470 research outputs found
A theoretical approach for the interpretation of pulsating PMS intermediate-mass stars
The investigation of the pulsation properties of pre-main-sequence
intermediate-mass stars is a promising tool to evaluate the intrinsic
properties of these stars and to constrain current evolutionary models. Many
new candidates of this class have been discovered during the last decade and
very accurate data are expected from space observations obtained for example
with the CoRoT satellite. In this context we aim at developing a theoretical
approach for the interpretation of observed frequencies, both from the already
available ground-based observations and from the future more accurate and
extensive CoRoT results. To this purpose we have started a project devoted to
the computations of fine and extensive grids of asteroseismic models of
intermediate mass pre-main-sequence stars. The obtained frequencies are used to
derive an analytical relation between the large frequency separation and the
stellar luminosity and effective temperature and to develop a tool to compare
theory and observations in the echelle diagram. The predictive capabilities of
the proposed method are verified through the application to two test stars. As
a second step, we apply the procedure to two true observations from multisite
campaigns and we are able to constrain their stellar parameters, in particular
the mass, in spite of the small number of frequencies. We expect that with a
significantly higher number of frequencies both the stellar mass and age could
be constrained and, at the same time, the physics of the models could be
tested.Comment: Accepted for publication on A&
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
Detection of delta Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348
We present time series observations of intermediate mass PMS stars belonging
to the young star cluster IC 348. The new data reveal that a young member of
the cluster, H254, undergoes periodic light variations with delta Scuti-like
characteristics. This occurrence provides an unambiguous evidence confirming
the prediction that intermediate-mass pre-main sequence (PMS) stars should
experience this transient instability during their approach to the
main-sequence. On the basis of the measured frequency f=7.406 c/d, we are able
to constrain the intrinsic stellar parameters of H254 by means of linear, non
adiabatic, radial pulsation models. The range of the resulting luminosity and
effective temperature permitted by the models is narrower than the
observational values. In particular, the pulsation analysis allows to derive an
independent estimate of the distance to IC 348 of about 320 pc. Further
observations could either confirm the monoperiodic nature of H254 or reveal the
presence of other frequencies.Comment: 7 pages, including 7 postscript figures, accepted for publication on
A&
Eclipsing binary stars in the Large Magellanic Cloud : results from the EROS-2, OGLE and VMC surveys
We present a catalogue of 1768 eclipsing binary stars (EBs) detected in the Large Magellanic Cloud (LMC) by the second generation of the EROS survey (hereinafter EROS-2); 493 of them are new discoveries located in outer regions (out of the central bar) of the LMC. These sources were originally included in a list of candidate classical Cepheids (CCs) extracted from the EROS-2 catalogue on the basis of the period (0.89 < 17.82 mag] diagram. After visual inspection of the light curves we reclassified them as eclipsing binaries. They have blue colours (B_EROS - R_EROS < 0.2 mag) hence we classed them as hot eclipsing binaries (HEBs) containing hot massive components: main sequence (MS) stars or blue giants. We present Ks-band light curves for 999 binaries from our sample that have a counterpart in the VISTA near-infrared ESO public survey of the Magellanic Clouds system (VMC). We provide spectral classifications of 13 HEBs with existing spectroscopy. We divided our sample into contact-like binaries and detached/semi-detached systems based on both visual inspection and the parameters of the Fourier decomposition of the light curves and analysed the period-luminosity (PL) relations of the contact-like systems using the R_EROS and Ks magnitudes at maximum light. The contact-like binaries in our sample do not follow PL relations. We analysed the sample of contact binaries from the OGLE III catalogue and confirmed that PL_I and PL_Ks sequences are defined only by eclipsing binaries containing a red giant component.Peer reviewe
Theoretical Models for Classical Cepheids: IV. Mean Magnitudes and Colors and the Evaluation of Distance, Reddening and Metallicity
We discuss the metallicity effect on the theoretical visual and near-infrared
PL and PLC relations of classical Cepheids, as based on nonlinear, nonlocal and
time--dependent convective pulsating models at varying chemical composition. In
view of the two usual methods of averaging (magnitude-weighted and
intensity-weighted) observed magnitudes and colors over the full pulsation
cycle, we briefly discuss the differences between static and mean quantities.
We show that the behavior of the synthetic mean magnitudes and colors fully
reproduces the observed trend of Galactic Cepheids, supporting the validity of
the model predictions. In the second part of the paper we show how the estimate
of the mean reddening and true distance modulus of a galaxy from Cepheid VK
photometry depend on the adopted metal content, in the sense that larger
metallicities drive the host galaxy to lower extinctions and distances.
