2,927 research outputs found
Observation of the stray field of thin film magnetic tips using electron holography
The stray field around thin film ferromagnetic tips employed for magnetic force microscopy has been revealed using electron holography. The experimental phase difference maps are in good agreement with simulations. Quantitative flux measurements of the leakage field are obtained
Delay time distribution of type Ia supernovae: theory vs. observation
Two formation scenarios are investigated for type Ia supernovae in elliptical
galaxies: the single degenerate scenario (a white dwarf reaching the
Chandrasekhar limit through accretion of matter transferred from its companion
star in a binary) and the double degenerate scenario (the inspiraling and
merging of two white dwarfs in a binary as a result of the emission of
gravitational wave radiation). A population number synthesis code is used,
which includes the latest physical results in binary evolution and allows to
differentiate between certain physical scenarios (such as the description of
common envelope evolution) and evolutionary parameters (such as the mass
transfer efficiency during Roche lobe overflow). The thus obtained theoretical
distributions of type Ia supernova delay times are compared to those that are
observed, both in morphological shape and absolute number of events. The
critical influence of certain parameters on these distributions is used to
constrain their values. The single degenerate scenario alone is found to be
unable in reproducing the morphological shape of the observational delay time
distribution, while use of the double degenerate one (or a combination of both)
does result in fair agreement. Most double degenerate type Ia supernovae are
formed through a normal, quasi-conservative Roche lobe overflow followed by a
common envelope phase, not through two successive common envelope phases as is
often assumed. This may cast doubt on the determination of delay times by using
analytical formalisms, as is sometimes done in other studies. The theoretical
absolute number of events in old elliptical galaxies lies a factor of at least
three below the rates that are observed. While this may simply be the result of
observational uncertainties, a better treatment of the effects of rotation on
stellar structure could mitigate the discrepancy.Comment: 5 pages, 4 figures, to appear in proceedings of "Binary Star
Evolution: Mass Loss, Accretion, and Mergers
Non-resonant background suppression in preresonance CARS spectra of flavin adenine dinucleotide: Demonstration of a background suppression technique using phase mismatching and comparison with the polarization-sensitive CARS technique
Polarization-sensitive CARS spectra of a 5.7 × 10-3 mol dm-3 flavin adenine dinucleotide (FAD) solution were recorded under preresonance conditions at a pump wavelength of 532 nm. The depolarization ratios of the vibrations are shown to be close to the depolarization ratio of the non-resonant background. This results in a severe reduction of the vibration resonant signal (a factor of 700-900) in the polarization CARS spectrum, and a poor improvement in the ratio of the resonant signal and the non-resonant background (<10). \ud
In this context, a non-resonant background suppression technique is discussed and demonstrated for 5.7 × 10-3 and 1.4 × 10-3 mol dm-3 FAD solutions excited at 532 nm; the non-resonant susceptibility of the walls of the cuvette, which contains the FAD solution, is used to compensate the non-resonant signal contribution of the solution. An improvement in the signal-to-noise ratio of ca. 50 is achieved at the cost of a factor of 30 in the resonant signal strength. Lorentzian-shaped spectral bands are obtained, facilitating the determination of band position, width and intensity. Line shape parameters and depolarization ratios for FAD are extracted from the presented spectra by curve fitting. The signal strength and background suppression achieved with these techniques and the resonance CARS technique (at a pump wavelength of 480 nm) are compared and discussed
Molecular gas in extreme star-forming environments: the starbursts Arp220 and NGC6240 as case studies
We report single-dish multi-transition measurements of the 12^CO, HCN, and
HCO^+ molecular line emission as well as HNC J=1-0 and HNCO in the two
ultraluminous infra-red galaxies Arp220 and NGC6240. Using this new molecular
line inventory, in conjunction with existing data in the literature, we
compiled the most extensive molecular line data sets to date for such galaxies.
