A new approach to the study of quasi-normal modes of rotating stars

We propose a new method to study the quasi-normal modes of rotating relativistic stars. Oscillations are treated as perturbations in the frequency domain of the stationary, axisymmetric background describing a rotating star. The perturbed quantities are expanded in circular harmonics, and the resulting 2D-equations they satisfy are integrated using spectral methods in the (r,theta)-plane. The asymptotic conditions at infinity, needed to find the mode frequencies, are implemented by generalizing the standing wave boundary condition commonly used in the non rotating case. As a test, the method is applied to find the quasi-normal mode frequencies of a slowly rotating star.Comment: 24 pages, 7 figures, submitted to Phys. Rev.

Vico o la monumentale memoria delle tenebre

Traducción del italiano por Jéssica Sánchez EspillaqueMi experiencia de Vico comienza con la sorpresa ocasionada por el poder expresado en la imagen del frontispicio. Es ahí donde aprendí de la importancia de la historia ideal eterna, y acerca de la oculta historia del asunto, la repetición y el procesamiento de la memoria, la fuerza generadora de la oscuridad, el injustificable significado del principio, la mutable acción del sentido.My experience of Vico begins with the surprise arisen from the power expressed by the frontispiece picture. This is where I learned about the importance of ideal eternal history, and about the hidden history of matter, the repetition and processing of memory, the generating force of the darkness, the unjustifiable meaning of the principle, the mutable action of sense.La mia esperienza dello studio di Vico inizia con la sorpresa generata dal potere espresso dalla dipintura iniziale. Qui ho appreso l’importanza della storia ideale eterna e della storia nascosta della materia, la ripetizione ed elaborazione della memoria, la forza generante delle tenebre, il significato ingiustificabile del principio, l’azione mutabile del senso

Formation of Dark Matter Haloes in a Homogeneous Dark Energy Universe

Several independent cosmological tests have shown evidences that the energy density of the Universe is dominated by a dark energy component, which cause the present accelerated expansion. The large scale structure formation can be used to probe dark energy models, and the mass function of dark matter haloes is one of the best statistical tools to perform this study. We present here a statistical analysis of mass functions of galaxies under a homogeneous dark energy model, proposed in the work of Percival (2005), using an observational flux-limited X-ray cluster survey, and CMB data from WMAP. We compare, in our analysis, the standard Press-Schechter (PS) approach (where a Gaussian distribution is used to describe the primordial density fluctuation field of the mass function), and the PL (Power Law) mass function (where we apply a nonextensive q-statistical distribution to the primordial density field). We conclude that the PS mass function cannot explain at the same time the X-ray and the CMB data (even at 99% confidence level), and the PS best fit dark energy equation of state parameter is $\omega=-0.58$, which is distant from the cosmological constant case. The PL mass function provides better fits to the HIFLUGCS X-ray galaxy data and the CMB data; we also note that the $\omega$ parameter is very sensible to modifications in the PL free parameter, $q$, suggesting that the PL mass function could be a powerful tool to constrain dark energy models.Comment: 4 pages, 2 figures, Latex. Accepted for publication in the International Journal of Modern Physics D (IJMPD)

La post-modernità e il soggetto del racconto

La modernità ha elaborato nozioni forti di soggettività esposte in vari modelli, dal razionalismo all’idealismo, in grado di confrontarsi con il mutare delle epoche e delle culture. Il postmoderno si caratterizza invece come l’epoca che mette in crisi il soggetto moderno e che non crede più nella capacità della storia di conferire un senso all’esistenza e un orientamento alle azioni. In particolare il postmoderno ha costretto a ripensare la nozione di persona, non più concepibile come unità di sostanza, uguaglianza o convenzione. In tale contesto si inserisce la proposta di Ricoeur di ripensare la nozione di identità tenendo conto delle sue formulazioni storiche e anche delle critiche a cui il postmoderno ha sottoposto ogni classica nozione di soggetto. Viene quindi indicata una nozione di identità narrativa che rimane fissata nelle storie personali e nelle loro relazioni all’interno di una storia comune, diventando una categoria di ontologia della prassi. L’identità umana si esprime perciò come permanenza nel tempo, fedeltà alla parola data, partecipazione a una origine comune che permette esperienze e scelte responsabili nei racconti che è in grado di generare e condividere

El recurso de las cosas humanas y la anáfora del "retornaron"

El ensayo se propone demostrar que la teoría del “recurso” no es atribuible ni a una visión cíclica ni a una visión lineal de la historia. Lo que Vico se propone es alcanzar una integración de las dos perspectivas clásicas. De esta manera, él se aparta de una abstracta filosofía general de la historia e inicia, en cambio, una investigación sobre lo que resulta constante en la experiencia de las cosas humanas. En este sentido, el ricorso es una posibilidad a la que la providencia se agarra para garantizar la continuidad de la humanidad, un remedio en épocas de crisis y de decadencia. Sin embargo, principalmente el “recurso” es el modo en que el principio de todo “curso”, que permanece oculto o diluido en el curso mismo, reaparece con nitidez en su descarte de toda declinación temporal y en su posible duplicidad de generación y de corrupción.This essay aims to show that the theory of “ricorso” is not attributable either to a cyclic or to a lineal conception of history. What Vico aims at is achieving an integration of those two classical perspectives. In this way, he abandons an abstract general philosophy of history and starts, in contrast, an inquiry into what remains permanent in the experience of human affairs. In this sense, the ricorso is a possibility that providence avails in order to guarantee the continuity of humanity, a remedy in times of crisis and decadence. Nevertheless, the “ricorso” is principally the way in which the principle of all “corso”, which remains hidden or diluted in the course itself, reappears clearly in its abandonment of all temporal declination, and in its possible duplicity of generation and corruption

