Asymptotic Giant Brach Stars as Astroparticle Laboratories

We show that the inclusion of axion emission during stellar evolution introduces important changes into the evolutionary behaviour of AGB stars. The mass of the resulting C/O white dwarf is much lower than the equivalent obtained from standard evolution. This implies a deficit in luminous AGB stars and in massive WDs. Moreover the total mass processed in the nuclear burning shells that is dredged-up to the surface ($3^{rd} D_{up}$) increases when axion emission is included, modifying the chemical composition of the photosphere. We conclude that the AGB is a promising phase to put constraints on particle physicsComment: 8 pages, 3 tables, 8 figures, accepted for publication in MNRA

Neural Network identification of halo white dwarfs

The white dwarf luminosity function has proven to be an excellent tool to study some properties of the galactic disk such as its age and the past history of the local star formation rate. The existence of an observational luminosity function for halo white dwarfs could provide valuable information about its age, the time that the star formation rate lasted, and could also constrain the shape of the allowed Initial Mass Functions (IMF). However, the main problem is the scarce number of white dwarfs already identified as halo stars. In this Letter we show how an artificial intelligence algorithm can be succesfully used to classify the population of spectroscopically identified white dwarfs allowing us to identify several potential halo white dwarfs and to improve the significance of its luminosity function.Comment: 15 pages, 3 postscript figures. Accepted for publication in ApJ Letters, uses aasms4.st

Detonations in white dwarf dynamical interactions

In old, dense stellar systems collisions of white dwarfs are a rather frequent phenomenon. Here we present the results of a comprehensive set of Smoothed Particle Hydrodynamics simulations of close encounters of white dwarfs aimed to explore the outcome of the interaction and the nature of the final remnants for different initial conditions. Depending on the initial conditions and the white dwarf masses, three different outcomes are possible. Specifically, the outcome of the interaction can be either a direct or a lateral collision or the interaction can result in the formation of an eccentric binary system. In those cases in which a collision occurs, the infalling material is compressed and heated such that the physical conditions for a detonation may be reached during the most violent phases of the merger. While we find that detonations occur in a significant number of our simulations, in some of them the temperature increase in the shocked region rapidly lifts degeneracy, leading to the quenching of the burning. We thus characterize under which circumstances a detonation is likely to occur as a result of the impact of the disrupted star on the surface of the more massive white dwarf. Finally, we also study which interactions result in bound systems, and in which ones the more massive white dwarf is also disrupted as a consequence of the dynamical interaction. The sizable number of simulations performed in this work allows to find how the outcome of the interaction depends on the distance at closest approach, and on the masses of the colliding white dwarfs, and which is the chemical pattern of the nuclearly processed material. Finally, we also discuss the influence of the masses and core chemical compositions of the interacting white dwarfs and the different kinds of impact in the properties of the remnants.Comment: 18 pages, 6 figures. Accepted for publication in MNRA

The effects of metallicity on the Galactic disk population of white dwarfs

It has been known for a long time that stellar metallicity plays a significant role in the determination of the ages of the different Galactic stellar populations, when main sequence evolutionary tracks are employed. Here we analyze the role that metallicity plays on the white dwarf luminosity function of the Galactic disk, which is often used to determine its age. We employ a Monte Carlo population synthesis code that accounts for the properties of the population of Galactic disk white dwarfs. Our code incorporates the most up-to-date evolutionary cooling sequences for white dwarfs with hydrogen-rich and hydrogen-deficient atmospheres for both carbon-oxygen and oxygen-neon cores. We use two different models to assess the evolution of the metallicity, one in which the adopted metallicity is constant with time, but with a moderate dispersion, and a second one in which the metallicity increases with time. We found that our theoretical results are in a very satisfactory agreement with the observational luminosity functions obtained from the Sloan Digital Sky Survey (SDSS) and from the SuperCOSMOS Sky Survey (SSS), independently of the adopted age-metallicity law. In particular, we found that the age-metallicity law has no noticeable impact in shaping the bright branch of the white dwarf luminosity function, and that the position of its cut-off is almost insensitive to the adopoted age-metallicity relationship. Because the shape of the bright branch of the white dwarf luminosity function is insensitive to the age-metallicity law, it can be safely employed to test the theoretical evolutionary sequences, while due to the limited sensitivity of the position of the drop-off to the distribution of metallicities, its location provides a robust indicator of the age of the Galactic disk.Comment: 7 pages, 5 figures, accepted for publication in A&

