health measures. The analyses suggest that differences in self-reported versus objective health measures may help to explain mixed results in the literature. Results suggest that individual’s health will suffer considerably more from U.S. migrations than from migration within Mexico which is consistent with the acculturation hypothesis. Not surprisingly, high levels of BMI and smoking are significant predictors of negative self-reported and objective health. There is also a troubling significant negative trend in health over time observed in the sample. Taken as a whole, these results suggest that even short trips to the United States can have a negative health effect on immigrants if they are repeated
Forming Disk Galaxies in Lambda CDM Simulations
We used fully cosmological, high resolution N-body + SPH simulations to
follow the formation of disk galaxies with rotational velocities between 135
and 270 km/sec in a Lambda CDM universe. The simulations include gas cooling,
star formation, the effects of a uniform UV background and a physically
motivated description of feedback from supernovae. The host dark matter halos
have a spin and last major merger redshift typical of galaxy sized halos as
measured in recent large scale N--Body simulations. The simulated galaxies form
rotationally supported disks with realistic exponential scale lengths and fall
on both the I-band and baryonic Tully Fisher relations. An extended stellar
disk forms inside the Milky Way sized halo immediately after the last major
merger. The combination of UV background and SN feedback drastically reduces
the number of visible satellites orbiting inside a Milky Way sized halo,
bringing it in fair agreement with observations. Our simulations predict that
the average age of a primary galaxy's stellar population decreases with mass,
because feedback delays star formation in less massive galaxies. Galaxies have
stellar masses and current star formation rates as a function of total mass
that are in good agreement with observational data. We discuss how both high
mass and force resolution and a realistic description of star formation and
feedback are important ingredients to match the observed properties of
galaxies.Comment: Revised version after the referee's comments. Conclusions unchanged.
2 new plots. MNRAS in press. 20 plots. 21 page
- …