Constraining modified theories of gravity with gravitational wave stochastic background

The direct discovery of gravitational waves has finally opened a new observational window on our Universe, suggesting that the population of coalescing binary black holes is larger than previously expected. These sources produce an unresolved background of gravitational waves, potentially observables by ground-based interferometers. In this paper we investigate how modified theories of gravity, modeled using the ppE formalism, affect the expected signal, and analyze the detectability of the resulting stochastic background by current and future ground-based interferometers. We find the constraints that AdLIGO would be able to set on modified theories, showing that they may significantly improve the current bounds obtained from astrophysical observations of binary pulsars.Comment: Results updated to match the version accepted on Phys. Rev. Let

Where does galactic dust come from?

Here we investigate the origin of the dust mass (Mdust) observed in the Milky Way (MW) and of dust scaling relations found in a sample of local galaxies from the DGS and KINGFISH surveys. To this aim, we model dust production from Asymptotic Giant Branch (AGB) stars and supernovae (SNe) in simulated galaxies forming along the assembly of a Milky Way-like halo in a well resolved cosmic volume of 4cMpc using the GAMESH pipeline. We explore the impact of different sets of metallicity and mass-dependent AGB and SN dust yields on the predicted Mdust. Our results show that models accounting for grain destruction by the SN reverse shock predict a total dust mass in the MW that is a factor of ~4 lower than observed, and can not reproduce the observed galaxy-scale relations between dust and stellar masses, and dust-to-gas ratios and metallicity, with a smaller discrepancy in galaxies with low metallicity (12 + log(O/H) < 7.5) and low stellar masses (Mstar < 10^7 Msun). In agreement with previous studies, we suggest that competing processes in the interstellar medium must be at play to explain the observed trends. Our result reinforces this conclusion by showing that it holds independently of the adopted AGB and SN dust yields.Comment: 6 pages, 4 figures. Accepted version for publication in MNRA

The origin of the most iron-poor star

We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting from the recently discovered $\rm [Fe/H]<-7.1$ star SMSS J031300 (Keller et al. 2014). We show that the elemental abundances observed on the surface of SMSS J031300 can be well fit by the yields of faint, metal free, supernovae. Using properly calibrated faint supernova explosion models, we study, for the first time, the formation of dust grains in such carbon-rich, iron-poor supernova ejecta. Calculations are performed assuming both unmixed and uniformly mixed ejecta and taking into account the partial destruction by the supernova reverse shock. We find that, due to the paucity of refractory elements beside carbon, amorphous carbon is the only grain species to form, with carbon condensation efficiencies that range between (0.15-0.84), resulting in dust yields in the range (0.025-2.25)M$_{\odot}$. We follow the collapse and fragmentation of a star forming cloud enriched by the products of these faint supernova explosions and we explore the role played by fine structure line cooling and dust cooling. We show that even if grain growth during the collapse has a minor effect of the dust-to-gas ratio, due to C depletion into CO molecules at an early stage of the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could be triggered by dust cooling and fragmentation. A comparison between model predictions and observations of a sample of C-normal and C-rich metal-poor stars supports the idea that a single common pathway may be responsible for the formation of the first low-mass stars.Comment: 14 pages, 8 figures, accepted for publication in ApJ. Rephrased sentence in section 5 to avoid text overlap with arXiv:1307.2239 in their model descriptio

The metal and dust yields of the first massive stars

We quantify the role of Population (Pop) III core-collapse supernovae (SNe) as the first cosmic dust polluters. Starting from a homogeneous set of stellar progenitors with masses in the range [13 - 80] Msun, we find that the mass and composition of newly formed dust depend on the mixing efficiency of the ejecta and the degree of fallback experienced during the explosion. For standard Pop III SNe, whose explosions are calibrated to reproduce the average elemental abundances of Galactic halo stars with [Fe/H] < -2.5, between 0.18 and 3.1 Msun (0.39 - 1.76 Msun) of dust can form in uniformly mixed (unmixed) ejecta, and the dominant grain species are silicates. We also investigate dust formation in the ejecta of faint Pop III SN, where the ejecta experience a strong fallback. By examining a set of models, tailored to minimize the scatter with the abundances of carbon-enhanced Galactic halo stars with [Fe/H ] < -4, we find that amorphous carbon is the only grain species that forms, with masses in the range 2.7 10^{-3} - 0.27 Msun (7.5 10^{-4} - 0.11 Msun) for uniformly mixed (unmixed) ejecta models. Finally, for all the models we estimate the amount and composition of dust that survives the passage of the reverse shock, and find that, depending on circumstellar medium densities, between 3 and 50% (10 - 80%) of dust produced by standard (faint) Pop III SNe can contribute to early dust enrichment.Comment: Accepted by MNRAS, 22 pages, 12 figures, 12 table