arXiv subjects

F. Caputo

Publications and source records attributed to F. Caputo.

At least 55 records · Page 3Linked to original sources

arXiv2005

The Carina dSph galaxy: where is the edge?

Recent cosmological N-body simulations suggest that current empirical estimates of tidal radii in dSphs might be underestimated by at least one order of magnitude. To constrain the plausibility of this theoretical framework, we undertook a multiband (U,B,V,I) survey of the Carina dSph. Deep B,V data of several fields located at radial distances from the Carina center ranging from 0.5 to 4.5 degrees show a sizable sample of faint blue objects with the same magnitudes and colors of old, Turn-Off stars detected across the center. We found that the (U-V,B-I) color-color plane is a robust diagnostic to split stars from background galaxies. Unfortunately, current U,I-band data are too shallow to firmly constrain the real extent of Carina.

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arXiv2005

Pulsation and evolutionary masses of classical Cepheids. I. Milky Way variables

We investigate a selected sample of Galactic classical Cepheids with available distance and reddening estimates in the framework of the theoretical scenario provided by pulsation models, computed with metal abundance Z=0.02, helium content in the range of Y=0.25 to 0.31, and various choices of the stellar mass and luminosity. After transforming the bolometric light curve of the fundamental models into BVRIJK magnitudes, we derived analytical relations connecting the pulsation period with the stellar mass, the mean (intensity-averaged) absolute magnitude, and the color of the pulsators. These relations are used together with the Cepheid observed absolute magnitudes in order to determine the "pulsation" mass -M_p- of each individual variable. The comparison with the "evolutionary" masses -M_(e,can)- given by canonical (no convective core overshooting, no mass-loss) models of central He-burning stellar structures reveals that the M_p/M_(e,can) ratio is correlated with the Cepheid period, ranging from about 0.8 at logP=0.5 to about 1 at logP=1.5. We discuss the effects of different input physics and/or assumptions on the evolutionary computations, as well as of uncertainties in the adopted Cepheid metal content, distance, and reddening. Eventually, we find that the pulsational results can be interpreted in terms of mass-loss during or before the Cepheid phase, whose amount increases as the Cepheid original mass decreases. It vanishes around 13M_(sun) and increases up to about 20% at 4M_(sun).

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arXiv2005

Predicted and empirical radii of RR Lyrae stars

We present new theoretical Period-Radius-Metallicity (PRZ) relations for RR Lyrae stars. Current predictions are based on a large set of nonlinear, convective models that cover a broad range of chemical abundances and input parameters. We also provide new and homogeneous estimates of angular diameters for a sample of field RR Lyrae stars using a recent calibration of the Barnes-Evans surface brightness relation. Predicted and empirical radii are, within the errors, in reasonable agreement, but in the short-period range the latter present a larger scatter. As a working hypothesis we suggest that this discrepancy might be due to the occurrence either of nonlinear features such as bumps or a steep rising branch. New distance determination for RR Lyr itself is in very good agreement with HST trigonometric parallax and with pulsation parallax.

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arXiv2004

Classical Cepheid Pulsation Models. X. The Period-Age Relation

We present new Period-Age (PA) and Period-Age-Color (PAC) relations for fundamental and first overtone classical Cepheids. Current predictions rely on homogeneous sets of evolutionary and pulsation models covering a broad range of stellar masses and chemical compositions. We found that PA and PAC relations present a mild dependence upon metal content. Moreover, the use of different PA and PAC relation for fundamental and first overtone Cepheids improves the accuracy of age estimates in the short-period (log P < 1) range (old Cepheids), because they present smaller intrinsic dispersions. At the same time, the use of the PAC relations improves the accuracy in the long-period (log P >= 1) range (young Cepheids), since they account for the position of individual objects inside the instability strip. We performed a detailed comparison between evolutionary and pulsation ages for a sizable sample of LMC (15) and SMC (12) clusters which host at least two Cepheids. In order to avoid deceptive uncertainties in the photometric absolute zero-point, we adopted the homogeneous set of B,V,I data for clusters and Cepheids collected by OGLE. We also adopted the same reddening scale. The different age estimates agree at the level of 20 % for LMC clusters and of 10 % for SMC clusters. We also performed the same comparison for two Galactic clusters (NGC6067, NGC7790) and the difference in age is smaller than 20 %. These findings support the use of PA and PAC relations to supply accurate estimates of individual stellar ages in the Galaxy and in external Galaxies. The main advantage of this approach is its independence from the distance.

