5 research outputs found
Flat galaxies with dark matter halos - existence and stability
We consider a model for a flat, disk-like galaxy surrounded by a halo of dark
matter, namely a Vlasov-Poisson type system with two particle species, the
stars which are restricted to the galactic plane and the dark matter particles.
These constituents interact only through the gravitational potential which
stars and dark matter create collectively. Using a variational approach we
prove the existence of steady state solutions and their nonlinear stability
under suitably restricted perturbations.Comment: 39 page
Is the model of one-armed oscillations able to explain the long-term
Context.Many scientists studying Be stars currently adopt the
model of one-armed oscillations as the correct explanation of the
cyclic long-term variations observed for a number of Be
stars. We test the ability of this model to be used for the
predictions of V/R variations in real observed Be stars.
Aims.The
behavior of the one-armed oscillations can be described as a
solution of linearized hydrodynamical equations with the presence
of “distorted” gravitational potential and a radiation force.
Methods.We
developed a new computer program to model one-armed oscillations
in Be star disks, which includes both the pressure force and the
quadrupole term in the gravitational potential, related to the
obliquity of a rapidly rotating star inside the disk. Moreover, we
slightly improved the model in an effort to decrease the number of
input parameters with the help of NLTE stellar atmosphere
models.
Results.We carried out detailed tests of the dependence of
“periods” predicted by the model on all individual input
parameters. We arrived at the following results:
(1) the model has great potential to
explain not only the cause of the cyclic long-term changes
but also some of the observed statistical properties of the
phenomenon.
(2) The model in its present linear form cannot be considered as
proven. Its ability to predict the duration of cycles for
individual well observed Be stars is insufficient. Changing some
of the input parameters of the model, which are still loosely
constrained by observations and/on current understanding of the
disks, like the radial density distribution in the disk, one can
easily arrive at any desired cycle length from, say, 1 to 20 years.
Conclusions. Clearly, a much more sophisticated non-linear and
self-consistent model of disk structure and its oscillations will
be needed before a truly quantitative test of a one-armed model
vs. observations will be possible
Properties and nature of Be stars
Context. One way to understand the still mysterious Be phenomenon is to study
the time variations of particular Be stars with a long observational history.
ζ Tau is one obvious candidate.
Aims. Using our rich series of spectral and photometric observations
and a critical compilation of available radial velocities,
spectrophotometry of Hα, and photometry, we characterize the pattern
of time variations of ζ Tau over about a century. Our goal is to find
the true timescales of its variability and confront them with the
existing models related to various aspects of the Be phenomenon.
Methods. Spectral reductions were carried out using the IRAF and SPEFO programs. The HEC22 program was used for both photometric reductions and transformations to . Orbital solutions were derived with the latest publicly available version of the program FOTEL, period analyses employed both the PDM and Fourier techniques – programs HEC27 and PERIOD04.
Results. We derived a new orbital ephemeris HJD (2447025.6±1.8) + (132987 ±0050) . The analysis of long-term spectral and light variations shows a clear correlation between the RV and changes, and a very complex behaviour of the light changes. The character of the orbital light and changes varies from season to season