4,753 research outputs found
On the Quantum Theory of the Autoelectric Field Currents
By the application of a sufficiently intense electric field a Bohr atom could be rendered unstable. For when the drop in potential across the electronic orbit reaches a value of the order of magnitude of the ionizing potential of the atom (10^9 volts/cm.), the electron, instead of remaining in the neighborhood of the nucleus will fall down the hill of potential energy, and the atom will be dissociated. This dissociation is explosive in character. For there is a critical field strength, below which the atom remains stable indefinitely and above which it dissociates in a time of the order of the orbital periods of the atom. The characteristic for the autoelectric current should accordingly show abrupt discontinuities
The Reionization of Carbon
Observations suggest that CII was more abundant than CIV in the intergalactic
medium towards the end of the hydrogen reionization epoch. This transition
provides a unique opportunity to study the enrichment history of intergalactic
gas and the growth of the ionizing background (UVB) at early times. We study
how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic
simulation that includes a self-consistent multifrequency UVB as well as a
well-constrained model for galactic outflows to disperse metals. Our predicted
UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement
given the uncertainties. Nonetheless, we use a calibration in post-processing
to account for Lyman-alpha forest measurements while preserving the predicted
spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way
that it always exceeds the volume average in regions where metals are found.
This implies both that a spatially-uniform UVB is a poor approximation and that
metal absorption is not sensitive to the epoch when HII regions overlap
globally even at column densites of 10^{12} cm^{-2}. We find, consistent with
observations, that the CII mass fraction drops to low redshift while CIV rises
owing the combined effects of a growing UVB and continued addition of carbon in
low-density regions. This is mimicked in absorption statistics, which broadly
agree with observations at z=6-3 while predicting that the absorber column
density distributions rise steeply to the lowest observable columns. Our model
reproduces the large observed scatter in the number of low-ionization absorbers
per sightline, implying that the scatter does not indicate a partially-neutral
Universe at z=6.Comment: 16 pages, 14 figures, accepted to MNRA
A cortical potential reflecting cardiac function
Emotional trauma and psychological stress can precipitate cardiac arrhythmia and sudden death through arrhythmogenic effects of efferent sympathetic drive. Patients with preexisting heart disease are particularly at risk. Moreover, generation of proarrhythmic activity patterns within cerebral autonomic centers may be amplified by afferent feedback from a dysfunctional myocardium. An electrocortical potential reflecting afferent cardiac information has been described, reflecting individual differences in interoceptive sensitivity (awareness of one's own heartbeats). To inform our understanding of mechanisms underlying arrhythmogenesis, we extended this approach, identifying electrocortical potentials corresponding to the cortical expression of afferent information about the integrity of myocardial function during stress. We measured changes in cardiac response simultaneously with electroencephalography in patients with established ventricular dysfunction. Experimentally induced mental stress enhanced cardiovascular indices of sympathetic activity (systolic blood pressure, heart rate, ventricular ejection fraction, and skin conductance) across all patients. However, the functional response of the myocardium varied; some patients increased, whereas others decreased, cardiac output during stress. Across patients, heartbeat-evoked potential amplitude at left temporal and lateral frontal electrode locations correlated with stress-induced changes in cardiac output, consistent with an afferent cortical representation of myocardial function during stress. Moreover, the amplitude of the heartbeat-evoked potential in the left temporal region reflected the proarrhythmic status of the heart (inhomogeneity of left ventricular repolarization). These observations delineate a cortical representation of cardiac function predictive of proarrhythmic abnormalities in cardiac repolarization. Our findings highlight the dynamic interaction of heart and brain in stress-induced cardiovascular morbidity
NICMOS2 hubble space telescope observations of the embedded cluster associated with Mon R2: Constraining the substellar initial mass function
