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

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