32,289 research outputs found

    Studying the small scale ISM structure with supernovae

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    AIMS. In this work we explore the possibility of using the fast expansion of a Type Ia supernova photosphere to detect extra-galactic ISM column density variations on spatial scales of ~100 AU on time scales of a few months. METHODS. We constructed a simple model which describes the expansion of the photodisk and the effects of a patchy interstellar cloud on the observed equivalent width of Na I D lines. Using this model we derived the behavior of the equivalent width as a function of time, spatial scale and amplitude of the column density fluctuations. RESULTS. The calculations show that isolated, small (<100 AU) clouds with Na I column densities exceeding a few 10^11 cm^-2 would be easily detected. In contrast, the effects of a more realistic, patchy ISM become measurable in a fraction of cases, and for peak-to-peak variations larger than ~10^12 cm^-2 on a scale of 1000 AU. CONCLUSIONS. The proposed technique provides a unique way to probe the extra-galactic small scale structure, which is out of reach for any of the methods used so far. The same tool can also be applied to study the sub-AU Galactic ISM structure.Comment: 6 pages, 3 figures. Accepted for publication in Astronomy & Astrophysic

    Triple mode Cepheid masses

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    Unconventional composition structures are proposed to explain the periods of the triple mode Cepheid aC And. A strong Cepheid wind appears to enrich helium in the convection zones down to about 60,000 K or 70,000 K. Then some downward partial mixing occurs to the bottom of a layer with about 1-q = .0005 of the stellar mass. It was found that AC And was not unlike anomalous Cepheids. However, masses of betwen one and two solar masses are suggested and the population is more likely a type two

    Fast algorithm for border bases of Artinian Gorenstein algebras

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    Given a multi-index sequence σ\sigma, we present a new efficient algorithm to compute generators of the linear recurrence relations between the terms of σ\sigma. We transform this problem into an algebraic one, by identifying multi-index sequences, multivariate formal power series and linear functionals on the ring of multivariate polynomials. In this setting, the recurrence relations are the elements of the kerne lII\sigma of the Hankel operator $H$\sigma associated to σ\sigma. We describe the correspondence between multi-index sequences with a Hankel operator of finite rank and Artinian Gorenstein Algebras. We show how the algebraic structure of the Artinian Gorenstein algebra AA\sigmaassociatedtothesequence associated to the sequence \sigma yields the structure of the terms $\sigma\alphaforall for all α\alpha ∈\in N n.Thisstructureisexplicitlygivenbyaborderbasisof. This structure is explicitly given by a border basis of Aσ\sigma,whichispresentedasaquotientofthepolynomialring, which is presented as a quotient of the polynomial ring K[x 1 ,. .. , xn]bythekernel] by the kernel Iσ\sigmaoftheHankeloperator of the Hankel operator Hσ\sigma.Thealgorithmprovidesgeneratorsof. The algorithm provides generators of Iσ\sigmaconstitutingaborderbasis,pairwiseorthogonalbasesof constituting a border basis, pairwise orthogonal bases of Aσ\sigma$ and the tables of multiplication by the variables in these bases. It is an extension of Berlekamp-Massey-Sakata (BMS) algorithm, with improved complexity bounds. We present applications of the method to different problems such as the decomposition of functions into weighted sums of exponential functions, sparse interpolation, fast decoding of algebraic codes, computing the vanishing ideal of points, and tensor decomposition. Some benchmarks illustrate the practical behavior of the algorithm

    Theory of One-Channel vs. Multi-Channel Kondo Effects for Ce3+^{3+} Impurities

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    We introduce a model for Ce3+^{3+} impurities in cubic metals which exhibits competition between the Fermi-liquid fixed point of the single channel Kondo model and the non-Fermi-liquid fixed point of the two- and three-channel Kondo models. Using the non-crossing approximation and scaling theory, we find: (i) A possible three-channel Kondo effect between the one- and two-channel regimes in parameter space. (ii) The sign of the thermopower is a fixed point diagnostic. (iii) Our results will likely survive the introduction of additional f2f^2 and conduction states. We apply this model to interpret the non-Fermi liquid alloy La1−x_{1-x}Cex_xCu2.2_{2.2}Si2_2.Comment: 13 pages, Revtex, To appear in Phys. Rev. Let

    The Wish-lists: some Comments

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    We provide brief comments on some common threads arising from the `wishlists' set out in some of the other papers in this volume. The discussion is necessarily incomplete: in particular we have dealt only with points for which a reasonably compact answer seems possible

    Blind Normalization of Speech From Different Channels

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    We show how to construct a channel-independent representation of speech that has propagated through a noisy reverberant channel. This is done by blindly rescaling the cepstral time series by a non-linear function, with the form of this scale function being determined by previously encountered cepstra from that channel. The rescaled form of the time series is an invariant property of it in the following sense: it is unaffected if the time series is transformed by any time-independent invertible distortion. Because a linear channel with stationary noise and impulse response transforms cepstra in this way, the new technique can be used to remove the channel dependence of a cepstral time series. In experiments, the method achieved greater channel-independence than cepstral mean normalization, and it was comparable to the combination of cepstral mean normalization and spectral subtraction, despite the fact that no measurements of channel noise or reverberations were required (unlike spectral subtraction).Comment: 25 pages, 7 figure
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