ADS (authors="CASTELLANI, M")

Bono, G.: Dwarf galaxies in the Local Group: cornerstones for stellar astrophysics and cosmology

Dwarf galaxies have been the crossroad of significant theoretical and observational efforts, but we still lack firm constraints concerning their formation and evolution. They are also fundamental laboratories to investigate the impact of the environment on star formation and on chemical evolution in stellar systems that are order of magnitudes smaller than giant galaxies. We present some recent results concerning the dwarf spheroidal Carina and the dwarf irregular IC10. In particular, we focus our attention on the evolutionary properties of their stellar populations using accurate and deep color-magnitude diagrams. We also briefly discuss the impact that the transition from old, low-mass (horizontal branch) to intermediate-age (red clump) helium burning stars has in constraining the star formation history of complex stellar systems.

Bono, G.: 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 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 Carina's 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 Carina's old stellar population is metal-poor and appears to have a limited spread in metallicity (Δ[Fe/H] = 0.2 - 0.3 dex). The Carina's 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 Carina's evolved stars indicate a metallicity of [Fe/H] = -1.70 ± 0.19 dex, which agrees quite well with spectroscopic measurements. Based on images collected with the MOSAIC II camera available at the CTIO 4 m Blanco telescope, La Serena; (2003B-0051, 2004B-0227, 2005B-0092, PI: A. R. Walker), and in part with the WFI available at the 2.2 m MPG/ESO telescope (A064.L-0327), and images obtained from the ESO/ST-ECF Science Archive Facility.

Di Cecco, A.: On the ΔV bump HB Parameter in Globular Clusters

We present new empirical estimates of the ΔV ^bump _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 <= [M/H] <= -0.58 dex). Adopting the homogeneous metallicity scale provided either by Kraft & Ivans or by Carretta et al., we found that the observed ΔV ^bump _HB parameters are larger than predicted. In the metal-poor regime ([M/H] <~ -1.7, -1.6 dex) 40% of GCs show discrepancies of 2σ (≈0.40 mag) or more. Evolutionary models that account either for α- and CNO-enhancement or for helium enhancement do not alleviate the discrepancy between theory and observations. The outcome is the same if we use the new solar heavy-element mixture. The comparison between α- and CNO-enhanced evolutionary models and observations in the Carretta et al. metallicity scale also indicates that observed ΔV ^bump _HB parameters, in the metal-rich regime ([M/H] >= 0), might be systematically smaller than predicted. Based in part on data obtained from the ESO/ST-ECF Science Archive Facility, from the Isaac Newton Group Archive which is maintained as part of the CASU Astronomical Data Centre at the Institute of Astronomy, Cambridge, and from the Canadian Astronomy Data Centre operated by the National Research Council of Canada with the support of the Canadian Space Agency.

Giuffrida, G.: Reconstruction of Gaia BP/RP spectra in crowded fields

Gaia will produce a catalogue of 1 billion of Milky Way stars, along with huge number of extragalactic and solar system's objects, changing drastically our vision of the sky. The photometric cameras will supply low resolution spectra (from 3 to 29 nm/pixel) in two bands, BP ( 300-660 nm) and RP (650-1000 nm) for all targets with magnitude G<20. This means that a large fraction of observed stars will be crowded. In this context, the development of an algorithm for the deblending of crowded images is a fundamental task and a real challenge.

Calamida, A.: Strömgren photometry of omega Cen Red Giants

We present new intermediate-band Strömgren photometry based on more than 300 u,v,b,y images of the Galactic globular cluster omega Cen p, covering a region of more than 20×20 arcmin squared across the cluster center. By adopting different Strömgren metallicity indices we estimate the photometric metallicity for ≈ 4,000 Red Giants, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]_phot= -1.69±0.08, -1.28±0.03, -1.07±0.02, -0.88±0.04, -0.71±0.12 and -0.03±0.08 dex) and a sharp cut-off in the metal-poor tail ([Fe/H]_phot ≲ -2 dex) that agree quite well with spectroscopic measurements. We identify four distinct sub-populations, namely metal-poor (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 (SM, [Fe/H] ≈ 0). The last group includes only a small fraction of stars (˜; 8±5%) and should be confirmed spectroscopically.

Calamida, A.: Strömgren Photometry of Galactic Globular Clusters. II. Metallicity Distribution of Red Giants in ω Centauri

We present new intermediate-band Strömgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster ω Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J, H, K_s 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 Strömgren 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 lsim -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 Strömgren metallicity indices display a robust correlation with α-elements ([Ca+Si/H]) when moving from the MI to the MR regime ([Fe/H] gsim -1.7 dex). Based on observations collected in part with the 1.54 m Danish Telescope and with the NTT@ESO Telescope operated in La Silla, and in part with the VLT@ESO Telescope operated in Paranal. The Strömgren data were collected with DFOSC2@Danish (proprietary data), while the NIR data were collected with SOFI@NTT, proposals: 66.D-0557 and 68D-0545 (proprietary data), 073.D-0313 and 59.A-9004 (ESO Science archive) and with ISAAC@VLT, proposal 075.D-0824 (proprietary data).

