2,806 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
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
Vector-soliton collision dynamics in nonlinear optical fibers
We consider the interactions of two identical, orthogonally polarized vector
solitons in a nonlinear optical fiber with two polarization directions,
described by a coupled pair of nonlinear Schroedinger equations. We study a
low-dimensional model system of Hamiltonian ODE derived by Ueda and Kath and
also studied by Tan and Yang. We derive a further simplified model which has
similar dynamics but is more amenable to analysis. Sufficiently fast solitons
move by each other without much interaction, but below a critical velocity the
solitons may be captured. In certain bands of initial velocities the solitons
are initially captured, but separate after passing each other twice, a
phenomenon known as the two-bounce or two-pass resonance. We derive an analytic
formula for the critical velocity. Using matched asymptotic expansions for
separatrix crossing, we determine the location of these "resonance windows."
Numerical simulations of the ODE models show they compare quite well with the
asymptotic theory.Comment: 32 pages, submitted to Physical Review
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&
Effects of rotational mixing on the asteroseismic properties of solar-type stars
The influence of rotational mixing on the evolution and asteroseismic
properties of solar-type stars is studied. Rotational mixing changes the global
properties of a solar-type star with a significant increase of the effective
temperature resulting in a shift of the evolutionary track to the blue part of
the HR diagram. These differences are related to changes of the chemical
composition, because rotational mixing counteracts the effects of atomic
diffusion leading to larger helium surface abundances for rotating models than
for non-rotating ones. Higher values of the large frequency separation are then
found for rotating models than for non-rotating ones at the same evolutionary
stage, because the increase of the effective temperature leads to a smaller
radius and hence to an increase of the stellar mean density. Rotational mixing
also has a considerable impact on the structure and chemical composition of the
central stellar layers by bringing fresh hydrogen fuel to the core, thereby
enhancing the main-sequence lifetime. The increase of the central hydrogen
abundance together with the change of the chemical profiles in the central
layers result in a significant increase of the values of the small frequency
separations and of the ratio of the small to large separations for models
including shellular rotation. This increase is clearly seen for models with the
same age sharing the same initial parameters except for the inclusion of
rotation as well as for models with the same global stellar parameters and in
particular the same location in the HR diagram. By computing rotating models of
solar-type stars including the effects of a dynamo that possibly occurs in the
radiative zone, we find that the efficiency of rotational mixing is strongly
reduced when the effects of magnetic fields are taken into account, in contrast
to what happens in massive stars.Comment: 11 pages, 15 figures, accepted for publication in A&
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