I. Musella

Cepheid calibration of Type Ia supernovae and the Hubble constant

We investigate how a different calibration of the Cepheid period-luminosity (PL) relation, taking into account metallicity corrections, affects the absolute magnitude calibration of Type Ia supernovae (SNe Ia) and, in turn, the determination of the Hubble constant H 0 . We use SN Ia light curves from the literature and previously unpublished data to establish the M B -Δm 15 (B) relation, and calibrate the zero point by means of nine SNe Ia with Cepheid-measured distances. This relation is then used to establish the Hubble diagram, and in turn to derive H 0 . In the attempt to correct for the host-galaxy extinction, we find that the data suggest a value for the total to selective absorption ratio of R B = 3.5, which is smaller than the standard value for our own Galaxy of R B = 4.315. Depending on the metallicity correction for the Cepheid PL relation, the value of R B , and SN sample selection criteria, the value of the Hubble constant H 0 takes a value in the range 68-74 km s -1 Mpc -1 , with associated uncertainties of the order of 10 per cent. Unpublished photometry is also presented for 18 SNe of our sample (1991S, 1991T, 1992A, 1992K, 1993H, 1993L, 1994D, 1994M, 1994ae, 1995D, 1995ac, 1995bd, 1996bo, 1997bp, 1997br, 1999aa, 1999dk, 2000cx). These data are the results of a long-standing effort in supernova monitoring at ESO - La Silla and Asiago observatories.

Cepheid Pulsation Models at Varying Metallicity and ΔY/ΔZ

In this paper we present an extended set of nonlinear convective pulsation models at varying metallicity and ΔY/ΔZ ratio. The predicted instability strip and bolometric light curves are discussed by comparing the new models with our previous ones. In particular, the dependence on both metal and helium abundances is investigated. By transforming the bolometric light curves into the observational bands, we are able to derive both period-color-luminosity and Wesenheit relations for each selected chemical composition. Synthetic period-luminosity relations are obtained by populating the instability strip according to specific assumptions on the number of pulsators and the mass distribution. These theoretical results are compared with recent accurate data by Sandage et al. and Kervella et al. in order to test the predictive capabilities of the models. We confirm our previous results that the theoretical metallicity correction to the Key Project Cepheid distance scale depends on both the period range and ΔY/ΔZ ratio, becoming important for periods longer than 20 days and ΔY/ΔZ > 1.5.

I Zw 18 Revisited with HST ACS and Cepheids: New Distance and Age*

We present new V- and I-band HST ACS photometry of I Zw 18, the most metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been argued in the past that I Zw 18 is a very young system that started forming stars only 500 Myr ago, but other work has hinted that older (1 Gyr) red giant branch (RGB) stars may also exist. Our new data, once combined with archival HST ACS data, provide a deep and uncontaminated optical color-magnitude diagram (CMD) that now strongly indicates an RGB. The RGB tip (TRGB) magnitude yields a distance modulus (m - M)0 = 31.30 ± 0.17, i.e., D = 18.2 ± 1.5 Mpc. The time-series nature of our observations allows us to also detect and characterize for the first time three classical Cepheids in I Zw 18. The time-averaged Cepheid V and I magnitudes are compared to the VI reddening-free Wesenheit relation predicted from new nonlinear pulsation models specifically calculated at the metallicity of I Zw 18. For the one bona fide classical Cepheid with a period of 8.63 days this implies a distance modulus (m - M)0 = 31.42 ± 0.26. The other two Cepheids have unusually long periods (125.0 and 129.8 days) but are consistent with this distance. The coherent picture that emerges is that I Zw 18 is farther away than previously assumed and older than suggested by some previous works. The presence of an RGB population rules out the possibility that I Zw 18 is a truly primordial galaxy formed recently (z 0.1) in the local universe.

Cepheid pulsation models at varying metallicity and DY/DZ

In this paper we present an extended set of nonlinear convective pulsation models at varying metallicity and ΔY/ΔZ ratio. The predicted instability strip and bolometric light curves are discussed by comparing the new models with our previous ones. In particular, the dependence on both metal and helium abundances is investigated. By transforming the bolometric light curves into the observational bands, we are able to derive both period-color-luminosity and Wesenheit relations for each selected chemical composition. Synthetic period-luminosity relations are obtained by populating the instability strip according to specific assumptions on the number of pulsators and the mass distribution. These theoretical results are compared with recent accurate data by Sandage et al. and Kervella et al. in order to test the predictive capabilities of the models. We confirm our previous results that the theoretical metallicity correction to the Key Project Cepheid distance scale depends on both the period range and ΔY/ΔZ ratio, becoming important for periods longer than 20 days and ΔY/ΔZ > 1.5.

Theoretical Models for Classical Cepheids. VIII. Effects of Helium and Heavy-Element 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 supersolar metallicity (Z = 0.03, 0.04) and selected choices of the helium-to-metal enrichment ratio ?Y/?Z. On this basis, we show that the location into the H-R diagram of the Cepheid instability strip is dependent on both metal and helium abundance, moving toward higher effective temperatures with decreasing metal content (at fixed Y) or with increasing 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. We suggest that the adoption of empirical V and I period-luminosity relations, as inferred by Cepheids at the LMC, to get distance moduli with an uncertainty of ?0.10 mag is fully justified for variables in the short-period range (P ? 10 days), at least with Z ? 0.04 and ?Y/?Z in the range of 2-4. Conversely, at longer periods (P > 10 days) a correction to LMC-based distance moduli may be needed, whose sign and amount depend on the helium and metal content of the Cepheids. Specifically, from fundamental pulsators with Z > 0.008 we derive that the correction (in magnitude) may be approximated as c = -6.03 + 17.80Y - 2.80 log Z + 8.19Y log Z, with a total intrinsic uncertainty of ?0.05 mag, whereas c = -0.23(?0.03) log(Z/0.008) if Z < 0.008. Based on these new results, we 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 (?Y/?Z ~ 3.5).

