arXiv subjects

F. Caputo

Publications and source records attributed to F. Caputo.

At least 73 records · Page 4Linked to original sources

arXiv2002

Theoretical models for classical Cepheids. VIII. Effects of helium and heavy elements abundance on the Cepheid distance scale

Previous nonlinear fundamental pulsation models for classical Cepheids with metal content Z <= 0.02 are implemented with new computations at super-solar metallicity (Z=0.03, 0.04) and selected choices of the helium-to-metal enrichment ratio DeltaY/Delta Z. On this basis, we show that the location into the HR diagram of the Cepheid instability strip is dependent on both metal and helium abundance, moving towards higher effective temperatures with decreasing the metal content (at fixed Y) or with increasing the helium content (at fixed Z). The contributions of helium and metals to the predicted Period-Luminosity and Period-Luminosity-Color relations are discussed, as well as the implications on the Cepheid distance scale. Based on these new results, we finally show that the empirical metallicity correction suggested by Cepheid observations in two fields of the galaxy M101 may be accounted for, provided that the adopted helium-to-metal enrichment ratio is reasonably high (Delta Y/Delta Z ~ 3.5).

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arXiv2002

On the pulsation parallax of the variable star RR Lyr

We show that a straightforward application of the predicted relation connecting the absolute K magnitude of fundamental RR Lyrae variables to their period and metal content, together with current evolutionary predictions on the mass and luminosity of Horizontal Branch stars, supply a distance estimate to the prototype star RR Lyr in close agreement with the recent parallax determination by HST (Benedict et al. 2002), largely unaffected by the interstellar extinction correction. Adopting =0.12+-0.10 mag as a safe estimate of the extinction correction to RR Lyr, we obtain a "pulsation" parallax of 3.858+-0.131 mas which agrees quite well, and with a smaller formal error, with the HST measurement (3.82+-0.20 mas) and with the weighted average of HST, Hipparcos, and ground-based determinations (3.87+-0.19 mas). This result suggests that near-infrared magnitudes and pulsation models could constrain the distance to well-studied RR Lyrae stars, both in the field and in globular clusters, with an accuracy better than current direct trigonometric measurements.

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arXiv2002

Global metallicity of globular cluster stars from colour-magnitude diagrams

We have developed an homogeneous evolutionary scenario for H- and He-burning low-mass stars by computing updated stellar models for a wide metallicity and age range (0.0002$\le Z \le$0.004 and 9$\le t(Gyr) \le$15, respectively) suitable to study globular clusters. This theoretical scenario allows us to provide self-consistent predictions about the dependence of selected observational features of the colour-magnitude diagram, such as the brightness of the Turn Off (TO), Zero Age Horizontal Branch (ZAHB) and Red Giant Branch bump (BUMP), on the cluster metallicity and age. Taking into account these predictions, we introduce a new observable based on the visual magnitude difference between the TO and the ZAHB, and the TO and the RGB-bump, given by $A=ΔM_V$(TO-BUMP)$-0.566ΔM_V$(TO-ZAHB). We show that the parameter $A$ does not depend at all on the cluster age, whereas it does strongly depend on the cluster global metallicity. The calibration of the parameter $A$ as a function of $Z$ is then provided, as based on our evolutionary models. We tested the reliability of this result by also considering stellar models computed by other authors,employing different input physics. Eventually, we present clear evidence that the variation of $ΔM_V$(TO-BUMP) with $ΔM_V$(TO-ZAHB) does supply a powerful probe of the global metal abundance, at least when homogeneous theoretical frameworks are adopted. We provide an estimate of the global metallicity of 36 globular clusters in the Milky Way, based on our {\it A-Z} calibration, and a large observational database of Galactic GCs. By considering the empirical [Fe/H] scales by both Zinn & West (1984) and Carretta & Gratton (1997), we are also able to provide an estimate of the GC $α-$element enhancement.

