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Search Results: 1 - 10 of 191320 matches for " D. Bahena "
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The final fate of very massive first stars
Klapp, J;Bahena, D;
Revista mexicana de física , 2008,
Abstract: a generation of stars which formed from primordial nearly pure h/he gas, the so-called first stars or population iii stars, must have existed since heavy elements can only be synthesized in the interior of the stars. these stars were responsible for the initial heavy elements enrichment of the intergalactic medium. in this work we analyze the possible outcome of the evolution of very massive population iii stars and whether their final fate can avoid the pair instability supernovae explosion. we have recently calculated the evolution and nucleosynthesis of mass losing very massive population iii stars during the hydrogen and helium burning phases, and proposed a new scenario for the first stars in the universe. according to this scenario, the first stars were born very massive, but evolve with mass loss, and its possible endpoint is a hypernovae stage. at low metallicity the effects on the presupernova structure depends on the initial mass and the mass loss rate during the main sequence evolution. presupernova stars of lower metallicity have different characteristics depending if they are galactic or pregalactic, and if they evolve without or with mass loss. when these stars evolve with mass loss, their convective core size increase and their helium- or carbon-oxygen core mass decreases. then, the stars could explode like hypernovae or supernovae.
The final fate of very massive first stars
J. Klapp,D. Bahena
Revista mexicana de física , 2008,
Abstract: Una primera generación de estrellas formada de gas primordial casi puro de H y He, las llamadas primeras estrellas o estrellas de Población III, debió haber existido ya que los elementos pesados solo pueden ser sintetizados en el interior de las estrellas. Estas estrellas fueron responsables del enriquecimiento inicial del medio intergaláctico en elementos pesados. En este trabajo, analizamos el posible desenlace de la evolución de estrellas muy masivas de la Población III y cuando, su destino final, puede evitar la explosión de supernova por inestabilidad de pares. Recientemente hemos calculado la evolución y nucleosíntesis de estrellas muy masivas de población III con pérdida de masa durante los quemados de hidrógeno y helio, y propuesto un nuevo escenario para las primeras estrellas del universo. De acuerdo a este escenario, las primeras estrellas nacieron muy masivas pero evolucionaron con perdida de masa terminando su vida como hipernovas. A baja metalicidad los efectos de la estructura presupernova depende de la masa inicial y de la tasa de perdida de masa durante la secuencia principal. Las estrellas de presupernova de baja metalicidad tienen diferentes características dependiendo de si son galácticas o pregalácticas y si evolucionan con o sin pérdida de masa. Cuando evolucionan con pérdida de masa, el tama o de su núcleo convectivo aumenta y las masas de sus nucleos de helio o de carbono-oxígeno disminuyen. Por consiguiente, estas estrellas podrían explotar como supernovas o hipernovas.
First Stars. I. Evolution without mass loss
D. Bahena,J. Klapp
Physics , 2010, DOI: 10.1007/s10509-010-0322-1
Abstract: The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M > 100 Mo. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300 Mo galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10E-6 and 10E-9, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low metallicity massive stars are hotter and more compact and luminous than their metal enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have an important influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.
