7,562 research outputs found
Pulsational M_V versus [Fe/H] relation(s) for globular cluster RR Lyrae variables
We use the results from recent computations of updated non-linear convective
pulsating models to constrain the distance modulus of Galactic globular
clusters through the observed periods of first overtone RRc pulsators. The
resulting relation between the mean absolute magnitude of RR Lyrae stars
and the heavy element content [Fe/H] appears well in the range of
several previous empirical calibrations, but with a non linear dependence on
[Fe/H] so that the slope of the relation increases when moving towards larger
metallicities. On this ground, our results suggest that metal-poor
([Fe/H]-1.5) variables follow two different
linear -[Fe/H] relations. Application to RR Lyrae stars in the
metal-poor globular clusters of the Large Magellanic Cloud provides a LMC
distance modulus of the order of 18.6 mag, thus supporting the "long" distance
scale. The comparison with recent predictions based on updated stellar
evolution theory is shortly presented and discussed.Comment: 8 pages, 4 postscript figures, accepted for publication on MNRA
RR Lyrae variables in M5 as a test of pulsational theory
We present B and V CCD photometry for variables in the cluster central
region, adding new data for 32 variables and giving suitable light curves, mean
magnitudes and corrected colors for 17 RR Lyrae. Implementing the data given in
this paper with similar data already appeared in the literature we discuss a
sample of 42 variables, as given by 22 RRab and 20 RRc, to the light of recent
predictions from pulsational theories. We find that the observational evidence
concerning M5 pulsators appears in marginal disagreement with predictions
concerning the color of the First Overtone Blue Edge (FOBE), whereas a clear
disagreement appears between the ZAHB luminosities predicted through
evolutionary or pulsational theories.Comment: 7 pages, 7 postscript figures, accepted for publication on MNRA
Theoretical Zero Age Main Sequences revisited
Zero Age Main Sequence (ZAMS) models with updated physical inputs are
presented for selected assumptions about the chemical composition, covering the
ranges 0.6 < M/Mo < 1.2, 0.0001 < Z < 0.04, 0.23 < Y < 0.34.The HR diagram
location of the ZAMS as a function of Y and Z is discussed both in the
theoretical and in the observational HR diagrams, showing that the V magnitude
presents an increased dependence on Z to be taken into account when discussing
observational evidences. Analytical relations quantifying both these
dependences are derived. Implications for the galactic helium to heavier
elements enrichment are finally discussed.Comment: 4 pages, 4 postscript figures, accepted for publication on Astronomy
& Astrophysic
Wide-field multi-color photometry of the Galactic globular cluster NGC 1261
(Abriged)This work studies in more detail the stellar population, including
its photometric properties and characteristics, in the rarely studied southern
Galactic globular cluster NGC 1261. We focus on the brighter sequences of the
cluster's color-magnitude diagram (CMD). Like in our previous works, we rely
upon photometry in several passbands to achieve more reliable results and
conclusions. We carried out and analyzed new multi-color photometry of NGC 1261
in UBVI reaching below the turnoff point in all passbands in a fairly extended
cluster field, about 14'x14'. We found several signs of the inhomogeneity
("multiplicity") in the stellar population. The most prominent of them are: (1)
the dependence of the radial distribution of sub-giant branch (SGB) stars in
the cluster on their U magnitude, with brighter stars less centrally
concentrated at the 99.9 \% level than their fainter counterparts; (2) the
dependence of the location of red giant branch (RGB) stars in the U-(U-B) CMD
on their radial distance from the cluster center, with the portion of stars
bluer in the (U-B) color increasing towards the cluster outskirts.
Additionally, the radial variation of the RGB luminosity function in the bump
region is suspected. We assume that both the SGB stars brighter in the U and
the RGB stars bluer in the (U-B) color are probably associated with blue
horizontal branch stars, because of a similarity in their radial distribution
in the cluster. We estimated the metalicity of NGC 1261 from the slope of the
RGB in U-based CMDs and the location of the RGB bump on the branch. These
metallicity indicators give [Fe/H]zw = -1.34 +/- 0.16 dex and [Fe/H]zw = -1.41
+/- 0.10 dex, respectively. We isolated 18 probable blue straggler candidates.
They are more centrally concentrated than the lower red giants of comparable
brightness at the 97.9 \% level.Comment: 8 pages, 9 figures, accepted for publication in A&
Theoretical insights into the RR Lyrae K-band Period-Luminosity relation
Based on updated nonlinear, convective pulsation models computed for several
values of stellar mass, luminosity and metallicity, theoretical constraints on
the K-band Period-Luminosity (PLK) relation of RR Lyrae stars are presented. We
show that for each given metal content the predicted PLK is marginally
dependent on uncertainties of the stellar mass and/or luminosity. Then, by
considering the RR Lyrae masses suggested by evolutionary computations for the
various metallicities, we obtain that the predicted infrared magnitude M_K over
the range 0.0001< Z <0.02 is given by the relation
MK=0.568-2.071logP+0.087logZ-0.778logL/Lo, with a rms scatter of 0.032 mag.
