8,276 research outputs found
Long-term photometry of the eclipsing dwarf nova V893 Scorpii: Orbital period, oscillations, and a possible giant planet
The cataclysmic variable V893 Sco is an eclipsing dwarf nova which, apart
from outbursts with comparatively low amplitudes, exhibits a particularly
strong variability during quiescence on timescales of days to seconds.The
present study aims to update the outdated orbital ephemerides published
previously, to investigate deviations from linear ephemerides, and to
characterize non-random brightness variations in a range of timescales. Light
curves of V893 Sco were observed on 39 nights, spanning a total time base of
about 14 years. They contain 114 eclipses which were used to significantly
improve the precision of the orbital period and to study long-term variations
of the time of revolution. Oscillations and similar brightness variations were
studied with Fourier techniques in the individual light curves. The orbital
period exhibits long-term variations with a cycle time of 10.2 years. They can
be interpreted as a light travel time effect caused by the presence of a giant
planet with approximately 9.5 Jupiter masses in a 4.5 AU orbit around V893 Sco.
On some nights transient semi-periodic variations on timescales of several
minutes can be seen which may be identified as quasi-periodic oscillations.
However, it is difficult to distinguish whether they are caused by real
physical mechanisms or if they are the effect of an accidental superposition of
unrelated flickering flares. Simulations to investigate this question are
presented.Comment: Accepted for publication in Astronomy & Astrophysics (acceptance
date: 2014, March 31
Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence
We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap
A unified model for the dynamics of driven ribbon with strain and magnetic order parameters
We develop a unified model to explain the dynamics of driven one dimensional
ribbon for materials with strain and magnetic order parameters. We show that
the model equations in their most general form explain several results on
driven magnetostrictive metallic glass ribbons such as the period doubling
route to chaos as a function of a dc magnetic field in the presence of a
sinusoidal field, the quasiperiodic route to chaos as a function of the
sinusoidal field for a fixed dc field, and induced and suppressed chaos in the
presence of an additional low amplitude near resonant sinusoidal field. We also
investigate the influence of a low amplitude near resonant field on the period
doubling route. The model equations also exhibit symmetry restoring crisis with
an exponent close to unity. The model can be adopted to explain certain results
on magnetoelastic beam and martensitic ribbon under sinusoidal driving
conditions. In the latter case, we find interesting dynamics of a periodic one
orbit switching between two equivalent wells as a function of an ac magnetic
field that eventually makes a direct transition to chaos under resonant driving
condition. The model is also applicable to magnetomartensites and materials
with two order parameters.Comment: 11 pages, 18 figure
Secular evolution of compact binaries near massive black holes: gravitational wave sources and other exotica
The environment near super massive black holes (SMBHs) in galactic nuclei
contain a large number of stars and compact objects. A fraction of these are
likely to be members of binaries. Here we discuss the binary population of
stellar black holes and neutron stars near SMBHs and focus on the secular
evolution of such binaries, due to the perturbation by the SMBH. Binaries with
highly inclined orbits in respect to their orbit around the SMBH are strongly
affected by secular Kozai processes, which periodically change their
eccentricities and inclinations (Kozai-cycles). During periapsis approach, at
the highest eccentricities during the Kozai-cycles, gravitational wave emission
becomes highly efficient. Some binaries in this environment can inspiral and
coalesce at timescales much shorter than a Hubble time and much shorter than
similar binaries which do not reside near a SMBH. The close environment of
SMBHs could therefore serve as catalyst for the inspiral and coalescence of
binaries, and strongly affect their orbital properties. Such compact binaries
would be detectable as gravitational wave (GW) sources by the next generation
of GW detectors (e.g. advanced- LIGO). About 0.5% of such nuclear merging
binaries will enter the LIGO observational window while on orbit that are still
very eccentric (e>~0.5). The efficient gravitational wave analysis for such
systems would therefore require the use of eccentric templates. We also find
that binaries very close to the MBH could evolve through a complex dynamical
(non-secular) evolution leading to emission of several GW pulses during only a
few yrs (though these are likely to be rare). Finally, we note that the
formation of close stellar binaries, X-ray binaries and their merger products
could be induced by similar secular processes, combined with tidal friction
rather than GW emission as in the case of compact object binaries.Comment: 15 pages, 7 Figures. ApJ accepte
Variable stars with the Kepler space telescope
The Kepler space telescope has revolutionised our knowledge about exoplanets
and stars and is continuing to do so in the K2 mission. The exquisite
photometric precision, together with the long, uninterrupted observations
opened up a new way to investigate the structure and evolution of stars.
Asteroseismology, the study of stellar oscillations, allowed us to investigate
solar-like stars and to peer into the insides of red giants and massive stars.
But many discoveries have been made about classical variable stars too, ranging
from pulsators like Cepheids and RR Lyraes to eclipsing binary stars and
cataclysmic variables, and even supernovae. In this review, which is far from
an exhaustive summary of all results obtained with Kepler, we collected some of
the most interesting discoveries, and ponder on the role for amateur observers
in this golden era of stellar astrophysics.Comment: 12 pages, 6 figures, published in the Journal of the AAVSO:
https://www.aavso.org/apps/jaavso/article/3235/, v2: fixed a bad a reference.
arXiv admin note: text overlap with arXiv:1108.3083 by other author
A new deterministic model for chaotic reversals
We present a new chaotic system of three coupled ordinary differential
equations, limited to quadratic nonlinear terms. A wide variety of dynamical
regimes are reported. For some parameters, chaotic reversals of the amplitudes
are produced by crisis-induced intermittency, following a mechanism different
from what is generally observed in similar deterministic models. Despite its
simplicity, this system therefore generates a rich dynamics, able to model more
complex physical systems. In particular, a comparison with reversals of the
magnetic field of the Earth shows a surprisingly good agreement, and highlights
the relevance of deterministic chaos to describe geomagnetic field dynamics.Comment: 12 pages, 14 figures, accepted in EPJ
Orbital Instabilities in a Triaxial Cusp Potential
This paper constructs an analytic form for a triaxial potential that
describes the dynamics of a wide variety of astrophysical systems, including
the inner portions of dark matter halos, the central regions of galactic
bulges, and young embedded star clusters. Specifically, this potential results
from a density profile of the form , where the radial
coordinate is generalized to triaxial form so that . Using the resulting analytic form of the potential, and the
corresponding force laws, we construct orbit solutions and show that a robust
orbit instability exists in these systems. For orbits initially confined to any
of the three principal planes, the motion in the perpendicular direction can be
unstable. We discuss the range of parameter space for which these orbits are
unstable, find the growth rates and saturation levels of the instability, and
develop a set of analytic model equations that elucidate the essential physics
of the instability mechanism. This orbit instability has a large number of
astrophysical implications and applications, including understanding the
formation of dark matter halos, the structure of galactic bulges, the survival
of tidal streams, and the early evolution of embedded star clusters.Comment: 50 pages, accepted for publication in Ap
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