arXiv subjects

M. Marconi

Publications and source records attributed to M. Marconi.

At least 217 records · Page 12Linked to original sources

arXiv2002

Anomalous RR Lyrae stars(?). III. CM Leonis

Time series of B,V,I CCD photometry and radial velocity measurements from high resolution spectroscopy (R=30,000) covering the full pulsation cycle are presented for the field RR Lyrae star CM Leonis. The photometric data span a 6 year interval from 1994 to 1999, and allow us to firmly establish the pulsation mode and periodicity of the variable. The derived period P=0.361699 days (+/- 0.000001) is very close to the value published in the Fourth Edition of the General Catalogue of Variable Stars (P=0.361732 days). However, contrary to what was previously found, the amplitude and shape of the light curve qualify CM Leo as a very regular first overtone pulsator with a prominent hump on the rising branch of its multicolour light curves. According to an abundace analysis performed on three spectra taken near minimum light (0.42 < phase < 0.61), CM Leo is a metal-poor star with metal abundance [Fe/H]=-1.93 +/- 0.20. The photometric and radial velocity curves of CM Leo have been compared with the predictions of suitable pulsational models to infer tight constraints on the stellar mass, effective temperature, and distance modulus of the star. We derive a true distance modulus of CM Leo of (m-M)0=13.11 +/- 0.02 mag and a corresponding absolute magnitude of Mv=0.47 +/- 0.04. This absolute magnitude, once corrected for evolutionary and metallicity effects, leads to a true distance modulus of the Large Magellanic Cloud of (m-M)0=18.43 +/- 0.06 mag, in better agreement with the long astronomical distance scale.

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arXiv2002

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.

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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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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

Theoretical models for Bump Cepheids

We present the results of a theoretical investigation aimed at testing whether full amplitude, nonlinear, convective models account for the I-band light curves of Bump Cepheids in the Large Magellanic Cloud (LMC). We selected two objects from the OGLE sample that show a well-defined bump along the decreasing (short-period) and the rising (long-period) branch respectively. We find that current models do reproduce the luminosity variation over the entire pulsation cycle if the adopted stellar mass is roughly 15 % smaller than predicted by evolutionary models that neglect both mass loss and convective core overshooting. Moreover, we find that the fit to the light curve of the long-period Cepheid located close to the cool edge of the instability strip requires an increase in the mixing length from 1.5 to 1.8 Hp. This suggests an increase in the efficiency of the convective transport when moving toward cooler effective temperatures. Current pulsation calculations supply a LMC distance modulus ranging from 18.48 to 18.58 mag.

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arXiv2001

The tricky galactic cluster NGC2420

We discuss the CM diagram of the galactic cluster NGC2420 to the light of current theoretical predictions. By relying on the most recent updating of the physical input, one finds too luminous theoretical He burning stars together with the evidence for a misfitting of the lower portion of the MS. Moreover one finds two well known overshooting signatures, as given by i) the large extension of the "hook" preceding the overall contraction gap, and ii) the scarcity of stars just at the end of the gap. We show that the overluminosity of He burning stars appears as a constant prediction of models based on updated physics, whereas alternative assumptions about the Equation of State can account for the MS fitting. Moreover, due to the scarse statistical significance of the observational sample, one finds that overshooting signatures can be present also in canonical (without overshooting) predictions. We conclude that, unfortunately, NGC2420 does not keep the promise to be of help in costraining the actual dimensions of convective cores in H burning MS stars, suggesting in the meantime that using clumping He burning stars as theoretical standard candle is at least a risky procedure. In this context the need for firmer constraints about the reddening of galactic clusters is shortly discussed.

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arXiv2001

RR Lyrae variables in Omega Centauri: a theoretical route

We present a theoretical approach to infer information about RR Lyrae variables from the morphology of their light curves. The method, already successfully applied to the field first overtone variable U Comae, is now tested on a RR$_{\mathrm ab}$ variable in the globular cluster $ω$ Cen. We show that, with this method, it could be possible to give an estimate of the distance modulus of the cluster with an uncertainty not larger than $\pm$0.1 mag. The predicted variable luminosity and mass are well within the range of theoretical evolutionary expectations giving the ``feeling'' of a full compatibility between pulsational and evolutionary theory. However, before completely relying on the method, further tests are needed; the goal is that a well tested and well calibrated pulsational theory could provide reliable distance moduli from just one (or few) RR Lyrae stars.

