Articles de revista
http://hdl.handle.net/2117/1137
2015-09-05T15:17:40ZSpiral instability can drive thermonuclear explosions in binary white dwarf mergers
http://hdl.handle.net/2117/28120
Spiral instability can drive thermonuclear explosions in binary white dwarf mergers
kashyap, Rahul; Fisher, Robert T.; García-Berro Montilla, Enrique; Aznar Siguan, Gabriela; Ji, Suoqing; Loren Aguilar, Pablo
Thermonuclear, or Type Ia supernovae (SNe Ia), originate from the explosion of carbon-oxygen white dwarfs, and serve as standardizable cosmological candles. However, despite their importance, the nature of the progenitor systems that give rise to SNe Ia has not been hitherto elucidated. Observational evidence favors the double-degenerate channel in which merging white dwarf binaries lead to SNe Ia. Furthermore, significant discrepancies exist between observations and theory, and to date, there has been no self-consistent merger model that yields a SNe Ia. Here we show that a spiral mode instability in the accretion disk formed during a binary white dwarf merger leads to a detonation on a dynamical timescale. This mechanism sheds light on how white dwarf mergers may frequently yield SNe Ia.
2015-02-10T00:00:00ZEstrategias e indicadores para la evaluación de la docencia en el marco del EEES
http://hdl.handle.net/2117/28115
Estrategias e indicadores para la evaluación de la docencia en el marco del EEES
García-Berro Montilla, Enrique; Dapia de Dios, Francesc; Amblas Marondo, Gemma; Bugeda Castelltort, Gabriel; Roca Martín, Santiago
En este artículo se describen las estrategias que ha emprendido la
Universidad Politécnica de Cataluña (UPC) para implantar las nuevas titulaciones
de Ingeniería adaptadas al Espacio Europeo de Enseñanza Superior (EEES). Estas
estrategias pueden ser de utilidad para otras universidades. Partiendo de un análisis
detallado del contexto general de la Universidad española y de la UPC en
particular, se muestra el sistema de planificación estratégica de nuestra institución.
Asimismo se describe en detalle el sistema de indicadores estadísticos que se usa
para medir el rendimiento de centros y departamentos en el referido marco de
planificación estratégica. Por último, también se detallan las acciones que se han
emprendido para evaluar la actividad docente del profesorado.
2009-01-01T00:00:00ZThe variation of the fine structure constant: testing the dipole model with thermonuclear supernovae
http://hdl.handle.net/2117/28112
The variation of the fine structure constant: testing the dipole model with thermonuclear supernovae
Kraiselburd, L; Landau, S. J.; García-Berro Montilla, Enrique; Negrelli, C
The large-number hypothesis conjectures that fundamental constants may vary. Accordingly, the space-time variation of fundamental constants has been an active subject of research for decades. Recently, using data obtained with large telescopes a phenomenological model in which the fine structure constant might vary spatially has been proposed. We test whether this hypothetical spatial variation of alpha, which follows a dipole law, is compatible with the data of distant thermonuclear supernovae. Unlike previous works, in our calculations we consider not only the variation of the luminosity distance when a varying alpha is adopted, but we also take into account the variation of the peak luminosity of Type Ia supernovae resulting from a variation of alpha. This is done using an empirical relation for the peak bolometric magnitude of thermonuclear supernovae that correctly reproduces the results of detailed numerical simulations. We find that there is no significant difference between the several phenomenological models studied here and the standard one, in which alpha does not vary spatially. We conclude that the present set of data of Type Ia supernovae is not able to distinguish the standard model from the dipole models, and thus cannot be used to discard nor to confirm the proposed spatial variation of alpha.
