10,782 research outputs found
A 6-12 GHz Analogue Lag-Correlator for Radio Interferometry
Aims: We describe a 6-12 GHz analogue correlator that has been developed for
use in radio interferometers. Methods: We use a lag-correlator technique to
synthesis eight complex spectral channels. Two schemes were considered for
sampling the cross-correlation function, using either real or complex
correlations, and we developed prototypes for both of them. We opted for the
``add and square'' detection scheme using Schottky diodes over the more
commonly used active multipliers because the stability of the device is less
critical. Results: We encountered an unexpected problem, in that there were
errors in the lag spacings of up to ten percent of the unit spacing. To
overcome this, we developed a calibration method using astronomical sources
which corrects the effects of the non-uniform sampling as well as gain error
and dispersion in the correlator.Comment: 14 pages, 21 figures, accepted for publication in A&
A 6-12 GHz Analogue Lag-Correlator for Radio Interferometry
Aims: We describe a 6-12 GHz analogue correlator that has been developed for
use in radio interferometers. Methods: We use a lag-correlator technique to
synthesis eight complex spectral channels. Two schemes were considered for
sampling the cross-correlation function, using either real or complex
correlations, and we developed prototypes for both of them. We opted for the
``add and square'' detection scheme using Schottky diodes over the more
commonly used active multipliers because the stability of the device is less
critical. Results: We encountered an unexpected problem, in that there were
errors in the lag spacings of up to ten percent of the unit spacing. To
overcome this, we developed a calibration method using astronomical sources
which corrects the effects of the non-uniform sampling as well as gain error
and dispersion in the correlator.Comment: 14 pages, 21 figures, accepted for publication in A&
The Galactic distribution of magnetic fields in molecular clouds and HII regions
{Magnetic fields exist on all scales in our Galaxy. There is a controversy
about whether the magnetic fields in molecular clouds are preserved from the
permeated magnetic fields in the interstellar medium (ISM) during cloud
formation. We investigate this controversy using available data in the light of
the newly revealed magnetic field structure of the Galactic disk obtained from
pulsar rotation measures (RMs).} % {We collected measurements of the magnetic
fields in molecular clouds, including Zeeman splitting data of OH masers in
clouds and OH or HI absorption or emission lines of clouds themselves.} % {The
Zeeman data show structures in the sign distribution of the line-of-sight
component of the magnetic field. Compared to the large-scale Galactic magnetic
fields derived from pulsar RMs, the sign distribution of the Zeeman data shows
similar large-scale field reversals. Previous such examinations were flawed in
the over-simplified global model used for the large-scale magnetic fields in
the Galactic disk.} % {We conclude that the magnetic fields in the clouds may
still ``remember'' the directions of magnetic fields in the Galactic ISM to
some extent, and could be used as complementary tracers of the large-scale
magnetic structure. More Zeeman data of OH masers in widely distributed clouds
are required.}Comment: Typo fixed in this new versio
Rotation and differential rotation of active Kepler stars
We present rotation periods for thousands of active stars in the Kepler field
derived from Q3 data. In most cases a second period close to the rotation
period was detected, which we interpreted as surface differential rotation
(DR). Active stars were selected from the whole sample using the range of the
variability amplitude. To detect different periods in the light curves we used
the Lomb-Scargle periodogram in a pre-whitening approach to achieve parameters
for a global sine fit. The most dominant periods from the fit were ascribed to
different surface rotation periods, but spot evolution could also play a role.
Due to the large number of stars the period errors were estimated in a
statistical way. We thus cannot exclude the existence of false positives among
our periods. In our sample of 40.661 active stars we found 24.124 rotation
periods between 0.5-45 days. The distribution of stars with 0.5 < B-V <
1.0 and ages derived from angular momentum evolution that are younger than 300
Myr is consistent with a constant star-formation rate. A second period
within % of the rotation period was found in 18.619 stars (77.2%).