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arXiv2004

RR Lyrae variables in Galactic globular clusters: IV. Synthetic HB and RR Lyrae predictions

We present theoretical predictions concerning horizontal branch stars in globular clusters, including RR Lyrae variables, as derived from synthetic procedures collating evolutionary and pulsational constraints. On this basis, we explore the predicted behavior of the pulsators as a function of the horizontal branch morphology and over the metallicity range Z=0.0001 to 0.006, revealing an encouraging concordance with the observed distribution of fundamentalised periods with metallicity. Theoretical relations connecting periods to K magnitudes and BV or VI Wesenheit functions are presented, both appearing quite independent of the horizontal branch morphology only with Z greater or equal than 0.001. Predictions concerning the parameter R are also discussed and compared under various assumptions about the horizontal branch reference luminosity level.

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arXiv2004

RR Lyrae stars in Galactic globular clusters.III. Pulsational predictions for metal content Z=0.0001 to Z=0.006

The results of nonlinear, convective models of RR Lyrae pulsators with metal content Z=0.0001 to 0.006 are discussed and several predicted relations connecting pulsational (period and amplitude of pulsation) and evolutionary parameters (mass, absolute magnitude and color of the pulsator) are derived. These relations, when linked with the average mass of RR Lyrae stars, as suggested by horizontal branch evolutionary models, provide a ``pulsational'' route to the determination of the distance modulus, both apparent and intrinsic, of RR Lyrae rich globular clusters. Based on a preliminary set of synthetic horizontal branch simulations, we compare the predicted relations with observed variables in selected globular clusters (M2, M3, M5, M15, M55, M68, NGC 1851, NGC 3201, NGC 5466, NGC 6362, NGC 6934 and IC 4499). We show that the distance moduli inferred by the various theoretical relations are mutually consistent within the errors, provided that the value of the mixing-length parameter slightly increases from the blue to the red edge of the pulsation region. Moreover, we show that the relative ``pulsational'' distance moduli fit well previous empirical results and that the parallax of the prototype variable RR Lyr, as inferred by the predicted Period-Wasenheit relation, is in close agreement with the HST astrometric measurement.

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arXiv2004

Bright metal-poor variables: why "Anomalous" Cepheids?

We investigate the theoretical scenario concerning the large sample of variables recently discovered in the dwarf, metal-poor irregular galaxy Leo A, focusing the attention on the "Anomalous" Cepheid phenomenon and its correlation with RR Lyrae stars, Classical and Population II Cepheids. To this purpose, we make use of suitable stellar and pulsation models to depict the pulsational history of evolutionary structures with metallicity Z=0.0004, finding that He-burning pulsators are expected only outside the mass interval ~ 0.8-1.7 Mo. The dependence on metal content of this scenario has been investigated over the range Z=0.0002 to 0.008. One finds that the edges of the pulsational strip for the more massive class of pulsators appear independent of metallicity, but with the minimum mass of these bright pulsators which decreases when decreasing the metallicity, thus decreasing the predicted minimum luminosity and period. Comparison with data for Cepheids in Leo A and in the moderately metal rich extragalactic stellar system Sextans A discloses an encouraging agreement with the predicted pulsational scenario. On this basis, we predict that in a stellar system where both RR Lyrae stars and Cepheids are observed, their magnitude difference may help in constraining both the metal content and the distance. The current classification of metal-poor Cepheids is shortly discussed, advancing the suggestion for an updated terminology abreast of the current knowledge of stellar evolution.

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arXiv2004

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.

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arXiv2004

Updated pulsation models for anomalous Cepheids

A theoretical investigation of the pulsation behavior of so-named ``anomalous'' Cepheids is presented. The study is based on nonlinear convective pulsation models with $Z$=0.0001 and 0.0004, mass in the range 1.3-2.2 Mo and various luminosity levels. Based on these computations, we derive period, bolometric light curves and the edges of the instability strip, showing that a variation of the metal abundance from Z=0.0001 to 0.0004 has quite small effects on these quantities. Then, using bolometric corrections and color-temperature transformations, we are able to provide the predicted relations connecting pulsational properties (periods, amplitudes) with magnitudes and colors in the various photometric bands. The theoretical pulsational scenario is compared to observed anomalous Cepheids in dwarf spheroidal galaxies and, in particular, the predicted mass-dependent Period-Magnitude-Amplitude and Period-Magnitude-Color relations are used to estimate individual mass values, as well as to discriminate between fundamental (F) and first-overtone (FO) pulsators.