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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

Metallicities for Double Mode RR Lyrae in the Large Magellanic Cloud

Metallicities for six double mode RR Lyrae's (RRd's) in the Large Magellanic Cloud have been estimated using the DeltaS method. The derived [Fe/H] values are in the range [Fe/H] = -1.09 to -1.78 (or -0.95 to -1.58, adopting a different calibration of [Fe/H] vs DeltaS). Two stars in our sample are at the very metal rich limit of all RRd's for which metal abundance has been estimated, either by direct measure (for field objects) or on the basis of the hosting system (for objects in globular clusters or external galaxies). These metal abundances, coupled with mass determinations from pulsational models and the Petersen diagram, are used to compare the mass-metallicity distribution of field and cluster RR Lyrae variables. We find that field and cluster RRd's seem to follow the same mass-metallicity distribution, within the observational errors, strengthening the case for uniformity of properties between field and cluster variables At odds to what is usually assumed, we find no significative difference in mass for RR Lyrae's in globular clusters of different metallicity and Oosterhoff types, or there may even be a difference contrary to the commonly accepted one, depending on the metallicity scale adopted to derive masses. This ``unusual'' result for the mass-metallicity relation is probably due, at least in part, to the inclusion of updated opacity tables in the computation of metal-dependent pulsation models.

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arXiv2001

On the pulsation mode identification of short-period Galactic Cepheids

We present new theoretical Period-Radius (PR) relations for first overtone Galactic Cepheids. Current predictions are based on several sequences of nonlinear, convective pulsation models at solar chemical composition (Y=0.28, Z=0.02) and stellar masses ranging from 3.0 to 5.5 Mo. The comparison between predicted and empirical radii of four short-period Galactic Cepheids suggests that QZ Nor and EV Sct are pulsating in the fundamental mode, whereas Polaris and SZ Tau pulsate in the first overtone. This finding supports the mode identifications that rely on the comparison between direct and Period-Luminosity (PL) based distance determinations but it is somewhat at variance with the mode identification based on Fourier parameters. In fact, we find from our models that fundamental and first overtone pulsators attain, for periods ranging from 2.7 to 4 d, quite similar phi_21 values, making mode discrimination from this parameter difficult. The present mode identifications for our sample of Cepheids are strengthened by the accuracy of their empirical radius estimates, as well as by the evidence that predicted fundamental and first overtone radii do not show, within the current uncertainty on the Mass-Luminosity (ML) relation, any degeneracy in the same period range. Accurate radius determinations are therefore an excellent tool to unambiguously determine the pulsation modes of short-period Cepheids.

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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

Classical Cepheid pulsation models. VI. The Hertzsprung progression

We present the results of an extensive theoretical investigation on the pulsation behavior of Bump Cepheids. We constructed several sequences of full amplitude, nonlinear, convective models by adopting a chemical composition typical of Large Magellanic Cloud (LMC) Cepheids (Y=0.25, Z=0.008) and stellar masses ranging from 6.55 to 7.45 solar masses. We find that theoretical light and velocity curves reproduce the HP, and indeed close to the blue edge the bump is located along the descending branch, toward longer periods it crosses at first the luminosity/velocity maximum and then it appears along the rising branch. In particular, we find that the predicted period at the HP center is 11.24 +- 0.46 d and that such a value is in very good agreement with the empirical value estimated by adopting the Fourier parameters of LMC Cepheid light curves. We also find that the models at the HP center are located within the resonance region but not on the 2:1 resonance line. Interestingly enough, the predicted Bump Cepheid masses, based on a Mass-Luminosity (ML) relation which neglects the convective core overshooting, are in good agreement with the empirical masses of Galactic Cepheids estimated by adopting the Baade-Wesselink method. Even by accounting for the metallicity difference between Galactic and LMC Cepheids, this result seems to settle down the long-standing problem of the Bump mass discrepancy. Finally, the dynamical behavior of a cool Bump Cepheid model provides a plain explanation of an ill-understood empirical evidence.

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