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arXiv2001

The Cepheid Period-Luminosity relation and the maser distance to NGC 4258

In a recent paper describing HST observations of Cepheids in the spiral galaxy NGC 4258, Newman et al. (2001) report that the revised calibrations and methods for the Key Project on the Extragalactic Distance Scale yield that the true distance modulus of this galaxy is 29.40+-0.09 mag, corresponding to a metric distance of 7.6+-0.3 Mpc. This Cepheid distance, which holds for 18.50 mag as the true distance modulus of the LMC, is not significantly larger than 7.2+-0.5 Mpc, the value determined by Herrnstein et al. (1999) from purely geometric considerations on the orbital motions of water maser sources. However, if the metallicity difference D[O/H]~0.35 between NGC 4258 and LMC is taken into account, then the Key Project methods lead to a metallicity-corrected value of 29.47+-0.09 mag, with 18.50 mag for the LMC, namely to a Cepheid distance of 7.8+-0.3 Mpc, which is 1.2 sigma from the maser determination. In this paper we show that the metallicity correction on Cepheid distance determinations, as suggested by pulsation models, might provide the natural way of reaching a close agreement between Cepheid and maser distance to NGC 4258 for a wide variety of LMC distance determinations.

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arXiv2001

Improving the mass determination of Galactic Cepheids

We have selected a sample of Galactic Cepheids for which accurate estimates of radii, distances, and photometric parameters are available. The comparison between their pulsation masses, based on new Period-Mass-Radius (PMR) relations, and their evolutionary masses, based on both optical and NIR Color-Magnitude (CM) diagrams, suggests that pulsation masses are on average of the order of 10% smaller than the evolutionary masses. Current pulsation masses show, at fixed radius, a strongly reduced dispersion when compared with values published in literature.The increased precision in the pulsation masses is due to the fact that our predicted PMR relations based on nonlinear, convective Cepheid models present smaller standard deviations than PMR relations based on linear models. At the same time, the empirical radii of our Cepheid sample are typically accurate at the 5% level. Our evolutionary mass determinations are based on stellar models constructed by neglecting the effect of mass-loss during the He burning phase. Therefore, the difference between pulsation and evolutionary masses could be intrinsic and does not necessarily imply a problem with either evolutionary and/or nonlinear pulsation models. The marginal evidence of a trend in the difference between evolutionary and pulsation masses when moving from short to long-period Cepheids is also briefly discussed. The main finding of our investigation is that the long-standing Cepheid mass discrepancy seems now resolved at the 10% level either if account for canonical or mild convective core overshooting evolutionary models.

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arXiv2001

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).

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arXiv2001

On the second overtone stability among SMC Cepheids

We present a new set of Cepheid, full amplitude, nonlinear, convective models which are pulsationally unstable in the second overtone (SO). Hydrodynamical models were constructed by adopting a chemical composition typical for Cepheids in the Small Magellanic Cloud (SMC) and stellar masses ranging from 3.25 to 4 solar masses. Predicted phi{21} Fourier parameters agree, within current uncertainties, with empirical data for pure first and second overtone variables as well as for first/second overtone (FO/SO) double-mode Cepheids collected by Udalski et al. (1999a,b) in the SMC. On the other hand, predicted I band amplitudes are systematically larger than the observed ones in the short period range. We also find, in agreement with empirical evidence, that the region within which both second and first overtones attain a stable limit cycle widens when moving toward lower luminosities. Moreover, predicted P{SO}/P{FO} and P{FO}/P{F} period ratios agree quite well with empirical period ratios for FO/SO and F/FO double-mode SMC Cepheids. Finnaly, current models support the evidence that pure SO Cepheids and SO components in FO/SO Cepheids are good distance indicators.

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arXiv2001

Linear nonadiabatic properties of SX Phoenicis variables

We present a detailed linear, nonadiabatic pulsational scenario for oscillating Blue Stragglers (BSs)/SX Phoenicis (SX Phe) in Galactic Globular Clusters (GGCs) and in Local Group (LG) dwarf galaxies. The sequences of models were constructed by adopting a wide range of input parameters and properly cover the region of the HR diagram in which these objects are expected to be pulsationally unstable. Current calculations together with more metal-rich models already presented by Gilliland et al. suggest that the pulsation properties of SX Phe are partially affected by metal content. Our calculations also suggest that the secular period change seems to be a good observable to identify the pulsation mode of cooler SX Phe variables. The dependence of pulsation properties on the helium envelope content is discussed and we find that the secular period change for He-enhanced models is approximately a factor of two larger than for canonical ones. According to this evidence we suggest that the pulsation properties of SX Phe can be soundly adopted to constrain the evolutionary history of BSs, and in turn to single out the physical mechanisms which trigger their formation.