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arXiv2003

RR Lyrae variables in Galactic globular clusters - I: The observational scenario

In this paper we revisit observational data concerning RR Lyrae stars in Galactic globular clusters, presenting frequency histograms of fundamentalized periods for the 32 clusters having more than 12 pulsators with well recognized period and pulsation mode. One finds that the range of fundamentalized periods covered by the variables in a given cluster remains fairly constant in varying the cluster metallicity all over the metallicity range spanned by the cluster sample, with the only two exceptions given by M15 and NGC6441. We conclude that the width in temperature of the RR Lyrae instability strip appears largely independent of the cluster metallicity. At the same time, it appears that the fundamentalized periods are not affected by the predicted variation of pulsators luminosity with metal abundance, indicating the occurrence of a correlated variation in the pulsator mass. We discuss mean periods in a selected sample of statistically significant "RR rich" clusters with no less than 10 RRab and 5 RRc variables. One finds a clear evidence for the well known Oosterhoff dichotomy in the mean period of ab-type variables, together with a similarly clear evidence for a constancy of the mean fundamentalized period in passing from Oosterhoff type II to type I clusters. On this basis, the origin of the Oosterhoff dichotomy is discussed, presenting evidence against a strong dependence of the RR Lyrae luminosity on the metal content.

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arXiv2003

RR Lyrae stars in Galactic globular clusters. II. A theoretical approach to variables in M3

We present predicted relations connecting pulsational and evolutionary parameters, as based on a wide set of convective pulsating models of RR Lyrae stars with Z=0.001, Y=0.24 and mass and luminosity suitable for the ``old'' (age $>$ 8 Gyr) variables observed in globular clusters. The relations are collated with sound constraints on the mass of pulsators, as inferred from up-to-date evolutionary models of horizontal branch stars, in order to provide a self-consistent theoretical framework for the analysis of observed variables. The theoretical predictions are tested through a detailed comparison with measurements of RR Lyrae stars in the globular cluster M3. We show that the predicted relations satisfy a variety of observed data, thus providing a "pulsational" route to the determination of accurate distances to RR Lyrae-rich globular clusters with intermediate metal content. We discuss the effect of different physical inputs in evolutionary computations, as well as of different bolometric corrections adopted to convert bolometric luminosity into absolute magnitude. We show that the constraints inferred by pulsation theory support the large value of the mixing-length parameter adopted to fit observed RGB, but, at the same time, they would yield that the luminosity of updated HB models is too bright. Such discrepancy is significantly reduced if element diffusion is properly taken into account.

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arXiv2003

A pulsational approach to near infrared and visual magnitudes of RR Lyrae stars

In this paper we present an improved theoretical scenario concerning near infrared and visual magnitudes of RR Lyrae variables, as based on up-to-date pulsating models. On this basis, we revisit the case of the prototype variable RR Lyr, showing that the parallax inferred by this new pulsational approach appears in close agreement with HST absolute parallax. Moreover, available K and V measurements for field and cluster RR Lyrae variables with known reddening and metal content are used to derive a relation connecting the K absolute magnitude to period and metallicity, as well as a new calibration of the M_V-[Fe/H] relation. The comparison between theoretical prescriptions and observations suggests that RR Lyrae stars in the field and in Galactic Globular Clusters should have quite similar evolutionary histories. The comparison between theory and observations also discloses a general agreement that supports the reliability of current pulsational scenario. On the contrary, current empirical absolute magnitudes based on the Baade-Wesselink (BW) method suggest relations with a zero-point that is fainter than predicted by pulsation models, together with a milder metallicity dependence. However, preliminary results based on a new calibration of the BW method provided by Cacciari et al. (2000) for RR Cet and SW And appear in a much better agreement with the pulsational predictions.

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arXiv2003

Near-Infrared photometry of LMC cluster Reticulum

We present near-infrared (JKs) time series data of the Large Magellanic Cloud (LMC) cluster Reticulum. The observing strategy and data reduction (DAOPHOTII/ALLFRAME) allowed us to reach a photometry accuracy of the order of 0.02 mag at limiting magnitudes typical of RR Lyrae stars. We are interested in Reticulum, since it hosts a sizable sample of RR Lyrae (32), and therefore the use of the K-band Period-Luminosity-Metallicity (PLZ_K) relation will allow us to supply an accurate LMC distance evaluation. The main advantages in using this method is that it is marginally affected by off-ZAHB evolutionary effects and by reddening corrections. As a preliminary but robust result we find a true distance in good agreement with the LMC Cepheid distance scale, i.e. DM = 18.47 +- 0.07 mag.

