6,813 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
Kepler observations of A-F pre-main sequence stars in Upper Scorpius: Discovery of six new ~Scuti and one ~Doradus stars
We present light curves and periodograms for 27 stars in the young Upper
Scorpius association (age=\,Myr) obtained with the Kepler spacecraft.
This association is only the second stellar grouping to host several pulsating
pre-main sequence (PMS) stars which have been observed from space. From an
analysis of the periodograms, we identify six ~Scuti variables and one
~Doradus star. These are most likely PMS stars or else very close to
the zero-age main sequence. Four of the ~Scuti variables were observed
in short-cadence mode, which allows us to resolve the entire frequency
spectrum. For these four stars, we are able to infer some qualitative
information concerning their ages. For the remaining two ~Scuti stars,
only long-cadence data are available, which means that some of the frequencies
are likely to be aliases. One of the stars appears to be a rotational variable
in a hierarchical triple system. This is a particularly important object, as it
allows the possibility of an accurate mass determination when radial velocity
observations become available. We also report on new high-resolution echelle
spectra obtained for some of the stars of our sample.Comment: 19 pages, 9 figures. Accepted for publication on MNRA
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
The Galactic Halo density distribution from photometric survey data: results of a pilot study
Our goal is to recover the Galactic Halo spatial density by means of field
stars. To this aim, we apply a new technique to the Capodimonte Deep Field
(OACDF, Alcala' et al. 2004), as a pilot study in view of the VLT Survey
Telescope (VST) stellar projects. Considering the unique chance to collect deep
and wide-field photometry with the VST, our method may represent a useful tool
towards a definitive mapping of the Galactic Halo. In the framework of
synthetic stellar populations, turn-off stars are used to reconstruct the
spatial density. The determination of the space density is achieved by
comparing the data with synthetic color-magnitude diagrams (CMDs). The only
assumptions involve the IMF, age and metallicity of the synthetic halo
population. Stars are randomly placed in the solid angle. The contributions of
the various Monte Carlo distributions (with a step of 4 kpc) along the line of
sight are simultaneously varied to reproduce the observed CMD. Our result on
the space density is consistent with a power-law exponent n~3 over a range of
Galactocentric distances from 8 to 40 kpc.Comment: 5 pages. Accepted for publication in Astronomy and Astrophysic
The distance to the LMC cluster Reticulum from the K-band Period-Luminosity-Metallicity relation of RR Lyrae stars
We present new and accurate Near-Infrared J and Ks-band data of the Large
Magellanic Cloud cluster Reticulum. Data were collected with SOFI available at
NTT and covering an area of approximately (5 x 5) arcmin^2 around the center of
the cluster. Current data allowed us to derive accurate mean K-band magnitudes
for 21 fundamental and 9 first overtone RR Lyrae stars. On the basis of the
semi-empirical K-band Period-Luminosity-Metallicity relation we have recently
derived, we find that the absolute distance to this cluster is 18.52 +- 0.005
(random) +- 0.117 (systematic). Note that the current error budget is dominated
by systematic uncertainty affecting the absolute zero-point calibration and the
metallicity scale.Comment: 14 pages, 2 figures, ApJ accepted. Full resolution figure 1 on
request ([email protected]
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
- …