25,159 research outputs found
The sdA problem - II. Photometric and Spectroscopic Follow-up
Subdwarf A star (sdA) is a spectral classification given to objects showing
H-rich spectra and sub-main sequence surface gravities, but effective
temperature lower than the zero-age horizontal branch. Their evolutionary
origin is an enigma. In this work, we discuss the results of follow-up
observations of selected sdAs. We obtained time resolved spectroscopy for 24
objects, and time-series photometry for another 19 objects. For two targets, we
report both spectroscopy and photometry observations. We confirm seven objects
to be new extremely-low mass white dwarfs (ELMs), one of which is a known
eclipsing star. We also find the eighth member of the pulsating ELM class.Comment: Accepted for publication in MNRAS. 19 pages, 30 figures, 6 table
Study program to develop and evaluate die and container materials for the growth of silicon ribbons
The development and evaluation of proprietary coatings of pure silicon carbide, silicon nitride, and aluminum nitride on less pure hot pressed substrates of the respective ceramic materials, is described. Silicon sessile drop experiments were performed on coated test specimens under controlled oxygen partial pressure. Prior to testing, X-ray diffraction and SEM characterization was performed. The reaction interfaces were characterized after testing with optical and scanning electron microscopy and Auger electron spectroscopy. Increasing the oxygen partial pressure was found to increase the molten silicon contact angle, apparently because adsorbed oxygen lowers the solid-vapor interfacial free energy. It was also found that adsorbed oxygen increased the degree of attack of molten silicon upon the chemical vapor deposited coatings. Cost projections show that reasonably priced, coated, molten silicon resistant refractory material shapes are obtainable
Ionic calcium content of light dense human red cells separated by percoll density gradients
AbstractIn this paper we have compared the adequacy of two methods using Percoll density gradients to separate light and dense erythrocytes from fresh human blood. After measuring the distribution of some classical age-markers such as haemoglobin, potassium and creatine contents, it was found that preformed gradients generated more stringent conditions for age-related density separations. Employing such gradients, the free Ca2+ content of above sub-populations was assessed with Fura-2, under conditions where the viscosity effect was abolished. In five experiments, the free Ca2+ content (mean value±1 S.D.) was 8.4±2.82 nM and 31.2±13.0 nM for the 7–10% lightest and densest cells, respectively. These results are discussed in connection to red cell senescence
Global aspects of gravitomagnetism
We consider global properties of gravitomagnetism by investigating the
gravitomagnetic field of a rotating cosmic string. We show that although the
gravitomagnetic field produced by such a configuration of matter vanishes
locally, it can be detected globally. In this context we discuss the
gravitational analogue of the Aharonov-Bohm effect.Comment: 10 pages - Typeset using REVTE
Microquasar models for 3EG J1828+0142 and 3EG J1735-1500
Microquasars are promising candidates to emit high-energy gamma-rays.
Moreover, statistical studies show that variable EGRET sources at low galactic
latitudes could be associated with the inner spiral arms. The variable nature
and the location in the Galaxy of the high-mass microquasars, concentrated in
the galactic plane and within 55 degrees from the galactic center, give to
these objects the status of likely counterparts of the variable low-latitude
EGRET sources. We consider in this work the two most variable EGRET sources at
low-latitudes: 3EG J1828+0142 and 3EG J1735-1500, proposing a microquasar model
to explain the EGRET data in consistency with the observations at lower
energies (from radio frequencies to soft gamma-rays) within the EGRET error
box.Comment: (1)Universitat de Barcelona, (2)Instituto Argentino de
Radioastronomia (3) Facultad de Ciencias Astronomicas y Geofisicas
(4)Lawrence Livermore National Laboratory 6 pages, 2 figures. Presented as a
poster at the V Microquasar Workshop, Beijing, June 2004. Accepted for
publication in the Chinese Journal of Astronomy & Astrophysic
Dynamics of open quantum systems initially entangled with environment: Beyond the Kraus representation
We present a general analysis of the role of initial correlations between the
open system and an environment on quantum dynamics of the open system.Comment: 5 revtex pages, no figures, accepted for publication in Phys. Rev.
Comment on "On the temperature dependence of the Casimir effect"
Recently, Brevik et al. [Phys. Rev. E 71, 056101 (2005)] adduced arguments
against the traditional approach to the thermal Casimir force between real
metals and in favor of one of the alternative approaches. The latter assumes
zero contribution from the transverse electric mode at zero frequency in
qualitative disagreement with unity as given by the thermal quantum field
theory for ideal metals. Those authors claim that their approach is consistent
with experiments as well as with thermodynamics. We demonstrate that these
conclusions are incorrect. We show specifically that their results are
contradicted by four recent experiments and also violate the third law of
thermodynamics (the Nernst heat theorem).Comment: 11 pages, 3 figures, changed in accordance with the final published
versio
The polar ring galaxy AM1934-563 revisited
We report long-slit spectroscopic observations of the dust-lane polar-ring
galaxy AM1934-563 obtained with the Southern African Large Telescope (SALT)
during its performance-verification phase. The observations target the spectral
region of the Ha, [NII] and [SII] emission-lines, but show also deep NaI
stellar absorption lines that we interpret as produced by stars in the galaxy.
We derive rotation curves along the major axis of the galaxy that extend out to
about 8 kpc from the center for both the gaseous and the stellar components,
using the emission and absorption lines. We derive similar rotation curves
along the major axis of the polar ring and point out differences between these
and the ones of the main galaxy. We identify a small diffuse object visible
only in Ha emission and with a low velocity dispersion as a dwarf HII galaxy
and argue that it is probably metal-poor. Its velocity indicates that it is a
fourth member of the galaxy group in which AM1934-563 belongs. We discuss the
observations in the context of the proposal that the object is the result of a
major merger and point out some observational discrepancies from this
explanation. We argue that an alternative scenario that could better fit the
observations may be the slow accretion of cold intergalactic gas, focused by a
dense filament of galaxies in which this object is embedded (abridged).Comment: 19 pages, 13 figures, submitted to MNRAS. Some figures were bitmapped
to reduce the size. Full resolution version is available from
http://www.saao.ac.za/~akniazev/pub/AM1934_563.pd
Late stage kinetics for various wicking and spreading problems
The kinetics of spreading of a liquid drop in a wedge or V-shaped groove, in
a network of such grooves, and on a hydrophilic strip, is re-examined. The
length of a droplet of volume Omega spreading in a wedge after a time t is
predicted to scale as Omega^(1/5) * t^(2/5), and the height profile is
predicted to be a parabola in the distance along the wedge. If the droplet is
spreading radially in a sparse network of V-shaped grooves on a surface, the
radius is predicted to scale as Omega^(1/6) * t^(1/3), provided the liquid is
completely contained within the grooves. A number of other results are also
obtained.Comment: 5 pages, 2 figures, RevTeX
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