M. Castellani

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.

A Pulsational Distance to ω Centauri Based on Near-Infrared Period-Luminosity Relations of RR Lyrae Stars*

We present new near-infrared (J and K) magnitudes for 114 RR Lyrae stars in the globular cluster ω Centauri (NGC 5139), which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is currently the largest such sample in any stellar system. We also present updated predictions for J- and K-band period-luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z = 0.0001 to 0.004. By adopting for the ω Cen variables with measured metal abundances an α-element enhancement of a factor of 3 (≈0.5 dex) with respect to iron, we find a true distance modulus μ0 = 13.70 ± 0.06 ± 0.06 (random and systematic errors, respectively), corresponding to a distance d = 5.5 ± 0.03 ± 0.03 kpc. Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC 17, but differ significantly from the recent distance estimates based on cluster dynamics and on high-amplitude δ Scuti stars.

Strömgren Photometry of Galactic Globular Clusters. I. New Calibrations of the Metallicity Index

We present a new calibration of the Stromgren metallicity index m(1) using red giant (RG) stars in four globular clusters (GCs: M92, M13, NGC 1851, 47 Tuc) with metallicity ranging from - 2.2 to - 0.7, marginally affected by reddening [E( B - V) <= 0: 04] and with accurate (u, v, b, y) photometry. The main difference between the new metallicity-index-color (MIC) relations and similar relations available in the literature is that we have adopted the u - y and v - y colors instead of b - y. These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear slope. The difference between photometric estimates and spectroscopic measurements for RGs in M71, NGC 288, NGC 362, NGC 6397, and NGC 6752 is 0: 04 +/- 0: 03 dex (sigma = 0: 11 dex). We also apply the new MIC relations to 85 field RGs with metallicity ranging from - 2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is -0.14 +/- 0.01 dex (sigma = 0.17 dex). We also provide two sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semiempirical or theoretical color-temperature relations. We apply the semiempirical relations to the nine GCs and find that the difference between photometric and spectroscopic metallicities is 0.04 +/- 0.03 dex (sigma = 0.10 dex). A similar agreement is found for the sample of field RGs, with a difference of -0.09 +/- 0.03 dex (with sigma = 0.19 dex). The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11 +/- 0.03 dex (sigma = 0.14 dex) for the nine GCs and -0.24 +/- 0.03 dex (sigma = 0.15 dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2 dex.

STROMGREN PHOTOMETRY OF GALACTIC GLOBULAR CLUSTERS. II. METALLICITY DISTRIBUTION OF RED GIANTS IN ω CENTAURI

We present new intermediate-band Str?mgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster ? Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J, H, Ks images). The final optical-NIR catalog covers a region of more than 20 ? 20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper-motion data available in the literature to identify candidate cluster red-giant (RG) stars. By adopting different Str?mgren metallicity indices, we estimate the photometric metallicity for 4000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]phot = ?1.73 ? 0.08, ?1.29 ? 0.03, ?1.05 ? 0.02, ?0.80 ? 0.04, ?0.42 ? 0.12, and ?0.07 ? 0.08 dex) and a sharp cutoff in the metal-poor (MP) tail ([Fe/H]phot ?2 dex) that agree quite well with spectroscopic measurements. We identify four distinct subpopulations, namely, MP ([Fe/H] ? ?1.49), metal-intermediate (MI; ?1.49 < [Fe/H] ? ?0.93), metal-rich (MR; ?0.95 < [Fe/H] ? ?0.15), and solar metallicity ([Fe/H] 0). The last group includes only a small fraction of stars (~8% ? 5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19% ? 1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Str?mgren metallicity indices display a robust correlation with ?-elements ([Ca+Si/H]) when moving from the MI to the MR regime ([Fe/H] ?1.7 dex).

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 vari- ables, 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 ≥ 0.001. Predictions concerning the parameter R are also discussed and compared under various assumptions about the horizontal branch reference luminosity level.

The Carina Project. I. Bright Variable Stars

We present new BV time series data of the Carina dwarf spheroidal galaxy (dSph). Current data cover an area of ?0.3 deg2 around the center of the galaxy and allow 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 star, and one is located along the Carina red giant branch. Expanding upon the seminal photographic investigation by Saha, Monet, & Seitzer we supply, for the first time, accurate estimates of their pulsation parameters (periods, amplitudes, mean magnitudes, and colors) on the basis of CCD photometry. Approximately 50% of both RR Lyrae stars and anomalous Cepheids are new identifications. Among the RR Lyrae sample, six objects are new candidate double-mode (RRd) variables. On the basis of their pulsation properties we estimate that two variables (V158, V182) are about 50% more massive than typical RR Lyrae stars, while the bulk of the anomalous Cepheids are roughly a factor of 2 more massive than fundamental-mode (RRab) RR Lyrae stars. This finding supports the evidence that these objects are intermediate-mass stars during central He-burning phases. We adopted three different approaches to estimate the Carina distance modulus, namely, the first-overtone blue edge method, the period-luminosity-amplitude relation, and the period-luminosity-color relation. We found DM = 20.19 ? 0.12, a result that agrees quite well with similar estimates based on different distance indicators. The data for Carina, together with data available in the literature, strongly support the conclusion that dSphs can barely be classified into the classical Oosterhoff dichotomy. The mean period of RRabs in Carina resembles that found for Oosterhoff type II clusters, whereas the ratio between first-overtone (RRc) pulsators and the total number of RR Lyrae stars is quite similar to that found in Oosterhoff type I clusters.

