3,127 research outputs found
Binaries among Ap and Am stars
The results of long-term surveys of radial velocities of cool Ap and Am stars
are presented. There are two samples, one of about 100 Ap stars and the other
of 86 Am stars. Both have been observed with the CORAVEL scanner from
Observatoire de Haute-Provence (CNRS), France.
The conspicuous lack of short-period binaries among cool Ap stars seems
confirmed, although this may be the result of an observational bias; one system
has a period as short as 1.6 days. A dozen new orbits could be determined,
including that of one SB2 system. Considering the mass functions of 68 binaries
from the literature and from our work, we conclude that the distribution of the
mass ratios is the same for the Bp-Ap stars than for normal G dwarfs.
Among the Am stars, we found 52 binaries, i.e. 60%; an orbit could be
computed for 29 of them. Among these 29, there are 7 SB2 systems, one triple
and one quadruple system. The 21 stars with an apparently constant radial
velocity may show up later as long-period binaries with a high eccentricity.
The mass functions of the SB1 systems are compatible with cool main-sequence
companions, also suggested by ongoing spectral observations.Comment: 5 pages, 2 figures, to appear in: Proc. of the 26th workshop of the
European Working Group on CP stars, Contrib. Astr. Obs. Skalnate Pleso Vol.
27, No
Searching for solar-like oscillations in the delta Scuti star rho Puppis
Despite the shallow convective envelopes of delta Scuti pulsators, solar-like
oscillations are theoretically predicted to be excited in those stars as well.
To search for such stochastic oscillations we organised a spectroscopic
multi-site campaign for the bright, metal-rich delta Sct star rho Puppis. We
obtained a total of 2763 high-resolution spectra using four telescopes. We
discuss the reduction and analysis with the iodine cell technique, developed
for searching for low-amplitude radial velocity variations, in the presence of
high-amplitude variability. Furthermore, we have determined the angular
diameter of rho Puppis to be 1.68 \pm 0.03 mas, translating into a radius of
3.52 \pm 0.07Rsun. Using this value, the frequency of maximum power of possible
solar-like oscillations, is expected at ~43 \pm 2 c/d (498 \pm 23 muHz). The
dominant delta Scuti-type pulsation mode of rho Puppis is known to be the
radial fundamental mode which allows us to determine the mean density of the
star, and therefore an expected large frequency separation of 2.73 c/d (31.6
muHz). We conclude that 1) the radial velocity amplitudes of the delta Scuti
pulsations are different for different spectral lines; 2) we can exclude
solar-like oscillations to be present in rho Puppis with an amplitude per
radial mode larger than 0.5 m/s.Comment: 14 pages, 12 figure, accepted for MNRA
Oscillating red giants in the CoRoT exo-field: Asteroseismic mass and radius determination
Context. Observations and analysis of solar-type oscillations in red-giant
stars is an emerging aspect of asteroseismic analysis with a number of open
questions yet to be explored. Although stochastic oscillations have previously
been detected in red giants from both radial velocity and photometric
measurements, those data were either too short or had sampling that was not
complete enough to perform a detailed data analysis of the variability. The
quality and quantity of photometric data as provided by the CoRoT satellite is
necessary to provide a breakthrough in observing p-mode oscillations in red
giants. We have analyzed continuous photometric time-series of about 11 400
relatively faint stars obtained in the exofield of CoRoT during the first 150
days long-run campaign from May to October 2007. We find several hundred stars
showing a clear power excess in a frequency and amplitude range expected for
red-giant pulsators. In this paper we present first results on a sub-sample of
these stars. Aims. Knowing reliable fundamental parameters like mass and radius
is essential for detailed asteroseismic studies of red-giant stars. As the
CoRoT exofield targets are relatively faint (11-16 mag) there are no (or only
weak) constraints on the star's location in the H-R diagram. We therefore aim
to extract information about such fundamental parameters solely from the
available time series. Methods. We model the convective background noise and
the power excess hump due to pulsation with a global model fit and deduce
reliable estimates for the stellar mass and radius from scaling relations for
the frequency of maximum oscillation power and the characteristic frequency
separation.Comment: 10 pages, 7 figures, accepted for publication in A&
Quasiharmonic elastic constants corrected for deviatoric thermal stresses
The quasiharmonic approximation (QHA), in its simplest form also called the
statically constrained (SC) QHA, has been shown to be a straightforward method
to compute thermoelastic properties of crystals. Recently we showed that for
non-cubic solids SC-QHA calculations develop deviatoric thermal stresses at
high temperatures. Relaxation of these stresses leads to a series of
corrections to the free energy that may be taken to any desired order, up to
self-consistency. Here we show how to correct the elastic constants obtained
using the SC-QHA. We exemplify the procedure by correcting to first order the
elastic constants of MgSiO-perovskite and MgSiO-post-perovskite, the
major phases of the Earth's lower mantle. We show that this first order
correction is quite satisfactory for obtaining the aggregated elastic averages
of these minerals and their velocities in the lower mantle. This type of
correction is also shown to be applicable to experimental measurements of
elastic constants in situations where deviatoric stresses can develop, such as
in diamond anvil cells.Comment: 4 figures, 1 table, submitted to Phys. Rev. B, July 200
Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
A precise characterisation of the red giants in the seismology fields of the
CoRoT satellite is a prerequisite for further in-depth seismic modelling.
