Study of sdO models. Pulsation Analysis

We have explored the possibility of driving pulsation modes in models of sdO stars in which the effects of element diffusion, gravitational settling and radiative levitation have been neglected so that the distribution of iron-peak elements remains uniform throughout the evolution. The stability of these models was determined using a non-adiabatic oscillations code. We analysed 27 sdO models from 16 different evolutionary sequences and discovered the first ever sdO models capable of driving high-radial order g-modes. In one model, the driving is by a classical kappa-mechanism due to the opacity bump from iron-peak elements at temperature ~200,000 K. In a second model, the driving result from the combined action of kappa-mechanisms operating in three distinct regions of the star: (i) a carbon-oxygen partial ionization zone at temperature ~2 10^6 K, (ii) a deeper region at temperature ~2 10^7 K, which we attribute to ionization of argon, and (iii) at the transition from radiative to conductive opacity in the core of the star.Comment: 13 pages, 19 figures, accepted for publication in MNRAS, 2009 September 1

Study of sdO models: mode trapping

We present the first description of mode trapping for sdO models. Mode trapping of gravity modes caused by the He/H chemical transition is found for a particular model, providing a selection effect for high radial order trapped modes. Low- and intermediate-radial order {\em p}-modes (mixed modes with a majority of nodes in the P-mode region) are found to be trapped by the C-O/He transition, but with no significant effects on the driving. This region seems to have also a subtle effect on the trapping of low radial order {\em g}-modes (mixed modes with a majority of nodes in the G-mode region), but again with no effect on the driving. We found that for mode trapping to have an influence on the driving of sdO modes (1) the mode should be trapped in a way that the amplitude of the eigenfunctions is lower in a damping region and (2) in this damping region significant energy interchange has to be produced.Comment: 10 pages, 13 figures, accepted for publication in MNRAS, 2009 December 1

Estudio de correlación y variabilidad de los métodos de inmersión en mercurio y el método capacitivo resonante para determinar densidad aparente de material cerámico crudo

En el siguiente trabajo, se presentan los resultados del estudio de correlación de las variables y parámetros asociados a la medición de la Densidad Aparente (DAP) para material cerámico crudo utilizado en la fabricación industrial de baldosas para piso, frente al método de inmersión en mercurio. Los anteriores resultados se obtuvieron teniendo en cuenta la aplicación de un modelo matemático al DAP como función no lineal de parámetros como la temperatura, permitividad eléctrica relativa y variables como la humedad relativa HR, voltaje y frecuencia de resonancia, medidos con un dispositivo electrónico diseñado en la Universidad Central por el grupo de investigación en Estado Sólido y Optoelectrónica ESSOPTO y utilizado en el Centro de Investigación en Materiales Cerámicos de la UFPS – CIMAC de la Universidad Francisco de Paula Santander – Cúcuta Colombia.Peer reviewe

A search for new hot subdwarf stars by means of Virtual Observatory tools

Hot subdwarf stars are faint, blue objects, and are the main contributors to the far-UV excess observed in elliptical galaxies. They offer an excellent laboratory to study close and wide binary systems, and to scrutinize their interiors through asteroseismology, as some of them undergo stellar oscillations. However, their origins are still uncertain, and increasing the number of detections is crucial to undertake statistical studies. In this work, we aim at defining a strategy to find new, uncatalogued hot subdwarfs. Making use of Virtual Observatory tools we thoroughly search stellar catalogues to retrieve multi-colour photometry and astrometric information of a known sample of blue objects, including hot subdwarfs, white dwarfs, cataclysmic variables and main sequence OB stars. We define a procedure to discriminate among these spectral classes, particularly designed to obtain a hot subdwarf sample with a low contamination factor. In order to check the validity of the method, this procedure is then applied to two test sky regions: the Kepler FoV and to a test region of around (RA:225, DEC:5) deg. As a result, we obtained 38 hot subdwarf candidates, 23 of which had already a spectral classification. We have acquired spectroscopy for three other targets, and four additional ones have an available SDSS spectrum, which we used to determine their spectral type. A temperature estimate is provided for the candidates based on their spectral energy distribution, considering two-atmospheres fit for objects with clear infrared excess. Eventually, out of 30 candidates with spectral classification, 26 objects were confirmed to be hot subdwarfs, yielding a contamination factor of only 13%. The high rate of success demonstrates the validity of the proposed strategy to find new uncatalogued hot subdwarfs. An application of this method to the entire sky will be presented in a forthcoming work.Comment: 13 pages, 7 figure

Pulsations in M dwarf stars

We present the results of the first theoretical non-radial non-adiabatic pulsational study of M dwarf stellar models with masses in the range 0.1 to 0.5M_solar. We find the fundamental radial mode to be unstable due to an \epsilon mechanism caused by deuterium (D-) burning for the young 0.1 and 0.2M_solar models, by non-equilibrium He^3 burning for the 0.2 and 0.25M_solar models of 10^4Myr, and by a flux blocking mechanism for the partially convective 0.4 and 0.5M_solar models once they reach the age of 500Myr. The periods of the overstable modes excited by the D-burning are in the range 4.2 to 5.2h for the 0.1M_solar models and is of order 8.4h for the 0.2M_solar models. The periods of the modes excited by He^3 burning and flux blocking are in the range 23 to 40min. The more massive and oldest models are more promising for the observational detection of pulsations, as their ratio of instability e-folding time to age is more favourable.Comment: 5 pages, 4 figures, 2 tables. Accepted for publication in MNRAS Letters on 2011 October 11 Version 2 is the accepted manuscript with changes in boldfac

1/fα1/f^\alpha noise and integrable systems

An innovative test for detecting quantum chaos based on the analysis of the spectral fluctuations regarded as a time series has been recently proposed. According to this test, the fluctuations of a fully chaotic system should exhibit 1/f noise, whereas for an integrable system this noise should obey the 1/f^2 power law. In this letter, we show that there is a family of well-known integrable systems, namely spin chains of Haldane-Shastry type, whose spectral fluctuations decay instead as 1/f^4. We present a simple theoretical justification of this fact, and propose an alternative characterization of quantum chaos versus integrability formulated directly in terms of the power spectrum of the spacings of the unfolded spectrum.Comment: 5 pages, 3 figures, RevTe