384 research outputs found
An Error Analysis of the Geometric Baade-Wesselink Method
We derive an analytic solution for the minimization problem in the geometric
Baade-Wesselink method. This solution allows deriving the distance and mean
radius of a pulsating star by fitting its velocity curve and angular diameter
measured interferometrically. The method also provide analytic solutions for
the confidence levels of the best fit parameters, and accurate error estimates
for the Baade-Wesselink solution. Special care is taken in the analysis of the
various error sources in the final solution, among which the uncertainties due
to the projection factor, the limb darkening and the velocity curve. We also
discuss the importance of the phase shift between the stellar lightcurve and
the velocity curve as a potential error source in the geometric Baade-Wesselink
method. We finally discuss the case of the Classical Cepheid zeta Gem, applying
our method to the measurements derived with the Palomar Testbed Interferometer.
We show how a careful treatment of the measurement errors can be potentially
used to discriminate between different models of limb darkening using
interferometric techniques.Comment: 24 pages, to be published on the Astrophysical Journal, vol. 603
March 200
Electronic, dynamical and superconducting properties of CaBeSi
We report first-principles calculations on the normal and superconducting
state of CaBe(x)Si(2-x) (x=1), in the framework of density functional theory
for superconductors (SCDFT). CaBeSi is isostructural and isoelectronic to MgB2
and this makes possible a direct comparison of the electronic and vibrational
properties and the electron-phonon interaction of the two materials. Despite
the many similarities with MgB2 (e.g. sigma bands at the Fermi level and a
larger Fermi surface nesting), according to our calculations CaBeSi has a very
low critical temperature (Tc ~ 0.4 K, consistent with the experiment). CaBeSi
exhibits a complex gap structure, with three gaps at Fermi level: besides the
two sigma and pi gaps, present also in MgB2, the appearance of a third gap is
related to the anisotropy of the Coulomb repulsion, acting in different way on
the bonding and antibonding electronic pi states.Comment: 6 pages, 5 figure
First principles study of local electronic and magnetic properties in pure and electron-doped NdCuO
The local electronic structure of Nd2CuO4 is determined from ab-initio
cluster calculations in the framework of density functional theory.
Spin-polarized calculations with different multiplicities enable a detailed
study of the charge and spin density distributions, using clusters that
comprise up to 13 copper atoms in the CuO2plane. Electron doping is simulated
by two different approaches and the resulting changes in the local charge
distribution are studied in detail and compared to the corresponding changes in
hole doped La2CuO4. The electric field gradient (EFG) at the copper nucleus is
investigated in detail and good agreement is found with experimental values. In
particular the drastic reduction of the main component of the EFG in the
electron-doped material with respect to LaCuO4 is explained by a reduction of
the occupancy of the 3d3z^2-r^2 atomic orbital. Furthermore, the chemical
shieldings at the copper nucleus are determined and are compared to results
obtained from NMR measurements. The magnetic hyperfine coupling constants are
determined from the spin density distribution
Theoretical fits of the \delta Cephei light, radius and radial velocity curves
We present a theoretical investigation of the light, radius and radial
velocity variations of the prototype Cephei. We find that the best fit
model accounts for luminosity and velocity amplitudes with an accuracy better
than , and for the radius amplitude with an accuracy of .
