63 research outputs found
On-board processing for telecommunications satellites
In this decade, communications satellite systems will probably face dramatic challenges from alternative transmission means. To balance and overcome such competition, and to prepare for new requirements, INTELSAT has developed several on-board processing techniques, including Satellite-Switched TDMA (SS-TDMA), Satellite-Switched FDMA (SS-FDMA), several Modulators/Demodulators (Modem), a Multicarrier Multiplexer and Demodulator MCDD), an International Business Service (IBS)/Intermediate Data Rate (IDR) BaseBand Processor (BBP), etc. Some proof-of-concept hardware and software were developed, and tested recently in the INTELSAT Technical Laboratories. These techniques and some test results are discussed
On the modulation of RR Lyrae stars in the globular cluster M3
A new, extended time-series photometry of M3 RR Lyrae stars has revealed that
four of the ten double-mode stars show large-amplitude Blazhko modulation of
both radial modes. The first, detailed analysis of the peculiar behavior of the
unique, Blazhko RRd stars is given. While the P1/P0 period ratio is normal, and
the overtone mode is dominant in the other RRd stars of the cluster, the period
ratio is anomalous and the fundamental mode has a larger (or similar) mean
amplitude than the overtone has in Blazhko RRd stars. The modulations of the
fundamental and overtone modes are synchronized only in one of the Blazhko RRd
stars. No evidence of any connection between the modulations of the modes in
the other three stars is found. The Blazhko modulation accounts, at least
partly, for the previously reported amplitude and period changes of these
stars. Contrary to the Blazhko statistics of RRab and RRd stars,
Blazhko modulation occurs only in 10 of the overtone variables in M3. Four
of the five Blazhko RRc stars are bright, evolved objects, and one has a
similar period and brightness as Blazhko RRd stars have. The regions of the
instability strip with high and low occurrence rate of the Blazhko modulation
overlap with the regions populated by first- and second-generation stars
according to theoretical and observational studies, raising up the possibility
that the Blazhko modulation occurs preferentially in first-generation RR Lyrae
stars.Comment: 6 pages, 4 figures, accepted for publication in ApJ
Overtone and multi-mode RR Lyrae stars in the globular cluster M3
The overtone and multi-mode RR Lyrae stars in the globular cluster M3 are
studied using a 200-d long, and time-series photometry
obtained in 2012. 70\% of the 52 overtone variables observed show some kind of
multi-periodicity (additional frequency at
frequency ratio, Blazhko effect, double/multi-mode pulsation, period doubling).
A signal at 0.587 frequency ratio to the fundamental-mode frequency is detected
in the double-mode star, V13, which may be identified as the second radial
overtone mode. If this mode-identification is correct, than V13 is the first RR
Lyrae star showing triple-mode pulsation of the first three radial modes.
Either the Blazhko effect or the frequency (or both of these
phenomena) appear in 7 double-mode stars. The
period ratio of RRd stars showing the Blazhko effect are anomalous. A
displacement of the main frequency component at the fundamental-mode with the
value of modulation frequency (or its half) is detected in three Blazhko RRd
stars parallel with the appearance of the overtone-mode pulsation. The
frequency appears in RRc stars that lie at the blue side of the
double-mode region and in RRd stars, raising the suspicion that its occurrence
may be connected to double-mode pulsation. The changes of the Blazhko and
double-mode properties of the stars are also reviewed using the recent and
archive photometric data.Comment: accepted for publication in ApJ Suppl. 26 pages, 25 figure
Bonding in MgSi and AlMgSi Compounds Relevant to AlMgSi Alloys
The bonding and stability of MgSi and AlMgSi compounds relevant to AlMgSi
alloys is investigated with the use of (L)APW+(lo) DFT calculations. We show
that the and phases found in the precipitation sequence are
characterised by the presence of covalent bonds between Si-Si nearest neighbour
pairs and covalent/ionic bonds between Mg-Si nearest neighbour pairs. We then
investigate the stability of two recently discovered precipitate phases, U1 and
U2, both containing Al in addition to Mg and Si. We show that both phases are
characterised by tightly bound Al-Si networks, made possible by a transfer of
charge from the Mg atoms.Comment: 11 pages, 30 figures, submitted to Phys. Rev.
Kepler observations of variability in B-type stars
The analysis of the light curves of 48 B-type stars observed by Kepler is
presented. Among these are 15 pulsating stars, all of which show low
frequencies characteristic of SPB stars. Seven of these stars also show a few
weak, isolated high frequencies and they could be considered as SPB/beta Cep
hybrids. In all cases the frequency spectra are quite different from what is
seen from ground-based observations. We suggest that this is because most of
the low frequencies are modes of high degree which are predicted to be unstable
in models of mid-B stars. We find that there are non-pulsating stars within the
beta Cep and SPB instability strips. Apart from the pulsating stars, we can
identify stars with frequency groupings similar to what is seen in Be stars but
which are not Be stars. The origin of the groupings is not clear, but may be
related to rotation. We find periodic variations in other stars which we
attribute to proximity effects in binary systems or possibly rotational
modulation. We find no evidence for pulsating stars between the cool edge of
the SPB and the hot edge of the delta Sct instability strips. None of the stars
show the broad features which can be attributed to stochastically-excited modes
as recently proposed. Among our sample of B stars are two chemically peculiar
stars, one of which is a HgMn star showing rotational modulation in the light
curve.Comment: 19 pages, 11 figures, 4 table
Multisite campaign on the open cluster M67. III. Delta Scuti pulsations in the blue stragglers
We have made an asteroseismic analysis of the variable blue stragglers in the
open cluster M67. The data set consists of photometric time series from eight
sites using nine 0.6-2.1 meter telescopes with a time baseline of 43 days. In
two stars, EW Cnc and EX Cnc, we detect the highest number of frequencies (41
and 26) detected in delta Scuti stars belonging to a stellar cluster, and EW
Cnc has the second highest number of frequencies detected in any delta Scuti
star. We have computed a grid of pulsation models that take the effects of
rotation into account. The distribution of observed and theoretical frequencies
show that in a wide frequency range a significant fraction of the radial and
non-radial low-degree modes are excited to detectable amplitudes. Despite the
large number of observed frequencies we cannot constrain the fundamental
parameters of the stars. To make progress we need to identify the degrees of
some of the modes either from multi-colour photometry or spectroscopy.Comment: 15 pages, 14 figs, one appendix. Part three in a series of papers
describing results from an extensive multi-site campaign on the open cluster
M6
Multi-site campaign on the open cluster M67. I. Observations and photometric reductions
We report on an ambitious multi-site campaign aimed at detecting stellar
variability, particularly solar-like oscillations, in the red giant stars in
the open cluster M67 (NGC 2682). During the six-week observing run, which
comprised 164 telescope nights, we used nine 0.6-m to 2.1-m class telescopes
located around the world to obtain uninterrupted time-series photometry. We
outline here the data acquisition and reduction, with emphasis on the
optimisation of the signal-to-noise of the low amplitude (50-500 micromag)
solar-like oscillations. This includes a new and efficient method for obtaining
the linearity profile of the CCD response at ultra high precision (~10 parts
per million). The noise in the final time series is 0.50 mmag per minute
integration for the best site, while the noise in the Fourier spectrum of all
sites combined is 20 micromag. In addition to the red giant stars, this data
set proves to be very valuable for studying high-amplitude variable stars such
as eclipsing binaries, W UMa systems and delta Scuti stars.Comment: 12 pages, 13 figures, accepted by MNRA
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