arXiv subjects

M. Marconi

Publications and source records attributed to M. Marconi.

At least 235 records · Page 13Linked to original sources

arXiv2000

CU Comae: a new field double-mode RR Lyrae, the most metal poor discovered to date

We report the discovery of a new double-mode RR Lyrae variable (RRd) in the field of our Galaxy: CU Comae. CU Comae is the sixth such RRd identified to date and is the most metal-poor RRd ever detected. Based on BVI CCD photometry spanning eleven years of observations, we find that CU Comae has periods P0=0.5441641 +/-0.0000049d and P1=0.4057605 +/-0.0000018d. The amplitude of the primary (first-overtone) period of CU Comae is about twice the amplitude of the secondary (fundamental) period. The combination of the fundamental period of pulsation P0 and the period ratio of P1/P0=0.7457 places the variable on the metal-poor side of the Petersen diagram, in the region occupied by M68 and M15 RRd's. A mass of 0.83 solar masses is estimated for CU Comae using an updated theoretical calibration of the Petersen diagram. High resolution spectroscopy (R=30,000) covering the full pulsation cycle of CU Comae was obtained with the 2.7 m telescope of the Mc Donald Observatory, and has been used to build up the radial velocity curve of the variable. Abundance analysis done on the four spectra taken near minimum light (phase: 0.54 -- 0.71) confirms the metal poor nature of CU Comae, for which we derive [Fe/H]=-2.38 +/-0.20. This value places this new RRd at the extreme metal-poor edge of the metallicity distribution of the RR Lyrae variables in our Galaxy.

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arXiv2000

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) and metal-rich ([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.

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arXiv2000

Pulsation in two Herbig Ae stars: HD 35929 and V351 Ori

New photometric observations of seven intermediate mass pre-main sequence delta Scuti candidates are presented. The periods and pulsation modes are derived for two of these stars, namely HD 35929 and V351 Ori. The comparison between observations and nonlinear pulsational models allows us to provide some initial constraints on their mass and evolutionary state. As an illustration we discuss the use of periods to identify the mode of pulsation in these two stars and to have an independent estimate of their distances.

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arXiv2000

The RR Lyrae U Com as a test for nonlinear pulsation models

We use high precision multiband photometric data of the first overtone RR Lyrae U Com to investigate the predictive capability of full amplitude, nonlinear, convective hydrodynamical models. The main outcome of this investigation is that theoretical predictions properly account for the luminosity variations along a full pulsation cycle. Moreover, we find that this approach, due to the strong dependence of this observable and of the pulsation period on stellar parameters, supply tight constraints on stellar mass, effective temperature, and distance modulus. Pulsational estimates of these parameters appear in good agreement with empirical ones. Finally, the occurrence of a well-defined bump just before the luminosity maximum gave the unique opportunity to calibrate the turbulent convection model adopted for handling the coupling between pulsation and convection.

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arXiv1999

An investigation of the revised CORS method based on theoretical models

Direct measurements of Cepheid radii are a key for understanding the physical structure of these variables, and in turn for constraining their pulsation properties. In this paper we discuss the numerical experiments we performed for testing the accuracy of Cepheid radii obtained by adopting both the pure Baade-Wesselink (BW) method and the revised CORS method, as well as the consistency of the physical assumptions on which these methods are based. We applied both the BW and the revised CORS methods to the synthetic light, color and radial velocity curves predicted by Cepheid full amplitude, nonlinear, convective models at solar chemical composition. We found that these methods systematically either underestimate or overestimate "theoretical" radii if radius determinations are based on optical (BVR) bands or on (VJK) bands, respectively. At the same time, current simulations suggest that CORS radii are in very good agreement with "theoretical" radii if the surface brightness is calibrated by adopting a bidimensional fit of atmosphere models which accounts for temperature, gravity, and bolometric correction variations along the pulsation cycle.

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arXiv1999

The puzzling theoretical predictions for the luminosity of clumping He burning stars

This paper deals with theoretical predictions for He burning models in a range of masses covering the so-called Red Giant Branch phase transition. Taking as a guideline the observational constraints given by Hipparcos parallaxes to the predicted luminosity of models originated from Red Giant progenitors with He core undergoing electron degeneracy, we compare models by various authors as recently appeared in the literature, disclosing sensitive differences in the predicted luminosity. The solidity of these theoretical predictions is investigated by exploring the effects of varying the assumptions about the efficiency of core overshooting or the amount of mass loss, giving quantitative estimates of the related uncertainties. However, one finds that theoretical predictions concerning the luminosity of the red giant clump in the Hipparcos sample is scarcely affected by these mechanisms.A comparison among theoretical predictions as recently given by different authors convincingly demonstrates that the different luminosity predictions are the natural results of evolutionary codes with different - but all reasonable- input physics. In this context observations suggests that stellar models based on the most updated input physics are possibly overestimating the luminosity of these structures, raising doubts on several current predictions concerning the luminosity of HB stars in galactic globulars.

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arXiv1999

Theoretical models for classical cepheids: V. Multiwavelength relations

From a theoretical study based on nonlinear, nonlocal and time-dependent convective pulsating models at varying mass and chemical composition, we present the predicted Period-Luminosity, Period-Color, Color-Color and Period-Luminosity-Color relations in the BVRIJK bands for Classical Cepheids. All the theoretical relations are, in various degrees, metallicity dependent and the comparison with observed data for MC and Galactic Cepheids show a fair agreement which supports the validity of the pulsating models.

