2,883 research outputs found
Echo Emission From Dust Scattering and X-Ray Afterglows of Gamma-Ray Bursts
We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs).
We find that the echo emission can provide an alternative way of understanding
X-ray shallow decays and jet breaks. In particular, a shallow decay followed by
a "normal" decay and a further rapid decay of X-ray afterglows can be together
explained as being due to the echo from prompt X-ray emission scattered by dust
grains in a massive wind bubble around a GRB progenitor. We also introduce an
extra temporal break in the X-ray echo emission. By fitting the afterglow light
curves, we can measure the locations of the massive wind bubbles, which will
bring us closer to finding the mass loss rate, wind velocity, and the age of
the progenitors prior to the GRB explosions.Comment: 25 pages, 3 figures, 2 tables. Accepted for publication in Ap
Distribution of cytochemically detectable cholesterol in the electric organ of Torpedo marmorata.
A Compact X-ray Source and Possible X-ray Jets within the Planetary Nebula Menzel 3
We report the discovery, by the Chandra X-ray Observatory, of X-ray emission
from the bipolar planetary nebula Menzel 3. In Chandra CCD imaging, Mz 3
displays hot (3-6x10^6 K) gas within its twin, coaxial bubbles of optical
nebulosity, as well as a compact X-ray source at the position of its central
star(s). The brightest diffuse X-ray emission lies along the polar axis of the
optical nebula, suggesting a jet-like configuration. The observed combination
of an X-ray-emitting point source and possible X-ray jet(s) is consistent with
models in which accretion disks and, potentially, magnetic fields shape bipolar
planetary nebulae via the generation of fast, collimated outflows.Comment: 12 pages, 3 figures; to appear in Astrophysical Journal (Letters
Physical Structure of Small Wolf-Rayet Ring Nebulae
We have selected the seven most well-defined WR ring nebulae in the LMC (Br
2, Br 10, Br 13, Br 40a, Br 48, Br 52, and Br 100) to study their physical
nature and evolutionary stages. New CCD imaging and echelle observations have
been obtained for five of these nebulae; previous photographic imaging and
echelle observations are available for the remaining two nebulae. Using the
nebular dynamics and abundances, we find that the Br 13 nebula is a
circumstellar bubble, and that the Br 2 nebula may represent a circumstellar
bubble merging with a fossil main-sequence interstellar bubble. The nebulae
around Br 10, Br 52, and Br 100 all show influence of the ambient interstellar
medium. Their regular expansion patterns suggest that they still contain
significant amounts of circumstellar material. Their nebular abundances would
be extremely interesting, as their central stars are WC5 and WN3-4 stars whose
nebular abundances have not been derived previously. Intriguing and tantalizing
implications are obtained from comparisons of the LMC WR ring nebulae with ring
nebulae around Galactic WR stars, Galactic LBVs, LMC LBVs, and LMC BSGs;
however, these implications may be limited by small-number statistics. A SNR
candidate close to Br 2 is diagnosed by its large expansion velocity and
nonthermal radio emission. There is no indication that Br 2's ring nebula
interacts dynamically with this SNR candidate.Comment: 20 pages, Latex (aaspp4.sty), 2 figures, accepted by the Astronomical
Journal (March 99 issue
GSH23.0-0.7+117, a neutral hydrogen shell in the inner Galaxy
GSH23.0-0.7+117 is a well-defined neutral hydrogen shell discovered in the
VLA Galactic Plane Survey (VGPS). Only the blueshifted side of the shell was
detected. The expansion velocity and systemic velocity were determined through
the systematic behavior of the HI emission with velocity. The center of the
shell is at (l,b,v)=(23.05,-0.77,+117 km/s). The angular radius of the shell is
6.8', or 15 pc at a distance of 7.8 kpc. The HI mass divided by the volume of
the half-shell implies an average density n_H = 11 +/- 4 cm^{-3} for the medium
in which the shell expanded. The estimated age of GSH23.0-0.7+117 is 1 Myr,
with an upper limit of 2 Myr. The modest expansion energy of 2 * 10^{48} erg
can be provided by the stellar wind of a single O4 to O8 star over the age of
the shell. The 3 sigma upper limit to the 1.4 GHz continuum flux density
(S_{1.4} < 248 mJy) is used to derive an upper limit to the Lyman continuum
luminosity generated inside the shell. This upper limit implies a maximum of
one O9 star (O8 to O9.5 taking into account the error in the distance) inside
the HI shell, unless most of the incident ionizing flux leaks through the HI
shell. To allow this, the shell should be fragmented on scales smaller than the
beam (2.3 pc). If the stellar wind bubble is not adiabatic, or the bubble has
burst (as suggested by the HI channel maps), agreement between the energy and
ionization requirements is even less likely. The limit set by the non-detection
in the continuum provides a significant challenge for the interpretation of
GSH23.0-0.7+117 as a stellar wind bubble. A similar analysis may be applicable
to other Galactic HI shells that have not been detected in the continuum.Comment: 18 pages, 6 figures. Figures 1 and 4 separately in GIF format.