Electrochemical Noise Chaotic Analysis of NiCoAg Alloy in Hank Solution
D. Bahena,I. Rosales,O. Sarmiento,R. Guardián,C. Menchaca,J. Uruchurtu
International Journal of Corrosion , 2011, DOI: 10.1155/2011/491564
Abstract: The potential and current oscillations during corrosion of NiCoAg alloy in Hank solution were studied. Detailed nonlinear fractal analyses were used to characterize complex time series clearly showing that the irregularity in these time series corresponds to deterministic chaos rather than to random noise. The chaotic oscillations were characterized by power spectral densities, phase space, and Lyapunov exponents. Electrochemical impedance was also applied the fractal dimensions for the corroded surface was obtained, and a corrosion mechanism was proposed. 1. Introduction The wide diversity and sophistication of materials currently used in medicine and biotechnology are a testimony to the significant technological advances which have occurred over the past 25 years; metallic implant materials have a significant economic and clinical impact on the biomaterials field. The demand for such medical device implants is expected to increase in the coming years. Stainless steels, cobalt, titanium and their alloys are widely used in the production of artificial joints and fixation devices where both titanium, and cobalt alloys demonstrate good biocompatibility. Many other alloys are being developed to improve performance and to lower their costs [1, 2]. For such new materials little is known about their electrochemical characteristics and dynamic corrosion behaviour. Spontaneous electrochemical potential and current oscillations generated by corrosion reactions are known as electrochemical noise (EN). These oscillations are commonly observed in many electrochemical processes, such as electropolishing, passivation, active and different forms of localized corrosion, which are strongly influenced by the growth and the dynamic film breakdown and repassivation events of the anodic film. For the interpretation of electrochemical noise measurements (ENMs) data, different approaches have been proposed for processing the experimental data records: visual, statistical, spectral, and wavelet transform-based analysis, and the less common chaos-fractal non-linear dynamic systems analysis. Reviews of the measurements interpretation of electrochemcal noise describing these analysis and their practical applications associated with corroding metals can be found in the literature [3–6]. Cottis [6] questions the usefulness of chaos theory regarding electrochemical noise analysis. Legat et al. [7, 8], and others carrying out chaotic analysis of electrochemical noise in various corrosion processes, concluded that the chaotic analysis of measured electrochemical noise can help to
Nucleosynthesis constraints on the First Stars
Klapp, J;Bahena, D;Corona-Galindo, M.G;Dehnen, H;Galindo, S;
Revista mexicana de física , 2006,
Abstract: star formation and accretion calculations have recently suggested that the first stars are composed of very massive stars (vms). on the other hand, very massive supernovae (sn) explosion calculations have suggested that the vms hypothesis is inconsistent with abundance determinations in extremely metal poor (emp) stars. as an alternative scenario, we propose that the first stars are born very massive but their mass reduced to the massive star (ms) range during their hydrogen and helium burning phases. in this paper we present some details of zero age main sequence (zams) models and evolutionary calculations of a mass losing 5oom pop iii star. the results indicate that vms have very high effective temperatures, a large ratio of radiation to total pressure and a luminosity close to the eddington luminosity and hence, we expect them to have significant radiation driven winds. for conservative evolution our evolutionary tracks are similar to those found in the literature but with the introduction of mass loss the evolution changes strongly and we have shown that vms can reduce its mass to the ms range if the mass loss parameter n is equal or greater than ~ 300. we have estimated the total amount of matter ejected through winds until the end of the helium burning phase. the proposed scenario suggests that the first stars are born vms but transformed into ms during their hydrogen and helium burning phases, end as black holes or hypernovae producing the fe-rich and r-poor abundances observed in emp stars, and that could be connected to low redshift gamma-ray bursts and the reionization of the universe.
Area selection for conservation of Mexican mammals
L. B. Vázquez,C. G. Bustamante–Rodríguez,D. G. Bahena Arce
Animal Biodiversity and Conservation , 2009,
Abstract: Area selection for conservation of Mexican mammalsThree sets of priority cells for mammal conservation in Mexico were identified using distributional data. A complementarity approach was implemented through linear integer programming. The minimum set of sites required for the representation of each mammal species varied between 38 (5.4%) grid cells for at least one occurrence, 110 (15.6%) grid cells for at least three occurrences, and 173 (24.5%) grid cells for at least five occurrences. The complementary analyses mainly highlighted three regions of particular concern for mammal conservation in Mexico: (i) the trans–Mexican Volcanic Belt and natural provinces of the Pacific Coast, (ii) Sierra Madre del Sur and the Highlands of Chiapas, and (iii) the northern portion of the Sierra Madre Occidental. The results reported here did not indicate absolute priority locations for conservation activities, but rather identified locations warranting further investigation at finer resolutions more appropriate to such activity.