Sanna, N.: On the Stellar Content of the Starburst Galaxy IC10

We investigate the stellar content of the starburst dwarf galaxy IC10 using accurate and deep optical data collected with the Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. The comparison between theory and observations indicates a clear change in age distribution when moving from the center toward the external regions. Moreover, empirical calibrators and evolutionary predictions suggest the presence of a spread in heavy element abundance of the order of one-half dex. The comparison between old and intermediate-age core He-burning models with a well defined overdensity in the color-magnitude diagram indicates the presence of both intermediate-age, red clump stars and of old, red horizontal branch stars. Based on observations collected with the ACS and the WFPC2 onboard the HST.

Iannicola, G.: On the Radial Distribution of Horizontal Branch Stars in NGC 2808

We present accurate new ultraviolet and optical BVI photometry for the Galactic globular cluster NGC 2808 based on both ground-based and archival Hubble Space telescope imagery. From this we have selected a sample of ~2000 Horizontal Branch (HB) stars; given the extensive wavelength range considered and the combination of both high angular resolution and wide-field photometric coverage, our sample should be minimally biased. We divide the HB stars into three radial bins and find that the relative fractions of cool, hot, and extreme HB stars do not change radically when moving from the center to the outskirts of the cluster: the difference is typically smaller than ~2σ. These results argue against the presence of strong radial differentiation among any stellar subpopulations having distinctly different helium abundances. The ratio between HB and red giant (RG) stars brighter than the ZAHB steadily increases when moving from the innermost to the outermost cluster regions. The difference is larger than ~4σ and indicates a deficiency of bright RGs in the outskirts of the cluster. Based on observations collected with the ACS, WFPC2 and STIS onboard the Hubble Space Telescope.

Giuffrida, G.: Deblending of Gaia BP/RP spectra: a progress report

With an expected catalogue of 1 billion MW stars in addition to a huge number of extragalactic and solar system's objects, Gaia mission will drastically change our vision of the sky. Considered that the photometric cameras will produce low resolution spectra in two bands, BP ( 300-660 nm) and RP (650-1000 nm), the expected fraction of crowded stars will be of the order of 20%. In this context, the deblending of crowded images will be a real challenge for the algorithms of flux extraction.

Castellani, M.: Galactic Globular Clusters Database: a progress report

The present status of Galactic Globular Clusters Database is briefly reviewed. The features implemented at the time writing are described, as well as plans for future improvements.

Calamida, A.: Strömgren metallicity calibration: the m_1, b-y relation.

We performed a new calibration of the Strömgren metallicity index m_1 based on the b-y color of cluster red giant stars. The current Metallicity-Index-Color (MIC) relation is not linear in the color range 0.40 ≲ b-y ≲ 1.0, but provides iron abundances of cluster and field red giants with an accuracy of ˜ 0.25 dex.

Calamida, A.: Strömgren Photometry of Galactic Globular Clusters. I. New Calibrations of the Metallicity Index

We present a new calibration of the Strömgren metallicity index m₁ 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 (σ=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 (σ=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 (σ=0.10 dex). A similar agreement is found for the sample of field RGs, with a difference of -0.09+/-0.03 dex (with σ=0.19 dex). The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11+/-0.03 dex (σ=0.14 dex) for the nine GCs and -0.24+/-0.03 dex (σ=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. Based in part on observations collected with the 1.54 m Danish Telescope operated at ESO (La Silla, Chile) and with the Nordic Optical Telescope (NOT) operated at La Palma (Spain).

Busso, G.: The peculiar horizontal branch morphology of the Galactic globular clusters NGC 6388 and NGC 6441: new insights from UV observations

Context: In this paper we present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC 6388 and NGC 6441.
Aims: We investigate the properties of their anomalous horizontal branches in different photometric planes in order to shed light on the nature of the physical mechanism(s) responsible for the existence of an extended blue tail and of a slope in the horizontal branch, visible in all the color-magnitude diagrams.
Methods: New photometric data have been collected and carefully reduced. Empirical data have been compared with updated stellar models of low-mass, metal-rich, He-burning structures, transformed to the observational plane with appropriate model atmospheres.
Results: We have obtained the first UV color-magnitude diagrams for NGC 6388 and NGC 6441. These diagrams confirm previous results, obtained in optical bands, about the presence of a sizeable stellar population of extremely hot horizontal branch stars. At least in NGC 6388, we find a clear indication that at the hot end of the horizontal branch the distribution of stars forms a hook-like feature, closely resembling those observed in NGC 2808 and Omega Cen. We briefly review the theoretical scenarios that have been suggested for interpreting this observational feature. We also investigate the tilted horizontal branch morphology and provide further evidence that supports early suggestions that this feature cannot be interpreted as an effect of differential reddening. We show that a possible solution of the puzzle is to assume that a small fraction - ranging between 10-20% - of the stellar population in the two clusters is strongly helium-enriched (Y˜ 0.40 in NGC 6388 and Y˜ 0.35 in NGC 6441). The occurrence of a spread in the He abundance between the canonical value (Y˜ 0.26) and the quoted upper limits can significantly help in explaining the “whole” morphology of the horizontal branch and the pulsational properties of the variable stars in the target clusters.

Castellani, V.: 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. During the revision of this manuscript, Vittorio Castellani passed away on 2006 May 19. His suggestions, ideas, and personality will be greatly missed. Based on data obtained from the ESO Science Archive Facility and the Hubble Space Telescope Archive Facility.

Del Principe, M.: 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. Based in part on observations collected with the New Technology Telescope, operated at ESO, La Silla, under programs 066D-0557 and 068D-0545.