Accepted for publication in Astrophysical Journa
Influence of gaseous pollutants and their synergistic effects on the aging of reflector materials for concentrating solar thermal technologies
ABSTRACT: Concentrating solar thermal technologies have experienced an important boost in the last few years. Besides the production of electricity, they are particularly useful for the supply of industrial process heat. The industrial atmospheres affecting these solar plants typically contain gaseous pollutants that are likely to promote corrosion on the components of the solar facility, specifically solar reflectors, thereby compromising their optimal performance and the overall system efficiency. Seven accelerated aging tests were designed to study the effects of three air pollutants (H2S, SO2 and NO2) on the durability of two commercially available reflector types (silvered glass and aluminum), both in single-gas tests and in multicomponent gas mixtures. Additionally, the same material types were exposed outdoors at five representative polluted sites, including industrial, urban and coastal environments. Reflectance and optical microscope monitoring corroborated which degree of corrosion was developed on a specific type of reflector in the different tests with gaseous pollutants, as well as the synergistic effects of gas combinations. For example, tests with sulfur were harmful for silvered-glass reflectors (up to a total of 16 corrosion spots), whereas aluminum was particularly affected by tests with NO2 (numerous micro spots of around 50 pm size). Moreover, comparisons of the corrosion patterns found in accelerated-aging and outdoor exposures revealed which laboratory test reproduced the different real polluted atmospheres in the most realistic way, which is the main goal of this work. For instance, the degradation found at Site 2 was reproduced by Test NO2+SO2, with an acceleration factor of 27.info:eu-repo/semantics/publishedVersio
Growth hormone and aging
Les alteracions vasculars i degeneratives del sistema nerviós central (SNC) són dues de les
causes més comunes de malaltia i de mort entre la gent gran; ambdues es correlacionen amb
l'edat, amb la deficiència en GH, i poden afectar les funcions fisiològiques de la població d'edat
avançada.
Amb la finalitat de clarificar els efectes de la GH en el metabolisme, en els vasos i en el SNC,
hem dut a terme un estudi in vivo utilitzant rates vellesWistar tractades crònicament amb GH.
Les rates velles varen presentar un augment en el pes de greix i una disminució de l'índex
específic de gravetat (SGI) (p < 0,05) en comparar-les amb les rates adultes no tractades. La
GH va reduir el pes en greix (p < 0,05), i va mostrar també una tendència a augmentar l'SGI.
Es va analitzar també la resposta de diverses substàncies vasoactives en els anells aòrtics, i es
va demostrar una disminució de la vasodilatació per acetilcolina i isoprenalina (p < 0,05) en
els animals vells. La contracció induïda per acetilcolina+L-NAME era més alta en els animals
vells que en els adults. L'administració de GH millorava les respostes vasodilatadores (p < 0,05)
mentre que tendia a reduïr les respostes vasoconstrictores. L'àrea aòrtica mitja augmentava
també en les rates velles, mentre que la GH reduïa aquest paràmetre (p < 0,05).
Les poblacions neuronals es reduïen en els hipocamps de les rates velles en comparar-les
amb les joves. Aquesta reducció estava asociada a un augment dels nucleosomes i a una reducció
de Bcl2 en el cervell. Les caspases 3 i 9 també varen augmentar. El tractament amb GH va
augmentar significativament el nombre de neurones i va reduir els nucleosomes i les caspases
i augmentar el Bcl2. En conclusió, el tractament perGHindueix l'aparició d'efectes beneficiosos
en la composició del cos i ha restablert també les funcions cerebrals i vasculars en les rates velles.Vascular and degenerative alterations of the central nervous system (CNS) are two of the
most common reasons for illness and death in elderly people; they exhibit an age-related GH
deficiency that can affect their physiological functions.
