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$_\odot$ 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 (M$>$1.3M$_\odot$), diagrams comparing the effective temperature ($T_{eff}$) against the small separation can provide an alternative to Christensen-Dalsgaard (C-D) diagrams. The impact of the metal abundance of intermediate mass stars (2.5-4.4M$_\odot$) 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