Conversely, self-consistent estimates of the Cepheid mean reddening, distance
and metallicity may be derived if three-filter data are taken into account. By
applying the theoretical PL and PLC relations to available BVK data of Cepheids
in the Magellanic Clouds we eventually obtain Z \sim 0.008, E(B-V) \sim 0.02
mag, DM \sim 18.63 mag for LMC and Z \sim 0.004, E(B-V) \sim 0.01 mag., DM \sim
19.16 mag. for SMC. The discrepancy between such reddenings and the current
values based on BVI data is briefly discussed.Comment: 16 pages, 11 postscript figures, accepted for publication on Ap
Seismic evolution of low/intermediate mass PMS stars
This article presents a study of the evolution of the internal structure and
seismic properties expected for low/intermediate mass Pre-Main Sequence (PMS)
stars. Seismic and non-seismic properties of PMS stars were analysed. This was
done using 0.8 to 4.4M stellar models at stages ranging from the end of
the Hayashi track up to the Zero-Age Main-Sequence (ZAMS). This research
concludes that, for intermediate-mass stars (M1.3M), diagrams
comparing the effective temperature () against the small separation
can provide an alternative to Christensen-Dalsgaard (C-D) diagrams. The impact
of the metal abundance of intermediate mass stars (2.5-4.4M) has over
their seismic properties is also evaluated.Comment: 4 pages, 5 figures, accepted for publication on A&
Metal abundances of RR Lyrae stars in the metal rich globular cluster NGC 6441
Low resolution spectra have been used to measure individual metal abundances
of RR Lyrae stars in NGC 6441, a Galactic globular cluster known to have very
unusual horizontal branch morphology and periods of the RR Lyrae stars for its
high metallicity. We find an average metal abundance of [Fe/H]=-0.69 +/- 0.06
(r.m.s.=0.33 dex) and [Fe/H]=-0.41 +/- 0.06 (r.m.s.=0.36 dex) on Zinn & West
and Carretta & Gratton metallicity scales, respectively, consistent with the
cluster metal abundance derived by Armandroff & Zinn. Most of the metallicities
were extrapolated from calibration relations defined for [Fe/H] < -1; however,
they are clearly high and contrast with the rather long periods of the NGC 6441
variables, thus confirming that the cluster does not fit in the general
Oosterhoff classification scheme. The r.m.s. scatter of the average is larger
than observational errors (0.15-0.16 dex) possibly indicating some spread in
metallicity. However, even the metal poor variables, if confirmed to be cluster
members, are still more metal rich than those commonly found in the Oosterhoff
type II globular clusters.Comment: Accepted for publication on ApJ Letter
CORS Baade-Wesselink method in the Walraven photometric system: the period-radius and the period-luminosity relation of classical Cepheids
We present a new derivation of the CORS Baade-Wesselink method in the
Walraven photometric system. We solved the complete Baade-Wesselink equation by
calibrating the surface brightness function with a recent grid of atmosphere
models. The new approach was adopted to estimate the mean radii of a sample of
Galactic Cepheids for which are available precise light curves in the Walraven
bands. Current radii agree, within the errors, quite well with Cepheid radii
based on recent optical and near-infrared interferometric measurements. We also
tested the impact of the projection factor on the Period-Radius relation using
two different values (p=1.36, p=1.27) that bracket the estimates available in
the literature. We found that the agreement of our Period-Radius relation with
similar empirical and theoretical Period-Radius relations in the recent
literature, improves by changing the projection factor from p=1.36 to p=1.27.
Our Period-Radius relation is log(R)=(0.75\pm 0.03)log(P)+(1.10 \pm 0.03), with
a rms=0.03 dex. Thanks to accurate estimates of the effective temperature of
the selected Cepheids, we also derived the Period-Luminosity relation in the V
band and we found Mv=(-2.78 \pm 0.11)log(P)+(-1.42 \pm 0.11) with rms=0.13 mag,
for p=1.27. It agrees quite well with recent results in the literature, while
the relation for p=1.36 deviates by more than 2sigma. We conclude that, even
taking into account the intrinsic dispersion of the obtained Period-Luminosity
relations, that is roughly of the same order of magnitude as the effect of the
projection factor, the results of this paper seem to favour the value p = 1.27.Comment: 17 pages, 14 figures, accepted for publication in MNRA
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
- …