978 research outputs found
The MUCHFUSS photometric campaign
Hot subdwarfs (sdO/Bs) are the helium-burning cores of red giants, which lost
almost all of their hydrogen envelopes. This mass loss is often triggered by
common envelope interactions with close stellar or even substellar companions.
Cool companions like late-type stars or brown dwarfs are detectable via
characteristic light curve variations like reflection effects and often also
eclipses. To search for such objects we obtained multi-band light curves of 26
close sdO/B binary candidates from the MUCHFUSS project with the BUSCA
instrument. We discovered a new eclipsing reflection effect system
(~d) with a low-mass M dwarf companion ().
Three more reflection effect binaries found in the course of the campaign were
already published, two of them are eclipsing systems, in one system only
showing the reflection effect but no eclipses the sdB primary is found to be
pulsating. Amongst the targets without reflection effect a new long-period sdB
pulsator was discovered and irregular light variations were found in two sdO
stars. The found light variations allowed us to constrain the fraction of
reflection effect binaries and the substellar companion fraction around sdB
stars. The minimum fraction of reflection effect systems amongst the close sdB
binaries might be greater than 15\% and the fraction of close substellar
companions in sdB binaries might be as high as . This would result in a
close substellar companion fraction to sdB stars of about 3\%. This fraction is
much higher than the fraction of brown dwarfs around possible progenitor
systems, which are solar-type stars with substellar companions around 1 AU, as
well as close binary white dwarfs with brown dwarf companions. This might be a
hint that common envelope interactions with substellar objects are
preferentially followed by a hot subdwarf phase.Comment: accepted for A&
GaBoDS: The Garching-Bonn Deep Survey -- II. Confirmation of EIS cluster candidates by weak gravitational lensing
We report the first confirmation of colour-selected galaxy cluster candidates
by means of weak gravitational lensing. Significant lensing signals were
identified in the course of the shear-selection programme of dark matter haloes
in the Garching-Bonn Deep Survey, which currently covers 20 square degrees of
deep, high-quality imaging data on the southern sky. The detection was made in
a field that was previously covered by the ESO Imaging Survey (EIS) in 1997. A
highly significant shear-selected mass-concentration perfectly coincides with
the richest EIS cluster candidate at z~0.2, thus confirming its cluster nature.
Several other shear patterns in the field can also be identified with cluster
candidates, one of which could possibly be part of a filament at z~0.45.Comment: 4 pages, 4 figures, submitted to A&A Letter
GaBoDS: The Garching-Bonn Deep Survey - III. Lyman-Break Galaxies in the Chandra Deep Field South
We present first results of our search for high-redshift galaxies in deep CCD
mosaic images. As a pilot study for a larger survey, very deep images of the
Chandra Deep Field South (CDFS), taken withWFI@MPG/ESO2.2m, are used to select
large samples of 1070 U-band and 565 B-band dropouts with the Lyman-break
method. The data of these Lyman-break galaxies are made public as an electronic
table. These objects are good candidates for galaxies at z~3 and z~4 which is
supported by their photometric redshifts. The distributions of apparent
magnitudes and the clustering properties of the two populations are analysed,
and they show good agreement to earlier studies. We see no evolution in the
comoving clustering scale length from z~3 to z~4. The techniques presented here
will be applied to a much larger sample of U-dropouts from the whole survey in
near future.Comment: 11 pages, 11 figures, replaced with version accepted by A&A. Minor
changes and tabular appendix with LBG catalogues. Version with full
resolution figures available at
http://www.astro.uni-bonn.de/~hendrik/2544.pd
The Frequency Evolution of Interstellar Pulse Broadening from Radio Pulsars
In this paper we report multi-frequency measurements of pulse broadening
times (tau_d) for nine medium dispersion measure (DM pc
cm) pulsars observed over a wide frequency range. The low frequency data
at 243, 325 and 610 MHz are new observations done with the Giant Metrewave
Radio Telescope (GMRT). The frequency dependence of tau_d for all but one (PSR
B1933+16) of our sources is consistent with the Kolmogorov spectrum of electron
density fluctuations in a turbulent medium. PSR B1933+16, however, shows a very
flat spectrum as previously observed for high DM pulsars. Our observations
combined with earlier published results enable us to study the spectral index
of tau_d over the whole observed DM range. While the spectral properties are
generally consistent with a Kolmogorov spectrum, pulsars seen along
line-of-sights towards the inner Galaxy or complex regions often show
deviations from this expected behaviour.Comment: 7 pages, 5 figures, accepted for publication in A&
Zitterbewegung and semiclassical observables for the Dirac equation
In a semiclassical context we investigate the Zitterbewegung of relativistic
particles with spin 1/2 moving in external fields. It is shown that the
analogue of Zitterbewegung for general observables can be removed to arbitrary
order in \hbar by projecting to dynamically almost invariant subspaces of the
quantum mechanical Hilbert space which are associated with particles and
anti-particles. This not only allows to identify observables with a
semiclassical meaning, but also to recover combined classical dynamics for the
translational and spin degrees of freedom. Finally, we discuss properties of
eigenspinors of a Dirac-Hamiltonian when these are projected to the almost
invariant subspaces, including the phenomenon of quantum ergodicity
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
Quantum ergodicity of C* dynamical systems
This paper contains a very simple and general proof that eigenfunctions of
quantizations of classically ergodic systems become uniformly distributed in
phase space. This ergodicity property of eigenfunctions f is shown to follow
from a convexity inequality for the invariant states (Af,f). This proof of
ergodicity of eigenfunctions simplifies previous proofs (due to A.I.