Nucleosynthesis constraints on the First Stars
J. Klapp,D. Bahena,M.G. Corona-Galindo,H. Dehnen
Revista mexicana de física , 2006,
Abstract: Cálculos recientes de formación estelar y acreción han sugerido que las primeras estrellas están compuestas de Estrellas Muy Masivas (VMS). Por otro lado, cálculos de explosiones de supernovas muy masivas (SN) han sugerido que la hipótesis VMS es inconsistente con determinaciones de abundancias en estrellas Extremadamente Pobres en Metales (EMP). Como un escenario alternativo proponemos que las primeras estrellas nacen como muy masivas, pero su masa es reducida al rango de las estrellas masivas (MS) durante sus fases de quemado de hidrógeno y helio. En este artículo presentamos algunos detalles de modelos de edad cero de la secuencia principal (ZAMS) y cálculos evolutivos de una estrella Pop III de 5OOM con pérdida de masa. Los resultados indican que las VMS tienen temperaturas efectivas muy altas, una alta razón de la presión de radiación a la presión total y una luminosidad cercana a la luminosidad de Eddington y por lo tanto, esperamos que estas estrellas tengan vientos significativos impulsados por radiación. Para evolución conservativa nuestras trayectorias evolutivas son similares a las encontradas en la literatura, pero cuando se introduce pérdida de masa, la evolución cambia fuertemente y hemos demostrado que las VMS pueden reducir su masa al rango de las MS si el parámetro TV de pérdida de masa es igual o mayor que ~ 300. Hemos estimado la cantidad total de materia expulsada a traves de vientos hasta el final de la fase de quemado de helio. El escenario propuesto sugiere que las primeras estrellas nacen como VMS pero son transformadas en MS durante las fases de quemado de hidrógeno y helio, acaban como agujeros negros o hipernovas produciendo las abundancias ricas en Fe y pobres en elementos r que son observadas en estrellas EMP, y que podrían estar relacionadas con ráfagas de rayos gamma con corrimiento al rojo bajo y la reionización del Universo.
First Stars. II. Evolution with mass loss
David Bahena,Petr Hadrava
Physics , 2011, DOI: 10.1007/s10509-011-0898-0
Abstract: The first stars are assumed to be predominantly massive. Although, due to the low initial abundances of heavy elements the line-driven stellar winds are supposed to be inefficient in the first stars, these stars may loose a significant amount of their initial mass by other mechanisms. In this work, we study the evolution with a prescribed mass loss rate of very massive, galactic and pregalactic, Population III stars, with initial metallicities $Z=10^{-6}$ and $Z=10^{-9}$, respectively, and initial masses 100, 120, 150, 200, and 250$\,M_{\odot}$ during the hydrogen and helium burning phases. The evolution of these stars depends on their initial mass, metallicity and the mass loss rate. Low metallicity stars are hotter, compact and luminous, and they are shifted to the blue upper part in the Hertzprung-Russell diagram. With mass loss these stars provide an efficient mixing of nucleosynthetic products, and depending on the He-core mass their final fate could be either pair-instability supernovae or energetic hypernovae. These stars contributed to the reionization of the universe and its enrichment with heavy elements, which influences the subsequent star formation properties.
Regulación del ciclo celular y desarrollo de cáncer: perspectivas terapéuticas
Salud Pública de México , 1997, DOI: 10.1590/S0036-36341997000500008
Abstract: several genetic alterations occur during the transformation process from normal to tumor cells, that involve the loss of fidelity of processes as replication, reparation, and segregation of the genomic material. although normal cells have defense mechanisms against cancer progression, in tumor cells different escape pathways are activated leading to tumor progression. recent advances have permitted cancer research to focus on the identification of some of its etiological factors. the knowledge of cell cycle reveals a precise mechanism achieved by the coordinated interactions and functions of cyclin-dependent kinases, control checkpoint, and repair pathways. furthermore, it has been demonstrated that this coordinated function can be abrogated by specific genetic changes. these findings suggest that the molecular mechanisms responsible for cellular transformation may help to identify potential targets to improve cancer therapies.
Regulación del ciclo celular y desarrollo de cáncer: perspectivas terapéuticas
Salud Pública de México , 1997,
Abstract: Durante el proceso de transformación de las células normales a células cancerosas, ocurren varias alteraciones genéticas. En este proceso se presenta la pérdida del control de los mecanismos de replicación y reparación del ADN, así como de la segregación del material genético. Aunque las células normales tienen estrategias de defensa contra el desarrollo del cáncer, las células tumorales activan diferentes vías de escape que permiten la progresión de la neoplasia. Avances recientes han permitido enfocar la investigación del cáncer hacia la identificación de algunos de sus factores etiológicos. El estudio del ciclo celular y su regulación han permitido conocer cómo la fidelidad y la integridad de la replicación del genoma son mantenidas por las funciones coordinadas de los puntos de control y de los sistemas de reparación del ADN. El funcionamiento adecuado de estos procesos puede ser alterado por mutaciones genéticas. Estos hallazgos sugieren que los mecanismos moleculares de regulación que participan en la transformación celular pueden ser empleados como sistemas potenciales para instrumentar nuevas terapias contra el desarrollo del cáncer.
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