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arXiv2000

Intermediate-mass star models with different helium and metal contents

We present a comprehensive theoretical investigation of the evolutionary properties of intermediate-mass stars. The evolutionary sequences were computed from the Zero Age Main Sequence up to the central He exhaustion and often up to the phases which precede the carbon ignition or to the reignition of the H-shell which marks the beginning of the thermal pulse phase. The evolutionary tracks were constructed by adopting a wide range of stellar masses ($3\leq$\msun$\leq15$) and chemical compositions. In order to account for current uncertainties on the He to heavy elements enrichment ratio, the stellar models were computed by adopting at Z=0.02 two different He contents (Y=0.27, 0.289) and at Z=0.04 three different He contents (Y=0.29, 0.34, and 0.37). To supply a homogeneous evolutionary scenario which accounts for young Magellanic stellar systems the calculations were also extended toward lower metallicities (Z=0.004, Z=0.01), by adopting different initial He abundances. We evaluated for both solar (Z=0.02) and super-metal-rich (SMR, Z=0.04) models the transition mass $M^{up}$ between the stellar structures igniting carbon and those which develop a full electron degeneracy inside the CO core. This evolutionary scenario allows us to investigate in detail the properties of classical Cepheids. In particular, we find that the range of stellar masses which perform the blue loop during the central He-burning phase narrows when moving toward metal-rich and SMR structures.

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arXiv2000

Theoretical Models for Classical Cepheids: VII. Metallicity effects on the Cepheid distance scale

We use theoretical Period-Luminosity and Period-Luminosity-Color relations in the VI passbands, as based on nonlinear, nonlocal and time-dependent convective pulsating models, to predict the reddening and true distance modulus of distant Cepheids observed with the Hubble Space Telescope. By relying on the pulsating models with metal content Z=0.008, we find that the theoretical predictions agree to the values obtained by the Extragalactic Distance Scale Key Project on the basis of empirical Period-Luminosity relations referenced to LMC variables. In the meantime, from the theoretical relations with Z=0.004 and 0.02 we find that the predicted E(B-V) and mu_0 decrease as the adopted metal content increases. This suggests a metallicity correction to LMC-based distances as given by about -0.27 mag/dex. Such a theoretical correction appears supported by an existing, although weak, correlation between the Cepheid distance and the [O/H] metallicity of galaxies within a given group or cluster, as well as by a similar correlation between the H_0 estimate and the [O/H] metallicity of the galaxies which calibrate the SNIa luminosity. On the contrary, the metallicity correction earlier suggested on empirical grounds seem to be excluded. Eventually we suggest that the average value ~ 67 km/s/Mpc provided by the Key Project team should increase at least up to ~ 69 km/s/Mpc.

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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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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 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

Stellar populations in the dwarf spheroidal galaxy Leo I

We present a detailed study of the color magnitude diagram (CMD) of the dwarf spheroidal galaxy Leo I, based on archival Hubble Space Telescope data. Our photometric analysis, confirming previous results on the brighter portion of the CMD, allow us to obtain an accurate sampling of the stellar populations also at the faint magnitudes corresponding to the Main Sequence. By adopting a homogeneous and consistent theoretical scenario for both hydrogen and central helium-burning evolutionary phases, the various features observed in the CMD are interpreted and reliable estimations for both the distance modulus and the age(s) for the main stellar components of Leo I are derived. More in details, from the upper luminosity of the Red Giant Branch and the lower luminosity of the Subgiant Branch we simultaneously constrain the galaxy distance and the age of the oldest stellar population in Leo I. In this way we obtain a distance modulus (m-M)_V=22.00$\pm$0.15 mag and an age of 10--15 Gyr or 9--13 Gyr, adopting a metallicity Z=0.0001 and 0.0004, respectively. The reliability of this distance modulus has been tested by comparing the observed distribution of the Leo I anomalous Cepheids in the period-magnitude diagram with the predicted boundaries of the instability strip, as given by convective pulsating models.

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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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arXiv1995

Star luminosity function as an age indicator for the Dwarf spheroidal Leo I

Star luminosity function, already recognized as an age indicator for old galactic globular clusters, can be used to contrains the age of younger stellar systems like the nearby dwarfs spheroidal Leo I. We compare the observed luminosity function of Leo I, presented by Lee et al. 1993, with theoretical expectations for three selected ages, 1, 1.5 and 2 billion years, deriving an age of about 1.5 Gyr. This result does not appear critically affected by assumptions about the cluster distance modulus or the Initial Mass Function.

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