A study was conducted under chronic in vivo conditions using old Wistar rats, in order to
clarify the effects of GH on the metabolism, vessels, and the CNS. The old rats showed an increased
fat weight and a decreased Specific Gravity Index (SGI) (p < 0.05), as compared to the
adult animals. GH reduced the fat weight (p < 0.05) and tended to increase the SGI (N.S.). The
response to several vasoactive substances in aortic rings showed impaired vasodilatation to
Acetylcholine and Isoprenaline (p < 0.05) in the old animals. Contraction, induced by Acetylcholine+
L-NAME, was higher in the old rats than in the adults. GH administration improved
the vasodilatory responses (p < 0.05) and tended to reduce the constrictory responses. The aortic
media area was increased in the old rats, and GH reduced this parameter (p < 0.05).
The neuronal populations were reduced in the hippocampi of the old rats as compared to the
young ones. This reduction was associated with an increase in nucleosomes and a reduction
in Bcl2 in the brain. An increase was also detected in caspases 3 and 9. GH treatment was able
to significantly enhance the number of neurons by reducing the nucleosomes and the caspases
and by increasing Bcl2. In conclusion,GHtreatment was able to show beneficial effects on body
composition and was able to restore both vascular and brain functions in the old rats
An Interstellar Conduction Front Within a Wolf-Rayet Ring Nebula Observed with the GHRS
With the High Resolution Spectrograph aboard the Hubble Space Telescope we
obtained high signal-to-noise (S/N > 200 - 600 per 17 km/s resolution element)
spectra of narrow absorption lines toward the Wolf-Rayet star HD 50896. The
ring nebula S308 that surrounds this star is thought to be caused by a
pressure-driven bubble bounded by circumstellar gas (most likely from a red
supergiant or luminous blue variable progenitor) pushed aside by a strong
stellar wind. Our observation has shown for the first time that blueshifted
(approximately 70 km/s relative to the star) absorption components of C IV and
N V arise in a conduction front between the hot interior of the bubble and the
cold shell of swept-up material. These lines set limits on models of the
conduction front. Nitrogen in the shell appears to be overabundant by a factor
~10. The P Cygni profiles of N V and C IV are variable, possibly due to a
suspected binary companion to HD 50896.Comment: 32 pages, Latex, to appear in the Astrophysical Journal, April, 199
The Evolution of Supernovae in Circumstellar Wind Bubbles II: Case of a Wolf-Rayet star
(Abridged) Mass-loss from massive stars leads to the formation of
circumstellar wind-blown bubbles surrounding the star, bordered by a dense
shell. When the star ends its life in a supernova (SN) explosion, the resulting
shock wave will interact with this modified medium. In a previous paper we
discussed the basic parameters of this interaction. In this paper we go a step
further and study the evolution of SNe in the wind blown bubble formed by a 35
\msun star that starts off as an O star, goes through a red supergiant phase,
and ends its life as a Wolf-Rayet star. We model the evolution of the CSM and
then the expansion of the SN shock wave within this medium. Our simulations
clearly reveal fluctuations in density and pressure within the surrounding
medium. The SN shock interacting with these fluctuations, and then with the
dense shell surrounding the wind-blown cavity, gives rise to a variety of
transmitted and reflected shocks in the wind bubble. The interactions between
these various shocks and discontinuities is examined, and its effects on the
X-ray emission is noted. Our simulations reveal the presence of several
hydrodynamic instabilities. They show that the turbulent interior, coupled with
the large fluctuations in density and pressure, gives rise to an extremely
corrugated SN shock wave. The shock shows considerable wrinkles as it impacts
the dense shell, and the impact occurs in a piecemeal fashion, with some parts
of the shock wave interacting with the shell before the others. Therefore
different parts of the shell will `light-up' at different times. The
non-spherical nature of the interaction means that it will occur over a
prolonged period of time, and the spherical symmetry of the initial shock wave
is destroyed.Comment: 50 pages, 19 figures. Accepted to the Astrophysical Journal. For a
version with the original high-resolution color figures please download from
http://astro.uchicago.edu/~vikram/sncsm.htm
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