2015-05-01T00:00:00ZConstraining the neutrino magnetic dipole moment from white dwarf pulsations
http://hdl.handle.net/2117/28064
Constraining the neutrino magnetic dipole moment from white dwarf pulsations
Corsico, Alejandro H.; Althaus, Leandro G.; Miller Bartolami, Marcelo M.; Kepler, S.O.; García-Berro Montilla, Enrique
Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment (mu(nu)) using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. We employ state-of-the-art evolutionary and pulsational codes which allow us to perform a detailed asteroseismological period fit based on fully DB white dwarf evolutionary sequences. Plasmon neutrino emission is the dominant cooling mechanism for this class of hot pulsating white dwarfs, and so it is the main contributor to the rate of change of period with time ((Pi) over dot) for the DBV class. Thus, the inclusion of an anomalous neutrino emission through a non-vanishing magnetic dipole moment in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DBV stars. By comparing the theoretical ((Pi) over dot) value with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment with an upper limit of mu(nu) less than or similar to 10(-11) mu(B). This bound is somewhat less restrictive than, but still compatible with, other limits inferred from the white dwarf luminosity function or from the color-magnitude diagram of the Globular cluster M5. Further improvements of the measurement of the rate of period change of the dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.
2014-08-01T00:00:00ZThe white dwarf cooling sequence of 47 Tucanae
http://hdl.handle.net/2117/28063
The white dwarf cooling sequence of 47 Tucanae
García-Berro Montilla, Enrique; Torres Gil, Santiago; Althaus, Leandro G.; Miller Bartolami, Marcelo M.
Context. 47 Tucanae is one of the most interesting, well-observed, and theoretically studied globular clusters. This allows us to determine the reliability of our understanding of white dwarf cooling sequences, to compare different methods of determining its age, and to assess other important characteristics, such as its star formation history. Aims. Here we present a population synthesis study of the cooling sequence of the globular cluster 47 Tucanae. In particular, we study the distribution of effective temperatures, the shape of the color-magnitude diagram, and the corresponding magnitude and color distributions. Methods. To do this, we used an up-to-date population synthesis code based on Monte Carlo techniques that incorporates the most recent and reliable cooling sequences and an accurate modeling of the observational biases. Results. Our theoretical models and the observed data agree well. Thus, our study disproves previous claims that there are still missing physics in the white dwarf cooling models at moderately high effective temperatures. We also derive the age of the cluster using the termination of the cooling sequence and obtain a good agreement with the age determinations made using the main-sequence turn-off. Finally, the star formation history of the cluster is compatible with that obtained using main-sequence stars, which predicts two distinct populations. Conclusions. We conclude that a correct modeling of the white dwarf population of globular clusters, used in combination with the number counts of main-sequence stars, provides a unique tool to model the properties of globular clusters.
2014-11-01T00:00:00ZConstraining the double-degenerate scenario for Type Ia supernovae from merger ejected matter
http://hdl.handle.net/2117/27814
Constraining the double-degenerate scenario for Type Ia supernovae from merger ejected matter
Levanon, Naveh; Soker, Noam; García-Berro Montilla, Enrique
We follow the mass expelled during the WD-WD merger process in a particular case of the double-degenerate (DD) scenario for Type Ia supernovae (SNe Ia), and find that the interaction of the SN ejecta with the resulting wind affects the early (first day) light-curve in a way that may be in conflict with some SN Ia observations, if the detonation occurs shortly after the merger, i.e. (10(3) s less than or similar to t(exp) less than or similar to 1 d). The main source of the expelled mass is a disc-wind, or jets that are launched by the accretion disc around the more massive white dwarf (WD) during the viscous phase of the merger. This disc-originated matter will be shocked and heated by the SN ejecta from an explosion, leading to additional radiation in the early light-curve. This enhanced early radiation could then be interpreted as an explosion originating from a progenitor having an inferred radius of one solar radius or more, in conflict with observations of SN 2011fe.