Therefore, by allowing the luminosities of RR Lyrae stars to vary within the
range covered by current evolutionary predictions for metal-deficient (0.0001<
Z <0.006) horizontal branch models, we eventually find that the infrared
Period-Luminosity- Metallicity (PLZK) relation is
MK=0.139-2.071(logP+0.30)+0.167logZ, with a total intrinsic dispersion of 0.037
mag. As a consequence, the use of such a PLZK relation should constrain within
+-0.04 mag the infrared distance modulus of field and cluster RR Lyrae
variables, provided that accurate observations and reliable estimates of the
metal content are available. Moreover, we show that the combination of K and V
measurements can supply independent information on the average luminosity of RR
Lyrae stars, thus yielding tight constraints on the input physics of stellar
evolution computations. Finally, for globular clusters with a sizable sample of
first overtone variables, the reddening can be estimated by using the PLZK
relation together with the predicted MV-logP relation at the blue edge of the
instability strip (Caputo et al. 2000).Comment: 8 pages, including 5 postscript figures, accepted for publication on
MNRA
Reversible Vortex Ratchet Effects and Ordering in Superconductors with Simple Asymmetric Potential Arrays
We demonstrate using computer simulations that the simplest vortex ratchet
system for type-II superconductors with artificial pinning arrays, an
asymmetric one-dimensional (1D) potential array, exhibits the same features as
more complicated two-dimensional vortex ratchets that have been studied in
recent experiments. We show that the 1D geometry, originally proposed by Lee et
al. [Nature 400, 337 (1999)], undergoes multiple reversals in the sign of the
ratchet effect as a function of vortex density, substrate strength, and ac
drive amplitude, and that the sign of the ratchet effect is related to the type
of vortex lattice structure present. When the vortex lattice is highly ordered,
an ordinary vortex ratchet effect occurs which is similar to the response of an
isolated particle in the same ratchet geometry. In regimes where the vortices
form a smectic or disordered phase, the vortex-vortex interactions are relevant
and we show with force balance arguments that the ratchet effect can reverse in
sign. The dc response of this system features a reversible diode effect and a
variety of vortex states including triangular, smectic, disordered and square.Comment: 10 pages, 12 postscript figures. Version to appear in Phys. Rev.
Spectroscopic properties of a two-dimensional time-dependent Cepheid model II. Determination of stellar parameters and abundances
Standard spectroscopic analyses of variable stars are based on hydrostatic
one-dimensional model atmospheres. This quasi-static approach has theoretically
not been validated. We aim at investigating the validity of the quasi-static
approximation for Cepheid variables. We focus on the spectroscopic
determination of the effective temperature , surface gravity
, microturbulent velocity , and a generic metal
abundance -- here taken as iron. We calculate a grid of 1D
hydrostatic plane-parallel models covering the ranges in effective temperature
and gravity encountered during the evolution of a two-dimensional
time-dependent envelope model of a Cepheid computed with the
radiation-hydrodynamics code CO5BOLD. We perform 1D spectral syntheses for
artificial iron lines in local thermodynamic equilibrium varying the
microturbulent velocity and abundance. We fit the resulting equivalent widths
to corresponding values obtained from our dynamical model. For the
four-parametric case, the stellar parameters are typically underestimated
exhibiting a bias in the iron abundance of \approx-0.2\,\mbox{dex}. To avoid
biases of this kind it is favourable to restrict the spectroscopic analysis to
photometric phases using additional
information to fix effective temperature and surface gravity. Hydrostatic 1D
model atmospheres can provide unbiased estimates of stellar parameters and
abundances of Cepheid variables for particular phases of their pulsations. We
identified convective inhomogeneities as the main driver behind potential
biases. For obtaining a complete view on the effects when determining stellar
parameters with 1D models, multi-dimensional Cepheid atmosphere models are
necessary for variables of longer period than investigated here.Comment: accepted for publication in Astronomy & Astrophysic
Electrokinetic Lattice Boltzmann solver coupled to Molecular Dynamics: application to polymer translocation
We develop a theoretical and computational approach to deal with systems that
involve a disparate range of spatio-temporal scales, such as those comprised of
colloidal particles or polymers moving in a fluidic molecular environment. Our
approach is based on a multiscale modeling that combines the slow dynamics of
the large particles with the fast dynamics of the solvent into a unique
framework. The former is numerically solved via Molecular Dynamics and the
latter via a multi-component Lattice Boltzmann. The two techniques are coupled
together to allow for a seamless exchange of information between the
descriptions. Being based on a kinetic multi-component description of the fluid
species, the scheme is flexible in modeling charge flow within complex
geometries and ranging from large to vanishing salt concentration. The details
of the scheme are presented and the method is applied to the problem of
translocation of a charged polymer through a nanopores. In the end, we discuss
the advantages and complexities of the approach
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