Castellani, M.: 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 M_ȯ with 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_ȯ, all models igniting the He-flash with a mass of the H-rich envelope as given by M_e = 0.00050 ±0.00002 M_ȯ. 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-manqué He white dwarf. We predict that these structures will cool down, reaching the luminosity log L/L_ȯ=-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.

Marconi, M.: STREGA@VST: Structure and Evolution of the Galaxy .

In this article we present the survey STREGA, that has been proposed as part of the VST GTO (VLT Survey Telescope Guarantee Time of Observation) that will be given to the INAF-OAC in exchange for the construction of the telescope (see Alcalàet al., this volume). This survey will cover an area of about 150 sq. deg., searching for the southern portion of the Fornax stream, also allowing to study the properties of Disk and Halo White Dwarfs and interacting binaries in different fields at increasing galactic latitudes and to provide an ideal database for star counts and for the comparison with Galactic model predictions. The final aim is to constrain the structure and the evolutionary properties of the Milky Way. For the Fornax Stream this proposal is coordinated with a LBT Science Verification Pilot Project (P.I.: G. Bono).

Castellani, M.: RR Lyrae variables in GGCs: distribution of periods and synthetic models. .

We present some applications of our Synthetic Horizontal Branches (SHB) simulations, aimed to reproduce the peculiar period distributions of RR Lyrae belonging to the Galactic Globular Clusters M3 and M5. We show some evidence, supporting the importance of SHBs in obtaining parameters such as the mass distribution inside the instability strip.

Cassisi, S.: BASTI: an interactive database of updated stellar evolution models

We present a new database of stellar evolution models for a large range of masses and chemical compositions, based on an up-to-date theoretical framework. We briefly discuss the physical inputs and the assumptions adopted in computing the stellar models. We explain how to access to the on-line archive and briefly discuss the interactive WEB tools that can be used to compute user-specified evolutionary tracks/isochrones/luminosity functions. The future developments of this database are also outlined.

Ventura, P.: Diffusive convective overshoot in core He-burning intermediate mass stars. I. The LMC metallicity

We present detailed evolutionary calculations focused on the evolution of intermediate mass stars with 3 M_ȯ ≤ M ≤ 9 M_ȯ of metallicity typical of the Large Magellanic Cloud (LMC), i.e. Z=0.008. We carefully compare the models calculated by adopting a diffusive scheme for chemical mixing, in which nuclear burning and mixing are self-consistently coupled, while the eddy velocities beyond the formal convective core boundary are treated to decay exponentially, and those calculated with the traditional instantaneous mixing approximation. We find that: i) the physical and chemical behaviour of the models during the H-burning phase is independent of the scheme used for the treatment of mixing inside the CNO burning core; ii) the duration of the He-burning phase relative to the MS phase is systematically longer in the diffusive models, due to a slower redistribution of helium to the core from the outer layers; iii) the fraction of time spent in the blue part of the clump, compared to the stay in the red, is larger in the diffusive models. The differences described in points ii) and iii) tend to vanish for M > 6 M_ȯ. In terms of the theoretical interpretation of an open cluster stellar population, the differences introduced by the use of a self-consistent scheme for mixing in the core with adjacent exponential decay are relevant for ages in the range 80 Myr < t < 200 Myr. These results are robust, since they are insensitive to the choice of the free-parameters regulating the extension of the extra-mixing region.

Castellani, M.: 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 often debated problem of how adequate canonical HB stellar models are to account for the observed peaked distribution of RR Lyrae fundamentalised periods in the globular cluster M 3. We find that by assuming a suitable bimodal mass distribution, canonical models do 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 to predict the relative distribution of variables in fundamental and first overtone pulsators, reaching a satisfactory agreement. However, one finds that canonical models outnumber the observed number of red HB stars by roughly a factor of two. Possible solutions for this discrepancy are outlined. Alternative evolutionary scenarios are also briefly discussed.

Ventura, P.: Time dependent mixing in He-burning cores: The case of NGC 1866

We examine the helium burning phase of the convective core of intermediate mass stars, and investigate the role of the coupling of nuclear burning and mixing on the extension and duration of the blue loop phase. We compare the theoretical scenario with the distribution of stars in the colour-magnitude (CM) diagram of the Large Magellanic Cloud (LMC) cluster NGC 1866, whose densely populated clump of He-burning stars is equally populated on the blue and red side. We compare the distributions expected by adopting either a diffusive scheme within the instability regions, in which nuclear burning and mixing are self-consistently coupled, or the traditional instantaneous mixing approximation. We analyze with particular care the sensitivity of the results to: a) the e-folding distance with which the velocity of convective eddies decays outside the formal border of the convective regions fixed by the Schwarzschild criterion; b) the convective model adopted to evaluate the temperature gradient; c) the rate of reaction ¹²C+α -> ¹⁶O. Models not including convective overshoot are also commented.

Cassisi, S.: 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 variables, 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.

Monelli, M.: Short and long period variable stars in the Carina dwarf Spheroidal galaxy

We present first results concerning the detection of variable stars in the Carina dwarf Spheroidal from B, V images collected with the 4-m CTIO telescope. We show a sample of candidate variables spanning from the tip of the Red Giant Branch down to the Main Sequence turn off. Finally, we discuss the future photometric and spectroscopic developments of this project.