Attributing these two periods to DR we found that the relative shear
increases with rotation period, and slightly
decreases with effective temperature. The absolute shear
slightly increases between K. Above 6000 K
shows much larger scatter. We found weak dependence of on
rotation period. Latitudinal differential rotation measured for the first time
in more than 18.000 stars provides a comprehensive picture of stellar surface
shear, consistent with major predictions from mean-field theory. To what extent
our observations are prone to false positives and selection bias is not fully
explored, and needs to be addressed using more Kepler data.Comment: 19 pages, 18 figures, accepted by A&A. A table containing all
periods, KIC number, etc. can be found here:
http://www.astro.physik.uni-goettingen.de/~reinhold/period_table.te
Rotational properties of the binary and non-binary populations in the Trans-Neptunian belt
We present results for the short-term variability of Binary Trans-Neptunian
Objects (BTNOs). We performed CCD photometric observations using the 3.58 m
Telescopio Nazionale Galileo, the 1.5 m Sierra Nevada Observatory telescope,
and the 1.23 m Centro Astronomico Hispano Aleman telescope at Calar Alto
Observatory. We present results based on five years of observations and report
the short-term variability of six BTNOs. Our sample contains three classical
objects: 2003MW12, or Varda, 2004SB60, or Salacia, and 2002 VT130; one detached
disk object: 2007UK126; and two resonant objects: 2007TY430 and 2000EB173, or
Huya. For each target, possible rotational periods and/or photometric
amplitudes are reported. We also derived some physical properties from their
lightcurves, such as density, primary and secondary sizes, and albedo. We
compiled and analyzed a vast lightcurve database for Trans-Neptunian Objects
(TNOs) including centaurs to determine the lightcurve amplitude and spin
frequency distributions for the binary and non-binary populations. The mean
rotational periods, from the Maxwellian fits to the frequency distributions,
are 8.63+/-0.52 h for the entire sample, 8.37+/-0.58 h for the sample without
the binary population, and 10.11+/-1.19 h for the binary population alone.
Because the centaurs are collisionally more evolved, their rotational periods
might not be so primordial. We computed a mean rotational period, from the
Maxwellian fit, of 8.86+/-0.58 h for the sample without the centaur population,
and of 8.64+/-0.67 h considering a sample without the binary and the centaur
populations. According to this analysis, regular TNOs spin faster than
binaries, which is compatible with the tidal interaction of the binaries.
Finally, we examined possible formation models for several systems studied in
this work and by our team in previous papers.Comment: Accepted for publication in Astronomy and Astrophysics (June 26th,
2014); minor changes with published version; 21 pages, 17 figures, 7 table
Automorphic Lie Algebras and Cohomology of Root Systems
A cohomology theory of root systems emerges naturally in the context of
Automorphic Lie Algebras, where it helps formulating some structure theory
questions. In particular, one can find concrete models for an Automorphic Lie
Algebra by integrating cocycles. In this paper we define this cohomology and
show its connection with the theory of Automorphic Lie Algebras. Furthermore,
we discuss its properties: we define the cup product, we show that it can be
restricted to symmetric forms, that it is equivariant with respect to the
automorphism group of the root system, and finally we show acyclicity at
dimension two of the symmetric part, which is exactly what is needed to find
concrete models for Automorphic Lie Algebras.
Furthermore, we show how the cohomology of root systems finds application
beyond the theory of Automorphic Lie Algebras by applying it to the theory of
contractions and filtrations of Lie algebras. In particular, we show that
contractions associated to Cartan -filtrations of simple Lie
algebras are classified by -cocycles, due again to the vanishing of the
symmetric part of the second cohomology group.Comment: 26 pages, standard LaTeX2
KPD 0422+5421: A New Short Period Subdwarf B/White Dwarf Binary
The sdB star KPD 0422+5421 was discovered to be a single-lined spectroscopic
binary with a period of P=0.0901795 +/- (3\times 10^{-7}) days (2 hours, 10
minutes). The U and B light curves display an ellipsoidal modulation with
amplitudes of about 0.02 magnitudes. The sdB star contributes nearly all of the
observed flux. This and the absence of any reflection effect suggest that the
unseen companion star is small (i.e. R_comp ~ 0.01 solar radii) and therefore
degenerate. We modeled the U and B light curves and derived i = 78.05 +/- 0.50
degrees and a mass ratio of q = M_comp/M_sdB = 0.87 +/- 0.15. The sdB star
fills 69% of its Roche lobe. These quantities may be combined with the mass
function of the companion (f(M) = 0.126 +/- 0.028 solar masses) to derive M_sdB
= 0.72 +/- 0.26 solar masses and M_comp = 0.62 +/- 0.18 solar masses. We used
model spectra to derive the effective temperature, surface gravity, and helium
abundance of the sdB star. We found T_eff = 25,000 +/- 1500K, log g = 5.4 +/-
0.1, and [He/H] = -1.0. With a period of 2 hours and 10 minutes, KPD 0422+5421
has one of the shortest known orbital periods of a detached binary. This system
is also one of only a few known binaries which contain a subdwarf B star and a
white dwarf. Thus KPD 0422+5421 represents a relatively unobserved, and
short-lived, stage of binary star evolution.Comment: 9 pages, 8 figures, to appear in MNRAS, LaTeX, uses mn.st
Frame Theory for Signal Processing in Psychoacoustics
This review chapter aims to strengthen the link between frame theory and
signal processing tasks in psychoacoustics. On the one side, the basic concepts
of frame theory are presented and some proofs are provided to explain those
concepts in some detail. The goal is to reveal to hearing scientists how this
mathematical theory could be relevant for their research. In particular, we
focus on frame theory in a filter bank approach, which is probably the most
relevant view-point for audio signal processing. On the other side, basic
psychoacoustic concepts are presented to stimulate mathematicians to apply
their knowledge in this field
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