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arXiv2003

The Carina Project: II. Stellar Populations

We present a new (V,B-V) Color-Magnitude Diagram (CMD) of the Carina dwarf spheroidal (dSph) that extends from the tip of the Red Giant Branch (RGB) down to about V=25 mag. Data were collected with the Wide Field Imager (WFI) available at 2.2m ESO/MPI telescope and cover an area of about 0.3 square degrees around the center of the galaxy. We confirm the occurrence of a substantial amount of old stars with ages around 11 Gyr, together with an intermediate-age population around 5 Gyr. Moreover, we also detected a new well-defined blue plume of young Main-Sequence (MS) stars with an age at most of the order of 1 Gyr. At odds with previous results available in the literature we found thatstars along the RGB of the old and of the intermediate-age stellar populationindicate a mean metallicity roughly equal to Z=0.0004 and a small dispersion around this value. We find evidence for a smooth spatial distribution of the "intermediate-age" stellar population, which appears more centrally concentrated than the oldest one.

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arXiv2003

The Carina Project: I. Bright Variable Stars

We present new BV time series data of the Carina dwarf spheroidal galaxy. Current data cover an area of about 0.3 square degrees around the center of the galaxy and allowed us to identify 92 variables. Among them 75 are RR Lyrae stars, 15 are "bona fide" Anomalous Cepheids, one might be a Galactic field RR Lyrae, and one is located along the Carina Red Giant Branch. Expanding upon the seminal photographic investigation by Saha, Monet & Seitzer (1986) we supply for the first time accurate estimates of their pulsation parameters (periods, amplitudes, mean magnitude and colors) on the basis of CCD photometry. Approximately 50% of both RR Lyrae and Anomalous Cepheids are new identifications. Among the RR Lyrae sample 6 objects are new candidate double-mode variables. On the basis of their pulsation properties we estimated that two variables (V152, V182) are about 50% more massive than typical RR Lyrae stars, while the bulk of the Anomalous Cepheids are roughly a factor of two more massive than fundamental mode RR Lyrae stars. This finding supports the evidence that these objects are intermediate-mass stars during central He burning phases. The data for Carina, together with data available in the literature, strongly support the conclusion that dSph galaxies can barely be classified into the classical Oosterhoff dichotomy. The mean period of fundamental mode RR Lyrae in Carina resembles that found for Oosterhoff type II clusters, whereas the ratio between first overtones and total number of RR Lyrae is quite similar to that found in Oosterhoff type I clusters.

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arXiv2003

"The Carina Project": color magnitude diagram and radial distribution

We present B, V photometric data of the Carina Dwarf Spheroidal Galaxy collected with the Wide Field Imager (WFI) available at the 2.2m MPI/ESO telescope. We briefly discuss the main features of the color magnitude diagram and in particular the mix of stellar populations present in this galaxy. A preliminary analysis of the spatial distribution of these populations over a substantial fraction of the body of the galaxy is also presented.

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arXiv2002

Multiwavelength Time Series Data of the LMC Cluster Reticulum

We present accurate multiwavelength UBVI time series data of the LMC cluster Reticulum. Data cover a time interval of ~ 6 yr and have been collected with SUSI1/2 and SOFI at NTT/ESO. For each band we collected approximately 30 short/long exposures and the total exposure times range from roughly 3500 (U, B), 6000s (V) to 8300s (I). The observing strategy and data reduction (DAOPHOTII/ALLFRAME) allowed us to reach a photometry accuracy of 0.02 magnitude from the tip of the Red Giant Branch well below the Turn-Off region. Even though this cluster presents a very low central density, we found a sizable sample of Blue Stragglers (BSs). We also selected stars with a variability index larger than 2, and interestingly enough we detected together with the RR Lyrae stars a large sample of variable stars around and below the TO region. Preliminary analysis on the luminosity variation indicate that these objects might be binary stars.

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arXiv2002

On the distance of Magellanic Clouds: first overtone Cepheids

We present a detailed comparison between predicted and empirical PL_{I,K} relations and Wesenheit function for Galactic and Magellanic Clouds (MCs) First Overtone (FO) Cepheids. We find that zero-points predicted by Galactic Cepheid models based on a noncanonical (mild overshooting) Mass-Luminosity (ML) relation are in very good agreement with empirical zero-points based on HIPPARCOS parallaxes, while those based on canonical (no overshooting) ML relation are about 0.2-0.3 mag brighter. We also find that predicted and empirical PL_K relation and Wesenheit function give, according to optical (V,I OGLE) and near-infrared (NIR, K, 2mass) data, mean distances to the MCs that agree at the 2% level. Individual distances to the Large and the Small Cloud are: 18.53+-0.08-19.04+-0.11 (theory) and 18.48+-0.13-19.01+-0.13 (empirical). Moreover, predicted and empirical FO relations do not present, within the errors, a metallicity dependence. Finaly, we find that the upper limit in the FO period distribution is a robust observable to constrain the accuracy of pulsation models. Current models agree within 0.1 in log P with the observed FO upper limits.

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