1,143 research outputs found
Dynamic Response of a fast near infra-red Mueller matrix ellipsometer
The dynamic response of a near infrared Ferroelectric Liquid Crystal based
Mueller matrix ellipsometer (NIR FLC-MME) is presented. A time dependent
simulation model, using the measured time response of the individual FLCs, is
used to describe the measured temporal response. Furthermore, the impulse
response of the detector and the pre-amplifier is characterized and included in
the simulation model. The measured time-dependent intensity response of the MME
is reproduced in simulations, and it is concluded that the switching time of
the FLCs is the limiting factor for the Mueller matrix measurement time of the
FLC-based MME. Based on measurements and simulations our FLC based NIR-MME
system is estimated to operate at the maximum speed of approximately 16 ms per
Mueller matrix measurement. The FLC-MME may be operated several times faster,
since the switching time of the crystals depends on the individual crystal
being switched, and to what state it is switched. As a demonstration, the
measured temporal response of the Mueller matrix and the retardance of a thick
liquid crystal variable retarder upon changing state is demonstrated.Comment: to be published in Journal of Modern Optics 20 pages, 6 figure
Methanol Masers as Tracers of Circumstellar Disks
We show that in many methanol maser sources the masers are located in lines,
with a velocity gradient along them which suggests that the masers are situated
in edge-on circumstellar, or protoplanetary, disks. We present VLBI
observations of the methanol maser source G309.92+0.48, in the 12.2 GHz
transition, which confirm previous observations that the masers in this source
lie along a line. We show that such sources are not only linear in space but,
in many cases, also have a linear velocity gradient. We then model these and
other data in both the 6.7 GHz and the 12.2 GHz transition from a number of
star formation regions, and show that the observed spatial and velocity
distribution of methanol masers, and the derived Keplerian masses, are
consistent with a circumstellar disk rotating around an OB star. We consider
this and other hypotheses, and conclude that about half of these methanol
masers are probably located in edge-on circumstellar disks around young stars.
This is of particular significance for studies of circumstellar disks because
of the detailed velocity information available from the masers.Comment: 38 pages, 13 figures accepted by Ap
12.2-GHz methanol maser MMB follow-up catalogue - II. Longitude range 186 to 330 degrees
We present the second portion of a catalogue of 12.2-GHz methanol masers
detected towards 6.7-GHz methanol masers observed in the unbiased Methanol
Multibeam (MMB) Survey. Using the Parkes radio telescope we have targeted all
207 6.7-GHz methanol masers in the longitude range 186 to 330 degrees for
12.2-GHz counterparts. We report the detection of 83 12.2-GHz methanol masers,
and one additional source which we suspect is thermal emission, equating to a
detection rate of 40 per cent. Of the 83 maser detections, 39 are reported here
for the first time. We discuss source properties, including variability and
highlight a number of unusual sources. We present a list of 45 candidates that
are likely to harbor methanol masers in the 107.0-GHz transition.Comment: Accepted MNRAS 19 July 201
Statistical properties of 12.2 GHz methanol masers associated with a complete sample of 6.7 GHz methanol masers
We present definitive detection statistics for 12.2 GHz methanol masers
towards a complete sample of 6.7 GHz methanol masers detected in the Methanol
Multibeam survey south of declination -20 degrees. In total, we detect 250 12.2
GHz methanol masers towards 580 6.7 GHz methanol masers. This equates to a
detection rate of 43.1%, which is lower than that of previous significant
searches of comparable sensitivity. Both the velocity ranges and the flux
densities of the target 6.7 GHz sources surpass that of their 12.2 GHz
companion in almost all cases. 80 % of the detected 12.2 GHz methanol maser
peaks are coincident in velocity with the 6.7 GHz maser peak. Our data support
an evolutionary scenario whereby the 12.2 GHz sources are associated with a
somewhat later evolutionary stage than the 6.7 GHz sources devoid of this
transition. Furthermore, we find that the 6.7 GHz and 12.2 GHz methanol sources
increase in luminosity as they evolve. In addition to this, evidence for an
increase in velocity range with evolution is presented. This implies that it is
not only the luminosity, but also the volume of gas conducive to the different
maser transitions, that increases as the sources evolve. Comparison with
GLIMPSE mid-infrared sources has revealed a coincidence rate between the
locations of the 6.7 GHz methanol masers and GLIMPSE point sources similar to
that achieved in previous studies. Overall, the properties of the GLIMPSE
sources with and without 12.2 GHz counterparts are similar. There is a higher
12.2 GHz detection rate towards those 6.7 GHz methanol masers that are
coincident with extended green objects.Comment: Accepted to ApJ March 2011. 28 pages, 9 figure
Casimir attraction in multilayered plane parallel magnetodielectric systems
A powerful procedure is presented for calculating the Casimir attraction
between plane parallel multilayers made up of homogeneous regions with
arbitrary magnetic and dielectric properties by use of the Minkowski
energy-momentum tensor. The theory is applied to numerous geometries and shown
to reproduce a number of results obtained by other authors. Although the
various pieces of theory drawn upon are well known, the relative ease with
which the Casimir force density in even complex planar structures may be
calculated, appears not to be widely appreciated, and no single paper to the
author's knowledge renders explicitly the procedure demonstrated herein.
Results may be seen as an important building block in the settling of issues of
fundamental interest, such as the long-standing dispute over the thermal
behaviour of the Casimir force or the question of what is the correct stress
tensor to apply, a discussion re-quickened by the newly suggested alternative
theory due to Raabe and Welsch.Comment: 13 pages, 6 figures. Version 2: Updated contact details. Minor
changes and correction
Multi-transition study and new detections of class II methanol masers
We have used the ATNF Mopra antenna and the SEST antenna to search in the
directions of several class II methanol maser sources for emission from six
methanol transitions in the frequency range 85-115 GHz. The transitions were
selected from excitation studies as potential maser candidates. Methanol
emission at one or more frequencies was detected from five of the maser
sources, as well as from Orion KL. Although the lines are weak, we find
evidence of maser origin for three new lines in G345.01+1.79, and possibly one
new line in G9.62+0.20.
The observations, together with published maser observations at other
frequencies, are compared with methanol maser modelling for G345.01+1.79 and
NGC6334F. We find that the majority of observations in both sources are
consistent with a warm dust (175 K) pumping model at hydrogen density ~10^6
cm^-3 and methanol column density ~5 x 10^17 cm^-2. The substantial differences
between the maser spectra in the two sources can be attributed to the geometry
of the maser region.Comment: 13 pages, 6 figures, Accepted for publication in MNRA
Cliophysics: Socio-political Reliability Theory, Polity Duration and African Political (In)stabilities
Quantification of historical sociological processes have recently gained
attention among theoreticians in the effort of providing a solid theoretical
understanding of the behaviors and regularities present in sociopolitical
dynamics. Here we present a reliability theory of polity processes with
emphases on individual political dynamics of African countries. We found that
the structural properties of polity failure rates successfully capture the risk
of political vulnerability and instabilities in which 87.50%, 75%, 71.43%, and
0% of the countries with monotonically increasing, unimodal, U-shaped and
monotonically decreasing polity failure rates, respectively, have high level of
state fragility indices. The quasi-U-shape relationship between average polity
duration and regime types corroborates historical precedents and explains the
stability of the autocracies and democracies.Comment: 4 pages, 3 figures, 1 tabl
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