733 research outputs found
WSO/UV: World Space Observatory/Ultraviolet
We summarize the capabilities of the World Space Observatory (UV) Project
(WSO/UV). An example of the importance of this project (with a planned launch
date of 2007/8) for the study of Classical Novae is given.Comment: 4 pages, To appear in the proceeedings of the "Classical Nova
Explosions" conference, eds. M. Hernanz and J. Jose, AI
New Pulsating White Dwarfs in Cataclysmic Variables
The number of discovered non-radially pulsating white dwarfs (WDs) in
cataclysmic variables (CVs) is increasing rapidly by the aid of the Sloan
Digital Sky Survey (SDSS). We performed photometric observations of two
additional objects, SDSS J133941.11+484727.5 (SDSS 1339), independently
discovered as a pulsator by Gansicke et al., and SDSS J151413.72+454911.9,
which we identified as a CV/ZZ Ceti hybrid. In this Letter we present the
results of the remote observations of these targets performed with the Nordic
Optical Telescope (NOT) during the Nordic-Baltic Research School at Moletai
Observatory, and follow-up observations executed by NOT in service mode. We
also present 3 candidates we found to be non-pulsating. The results of our
observations show that the main pulsation frequencies agree with those found in
previous CV/ZZ Ceti hybrids, but specifically for SDSS 1339 the principal
period differs slightly between individual observations and also from the
recent independent observation by Gansicke et al. Analysis of SDSS colour data
for the small sample of pulsating and non-pulsating CV/ZZ Ceti hybrids found so
far, seems to indicate that the r-i colour could be a good marker for the
instability strip of this class of pulsating WDs.Comment: 5 pages, 6 figures, 2 tables, accepted for publication in MNRAS
Letter
First Attempt at Spectroscopic Detection of Gravity Modes in a Long-Period Pulsating Subdwarf B Star -- PG 1627+017
In the first spectroscopic campaign for a PG 1716 variable (or long-period
pulsating subdwarf B star), we succeeded in detecting velocity variations due
to g-mode pulsations at a level of 1.0-1.5 km/s.The observations were obtained
during 40 nights on 2-m class telescopes in Arizona, South Africa,and
Australia. The target,PG1627+017, is one of the brightest and largest amplitude
stars in its class.It is also the visible component of a post-common envelope
binary.Our final radial velocity data set includes 84 hours of time-series
spectroscopy over a time baseline of 53 days. Our derived radial velocity
amplitude spectrum, after subtracting the orbital motion, shows three potential
pulsational modes 3-4 sigma above the mean noise level, at 7201.0s,7014.6s and
7037.3s.Only one of the features is statistically likely to be real,but all
three are tantalizingly close to, or a one day alias of, the three strongest
periodicities found in the concurrent photometric campaign. We further
attempted to detect pulsational variations in the Balmer line amplitudes. The
single detected periodicity of 7209 s, although weak, is consistent with
theoretical expectations as a function of wavelength.Furthermore, it allows us
to rule out a degree index of l= 3 or l= 5 for that mode. Given the extreme
weakness of g-mode pulsations in these stars,we conclude that anything beyond
simply detecting their presence will require larger telescopes,higher
efficiency spectral monitoring over longer time baselines,improved longitude
coverage, and increased radial velocity precision.Comment: 39 pages, 9 figures, 4 tables, ApJ accepted. See postscript for full
abtrac
Absence of "Ghost Images" Excludes Large Values of the Cosmological Constant
We used the 1.4 GHz NRAO NVSS survey to search for ghost images of radio
sources, expected in cosmologies with a positive cosmological constant and
positive space curvature. No statistically significant evidence for ghost
images was found, placing constraints on the values of L, the space curvature
or the duration of the radio-luminous phase of extragalactic radio sources.Comment: 11 pages 2 figure
An inhomogeneous alternative to dark energy?