Prospects for Type Ia Supernova explosion mechanism identification with gamma rays

The explosion mechanism associated with thermonuclear supernovae (SNIa) is still a matter of debate. There is a wide agreement that high amounts of of radioactive nuclei are produced during these events and they are expected to be strong gamma-ray emitters. In the past, several authors have investigated the use of this gamma-ray emission as a diagnostic tool. In this paper we have done a complete study of the gamma-ray spectra associated with all the different scenarios currently proposed. This includes detonation, delayed detonation, deflagration and the off-center detonation. We have performed accurate simulations for this complete set of models in order to determine the most promising spectral features that could be used to discriminate among the different models. Our study is not limited to qualitative arguments. Instead, we have quantified the differences among the spectra and established distance limits for their detection. The calculations have been performed considering the best current response estimations of the SPI and IBIS instruments aboard INTEGRAL in such a way that our results can be used as a guideline to evaluate the capabilities of INTEGRAL in the study of type Ia supernovae. For the purpose of completeness we have also investigated the nuclear excitation and spallation reactions as a possible secondary source of gamma-rays present in some supernova scenarios. We conclude that this mechanism can be neglected due to its small contribution.Comment: 10 pages, 12 figures, LaTeX with MNRAS style file,accepted for publication in The Monthly Notices of the Royal Astron.So

Vortices of the Mediterranean Sea: An Altimetric Perspective

17 pages, 12 figures, 2 tablesThe presence of coherent vortices makes observed mesoscale fields of the ocean resemble two-dimensional turbulence. Using this analogy, a common definition of a coherent structure has been used to study the statistical properties of Mediterranean Sea vortices observed by satellite altimeters over a 7-yr period. A vortex has been defined as the simply connected region with values of the Okubo–Weiss parameter W < −0.2σW, where σW is the spatial standard deviation of W, and the same sign of vorticity. This definition is shown to be appropriate to detect and characterize, statistically, properties such as size, mean kinetic energy, and amplitude of vortices in the Mediterranean basin from sea level anomaly maps corresponding to the period from October 1992 to October 1999. The distribution of such properties for the Mediterranean vortices suggests a heuristic criterion to extract and select very coherent and long-lived vortices from the whole set of structures identified in altimetric maps. Such coherent vortices appear to be selected for amplitudes greater than 2σW, where the amplitude has been defined in terms of the Okubo–Weiss parameter rather than vorticity, and strongly correspond to those reported from observations with independent data. Systematic locating and tracking of such vortices provide, for the first time, a general picture of their preferential paths in the Mediterranean basin, which are characterized by complex but rather well defined patternsThis is a contribution to the IMAGEN project funded by the Spanish R+D Plan (REN2001-0802-C02-02) and MERSEA project funded by the European Union 6th Framework Program (AIP3-CT-2003-502885). Jordi Isern-Fontanet has been partially supported by contracts from IMAGEN and MERSEA projects. Altimetric maps for the period analyzed were elaborated and provided by CLS (Toulouse, France) under contract of the MATER project funded by the European Union MAST Program (MAS3-CT96-0051)Peer reviewe

White Dwarfs constrain Dark Forces

The white dwarf luminosity function, which provides information about their cooling, has been measured with high precision in the past few years. Simulations that include well known Standard Model physics give a good fit to the data. This leaves little room for new physics and makes these astrophysical objects a good laboratory for testing models beyond the Standard Model. It has already been suggested that white dwarfs might provide some evidence for the existence of axions. In this work we study the constraints that the white dwarf luminosity function puts on physics beyond the Standard Model involving new light particles (fermions or bosons) that can be pair-produced in a white dwarf and then escape to contribute to its cooling. We show, in particular, that we can severely constrain the parameter space of models with dark forces and light hidden sectors (lighter than a few tens of keV). The bounds we find are often more competitive than those from current lab searches and those expected from most future searches.Comment: 25 pages, 8 figures, equivalent to published versio