V. Castellani

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

In this paper, we present an improved theoretical scenario concerning near-infrared and visual magnitudes of RR Lyr variables, as based on up-to-date pulsating models. New relations connecting V and K absolute magnitudeswith periods, mass, luminosity and metal content are discussed separately for fundamental and first-overtone pulsators. We also show that the V -K colours are predicted to supply tight constraints on the pulsator intrinsic luminosity. 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 Hubble Space Telescope absolute parallax. Moreover, available K and V measurements for field and cluster RR Lyr variables with known reddening and metal content are used to derive a relation connecting the K absolute magnitude to period and metallicity (M K -[Fe/H]-log P) as well as a new calibration of the M V -[Fe/H] relation. The comparison between theoretical prescriptions and observations suggests that RR Lyr stars in the field and in galactic globular clusters (GGCs) should have quite similar evolutionary histories. The comparison between theory and observations also discloses a general agreement that supports the reliability of the current pulsational scenario. On the contrary, current empirical absolute magnitudes based on the Baade-Wesselink (BW) method suggest relations with a zero-point which is fainter than is 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.

The Distance to the Large Magellanic Cloud 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.

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.

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.

On the horizontal branch of the galactic globular cluster NGC 2808

We present new UV (F218W) data for stars in the central region of the Galactic globular cluster NGC 2808, collected with the WFPC2 camera on board the Hubble Space Telescope. These data together with F439W and F555W-band data and previous ground-based observations provide a multifrequency coverage of the cluster stellar population extending up to a distance of 1.7 times the cluster core radius. We discuss this complete sample of stars, which includes 764 Red Giant Branch (RGB) stars brighter than the Horizontal Branch (HB) luminosity level, 1239 HB stars, 119 Asymptotic Giant Branch (AGB), and 22 AGB-manque stellar structures. As already known, we find that blue HB stars separate into three distinct groups. However, our multiband photometry indicates that several stars in the two hotter HB groups show a flat spectrum, thus suggesting the binarity of these objects. Artificial star experiments suggest that at most 50% of them might be photometric blends. Moreover, at variance with previous claims one finds that canonical Zero Age Horizontal Branch (ZAHB) models do reach effective temperatures typical of observed hot HB stars. We also show that the ratio between HB and RGB stars brighter than the HB luminosity level steadly increases when moving from the cluster center to the periphery, passing from R = 1.37 ± 0.14 in the cluster core to R = 1.95 ± 0.26 in the outer cluster regions. We discuss the possible origin of such a radial gradient in the context of the Blue Tails phenomenon, advancing some suggestions concerning the clumpy stellar distribution along the HB.

The Discovery of More than 2000 White Dwarfs in the Globular Cluster ω Centauri

We present deep multiband (F435W, F625W, and F658N) photometric data of the globular cluster ω Cen collected with the Advanced Camera for Surveys on board the Hubble Space Telescope. We identified in the (F435W - F625W, F435W)-plane more than 2000 white dwarf (WD) candidates using three out of nine available pointings. Such a large sample appears to be in agreement with predictions based on the ratio between WD and horizontal-branch evolutionary lifetimes. We also detected ≈1600 WDs in the (F658N - F625W, F625W)-plane, supporting the evidence that a large fraction of current cluster WDs are Hα-bright.

RR Lyrae variables in Galactic globular clusters - V. The case of M 3 pulsators

We use our synthetic Horizontal Branch (HB) procedure to approach the debated problem concern- ing the adequacy of canonical HB stellar models to account for the observed peaked distribution of RR Lyrae fundamentalised periods in the globular cluster M3. We find that by assuming a suitable bimodal mass distri- bution, canonical models account for the observed period distribution. In particular, the best fit model, out of nine random extractions, reaches a 99.9 % Kolmogorov-Smirnov (KS) probability. We also attempt a prediction of the relative distribution of variables in fundamental and first overtone pulsators, reaching a rather satisfactory agreement. However, one finds that canonical models outnumber by roughly a factor of two the observed number of red HB stars. Possible solutions for this discrepancy are outlined. Alternative evolutionary scenarios are also briefly discussed.

PULSATIONAL EVIDENCE FOR MASS LOSS IN NGC 1866 CEPHEIDS

Available observational data for the 20 known Cepheids in the LMC cluster NGC 1866 have been compared with Hubble Space Telescope observations, discovering in the cluster central region five additional variables, one of which appears to be a Cepheid candidate. We also reach the conclusion that only the photometric data for the seven variables in the cluster periphery appear accurate enough to allow a meaningful comparison with the results of pulsational theories. Out of these seven well-observed Cepheids, we find that the six probable cluster members are located in the color-magnitude diagram at the hot tip of the blue nose experienced by He-burning giants. Since evolutionary theory predicts for these giants an original mass on the order of 4 M⊙, we extend down to ~3 M⊙ the theoretical pulsational scenario already presented for M ≥ 5.0 M⊙. On this basis we discuss the four member Cepheids with VI magnitudes accurate enough to produce robust constraints on the pulsating structures. Among these variables, one finds evidence for a spread of masses by about 7%, with the structures following a tight mass-luminosity relation. Moreover, we show that periods and colors of the Cepheids give a robust indication of pulsator masses smaller than predicted by stellar evolution theory without mass loss, independently of the occurrence of core overshooting.

The age of old Magellanic Cloud clusters – I. NGC 2257

Deep CCD photometry down to V=24.5 is presented for the old Magellanic Globular cluster NGC 2257. The main result cna be summarized as follows: i) the RGB Bump has been detected at V=18.7+-0.1, and is compatible with determination of this feature in GGCs; ii) NGC 2257 seems to be 2-3 Gyrs younger than a typical GGC; iii) the helium abundance has been estimated to be Y=0.21+-0.03, using the R- method as calibrated by Buzzoni et al., 1983.

Hot flashers and He dwarfs in galactic globulars

Context. We revisit the evolutionary scenario for hot flasher low-mass structures, where mass loss delays the He flash till the initial phases of their white dwarf cooling sequence. Aims. Our aim has been to test the theoretical results vis-a-vis different assumptions about the efficiency of mass loss. Methods. To this purpose, we present evolutionary models covering a fine grid of masses, as obtained assuming a single episode of mass loss in a red giant model of 0.86 Mwith Z = 0.0015. Results. We find reasonable agreement with previous evolutionary investigations, showing that for the given metallicity late hot flashers are predicted to cover the mass range M = 0.4975 to M = 0.4845 (±0.0005) M� , all models igniting the He-flash with a mass of the H-rich envelope as given by Me = 0.00050 ±0.00002 M� . The ignition mechanism is discussed in some detail, showing the occurrence of a bifurcation in the evolutionary history of stellar structures at the lower-mass limit for He ignition. Below such a critical mass, the structures miss the He ignition, cooling down as a hot flasher-manque He white dwarf. We predict that these structures will cool down, reaching the luminosity log L/L� = −1 in a time five times longer at the very least than the corresponding cooling time of a normal CO white dwarf. Conclusions. On very general grounds, one expects that old stellar clusters with a sizeable population of hot flashers should produce at least a similar amount of slow-cooling He white dwarfs. According to this result, in a cluster where 20% of red giants escape the He burning phase, one expects roughly twice as white dwarfs than in a normal cluster where all red giants undergo their He flash.