Recently, there have been suggestions that the apparent accelerated expansion
of the universe is not caused by repulsive gravitation due to dark energy, but
is rather a result of inhomogeneities in the distribution of matter. In this
work, we investigate the behaviour of a dust dominated inhomogeneous
Lemaitre-Tolman-Bondi universe model, and confront it with various
astrophysical observations. We find that such a model can easily explain the
observed luminosity distance-redshift relation of supernovae without the need
for dark energy, when the inhomogeneity is in the form of an underdense bubble
centered near the observer. With the additional assumption that the universe
outside the bubble is approximately described by a homogeneous Einstein-de
Sitter model, we find that the position of the first CMB peak can be made to
match the WMAP observations. Whether or not it is possible to reproduce the
entire CMB angular power spectrum in an inhomogeneous model without dark
energy, is still an open question.Comment: 8 pages (REVTeX4), 4 figures. v2: Minor changes to text plus added
some references. Accepted for publication in PR
The Structure and Evolution of Circumbinary Disks in Cataclysmic Variable Systems
We investigate the structure and evolution of a geometrically thin viscous
Keplerian circumbinary (CB) disk, using detailed models of their
radiative/convective vertical structure. We use a simplified description for
the evolution of the cataclysmic binary and focus on cases where the
circumbinary disk causes accelerated mass transfer (> 1e-8 Msun/yr). The inner
edge of the disk is assumed to be determined by the tidal truncation radius and
the mass input rate into the disk is assumed to be a small fraction (1e-5-0.01)
of the mass transfer rate. Under the action of the viscous stresses in the disk
the matter drifts outward with the optically thick region extending to several
AU. The inner part of the disk is cool with maximum effective temperatures <
3,000 K while the outermost parts of the disk are < 30 K and optically thin. We
calculate the effects of thermal instability on a sufficiently massive CB disk.
It leads to outbursts reminiscent of those in thermally unstable accretion
disks, with the instability remaining confined to the inner regions of the CB
disk. However, for most of the evolutionary sequences the surface densities
required to trigger instability are not reached. The spectral energy
distributions from circumbinary disks are calculated, and the prospects for the
detection of such disks in the infrared and submm wavelength regions are
discussed.Comment: 16 pages, 12 figures, accepted for publication by Ap
The mass of the sdB primary of the binary HS 2333+3927
Short period sdB binaries with cool companions are crucial to understand
pre-CV evolution, because they will evolve into cataclysmic variables, when the
sdB will have left the extended horizontal branch. Recently we discovered the
sixth such system, HS 2333+3927, consisting of an sdB star and an M dwarf
(period: 0.172 d) with a very strong reflection effect, but no eclipses. The
reflection is stronger than in any of the other similar systems which renders a
quantitative spectral analysis very difficult because the Balmer line profiles
may be disturbed by the reflected light. A spectroscopic analysis results in
Teff = 36500 K, log g = 5.70, and log (n_He/n_H) = -2.15. Mass-radius relations
were derived from the results of the analysis of light and radial-velocity
curves. Comparison with the mass-radius relation derived from the surface
gravity of the sdB star favours a rather low mass of 0.38 Msun for the primary.
The mass of the companion is 0.29 M_sun. HS 2333+3927 is the only known sdB+dM
system with a period above the CV period gap.Comment: 6 pages, 3 Fig., to appear in 14th European Workshop on White Dwarfs,
ASP Conference Series, eds. D. Koester, S. Moehle
A survey for pulsating subdwarf B stars with the Nordic Optical Telescope
A search programme for pulsating subdwarf B stars was conducted with the
Nordic Optical Telescope on La Palma over 59 nights between 1999 and 2009. The
purpose of the programme was to significantly extend the number of rapidly
pulsating sdB stars to better understand the properties of this new group of
variable compact stars. Candidates were selected initially from the HS and HE
surveys, but were supplemented with additional objects from other surveys.
Short sequences of time-series photometry were made on the candidates to
determine the presence of rapid pulsations. In total twenty new pulsators were
found in this survey, most of which have already been published and some
extensively studied. We present four new short period pulsators, bringing the
total of such pulsators up to 49. We also give limits on pulsation amplitudes
for 285 objects with no obvious periodic variations, summarise the results of
the survey, and provide improved physical parameters on the composite pulsators
for which only preliminary estimates were published earlier.Comment: 17 pages, accepted for publication in A&
A time varying speed of light as a solution to cosmological puzzles
We consider the cosmological implications of light travelling faster in the
early Universe. We propose a prescription for deriving corrections to the
cosmological evolution equations while the speed of light is changing. We
then show how the horizon, flatness, and cosmological constant problems may be
solved. We also study cosmological perturbations in this scenario and show how
one may solve the homogeneity and isotropy problems. As it stands, our scenario
appears to most easily produce extreme homogeneity, requiring structure to be
produced in the Standard Big Bang epoch. Producing significant perturbations
during the earlier epoch would require a rather careful design of the function
. The large entropy inside the horizon nowadays can also be accounted for
in this scenario.Comment: To be published in Physical Review D. Note added referring to John
Moffat's early work on VSL theorie
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