Monelli, M.: Wide Field Optical Photometry of the Carina Dwarf Spheroidal Galaxy

We present optical photometry of the Carina Dwarf Spheroidal (dSph) Galaxy. Data have been collected with CCD mosaic cameras available at the 2.2m ESO/MPI and at the 4m CTIO Blanco telescopes. Our current (V,B-V) Color-Magnitude diagram (CMD) confirms the occurrence of an old (˜ 11 Gyr) and an intermediate-age (˜ 5 Gyr ) population. Furthermore, we found evidence of a well-defined blue plume of young Main Sequence (MS) stars with an age at most of the order of ≃ 1 Gyr. The radial distributions of the stellar populations of different age appear to be different: the intermediate-age population is flatter and more centrally concentrated when compared with the old one. Moreover, star counts of the old stellar component are asimmetric along both the major and the minor axis. This feature might be the signature of a tidal interaction with the Galaxy.

Castellani, M.: RR Lyrae variables in Galactic globular clusters. I. The observational scenario

In this paper we revisit observational data concerning RR Lyrae stars in Galactic globular clusters, presenting frequency histograms of fundamentalized periods for the 32 clusters having more than 12 pulsators with well recognized period and pulsation mode. One finds that the range of fundamentalized periods covered by the variables in a given cluster remains fairly constant in varying the cluster metallicity all over the metallicity range spanned by the cluster sample, with the only two exceptions given by M 15 and NGC 6441. We conclude that the width in temperature of the RR Lyrae instability strip appears largely independent of the cluster metallicity. At the same time, it appears that the fundamentalized periods are not affected by the predicted variation of pulsators luminosity with metal abundance, indicating the occurrence of a correlated variation in the pulsator mass. We discuss mean periods in a selected sample of statistically significant ``RR rich" clusters with no less than 10 RRab and 5 RRc variables. One finds a clear evidence for the well known Oosterhoff dichotomy in the mean period <P_ab> of ab-type variables, together with a similarly clear evidence for a constancy of the mean fundamentalized period <Pf> in passing from Oosterhoff type II to type I clusters. On this basis, the origin of the Oosterhoff dichotomy is discussed, presenting evidence against a strong dependence of the RR Lyrae luminosity on the metal content. On the contrary, i) the continuity of the mean fundamentalized period, ii) the period frequency histograms in the two prototypes M 3 (type I) and M 15 (type II), iii) the relative abundance of first overtone pulsators, and iv) the observed difference between mean fundamental <P_ab> and fundamentalized periods <Pf>, all agree in suggesting the dominant occurrence of a variation in the pulsation mode in a middle region of the instability strip (the ``OR" zone), where variables of Oosterhoff type I and type II clusters are pulsating in the fundamental or first overtone mode, respectively.

Marconi, M.: RR Lyrae Stars in Galactic Globular Clusters. II. A Theoretical Approach to Variables in M3

We present predicted relations connecting pulsational (period and amplitude of pulsation) and evolutionary (mass, absolute magnitude, and color) parameters, as based on a wide set of convective pulsating models of RR Lyrae stars with Z=0.001, Y=0.24, and mass and luminosity suitable for the ``old'' (age > 8 Gyr) variables observed in globular clusters. The relations are collated with sound constraints on the mass of pulsators, as inferred from up-to-date evolutionary models of horizontal-branch stars, in order to provide a self-consistent theoretical framework for the analysis of observed variables. The theoretical predictions are tested through a detailed comparison with measurements of RR Lyrae stars in the globular cluster M3. We show that the predicted relations satisfy a variety of observed data, thus providing a pulsational route to the determination of accurate distances to RR Lyrae-rich globular clusters with intermediate metal content. We show that current uncertainties on the intrinsic luminosity of up-to-date horizontal-branch models, as due to the input physics used in the computations by the different authors, have a quite low influence (~0.02-0.03 mag) on the pulsational distance modulus. On the contrary, the effect of the different bolometric corrections adopted to convert bolometric luminosity into absolute magnitude is of the order of ~0.05-0.06 mag. The pulsation models are also used to perform some valuable tests on the physics adopted in current stellar evolution computations. We show that the constraints inferred by pulsation theory support the large value of the mixing-length parameter (l/H_p=1.9-2.0) adopted to fit observed red giant branches, but that, at the same time, they would yield that the luminosity of the horizontal-branch updated models is too bright by ~0.08+/-0.05 mag, if helium diffusion and metal diffusion are neglected. Conversely, if element diffusion is properly taken into account, then there is a marginal discrepancy of ~0.04+/-0.05 mag between evolutionary and pulsational predictions.

Monelli, M.: The Carina Project. II. Stellar Populations

We present a new (V, B-V) color-magnitude diagram of the Carina dwarf spheroidal galaxy (dSph) that extends from the tip of the red giant branch (RGB) down to V~25 mag. Data were collected with the Wide Field Imager available at the 2.2 m ESO/MPI telescope and cover an area of ~0.3 deg² around the center of the galaxy. We confirm the occurrence of a substantial number of old stars with ages around 11 Gyr, together with an intermediate-age population around 5 Gyr. Moreover, we also detected a new, well-defined blue plume of young main-sequence stars with an age, at most, on the order of 1 Gyr. This finding is further supported by the detection of a sizable sample of anomalous Cepheids, whose occurrence can be understood in terms of stars with ages ~0.6 Gyr. The evidence for such a young population appears at odds with current cosmological models, which predict that the most recent star formation episodes in dSph's should have taken place 2-3 Gyr ago. At odds with previous results available in the literature, we found that stars along the RGB of old and intermediate-age stellar populations indicate a mean metallicity roughly equal to Z=0.0004 ([Fe/H]~-1.7) and a small dispersion around this value. This finding is further strengthened by the reduced spread in luminosity of RR Lyrae and horizontal-branch stars in the old stellar population and of the red clump in the intermediate-age group. We find evidence of a smooth spatial distribution of the intermediate-age stellar population (~5 Gyr), which appears more centrally concentrated than the oldest one (~11 Gyr). The radial distribution of the old population appears more clumpy, with a peak off-center by ~2' when compared with the Carina center. Star counts show a well-defined ``shoulder'' in the northeast direction along both the minor and major axes. Current data do not allow us to assess whether this feature is the break in the slope of star-count profiles predicted by Johnston, Sigurdsson, & Hernquist. Based on observations collected at the European Southern Observatory, La Silla, Chile, on Osservatorio Astronomico di Capodimonte guaranteed time.