Star counts in the globular cluster ω centauri. I. Bright stellar components

We present a photometric investigation on HB, RGB, and MSTO stars in ω Cen=NGC 5139. The center of the cluster was covered with a mosaic of F435W, F625W, and F658N band data collected with HST ACS. The outer reaches were covered with a mosaic of U-, B-, V-, and I-band data collected with the 2.2 m ESO/MPI telescope. The final catalog includes ~1.7 million stars. We identified more than 3200 likely HB stars, the largest sample ever collected in a globular cluster. We found that the HB morphology changes with the radial distance from the cluster center. The relative number of extreme HB stars decreases from ~30% to ~21% when moving from the center toward the outer reaches of the cluster, while the fraction of less hot HB stars increases from ~62% to ~72%. The comparison between theory and observations indicates that the empirical star counts of HB stars are on average larger (30%-40%) than predicted by canonical evolutionary models. Moreover, the rate of HB stars is ~43% larger than the MSTO rate. We also compared theory and observations by assuming a mix of stellar populations made with 70% of canonical He (Y = 0.23) stars and 30% of He-enhanced (Y = 0.33, 0.42) stars. We found that the observed RG/MSTO ratio agrees with the predicted lifetimes of He-mixed stellar populations. The discrepancy between theory and observations decreases by a factor of 2 when compared with rates predicted by canonical He content models, but still 15%-25% (Y = 0.42) and 15%-20% (Y = 0.33) higher than observed. Furthermore, the ratios between HB and MSTO star counts are ~24% (Y = 0.42) and 30% (Y = 0.33) larger than predicted lifetime ratios.

Gaia Data Release 1 - The photometric data

Context. This paper presents an overview of the photometric data that are part of the first Gaia data release. Aims. The principles of the processing and the main characteristics of the Gaia photometric data are presented. Methods. The calibration strategy is outlined briefly and the main properties of the resulting photometry are presented. Results. Relations with other broadband photometric systems are provided. The overall precision for the Gaia photometry is shown to be at the milli-magnitude level and has a clear potential to improve further in future releases.

On the Delta V_HB_bump parameter in Globular Clusters

We present new empirical estimates of the DELTAV {sup bump}{sub HB} parameter for 15 Galactic globular clusters (GGCs) using accurate and homogeneous ground-based optical data. Together with similar evaluations available in the literature, we ended up with a sample of 62 GGCs covering a very broad range in metal content (-2.16 dex = 0), might be systematically smaller than predicted.

On the Stellar Content of the Carina Dwarf Spheroidal Galaxy

We present deep, accurate, and homogeneous multiband optical (U, B, V, I) photometry of the Carina dwarf spheroidal galaxy, based on more than 4000 individual CCD images from three different ground-based telescopes. Special attention was given to the photometric calibration, and the precision for the B, V, and I bands is generally better than 0.01 mag. We have performed detailed comparisons in the V, B - V, and V, B - I color-magnitude diagrams (CMDs) between Carina and three old, metal-poor Galactic globular clusters (GGCs, M53, M55, M79). We find that only the more metal-poor GCs (M55, [Fe/H] = -1.85; M53, [Fe/H] = -2.02 dex) provide a good match with the Carina giant branch. We have performed a similar comparison in the V, V - I CMD with three intermediate-age clusters (IACs) of the Small Magellanic Cloud (Kron 3, NGC 339, Lindsay 38). We find that the color extent of the subgiant branch (SGB) of the two more metal-rich IACs (Kron 3, [Fe/H] = -1.08; NGC339, [Fe/H] = -1.36 dex) is smaller than the range among Carinas intermediate-age stars. Moreover, the slope of the RGB of these two IACs is shallower than the slope of the Carina RGB. However, the ridge line of the more metal-poor IAC (Lindsay 38, [Fe/H] = -1.59 dex) agrees quite well with the Carina intermediate-age stars. These findings indicate that Carinas old stellar population is metal-poor and appears to have a limited spread in metallicity (Δ[Fe/H] = 0.2-0.3 dex). The Carinas intermediate-age stellar population can hardly be more metal-rich than Lindsay 38, and its spread in metallicity also appears modest. We also find that the synthetic CMD constructed assuming a metallicity spread of 0.5 dex for the intermediate-age stellar component predicts evolutionary features not supported by observations. In particular, red clump stars should attain colors that are redder than red giant stars, but this is not seen. These results are at odds with recent spectroscopic investigations suggesting that Carina stars cover a broad range in metallicity (Δ[Fe/H] ~ 1-2 dex). We also present a new method to estimate the metallicity of complex stellar systems using the difference in color between the red clump and the middle of the RR Lyrae instability strip. The observed colors of Carinas evolved stars indicate a metallicity of [Fe/H] = -1.70 ± 0.19 dex, which agrees quite well with spectroscopic measurements.