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arXiv1999

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.

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arXiv1999

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.

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arXiv1999

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.

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arXiv1999

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.

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arXiv1998

Theoretical stellar models for old galactic clusters

We present new evolutionary stellar models suitable for old Population I clusters, discussing both the consequences of the most recent improvements in the input physics and the effect of element diffusion within the stellar structures. Theoretical cluster isochrones are presented, covering the range of ages from 1 to 9 Gyr for the four selected choices on the metallicity Z= 0.007, 0.010, 0.015 and 0.020. Theoretical uncertainties on the efficiency of superadiabatic convection are discussed in some details. Isochrone fitting to the CM diagrams of the two well observed galactic clusters NGC2420 and M67 indicates that a mixing length parameter alpha = 1.9 appears adequate for reproducing the observed color of cool giant stars. The problems in matching theoretical preditions to the observed slope of MS stars are discussed.

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arXiv1998

On the Period-Luminosity-Color Relation of Classical Cepheids

We present the comparison between theoretical and empirical PL relations in V and I bands for the Large Magellanic Cloud. We found that, within the current intrinsic dispersions, theoretical predictions are in remarkable agreement with observational data. We also discuss the PLC relations in (V,B-V) and (V,V-I) as well as the dependence of distance determinations on uncertainties in colors, in reddening corrections and in metal content.

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arXiv1998

The Instability Strip for Pre--Main-Sequence Stars

We investigate the pulsational properties of Pre--Main-Sequence (PMS) stars by means of linear and nonlinear calculations. The equilibrium models were taken from models evolved from the protostellar birthline to the ZAMS for masses in the range 1 to 4 solar masses. The nonlinear analysis allows us to define the instability strip of PMS stars in the HR diagram. These models are used to constrain the internal structure of young stars and to test evolutionary models. We compare our results with observations of the best case of a pulsating young star, HR~5999, and we also identify possible candidates for pulsational variability among known Herbig Ae/Be stars which are located within or close to the instability strip boundaries.

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arXiv1998

Theoretical Models for Classical Cepheids: II. Period-Luminosity, Period-Color and Period-Luminosity-Color Relations

We present and discuss theoretical pulsational predictions concerning the properties of Classical Cepheids. The computations have been performed for three different chemical compositions taken as representative of Cepheids in the Magellanic Clouds and in the Galaxy. The resulting PL relations are significantly dependent on metallicity, with the amplitude of the metallicity effect decreasing at the longer wavelength. At variance with several empirical suggestions appeared in the literature, we find that at fixed period the metal-rich pulsators should be fainter than the metal-poor ones. Tight Period-Luminosity-Color relations are derived for both visual and near-infrared photometric bands. We present a preliminary use of our relations to evaluate the distance modulus of the Magellanic Clouds.

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arXiv1996

Nonlinear Investigation of the Pulsational Properties of RR Lyrae Variables

We present a theoretical investigation on periods and amplitudes of RR Lyrae pulsators by adopting stellar parameters which cover the range of theoretical evolutionary expectations. Extensive grids of nonlinear, nonlocal and time-dependent convective RR Lyrae envelope models have been computed to investigate the pulsational behavior in both fundamental and first overtone modes at selected luminosity levels and over an effective temperature range which covers the whole instability region. Blue and red boundaries for pulsational instability into the HR diagram are presented for three different mass values M=0.75, 0.65 and 0.58 solar masses, together with an atlas of full amplitude theoretical light curves for both fundamental and first overtone pulsators. Theoretical periods have been combined with theoretical amplitudes in order to predict the location of the pulsators in the amplitude-period diagram. Comparison with observational data brings to light what we regard as a clear indication that the region where both fundamental and first overtone show a stable limit cycle, is populated by fundamental or first overtone pulsators in Oosterhoff I and Oosterhoff II clusters respectively. Some evident mismatches between theory and observation have also been found, and they are presented and discussed.

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arXiv1996

Metallicity Effects on RR Lyrae Variables

The effects of heavy elements abundance on theoretical observables is studied for both fundamental and first overtone RR Lyrae pulsators at fixed mass and luminosity level. Nonlinear models have been computed adopting three different metal content. It has been found that the bolometric amplitude of first overtone pulsators decreases as metallicity increases, whereas for fundamental pulsators the amplitude is positively correlated with the metal abundance.

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arXiv1995

The Topology of RR Lyrae Instability Strip and the Oosterhoff Dichotomy

Convective pulsating models with Y=0.24, stellar mass M=0.65Mo, 0.75Mo, under selected assumptions about luminosities and effective temperatures, are used together with stellar atmosphere computations to get the predicted dependence of RR Lyrae instability strip on stellar mass in the observative plane MV, B-V. The comparison with observed variables belonging to the globular clusters M3 and M15 is presented. It appears that in M3 (Oosterhoff type I) the transition between ab-type and c-type RR Lyrae takes place along the fundamental blue edge, whereas in M15 (Oosterhoff type II) it occurs near the first overtone red edge. This result supports the key role of the hysteresis mechanism originally proposed by van Albada & Baker for explaining the Oosterhoff dichotomy.

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