3,387 research outputs found
Distance Powers and Distance Matrices of Integral Cayley Graphs over Abelian Groups
It is shown that distance powers of an integral Cayley graph over an abelian
group are again integral Cayley graphs over that group. Moreover, it is proved
that distance matrices of integral Cayley graphs over abelian groups have
integral spectrum
The state space of short-range Ising spin glasses: the density of states
The state space of finite square and cubic Ising spin glass models is
analysed in terms of the global and the local density of states. Systems with
uniform and gaussian probability distribution of interactions are compared.
Different measures for the local state density are presented and discussed. In
particular the question whether the local density of states grows exponentially
or not is considered. The direct comparison of global and local densities leads
to consequences for the structure of the state space.Comment: 18 pages (including 6 figures); submitted to Z.f.Physik
The complex environment of the bright carbon star TX Psc as probed by spectro-astrometry
Context: Stars on the asymptotic giant branch (AGB) show broad evidence of
inhomogeneous atmospheres and circumstellar envelopes. These have been studied
by a variety of methods on various angular scales. In this paper we explore the
envelope of the well-studied carbon star TX Psc by the technique of
spectro-astrometry. Aims: We explore the potential of this method for detecting
asymmetries around AGB stars. Methods:We obtained CRIRES observations of
several CO v=1 lines near 4.6 m and HCN lines near 3 m in
2010 and 2013. These were then searched for spectro-astrometric signatures. For
the interpretation of the results, we used simple simulated observations.
Results: Several lines show significant photocentre shifts with a clear
dependence on position angle. In all cases, tilde-shaped signatures are found
where the positive and negative shifts (at PA 0deg) are associated with blue
and weaker red components of the lines. The shifts can be modelled with a
bright blob 70 mas to 210 mas south of the star with a flux of several percent
of the photospheric flux. We estimate a lower limit of the blob temperature of
1000 K. The blob may be related to a mass ejection as found for AGB stars or
red supergiants. We also consider the scenario of a companion object.
Conclusions: Although there is clear spectro-astrometric evidence of a rather
prominent structure near TX Psc, it does not seem to relate to the other
evidence of asymmetries, so no definite explanation can be given. Our data thus
underline the very complex structure of the environment of this star, but
further observations that sample the angular scales out to a few hundred
milli-arcseconds are needed to get a clearer picture
Catching the fish - Constraining stellar parameters for TX Psc using spectro-interferometric observations
Stellar parameter determination is a challenging task when dealing with
galactic giant stars. The combination of different investigation techniques has
proven to be a promising approach. We analyse archive spectra obtained with the
Short-Wavelength-Spectrometer (SWS) onboard of ISO, and new interferometric
observations from the Very Large Telescope MID-infrared Interferometric
instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The
aim of this work is to determine stellar parameters using spectroscopy and
interferometry. The observations are used to constrain the model atmosphere,
and eventually the stellar evolutionary model in the region where the tracks
map the beginning of the carbon star sequence. Two different approaches are
used to determine stellar parameters: (i) the 'classic' interferometric
approach where the effective temperature is fixed by using the angular diameter
in the N-band (from interferometry) and the apparent bolometric magnitude; (ii)
parameters are obtained by fitting a grid of state-of-the-art hydrostatic
models to spectroscopic and interferometric observations. We find a good
agreement between the parameters of the two methods. The effective temperature
and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the
H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star
just recently, which means that the star is still in a 'quiet' phase compared
to the subsequent strong-wind regime. This is in agreement with the C/O ratio
being only slightly larger than 1.Comment: 11 pages, 9 figures, 5 table
Hands-On TAROT: Intercontinental use of the TAROT for Education and Public Outreach
The TAROT telescope has for primary goal the search for the prompt optical
counterpart of Cosmic Gamma-Ray Bursts. It is a completely autonomous 25cm
telescope installed near Nice (France), able to point any location of the sky
within 1-2 seconds. The control, scheduling, and data processing activities are
completely automated, so the instrument is completely autonomous. In addition
to its un-manned modes, we added recently the possibility to remotely control
the telescope, as a request of the "Hands-On Universe" (HOU) program for
exchange of time within automatic telescopes for the education and public
outreach. To this purpose we developed a simple control interface. A webcam was
installed to visualize the telescope. Access to the data is possible through a
web interface. The images can be processed by the HOU software, a program
specially suited for use within the classroom. We experienced these feature
during the open days of the University of California Berkeley and the Astronomy
Festival of Fleurance (France). We plan a regular use for an astronomy course
of the Museum of Tokyo, as well as for French schools. Not only does Hands-On
TAROT gives the general public an access to professional astronomy, but it is
also a more general tool to demonstrate the use of a complex automated system,
the techniques of data processing and automation. Last but not least, through
the use of telescopes located in many countries over the globe, a form of
powerful and genuine cooperation between teachers and children from various
countries is promoted, with a clear educational goal.Comment: 4 pages, Based on a demonstration presented at the ADASS X
Conference, Boston, MA, USA, October 2000, to appear in ASP Conf. Serie
The maximal energy of classes of integral circulant graphs
The energy of a graph is the sum of the moduli of the eigenvalues of its
adjacency matrix. We study the energy of integral circulant graphs, also called
gcd graphs, which can be characterized by their vertex count and a set
of divisors of in such a way that they have vertex set
and edge set . For a fixed prime power and a fixed divisor set size , we analyze the maximal energy among all matching integral circulant
graphs. Let be the elements of .
It turns out that the differences between the exponents of
an energy maximal divisor set must satisfy certain balance conditions: (i)
either all equal , or at most the two differences
and may occur; %(for a certain depending on and ) (ii)
there are rules governing the sequence of consecutive
differences. For particular choices of and these conditions already
guarantee maximal energy and its value can be computed explicitly.Comment: Discrete Applied Mathematics (2012
Toward an optimal search strategy of optical and gravitational wave emissions from binary neutron star coalescence
Observations of an optical source coincident with gravitational wave emission
detected from a binary neutron star coalescence will improve the confidence of
detection, provide host galaxy localisation, and test models for the
progenitors of short gamma ray bursts. We employ optical observations of three
short gamma ray bursts, 050724, 050709, 051221, to estimate the detection rate
of a coordinated optical and gravitational wave search of neutron star mergers.
Model R-band optical afterglow light curves of these bursts that include a
jet-break are extrapolated for these sources at the sensitivity horizon of an
Advanced LIGO/Virgo network. Using optical sensitivity limits of three
telescopes, namely TAROT (m=18), Zadko (m=21) and an (8-10) meter class
telescope (m=26), we approximate detection rates and cadence times for imaging.
We find a median coincident detection rate of 4 yr^{-1} for the three bursts.
GRB 050724 like bursts, with wide opening jet angles, offer the most optimistic
rate of 13 coincident detections yr^{-1}, and would be detectable by Zadko up
to five days after the trigger. Late time imaging to m=26 could detect off-axis
afterglows for GRB 051221 like bursts several months after the trigger. For a
broad distribution of beaming angles, the optimal strategy for identifying the
optical emissions triggered by gravitational wave detectors is rapid response
searches with robotic telescopes followed by deeper imaging at later times if
an afterglow is not detected within several days of the trigger.Comment: 6 pages, 1 figure, Accepted for publication in MNRAS Letters (2011
April 22
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