High-resolution FEROS and HARPS spectra were obtained as part of the
ground-based follow-up campaigns for 19 targets holding great asteroseismic
potential. These data are used to accurately estimate their fundamental
parameters and the abundances of 16 chemical species in a self-consistent
manner. Some powerful probes of mixing are investigated (the Li and CNO
abundances, as well as the carbon isotopic ratio in a few cases). The
information provided by the spectroscopic and seismic data is combined to
provide more accurate physical parameters and abundances. The stars in our
sample follow the general abundance trends as a function of the metallicity
observed in stars of the Galactic disk. After an allowance is made for the
chemical evolution of the interstellar medium, the observational signature of
internal mixing phenomena is revealed through the detection at the stellar
surface of the products of the CN cycle. A contamination by NeNa-cycled
material in the most massive stars is also discussed. With the asteroseismic
constraints, these data will pave the way for a detailed theoretical
investigation of the physical processes responsible for the transport of
chemical elements in evolved, low- and intermediate-mass stars.Comment: Accepted for publication in A&A, 25 pages, 13 colour figures (revised
version after language editing
Stochastic Cutoff Method for Long-Range Interacting Systems
A new Monte-Carlo method for long-range interacting systems is presented.
This method consists of eliminating interactions stochastically with the
detailed balance condition satisfied. When a pairwise interaction of a
-particle system decreases with the distance as ,
computational time per one Monte Carlo step is for
and for , where is the spatial
dimension. We apply the method to a two-dimensional magnetic dipolar system.
The method enables us to treat a huge system of spins with reasonable
computational time, and reproduces a circular order originated from long-range
dipolar interactions.Comment: 18 pages, 9 figures, 1 figure and 1 reference are adde
Multiplicity among peculiar A stars I. The Ap stars HD 8441 and HD 137909, and the Am stars HD 43478 and HD 96391
We present the first results of a radial-velocity survey of cool Ap and Am
stars. HD 8441 is not only a double system with P = 106.357 days, but is a
triple one, the third companion having an orbital period larger than 5000 days.
Improved orbital elements are given for the classical Ap star HD 137909 = beta
CrB by combining our radial velocities with published ones. We yield new
orbital elements of the two Am, SB2 binaries HD 43478 and HD 96391. Good
estimates of the individual masses of the components of HD 43478 can be given
thanks to the eclipses of this system, for which an approximate photometric
solution is also proposed.Comment: 10 pages, 10 figures, accepted for publication in A&A
Experimental Success and the Revelation of Reality: The Miracle Argument for Scientific Realism
Carrier M. Experimental Success and the Revelation of Reality: The Miracle Argument for Scientific Realism. In: Carrier M, Roggenhofer J, Küppers G, Blanchard P, eds. Knowledge and the World: Challenges Beyond the Science Wars. The frontiers collection. Berlin [u.a.]: Springer; 2004: 137-161
Determining Training Needs for Cloud Infrastructure Investigations using I-STRIDE
As more businesses and users adopt cloud computing services, security
vulnerabilities will be increasingly found and exploited. There are many
technological and political challenges where investigation of potentially
criminal incidents in the cloud are concerned. Security experts, however, must
still be able to acquire and analyze data in a methodical, rigorous and
forensically sound manner. This work applies the STRIDE asset-based risk
assessment method to cloud computing infrastructure for the purpose of
identifying and assessing an organization's ability to respond to and
investigate breaches in cloud computing environments. An extension to the
STRIDE risk assessment model is proposed to help organizations quickly respond
to incidents while ensuring acquisition and integrity of the largest amount of
digital evidence possible. Further, the proposed model allows organizations to
assess the needs and capacity of their incident responders before an incident
occurs.Comment: 13 pages, 3 figures, 3 tables, 5th International Conference on
Digital Forensics and Cyber Crime; Digital Forensics and Cyber Crime, pp.
223-236, 201
A Matrix Hyperbolic Cosine Algorithm and Applications
In this paper, we generalize Spencer's hyperbolic cosine algorithm to the
matrix-valued setting. We apply the proposed algorithm to several problems by
analyzing its computational efficiency under two special cases of matrices; one
in which the matrices have a group structure and an other in which they have
rank-one. As an application of the former case, we present a deterministic
algorithm that, given the multiplication table of a finite group of size ,
it constructs an expanding Cayley graph of logarithmic degree in near-optimal
O(n^2 log^3 n) time. For the latter case, we present a fast deterministic
algorithm for spectral sparsification of positive semi-definite matrices, which
implies an improved deterministic algorithm for spectral graph sparsification
of dense graphs. In addition, we give an elementary connection between spectral
sparsification of positive semi-definite matrices and element-wise matrix
sparsification. As a consequence, we obtain improved element-wise
sparsification algorithms for diagonally dominant-like matrices.Comment: 16 pages, simplified proof and corrected acknowledging of prior work
in (current) Section
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