On a New Theoretical Framework for RR Lyrae Stars. I. The Metallicity Dependence

We present new nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio and a broad range in metal abundances (Z=0.0001--0.02). The stellar masses and luminosities adopted to construct the pulsation models were fixed according to detailed central He burning Horizontal Branch evolutionary models. The pulsation models cover a broad range in stellar luminosity and effective temperatures and the modal stability is investigated for both fundamental and first overtones. We predict the topology of the instability strip as a function of the metal content and new analytical relations for the edges of the instability strip in the observational plane. Moreover, a new analytical relation to constrain the pulsation mass of double pulsators as a function of the period ratio and the metal content is provided. We derive new Period-Radius-Metallicity relations for fundamental and first-overtone pulsators. They agree quite well with similar empirical and theoretical relations in the literature. From the predicted bolometric light curves, transformed into optical (UBVRI) and near-infrared (JHK) bands, we compute the intensity-averaged mean magnitudes along the entire pulsation cycle and, in turn, new and homogenous metal-dependent (RIJHK) Period-Luminosity relations. Moreover, we compute new dual and triple band optical, optical--NIR and NIR Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical Period-W(V,B-V) is independent of the metal content and that the accuracy of individual distances is a balance between the adopted diagnostics and the precision of photometric and spectroscopic datasets.

Cepheids in External Galaxies. I. The Maser-Host Galaxy NGC 4258 and the Metallicity Dependence of Period-Luminosity and Period-Wesenheit Relations

We perform a detailed analysis of Cepheids in NGC 4258, the Magellanic Clouds, and Milky Way in order to verify the reliability of the theoretical scenario based on a large set of nonlinear convective pulsation models. We derive Wesenheit functions from the synthetic -->BVI magnitudes of the pulsators, and we show that the sign and the extent of the metallicity effect on the predicted period-Wesenheit ( -->P − W) relations change according to the adopted passbands. These -->P − W relations are applied to measured -->BVI magnitudes of NGC 4258, Magellanic, and Galactic Cepheids available in the literature. We find that Magellanic and Galactic Cepheids agree with the metallicity dependence of the predicted -->P − W relations. Concerning the NGC 4258 Cepheids, the results strongly depend on the adopted metallicity gradient across the galactic disk. The most recent nebular oxygen abundances support a shallower gradient and provide a metallicity dependence that agrees well with current pulsation predictions. Moreover, the comparison of Cepheid distances based on -->VI magnitudes with distance estimates based on the revised TRGB method for external galaxies, on the HST trigonometric parallaxes for Galactic Cepheids, and on eclipsing binaries in the Magellanic Clouds seems to favor the metallicity correction predicted by pulsation models. The sign and the extent of the metallicity dependence of the -->P − W and of the period-luminosity ( -->P − L) relations change according to the adopted passbands. Therefore, distances based on different methods and/or bands should not be averaged. The use of extragalactic Cepheids to constrain the metallicity effect requires new accurate and extensive nebular oxygen measurements.

NGC 1866: a milestone for understanding the chemical evolution of stellar populations in the Large Magellanic Cloud★

We present new FLAMES@VLT spectroscopic observations of 30 stars in the field of the LMC stellar cluster NGC 1866. NGC 1866 is one of the few young and massive globular cluster that is close enough so that its stars can be individually studied in detail. Radial velocities have been used to separate stars belonging to the cluster and to the LMC field and the same spectra have been used to derive chemical abundances for a variety of elements, from [Fe/H] to the light (i.e. Na, O, Mg...) to the heavy ones. The average iron abundance of NGC 1866 turns out to be [Fe/H]= –0.43±0.01 dex (with a dispersion �= 0.04 dex), from the analysis of 14 cluster-member stars. Within our uncertainties, the cluster stars are homogeneous, as far as chemical composition is concerned, independent of the evolutionary status. The observed cluster stars do not show any sign of the light elements ’anti-correlation’ present in all the Galactic globular clusters so far studied, and also found in the old LMC stellar clusters. A similar lack of anti-correlations has been detected in the massive intermediate-age LMC clusters, indicating a different formation/evolution scenario for the LMC massive clusters younger than � 3 Gyr with respect to the old ones. Also opposite to the Galactic globulars, the chemical composition of the older RGB field stars and of the young post-MS cluster stars show robust homogeneity suggesting a quite similar process of chemical evolution. The field and cluster abundances are in agreement with recent chemical analysis of LMC stars, which show a distinctive chemical pattern for this galaxy with respect to the Milky Way. We discuss these findings in light of the theoretical scenario of chemical evolution of the LMC.

Classical Cepheid pulsation models XI. Effects of convection and chemical composition on the period-luminosity and period-Wesenheit relations

In spite of the relevance of classical Cepheids as primary distance indicators, a general consensus on the dependence of the period-luminosity ( PL ) relation on the Cepheid chemical composition has not yet been achieved. From the theoretical point of view, our previous investigations were able to reproduce some empirical tests for suitable assumptions on the helium-to-metal relative enrichment, but these results relied on specific assumptions concerning the mass-luminosity relation and the efficiency of the convective transfer in the pulsating envelopes. In this paper, we investigate the effects of the assumed value of the mixing-length parameter

Cepheid theoretical models and observations in HST/WFC3 filters: the effect on the Hubble constant H0

l/H_{\rm p}