Shnirelman, Colin de Verdiere and the author) and extends the result to the
much more general framework of C* dynamical systems.Comment: Only very minor differences with the published versio
Long duration radio transients lacking optical counterparts are possibly Galactic Neutron Stars
(abridged) Recently, a new class of radio transients in the 5-GHz band was
detected by Bower et al. We present new deep near-Infrared (IR) observations of
the field containing these transients, and find no counterparts down to a
limiting magnitude of K=20.4 mag. We argue that the bright (>1 Jy) radio
transients recently reported by Kida et al. are consistent with being
additional examples of the Bower et al. transients. We refer to these groups of
events as "long-duration radio transients". The main characteristics of this
population are: time scales longer than 30 minute but shorter than several
days; rate, ~10^3 deg^-2 yr^-1; progenitors sky surface density of >60 deg^-2
(95% C.L.) at Galactic latitude ~40 deg; 1.4-5 GHz spectral slopes, f_\nu ~
\nu^alpha, with alpha>0; and most notably the lack of any counterparts in
quiescence in any wavelength. We rule out an association with many types of
objects. Galactic brown-dwarfs or some sort of exotic explosions remain
plausible options. We argue that an attractive progenitor candidate for these
radio transients is the class of Galactic isolated old neutron stars (NS). We
confront this hypothesis with Monte-Carlo simulations of the space distribution
of old NSs, and find satisfactory agreement for the large areal density.
Furthermore, the lack of quiescent counterparts is explained quite naturally.
In this framework we find: the mean distance to events in the Bower et al.
sample is of order kpc; the typical distance to the Kida et al. transients are
constrained to be between 30 pc and 900 pc (95% C.L.); these events should
repeat with a time scale of order several months; and sub-mJy level bursts
should exhibit Galactic latitude dependence. We discuss possible mechanisms
giving rise to the observed radio emission.Comment: Submitted to ApJ, 17 pages, 10 figure
Topological features of massive bosons on two dimensional Einstein space-time
In this paper we tackle the problem of constructing explicit examples of
topological cocycles of Roberts' net cohomology, as defined abstractly by
Brunetti and Ruzzi. We consider the simple case of massive bosonic quantum
field theory on the two dimensional Einstein cylinder. After deriving some
crucial results of the algebraic framework of quantization, we address the
problem of the construction of the topological cocycles. All constructed
cocycles lead to unitarily equivalent representations of the fundamental group
of the circle (seen as a diffeomorphic image of all possible Cauchy surfaces).
The construction is carried out using only Cauchy data and related net of local
algebras on the circle.Comment: 41 pages, title changed, minor changes, typos corrected, references
added. Accepted for publication in Ann. Henri Poincare
Neutrino-Nucleon Interactions in Magnetized Neutron-Star Matter: The Effects of Parity Violation
We study neutrino-nucleon scattering and absorption in a dense, magnetized
nuclear medium. These are the most important sources of neutrino opacity
governing the cooling of a proto-neutron star in the first tens of seconds
after its formation. Because the weak interaction is parity violating, the
absorption and scattering cross-sections depend asymmetrically on the
directions of the neutrino momenta with respect to the magnetic field. We
develop the moment formalism of neutrino transport in the presence of such
asymmetric opacities and derive explicit expressions for the neutrino flux and
other angular moments of the Boltzmann transport equation. For a given neutrino
species, there is a drift flux of neutrinos along the magnetic field in
addition to the usual diffusive flux. This drift flux depends on the deviation
of the neutrino distribution function from thermal equilibrium. Hence, despite
the fact that the neutrino cross-sections are asymmetric throughout the star,
asymmetric neutrino flux can be generated only in the outer region of the
proto-neutron star where the neutrino distribution deviates significantly from
thermal equilibrium. In addition to the asymmetric absorption opacity arising
from nucleon polarization, we find the contribution of the electron (or
positron) ground state Landau level. For neutrinos of energy less than a few
times the temperature, this is the dominant source of asymmetric opacity.
Lastly, we discuss the implication of our result to the origin of pulsar kicks:
in order to generate kick velocity of a few hundred km/s from asymmetric
neutrino emission using the parity violation effect, the proto-neutron star
must have a dipole magnetic field of at least G.Comment: 35 pages, no figures, submitted to Phys.Rev.
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