Dall'Ora, M.: 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 dSph's can barely be classified into the classical Oosterhoff dichotomy. The mean period of RRab's 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. Based on observations collected at the European Southern Observatory, La Silla, Chile, on Osservatorio Astronomico di Capodimonte guaranteed time.

Monelli, M.: The Carina Project: Photometric Accuracy

We present B,V photometric data of the Carina dwarf Spheroidal (dSph) galaxy, collected with the Mosaic Camera (36×36 arcmin²) available at the 4m CTIO Blanco telescope. We discuss different reduction strategies and in particular the PSF extraction and internal accuracy reached by our measurements. We discuss a new deep Color-Magnitude Diagram (CMD) of the Carina galaxy. Finally, we show the samples of RR Lyrae, Anomalous Cepheids, and delta Scuti candidates we have detected.

Dall'Ora, M.: The Carina Project: bright variables

We present recent results on variable stars in the Carina Dwarf Galaxy based on B,V time series data collected over three consecutive nights with the Wide Field Imager available at the 2.2m ESO/MPI telescope. At present we have already detected and measured a sample of 92 variable stars. Among them 75 are RR Lyrae stars (54 fundamental, 15 first overtone, and 6 suspected double-mode pulsators), while 15 are Anomalous Cepheids. By adopting recent theoretical predictions we discuss evolutionary and pulsation properties of these objects. Finally, we estimate the Carina distance according to three different methods that use RR Lyrae stars as distance indicators. Based on Osservatorio di Capodimonte guaranteed time.

Corsi, C. E.: The Carina Project: Absolute and Relative Calibrations

We discuss the reduction strategy adopted to perform the relative and the absolute calibration of the Wide Field Imager (WFI) available at the 2.2m ESO/MPI telescope and of the Mosaic Camera (MC) available at the 4m CTIO Blanco telescope. To properly constrain the occurrence of deceptive systematic errors in the relative calibration we observed with each chip the same set of stars. Current photometry seems to suggest that the WFI shows a positional effect when moving from the top to the bottom of individual chips. Preliminary results based on an independent data set collected with the MC suggest that this camera is only marginally affected by the same problem. To perform the absolute calibration we observed with each chip the same set of standard stars. The sample covers a wide color range and the accuracy both in the B and in the V-band appears to be of the order of a few hundredths of magnitude. Finally, we briefly outline the observing strategy to improve both relative and absolute calibrations of mosaic CCD cameras.

Castellani, M.: The Carina Project: color magnitude diagram and radial distribution

We present B, V photometric data of the Carina Dwarf Spheroidal Galaxy (dSph), collected with the Wide Field Imager (WFI) available at the 2.2m MPI/ESO telescope. We briefly discuss the main features of the color magnitude diagram (CMD) and in particular the mix of stellar populations present in this galaxy. A preliminary analysis of the spatial distribution of these populations over a substantial fraction of the body of the galaxy is also presented.

Bono, G.: Foreword

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Bono, G.: Workshop on Variability with Wide-Field Imagers

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Monelli, M.: Multiwavelength Time Series Data of the LMC Cluster Reticulum

We present accurate multiwavelength UBVI time series data of the LMC cluster Reticulum. Data cover a time interval of ~ 6 yr and have been collected with SUSI1/2 and SOFI at NTT/ESO. For each band we collected approximately 30 short/long exposures and the total exposure times range from roughly 3500 (U, B), 6000s (V) to 8300s (I). The observing strategy and data reduction (DAOPHOTII/ALLFRAME) allowed us to reach a photometry accuracy of 0.02 magnitude from the tip of the Red Giant Branch well below the Turn-Off region. Even though this cluster presents a very low central density, we found a sizable sample of Blue Stragglers (BSs). We also selected stars with a variability index larger than 2, and interestingly enough we detected together with the RR Lyrae stars a large sample of variable stars around and below the TO region. Preliminary analysis on the luminosity variation indicate that these objects might be binary stars.

Ripepi, V.: The Carina Dwarf Galaxy Variable Star Population

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Castellani, M.: Antlia: an outskirt Local Group galaxy

Deep (I,V-I) and (I,B-I) color-magnitude diagrams (CMDs) of the Antlia dwarf galaxy, based on Science Verification (SV) data collected with the FORS I camera on the ESO Very Large Telescope (VLT) are presented. By adopting the new calibration of the Tip of the Red Giant Branch (TRGB) provided by Salaris & Cassisi (1998) we estimated that the Antlia distance modulus is (m-M)₀ = 25.98+/-0.10 mag. We suggest an improvement of the classical TRGB method based on infrared H and K magnitudes of TRGB stars. Such a method should overcome the well-known limit of the "classical" TRGB method when dealing with metal-rich stellar populations.