The many rotational transitions, with their different excitation requirements,
allow the study of the molecular gas over a wide range of different densities
and temperatures with significant redundancy, and thus allow good constraints
on the properties of the dense gas in these two systems. The mass (~(1-2) x
10^10 Msun) of dense gas (>10^5-6 cm^-3) found accounts for the bulk of their
molecular gas mass, and is consistent with most of their IR luminosities
powered by intense star bursts while self-regulated by O,B star cluster
radiative pressure onto the star-forming dense molecular gas. The highly
excited HCN transitions trace a gas phase ~(10-100)x denser than that of the
sub-thermally excited HCO^+ lines (for both galaxies). These two phases are
consistent with an underlying density-size power law found for Galactic GMCs
(but with a steeper exponent), with HCN lines tracing denser and more compact
regions than HCO^+. Whether this is true in IR-luminous, star forming galaxies
in general remains to be seen, and underlines the need for observations of
molecular transitions with high critical densities for a sample of bright
(U)LIRGs in the local Universe -- a task for which the HI-FI instrument on
board Herschel is ideally suited to do.Comment: 38 pages (preprint ApJ style), 3 figures, accepted for Ap
Thickness and dielectric constant determination of thin dielectric layers
We derive a method for the determination of the dielectric constant and thickness of a thin dielectric layer, deposited on top of a thick dielectric layer which is in turn present on a metal film. Reflection of p- and s-polarized light from the metal layer yields minima for certain angles of incidence where the light is absorbed by the metal. The thin dielectric layer causes shifts in the angles at which the minima occur, from which the thickness and dielectric constant can be obtained. The model is tested for 3.5 and 14 nm thick photoresist gratings
Determination of thickness and dielectric constant of thin transparent dielectric layers using surface plasmon resonance
The determination of the thickness and dielectric constant of thin dielectric layers by means of surface plasmon resonance is discussed. It appears to be impossible to determine these parameters from one surface plasmon response experiment. This is illustrated theoretically. Variation of the refractive index of the solution in which surface plasmon experiments were performed allowed us to determine these parameters separately
Two small-volume electrochemical cells for the measurement of surface enhanced Raman scattering
Two electrochemical cells, for performing surface enhanced Raman scattering (SERS), with submillilitre volumes are presented. One of the cells is especially developed for use in a Raman microspectrometer. The smallest cell uses only 80 mu l of sample. SER measurements are performed on 2*10-3 M adenine
Lens magnification by CL0024+1654 in the U and R band
[ABRIDGED] We estimate the total mass distribution of the galaxy cluster
CL0024+1654 from the measured source depletion due to lens magnification in the
R band. Within a radius of 0.54Mpc/h, a total projected mass of
(8.1+/-3.2)*10^14 M_sol/h (EdS) is measured, which corresponds to a mass-
to-light ratio of M/L(B)=470+/-180. We compute the luminosity function of
CL0024+1654 in order to estimate contamination of the background source counts
from cluster galaxies. Three different magnification-based reconstruction
methods are employed using both local and non-local techniques. We have
modified the standard single power-law slope number count theory to incorporate
a break and applied this to our observations. Fitting analytical magnification
profiles of different cluster models to the observed number counts, we find
that the cluster is best described either by a NFW model with scale radius
r_s=334+/-191 kpc/h and normalisation kappa_s=0.23+/-0.08 or a power-law
profile with slope xi=0.61+/-0.11, central surface mass density
kappa_0=1.52+/-0.20 and assuming a core radius of r_core=35 kpc/h. The NFW
model predicts that the cumulative projected mass contained within a radius R
scales as M(<R)=2.9*10^14*(R/1')^[1.3-0.5lg (R/1')] M_sol/h. Finally, we have
exploited the fact that flux magnification effectively enables us to probe
deeper than the physical limiting magnitude of our observations in searching
for a change of slope in the U band number counts. We rule out both a total
flattening of the counts with a break up to U_AB<=26.6 and a change of slope,
reported by some studies, from dlog N/dm=0.4->0.15 up to U_AB<=26.4 with 95%
confidence.Comment: 19 pages, 12 figures, submitted to A&A. New version includes more
robust U band break analysis and contamination estimates, plus new plot
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