Effects of nuclear molecular configurations on the astrophysical S-factor for 16^{16}O + 16^{16}O

The impact of nuclear molecular configurations on the astrophysical S-factor for $^{16}$O + $^{16}$O is investigated within the realistic two-center shell model based on Woods-Saxon potentials. These molecular effects refer to the formation of a neck between the interacting nuclei and the radial dependent collective mass parameter. It is demonstrated that the former is crucial to explain the current experimental data with high accuracy and without any free parameter, whilst in addition the latter predicts a pronounced maximum in the S-factor. In contrast to very recent results by Jiang et al., the S-factor does not decline towards extremely low values as energy decreases.Comment: In press in Physics Letters

Conformal mapping of ultrasonic crystals: confining ultrasound and cochlear-like wave guiding

Conformal mapping of a slab of a two-dimensional ultrasonic crystal generate a closed geometrical arrangement of ultrasonic scatterers with appealing acoustic properties. This acoustic shell is able to confine ultrasonic modes. Some of these internal resonances can be induced from an external wave source. The mapping of a linear defect produces a wave-guide that exhibits a spatial-frequency selection analogous to that characteristic of a synthetic "cochlea". Both, experimental and theoretical results are reported here.Comment: 4 pages, 4 figure

How does breakup influence the total fusion of 6,7^{6,7}Li at the Coulomb barrier?

Total (complete + incomplete) fusion excitation functions of $^{6,7}$Li on $^{59}$Co and $^{209}$Bi targets around the Coulomb barrier are obtained using a new continuum discretized coupled channel (CDCC) method of calculating fusion. The relative importance of breakup and bound-state structure effects on total fusion is particularly investigated. The effect of breakup on fusion can be observed in the total fusion excitation function. The breakup enhances the total fusion at energies just around the barrier, whereas it hardly affects the total fusion at energies well above the barrier. The difference between the experimental total fusion cross sections for $^{6,7}$Li on $^{59}$Co is notably caused by breakup, but this is not the case for the $^{209}$Bi target.Comment: 9 pages, 9 figures, Submitted to Phys. Rev.

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

Discrete-Time Fractional Variational Problems

We introduce a discrete-time fractional calculus of variations on the time scale $h\mathbb{Z}$, $h > 0$. First and second order necessary optimality conditions are established. Examples illustrating the use of the new Euler-Lagrange and Legendre type conditions are given. They show that solutions to the considered fractional problems become the classical discrete-time solutions when the fractional order of the discrete-derivatives are integer values, and that they converge to the fractional continuous-time solutions when $h$ tends to zero. Our Legendre type condition is useful to eliminate false candidates identified via the Euler-Lagrange fractional equation.Comment: Submitted 24/Nov/2009; Revised 16/Mar/2010; Accepted 3/May/2010; for publication in Signal Processing

SACY - a Search for Associations Containing Young stars

The scientific goal of the SACY (Search for Associations Containing Young-stars) was to identify possible associations of stars younger than the Pleiades Association among optical counterparts of the ROSAT X-ray bright sources. High-resolution spectra for possible optical counterparts later than G0 belonging to HIPPARCOS and/or TYCHO-2 catalogs were obtained in order to assess both the youth and the spatial motion of each target. More than 1000 ROSAT sources were observed, covering a large area in the Southern Hemisphere. The newly identified young stars present a patchy distribution in UVW and XYZ, revealing the existence of huge nearby young associations. Here we present the associations identified in this survey.Comment: 8 pages, 2 figures, to appear in the Proceedings of Open Issues in Local Formation and Early Stellar Evolution, Ouro Preto, Brazi

The light curve of the companion to PSR B1957+20

We present a new analysis of the light curve for the secondary star in the eclipsing binary millisecond pulsar system PSR B1957+20. Combining previous data and new data points at minimum from the Hubble Space Telescope, we have 100% coverage in the R-band. We also have a number of new K_s-band data points, which we use to constrain the infrared magnitude of the system. We model this with the Eclipsing Light Curve code (ELC). From the modelling with the ELC code we obtain colour information about the secondary at minimum light in BVRI and K. For our best fit model we are able to constrain the system inclination to 65 +/- 2 degrees for pulsar masses ranging from 1.3 -- 1.9 M_sun. The pulsar mass is unconstrained. We also find that the secondary star is not filling its Roche lobe. The temperature of the un-irradiated side of the companion is in agreement with previous estimates and we find that the observed temperature gradient across the secondary star is physically sustainable.Comment: 6 pages, 4 figures & 3tables. Accepted for publication in MNRA

Time-resolved spectroscopy of the pulsating CV GW Lib

We present time-resolved optical spectroscopy of the dwarf nova GW Librae during its rare 2007 April superoutburst and compare these with quiescent epochs. The data provide the first opportunity to track the evolution of the principal spectral features. In the early stages of the outburst, the optically thick disc dominates the optical and the line components show clear orbital radial velocity excursions. In the course of several weeks, optically thin regions become more prominent as strong emission lines replace the broad disc absorption. Post-outburst spectroscopy covering the I band illustrates the advantages of Ca II relative to the commonly used Balmer lines when attempting to constrain binary parameters. Due to the lower ionization energy combined with smaller thermal and shear broadening of these lines, a sharp emission component is seen to be moving in between the accretion disc peaks in the Ca II line. No such component is visible in the Balmer lines. We interpret this as an emission component originating on the hitherto unseen mass donor star. This emission component has a mean velocity of similar to -15 +/- 5 km s(-1) which is associated with the systemic velocity., and a velocity semi-amplitude of K-em = 82.2 +/- 4.9 km s(-1). Doppler tomography reveals an asymmetric accretion disc, with the S-wave mapping to a sharp spot in the tomogram with a velocity consistent to what is obtained with line profile fitting. A centre of symmetry analysis of the disc component suggests a very small value for the WD orbital velocity K-1 as is also inferred from double Gaussian fits to the spectral lines. While our conservative dynamical limits place a hard upper limit on the binary mass ratio of q < 0.23, we favour a significantly lower value near q similar to 0.06. Pulsation modelling suggests a white dwarf mass similar to 1 M-circle dot. This, paired with a low-mass donor, near the empirical sequence of an evolved cataclysmic variable close to the period bounce, appears to be consistent with all the observational constraints to date