Castellani, M.: Star Formation History of the Carina Dwarf Galaxy (Preliminary Results) (Poster)

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Marconi, G.: Sagittarius Dwarf Galaxy VI photometry (Marconi+, 1998)

We present V,I deep CCD photometry for three fields of the dwarf galaxy in Sagittarius (Sgr), located at l=5.6°, b=-14.1°. One of the fields is centered on the globular cluster NGC 6715 (M54), which lies in one of the dense clumps of the Sgr galaxy. Comparing the CMD of Sgr with those of globular clusters which are believed to be kinematically associated with the dwarf galaxy (Da Costa & Armandroff, 1995AJ....109.2533D), we conclude that the stellar population of Sgr presents a spread in metallicity of -0.71<=[Fe/H]<=-1.58, and that the dominant population (=~10Gyr old) is extremely similar to the star content of the associated globular cluster Terzan 7. The estimated distance to Sgr is d=~24.55Kpc. (3 data files).

Castellani, M.: Tucana dwarf galaxy VI photometry (Castellani+, 1996)

We present deep CCD photometry for the dwarf galaxy in Tucana (l=323, b=-47.4). The data indicate that the galaxy is dominated by an intermediate/old stellar population with metallicity similar to that of the galactic globular clusters NGC 6752 and NGC 7089 (M2) (we find [Fe/H]=~-1.56). The distance modulus we derived, (m-M)_V=~24.72+/-0.2, makes clear that this galaxy belongs to the Local Gro up. Tucana is clearly different from other dwarf galaxies, such as Leo I (Lee et al., 1993AJ....106.1420L), or Phoenix (Ortolani and Gratton, 1988PASP..100.1405O), because there is no evidence of a young stellar population (t<=5Gyrs) We suggest that the Tucana Galaxy is a genuine dwarf spheroidal in which no recent burst of star formation occurred. (1 data file).

Piersimoni, A. M.: A new investigation on the Antlia Dwarf Galaxy

We present deep (I,V-I) and (I,B-I) color-magnitude diagrams (CMDs) of the Antlia dwarf galaxy, based on Science Verification (SV) data collected with the FORS I camera on the ESO Very Large Telescope (VLT). The CMDs present two key features: a well-defined Red Giant Branch (RGB), and a sample of bright blue stars, belonging to a young stellar component. The comparison between theory and observations confirms that the bright stars are consistent with the occurrence of a star formation episode ~0.1 Gyr ago. In agreement with Sarajedini et al. (1997) and Aparicio et al. (1997), we find that the young stars are more centrally concentrated than the old ones. By adopting the new calibration of the Tip of the RGB (TRGB) provided by Salaris & Cassisi (1998) we estimated that the Antlia distance modulus is (m-M)_0=25.89+/-0.10 mag, and therefore a distance D=1.51+/- 0.07 Mpc. This distance determination is ~13% larger than the values suggested in previous investigations. We estimated that the mean metallicity of Antlia is of the order of [Fe/H]~-1.3. This estimate is at least 0.3 dex more metal-rich than similar evaluations available in the literature. The disagreement both in the distance and in the metallicity determinations are mainly due to difference in the calibration of the TRGB method and in the (V-I) color index vs. metallicity relation. The differential RGB luminosity function shows an excess in the observed counts -at the 2sigma level- when compared with theoretical predictions. This discrepancy might be due to a stellar component with an age approximately equal to 0.7 Gyr. Based on observations collected with VLT-UT1 telescope of ESO in Paranal, during the Science Verification Program

Buonanno, R.: Hubble Space Telescope Photometry of the Fornax Dwarf Spheroidal Galaxy: Cluster 4 and Its Field

Using observations from the Hubble Space Telescope archive, color-magnitude diagrams (CMDs) have been constructed for globular cluster 4 in the Fornax dwarf spheroidal galaxy and its surrounding field. These diagrams extend below the main-sequence turnoffs and have yielded measurements of the ages of the populations. The most prominent features of the CMD of the Fornax field population are a heavily populated red clump of horizontal-branch (HB) stars, a broad red giant branch (RGB), and a main sequence that spans a large range in luminosity. In this CMD, there are very few stars at the positions of the HBs of the five globular clusters in Fornax, which suggests that only a very small fraction of the field population resembles the clusters in age and chemical composition. The large span in luminosity of the main sequence suggests that star formation began in the field ~=12 Gyr ago and continued to ~=0.5 Gyr ago. There are separate subgiant branches in the CMD, which indicates that the star formation was not continuous but occurred in bursts. The CMD of cluster 4 has a steep RGB, from which we estimate [Fe/H]~=-2.0. This is considerably lower than estimates from the integrated light of the cluster, and the origins of this discrepancy are discussed. Cluster 4 has a very red HB and is, therefore, a prime example of the second-parameter effect. Comparisons of cluster 4 with the other Fornax clusters and with M68, a very metal-poor globular cluster of the Galactic halo, reveal that cluster 4 is ~=3 Gyr younger than these other clusters, which have much bluer HBs. This age difference is consistent with the prediction that age is the second parameter to within the uncertainties. The CMD of cluster 4 is virtually identical to that of the unusual globular cluster of the Galactic halo Ruprecht 106, which suggests that they have very similar ages and chemical compositions. We discuss the possibility that cluster 4 also resembles R106 in having a higher [Fe/H] than is indicated by its steep RGB and also a lower [α/Fe] ratio than is usual for a globular cluster, as indicated by some recent observations of R106. The CMDs of the five Fornax clusters indicate that cluster age is a major-but probably not the sole-second parameter. Buonanno et al. recently concluded that cluster density probably influenced the HB morphologies of clusters 1, 2, 3, and 5. Despite a very large difference in central density, the HBs of cluster 4 and R106 are very similar. This suggests that density may act as a second parameter in clusters that have HBs that are on the verge of moving toward the blue or are already blue for another reason, such as very old age.