The chemical composition of this model suggests a decrease in both helium (0.26
vs 0.28) and metal (0.01 vs 0.02) content in the solar neighborhood. Moreover,
distance determinations based on the fit of light curves agree at the
level with the trigonometric parallax measured by the Hubble Space
Telescope (HST). On the other hand, distance determinations based on angular
diameter variations, that are independent of interstellar extinction and of the
-factor value, indicate an increase of the order of 5% in the HST parallax.Comment: accepted for publication on ApJ Letter
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A New Calibration Of Galactic Cepheid Period-Luminosity Relations From B To K Bands, And A Comparison To LMC Relations
Context. The universality of the Cepheid period-luminosity (PL) relations has been under discussion since metallicity effects were assumed to play a role in the value of the intercept and, more recently, of the slope of these relations. Aims. The goal of the present study is to calibrate the Galactic PL relations in various photometric bands (from B to K) and to compare the results to the well-established PL relations in the LMC. Methods. We use a set of 59 calibrating stars, the distances of which are measured using five different distance indicators: Hubble Space Telescope and revised Hipparcos parallaxes, infrared surface brightness and interferometric Baade-Wesselink parallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes for Cepheids belonging to open clusters or OB stars associations. A detailed discussion of absorption corrections and projection factor to be used is given. Results. We find no significant difference in the slopes of the PL relations between LMC and our Galaxy. Conclusions. We conclude that the Cepheid PL relations have universal slopes in all photometric bands, not depending on the galaxy under study (at least for LMC and Milky Way). The possible zero-point variation with metal content is not discussed in the present work, but an upper limit of 18.50 for the LMC distance modulus can be deduced from our data.McDonald Observator
Static and Dynamical Susceptibility of LaO1-xFxFeAs
The mechanism of superconductivity and magnetism and their possible interplay
have recently been under debate in pnictides. A likely pairing mechanism
includes an important role of spin fluctuations and can be expressed in terms
of the magnetic susceptibility chi. The latter is therefore a key quantity in
the determination of both the magnetic properties of the system in the normal
state, and of the contribution of spin fluctuations to the pairing potential. A
basic ingredient to obtain chi is the independent-electron susceptibility chi0.
Using LaO1-xFxFeAs as a prototype material, in this report we present a
detailed ab-initio study of chi0(q,omega), as a function of doping and of the
internal atomic positions. The resulting static chi0(q,0) is consistent with
both the observed M-point related magnetic stripe phase in the parent compound,
and with the existence of incommensurate magnetic structures predicted by
ab-initio calculations upon doping.Comment: 15 pages, 8 figure
Radii and Distances of Cepheids, I., Method and Measurement Errors
We develop a formulation of the Baade-Wesselink method which uses the Fourier
coefficients of the observables. We derive an explicit, analytic expression to
determine the mean radius from each Fourier order. The simplicity of this
method allows us to derive the uncertainty in the mean radius due to
measurement errors.
Using simulations and a recent dataset we demonstrate that the precision of
the radius measurement with optical magnitudes is in most cases limited by the
accuracy of the measurement of the phase difference between the light and the
color index curve. In this case it is advantageous to determine the inverse
radius, because it has normal errors.Comment: 18 pages, postscript, accepted for publication in Ap
Classical Cepheid Pulsation Models. III. The Predictable Scenario
Within the current uncertainties in the treatment of the coupling between
pulsation and convection, limiting amplitude, nonlinear, convective models
appear the only viable approach for providing theoretical predictions about the
intrinsic properties of radial pulsators. In this paper we present the results
of a comprehensive set of Cepheid models computed within such theoretical
framework for selected assumptions on their original chemical composition.Comment: 24 pages, 1 latex file containing 6 tables, 10 postscript figures,
accepted for publication on Ap
Persist or Produce: A Community Trade-Off Tuned by Species Evenness
Understanding the effects of biodiversity on community persistence and productivity is key to managing both natural and production systems. Because rare species face greater danger of extinction, species evenness, a measure of how similar abundances are across species in a community, is seen as a key component of biodiversity. However, previous studies have failed to find a consistent association of species evenness with species survival and biomass production. Here we provide a theoretical framework for the relationship among these three elements. We demonstrate that the lack of consistent outcomes is not an idiosyncratic artifact of different studies but can be unified under one common framework. Applying a niche theory approach, we confirm that under demographic stochasticity evenness is a general indicator of the risk of future species extinctions in a community, in accordance with the majority of empirical studies. In contrast, evenness cannot be used as a direct indicator of the level of biomass production in a community. When a single species dominates, as expressed by the constraints imposed by the population dynamics, biomass production depends on the niche position of the dominating species and can increase or decrease with evenness. We demonstrate that high species evenness and an intermediate level of biomass production is the configuration that maximizes the average species survival probability in response to demographic stochasticity
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