2015-03-01T00:00:00ZWhite dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
http://hdl.handle.net/2117/27813
White dwarf evolutionary sequences for low-metallicity progenitors: The impact of third dredge-up
Althaus, Leandro G.; Camisassa, Maria E; Miller Bertolami, Marcelo M; Corsico, Alejandro H.; García-Berro Montilla, Enrique
Context. White dwarfs are nowadays routinely used as reliable cosmochronometers, allowing several stellar populations to be dated.; Aims. We present new white dwarf evolutionary sequences for low-metallicity progenitors. This is motivated by the recent finding that residual H burning in low-mass white dwarfs resulting from Z = 0.0001 progenitors is the main energy source over a significant part of their evolution.; Methods. White dwarf sequences have been derived from full evolutionary calculations that take the entire history of progenitor stars into account, including the thermally pulsing and the post-asymptotic giant branch (AGB) phases.; Results. We show that for progenitor metallicities in the range 0.00003 less than or similar to Z less than or similar to 0.001, and in the absence of carbon enrichment from the occurrence of a third dredge-up episode, the resulting H envelope of the low-mass white dwarfs is thick enough to make stable H burning the most important energy source even at low luminosities. This has a significant impact on white dwarf cooling times. This result is independent of the adopted mass-loss rate during the thermally-pulsing and post-AGB phases and in the planetary nebulae stage.; Conclusions. We conclude that in the absence of third dredge-up episodes, a significant part of the evolution of low-mass white dwarfs resulting from low-metallicity progenitors is dominated by stable H burning. Our study opens the possibility of using the observed white dwarf luminosity function of low-metallicity globular clusters to constrain the efficiency of third dredge up episodes during the thermally-pulsing AGB phase of low-metallicity progenitors.
2015-04-01T00:00:00Z20 anys de "Física i ciència-ficció"
http://hdl.handle.net/2117/27465
20 anys de "Física i ciència-ficció"
Moreno Lupiáñez, Manuel; José Pont, Jordi
2014-06-01T00:00:00ZAn efficient multi-step iterative method for computing the numerical solution of systems of nonlinear equations associated with ODEs
http://hdl.handle.net/2117/26312
An efficient multi-step iterative method for computing the numerical solution of systems of nonlinear equations associated with ODEs
Ullah, Malik Zaka; Serra Capizzano, Stefano; Ahmad, Fayyaz
We developed multi-step iterative method for computing the numerical solution of nonlinear systems, associated with ordinary differential equations (ODEs) of the form L(x(t)) + f(x(t)) = g(t) : here L(.) is a linear differential operator and f(.) is a nonlinear smooth function. The proposed iterative scheme only requires one inversion of Jacobian which is computationally very efficient if either LU-decomposition or GMRES-type methods are employed. The higher-order Frechet derivatives of the nonlinear system stemming from the considered ODEs are diagonal matrices. We used the higher-order Frechet derivatives to enhance the convergence-order of the iterative schemes proposed in this note and indeed the use of a multi-step method dramatically increases the convergence-order. The second-order Frechet derivative is used in the first step of an iterative technique which produced third-order convergence. In a second step we constructed matrix polynomial to enhance the convergence-order by three. Finally, we freeze the product of a matrix polynomial by the Jacobian inverse to generate the multi-step method. Each additional step will increase the convergence-order by three, with minimal computational effort. The convergence-order (CO) obeys the formula CO = 3m, where m is the number of steps per full-cycle of the considered iterative scheme. Few numerical experiments and conclusive remarks end the paper.
2015-01-01T00:00:00ZUnderground study of the 17O(p,alpha)18F reaction relevant for explosive hydrogen burning
http://hdl.handle.net/2117/25055
Underground study of the 17O(p,alpha)18F reaction relevant for explosive hydrogen burning
Di Leva, A; Scott, D.A.; Caciolli, A.; Formicola, A.; Strieder, F.; Aliotta, M.; Anders, M.; Bemmerer, D.; Broggini, C.; Corvisiero, P.; Elekes, Z.; Fulop, Zs.; Gervino, G.; Guglielmetti, G.; Gustavino, C.; Gyurky, Gy.; Imbriani, G.; José Pont, Jordi; Junker, M.; et al., .
The 17O(p,γ )18F reaction affects the production of key isotopes (e.g., 18F and 18O) in the explosive hydrogen burning that powers classical novae. Under these explosive conditions, the reaction rate is dominated by contributions from a narrow resonance at Ec.m.= 183 keV and by the combined contributions of direct capture
and low-energy tails of broad resonances. At present, the astrophysical reaction rate is not well constrained because of the lack of data in the energy region appropriate to classical novae.
2014-01-01T00:00:00Z