Buonanno, R.: HST Photometry of the Fornax dSph Galaxy: cluster 4 and its field

Using observations from the Hubble Space Telescope archive, color-magnitude diagrams (CMDs) have been constructed for globular cluster 4 in the Fornax dSph galaxy and its surrounding field. These diagrams extend below the main-sequence turnoffs and have yielded measurements of the ages of the populations. The most prominent features of the CMD of the Fornax field population are a heavily populated red clump of horizontal branch (HB) stars, a broad red giant branch (RGB), and a main sequence that spans a large range in luminosity. In this CMD, there are very few stars at the positions of the HBs of the five globular clusters in Fornax, which suggests that only a very small fraction of the field population resembles the clusters in age and chemical composition. The large span in luminosity of the main-sequence suggests that star formation began in the field about 12 Gyrs ago and continued to about 0.5 Gyr ago. There are separate subgiant branches in the CMD, which indicate that the star formation was not continuous but occurred in bursts. The CMD of cluster 4 has a steep RGB, from which we estimate [Fe/H] around -2.0. This is considerably lower than estimates from the integrated light of the cluster, and the origins of this discrepancy are discussed.

Caputo, F.: 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, also allow us to obtain an accurate sampling of the stellar populations 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 estimates for both the distance modulus and the age(s) of the main stellar components of Leo I are derived. In more detail, 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+/-0.15 mag and an age of 10-15 Gyr or 9-13 Gyr, adopting a metallicity of Z=0.0001 or 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. A detailed investigation of the age(s) of the Leo I stellar populations is then performed by comparing the CMD with a suitable set of theoretical isochrones and central helium-burning models. By taking into account all the various features, including the lack of RR Lyrae variables, we conclude that the star formation process in Leo I started ~10 Gyr (with Z=0.0001) or ~13 Gyr (with Z=0.0004) ago, and stopped about 1 Gyr ago. Some evidence is reported supporting the mild metal deficiency (Z=0.0004), whereas no clear indication has been found supporting a star formation history characterized by episodic bursts. The adoption of updated physics, including the inward diffusion of elements as recently presented for globular cluster stars, would yield a slightly larger distance modulus (~0.10 mag) and a slightly lower age for the most ancient stellar component (~1 Gyr).

Marconi, G.: Star Formation History in the Fornax Dwarf Galaxy from HST data

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Marconi, G.: The stellar content of the Sagittarius Dwarf Galaxy

We present V,I deep CCD photometry for three fields of the dwarf galaxy in Sagittarius (Sgr), located at l=5.6(o) , b=-14.1(o) . One of the fields is centered on the globular cluster NGC 6715 (M54), which lies in one of the dense clumps of the Sgr galaxy. Comparing the CMD of Sgr with those of globular clusters which are believed to be kinematically associated with the dwarf galaxy (Da Costa & Armandroff 1995), we conclude that the stellar population of Sgr presents a spread in metallicity of -0.71<=[Fe/H]<=-1.58, and that the dominant population ( =~ 10 Gyr old) is extremely similar to the star content of the associated globular cluster Terzan 7. The estimated distance to Sgr is d =~ 24.55 Kpc. Based on observations collected at the European Southern Observatory, La Silla, Chile.

Cassisi, S.: Intermediate-age metal deficient stellar populations: the case of metallicity Z=0.00001.

We investigate the evolution of metal deficient stars, presenting H-burning isochrones covering cluster ages from 800Myr to 7Gyr. Evolutionary evidences for selection effects in the metallicity distribution of very metal poor H-burning red giants are reported. The evolution of stars during central and shell He burning is further investigated, discussing the occurrence of He burning pulsators as a function of cluster age.

Castellani, M.: The Tucana dwarf Galaxy.

We present deep CCD photometry for the dwarf galaxy in Tucana (l=323, b=-47.4). The data indicate that the galaxy is dominated by an intermediate/old stellar population with metallicity similar to that of the galactic globular clusters NGC 6752 and NGC 7089 (M2) (we find [Fe/H]=~-1.56). The distance modulus we derived, (m-M)_V_=~24.72+/-0.2, makes clear that this galaxy belongs to the Local Group. Tucana is clearly different from other dwarf galaxies, such as Leo I (Lee et al. 1993b), or Phoenix (Ortolani and Gratton 1988), because there is no evidence of a young stellar population (t<=5Gyrs) We suggest that the Tucana Galaxy is a genuine dwarf spheroidal in which no recent burst of star formation occurred.

Castellani, M.: The Tucana Dwarf Galaxy

Deep CCD photometry for the dwarf galaxy in Tucana is presented. Distance modulus and metallicity are derived, together with an estimate of the age of the galaxy.