We have analyzed Hubble Space Telescope NICMOS2 F110W-, F160W-, F165M-, and F207M-band images covering the central 1' × 1' region of the cluster associated with Mon R2 in order to constrain the initial mass function (IMF) down to 20M_J. The flux ratio between the F165M and F160W bands was used to measure the strength of the water-band absorption feature and select a sample of 12 out of the total sample of 181 objects that have effective temperatures between 2700 and 3300 K. These objects are placed in the H-R diagram together with sources observed by Carpenter et al. to estimate an age of ~1 Myr for the low-mass cluster population. By constructing extinction-limited samples, we are able to constrain the IMF and the fraction of stars with a circumstellar disk in a sample that is 90% complete for both high- and low-mass objects. For stars with estimated masses between 0.1 and 1.0 M_☉ for a 1 Myr population with A_V ≤ 19 mag, we find that 27% ± 9% have a near-infrared excess indicative of a circumstellar disk. The derived fraction is similar to or slightly lower than the fraction found in other star-forming regions of comparable age. We constrain the number of stars in the mass interval 0.08-1.0 M_☉ to the number of objects in the mass interval 0.02-0.08 M_☉ by forming the ratio R^(**) = N(0.08-1 M_☉)/N(0.02-0.08 M_☉) for objects in an extinction-limited sample complete for A_V ≤ 7 mag. The ratio is found to be R^(**) = 2.2 ± 1.3, assuming an age of 1 Myr, consistent with the similar ratio predicted by the system IMF proposed by Chabrier. The ratio is similar to the ratios observed toward the Orion Nebula Cluster and IC 348, as well as the ratio derived in the 28 deg^2 survey of Taurus by Guieu et al
Astrometry and Photometry with Coronagraphs
We propose a solution to the problem of astrometric and photometric
calibration of coronagraphic images with a simple optical device which, in
theory, is easy to use. Our design uses the Fraunhofer approximation of Fourier
optics. Placing a periodic grid of wires (we use a square grid) with known
width and spacing in a pupil plane in front of the occulting coronagraphic
focal plane mask produces fiducial images of the obscured star at known
locations relative to the star. We also derive the intensity of these fiducial
images in the coronagraphic image. These calibrator images can be used for
precise relative astrometry, to establish companionship of other objects in the
field of view through measurement of common proper motion or common parallax,
to determine orbits, and to observe disk structure around the star
quantitatively. The calibrator spots also have known brightness, selectable by
the coronagraph designer, permitting accurate relative photometry in the
coronagraphic image. This technique, which enables precision exoplanetary
science, is relevant to future coronagraphic instruments, and is particularly
useful for `extreme' adaptive optics and space-based coronagraphy.Comment: To appear in ApJ August 2006, 27 preprint style pages 4 figure
A Data Cube Extraction Pipeline for a Coronagraphic Integral Field Spectrograph
Project 1640 is a high contrast near-infrared instrument probing the
vicinities of nearby stars through the unique combination of an integral field
spectrograph with a Lyot coronagraph and a high-order adaptive optics system.
The extraordinary data reduction demands, similar those which several new
exoplanet imaging instruments will face in the near future, have been met by
the novel software algorithms described herein. The Project 1640 Data Cube
Extraction Pipeline (PCXP) automates the translation of 3.8*10^4 closely
packed, coarsely sampled spectra to a data cube. We implement a robust
empirical model of the spectrograph focal plane geometry to register the
detector image at sub-pixel precision, and map the cube extraction. We
demonstrate our ability to accurately retrieve source spectra based on an
observation of Saturn's moon Titan.Comment: 35 pages, 15 figures; accepted for publication in PAS
The Simulated HI Sky at low redshift
Observations of intergalactic neutral hydrogen can provide a wealth of
information about structure and galaxy formation, potentially tracing accretion
and feedback processes on Mpc scales. Below a column density of NHI ~ 10^19
cm-2, the "edge" or typical observational limit for HI emission from galaxies,
simulations predict a cosmic web of extended emission and filamentary
structures. We study the distribution of neutral hydrogen and its 21cm emission
properties in a cosmological hydrodynamic simulation, to gain more insights
into the distribution of HI below NHI ~ 10^19 cm-2. Such Lyman Limit systems
are expected to trace out the cosmic web, and are relatively unexplored.
Beginning with a 32 h^-1 Mpc simulation, we extract the neutral hydrogen
component by determining the neutral fraction, including a post-processed
correction for self-shielding based on the thermal pressure. We take into
account molecular hydrogen, assuming an average density ratio Omega_H2 /
Omega_HI = 0.3 at z = 0. The statistical properties of the HI emission are
compared with observations, to assess the reliability of the simulation. The
simulated HI distribution robustly describes the full column density range
between NHI ~ 10^14 and NHI ~ 10^21 cm-2 and agrees very well with available
measurements from observations. Furthermore there is good correspondence in the
statistics when looking at the two-point correlation function and the HI mass
function. The reconstructed maps are used to simulate observations of existing
and future telescopes by adding noise and taking account of the sensitivity of
the telescopes. The general agreement in statistical properties of HI suggests
that neutral hydrogen as modeled in this hydrodynamic simulation is a fair
representation of that in the Universe. (abridged)Comment: 20 pages, 17 figures, Accepted for publication in A&A, figures
compressed to low resolution; high-resolution version available at:
http://www.astro.rug.nl/~popping/simulated_HI_sky.pd
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