Castellani, M.: More on the Properties of Star Models Evolving off the Hayashi Track: The Metallicity and Evolutionary Phase Dependence

Models of stars evolving along the Hayashi track during the red giant phase or the asymptotic giant phase have been induced to leave these phases and to evolve toward the blue by means of huge surface mass loss. It is found, in contrast with our previous suggestions, that metallicity and evolutionary phase affect the properties of these star models in such a way that it is impossible to define a unique relation linking the residual envelope mass with the luminosity at which blueward evolution is initiated. We describe the way in which this relation depends on metallicity and evolutionary phase.

Castellani, M.: Theoretical hot horizontal branch stars

Previous evolutionary computations of horizontal branch stars have been extended to the range of 'thin-envelope', very hot stellar models, in order to make available a homogeneous set of models covering the whole range of theoretical horizontal branch temperatures. We present evolutionary tracks for selected models of various metallicities, intended to represent the theoretical counterpart of hot stars populating extremely blue horizontal branches as well as to provide an input for detailed population synthesis programs devoted to study the origin of the ultraviolet light in old stellar systems. The role of metallicity in governing the evolutionary approach to the asymptotic giant branch and the contribution of the models to the UV flux are both briefly discussed.

Castellani, M.: A Progress Report on a Population Synthesis Program

It is briefly described a population synthesis code under development at the Teramo observatory, in cooperation with the Rome Observatory. Preliminary results are then shown, illustrating the influence of various parameters on the radiation produced in different passbands by a given stellar population.

Castellani, M.: Mass loss in globular cluster red giants - an evolutionary investigation

The evolution of Population II red giants has been studied adopting Reimer's formalism for the efficiency of mass loss. Evolutionary computations for a model of 0.8 solar mass, Y = 0.23, Z = 0.0002 are presented for selected assumptions on the value of the parameter eta governing the mass-loss rate. Comparison with canonical models indicates that (1) the evolution of the internal He core is unaffected by mass loss, and (2) the luminosity of the red giant clump follows the actual stellar mass. Models with eta greater than 1.0 fail to ignite He, crossing the HR diagram toward the final cooling as He white dwarfs. However, one finds that the stellar core keeps ignoring the surface conditions well beyond this phase, unsuccessfully trying to ignite He along the cooling tracks, when log L/L(solar) is about 0. Exploring such an evidence, we found that for eta = 0.75 the cooling dwarf does succeed in igniting He at the quoted luminosity. As a result, one expects an extremely blue horizontal-branch star which evolves in about 200 x 10 exp 6 yr from log L/L(solar) = 1.2 to log L/L(solar) = 2.4 before starting the final cooling as a CO white dwarf. The resulting observational scenario is finally discussed, also in connection with the problematic of UV fluxes from old stellar populations.

Limongi, M.: Evolutionary Properties of Post-AGB and Post-EAGB Stars

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Castellani, M.: The contribution of advanced post-Hayashi track evolutionary phases to the ultraviolet light of elliptical galaxies

An extensive analysis is presented of the post red giant branch (RGB), the post early asymptotic giant branch (EAGB), and the post asymptotic giant branch (AGB) evolutionary phases in order to derive information on their UV light contribution to the spectral energy distribution of old stellar systems. The effect of the surface chemical composition on the well-known correlation between the luminosity and the critical envelope mass, below which a star evolving along the Hayashi track is forced to turn back to the blue, is investigated. The surface metallicity is found to represent an additional parameter which should be accounted for in the critical hydrogen envelope mass luminosity relation. The correlation between the core mass and the hydrogen fuel to be burned above a certain temperature along the blueward excursion is discussed. For low core masses (post-RGB stars) and large core masses (post-AGB stars) there is one single correlation linking the fuel with the core mass, whereas for intermediate core masses (post-EAGB stars), the fuel correlates with the core quite differently, turning out to be much larger than in the two adjacent cases.

Tornambe, A.: The properties of post-Hayashi track evolutionary phases and the problem of the ultraviolet emission from elliptical galaxies

The evolutionary properties of shell-burning stars from the Asymptotic Giant Branch (AGB) or the Red Giant Branch which evolve toward the blue portion of the HR diagram. Attention is given to the phases following the star's departure from the Hayashi track, and possible sources of addiional UV emission are considered. It is shown that the enhanced UV emission from elliptical galaxies can be explained by the properties of stars that were early AGB objects.

Castellani, M.: Advanced evolutionary phases of large-metallicity low-mass stars and the problem of the ultraviolet excess of elliptical galaxies

A set of evolutionary tracks of large-metallicity low-mass stars, covering the advanced evolutionary phases from the zero-age horizontal branch to the final white dwarf cooling phase, at the onset of crystallization, are presented. Physical parameters of the evolved stars were selected to search for models leaving the AGB phase before the onset of the thermal pulses and evolving to the blue side of the H-R diagram according to an evolutionary scheme earlier defined as post-early AGB evolution. The maximum UV flux that post-AGB models can provide is found to be insufficient to account for the UV emission of most UV-emitting elliptical galaxies, whereas models following the post-EAGB evolutionary behavior produce much more UV flux. Among the star models evolving off the early AGB, before the onset of thermal pulses, those maximizing the production of the UV flux give enough UV light to explain even the most UV-upturned branches observed in the spectra of the most metal-rich elliptical galaxy.

Tornambé, A.: The UV excess versus metallicity relation in elliptical galaxies: A probe for stellar nucleosynthesis?

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