Using the Spitzer IRAC camera we have obtained mid-IR photometry of the red giant branch stars in the Galactic globular cluster 47 Tuc. About 100 stars show an excess of mid-infrared light above that expected from their photospheric emission. This is plausibly due to dust formation in mass flowing from these stars. This mass loss extends down to the level of the horizontal branch and increases with luminosity. The mass loss is episodic, occurring in only a fraction of stars at a given luminosity. Using a simple model and our observations we derive mass loss rates for these stars. Finally, we obtain the first empirical mass loss formula calibrated with observations of Population II stars. The dependence on luminosity of our mass loss rate is considerably shallower than the widely used Reimers Law. The results presented here are the first from our Spitzer survey of a carefully chosen sample of 17 Galactic Globular Clusters, spanning the entire metallicity range from about one hundredth up to almost solar.
We have used a set of archived Hubble Space Telescope/ACS images to probe the evolved populations of the globular cluster 47 Tucanae. We find an excess of Asymptotic Giant Branch (AGB) stars in the cluster core. We interpret this feature as the signature of an extra-population likely made by the progeny of massive stars originated by the evolution of binary systems. Indeed the comparison with theoretical tracks suggests that the AGB population of 47 Tuc can be significantly contaminated by more massive stars currently experiencing the first ascending Red Giant Branch.
There is increasing evidence of a local population of short duration Gamma-ray Bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible Luminosity Functions (LF) for both neutron star binary mergers and giant flares from Soft Gamma Repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall BATSE sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertai…
We present results from two high-contrast imaging surveys that exploit a novel technique, L-band angular differential imaging. Our first survey targeted 21 young stars in the Beta Pic and Tuc-Hor moving groups with VLT/NACO reaching typical sensitivities of <1 MJup at r > 20 AU. The statistical analysis of the null result demonstrates that the giant planet population is truncated at 30 AU or less (90% confidence level). Our second, on-going MMT/Clio survey utilizes the unique sensitivity achieved in the L-band for old planets to probe all M-dwarf stars within 6 pc. The proximity of these targe…
We present HST/WFPC2 single-star photometry for the blue dwarf galaxy UGCA 290, whose morphology is intermediate between classic iE Blue Compact Dwarfs and blue dwarfs which exhibit no red background sheet of older stars. The color-magnitude diagram of this galaxy in V and I, extending over six magnitudes, is remarkably similar to that of the star-forming region in the iE Blue Compact Dwarf VII Zw 403. There is no evidence for gaps in its star-formation history over the last billion years, and the color of its red giant branch indicates a very metal-poor stellar population. From the magnitude …
Newly obtained HST/WFPC2 images of the disturbed elliptical galaxy NGC 5018 show that the average amount of internal reddening due to the its complex ``dust web'' is as low as E(B-V)~0.02 within the IUE aperture, thus implying that its observed and intrinsic energy distributions do not differ significantly down to UV wavelengths. This, in turn, is quite relevant to the current debate on the age of its dominant stellar population.
We present new determinations of bolometric corrections and effective temperature scales as a function of infrared and optical colors, using a large database of photometric observations of about 6500 Population II giants in Galactic Globular Clusters (GGCs), covering a wide range in metallicity (-2.0<[Fe/H]<0.0). New relations for BC_K vs (V-K), (J-K) and BC_V vs (B-V), (V-I), (V-J), and new calibrations for T_eff, using both an empirical relation and model atmospheres, are provided. Moreover, an empirical relation to derive the R parameter of the Infrared Flux Method as a function of the stel…
We report the results of a photometric search for giant stars associated with the cores of four high velocity clouds (HVCs) -- two of which are compact HVCs -- using the Las Campanas Du Pont 2.5 meter and Cerro Tololo Blanco 4 meter telescopes in combination with a system of filters (Washington M, T_2 + DDO51) useful for identifying low surface gravity, evolved stars. Identical observations of nearby control fields provide a measure of the ``giant star'' background. Our data reach M_0=22 for three of the HVCs and M_0=21.25 for the fourth, depths that allow the detection of any giant stars with…
We have computed a grid of high resolution synthetic spectra for cool stars (2500<Teff<6000 K) in the wavelength range 6000 -- 10200A, by employing an updated line list of atomic and molecular lines, together with state-of-the-art model atmospheres. As a by-product, by fitting TiO bandheads in spectra of well-known M giants, we have derived the electronic oscillator strengths of the TiO gamma prime, delta, epsilon and phi systems. The derived oscillator strenghts for the gamma prime, epsilon and phi systems differ from the laboratory and ab initio values found in the literature, but are consis…
We explore the realm of post-Asymptotic Giant Branch (post-AGB) stars from a theoretical viewpoint, by constructing synthetic population of transition objects, proto-Planetary Nebulae, Planetary Nebulae Nuclei, and post-Planetary Nebulae objects. We use the Montecarlo procedure to filter out the populations accordingly to a given set of assumptions. We explore the parameter space by studying the effects of the Initial Mass Function (IMF), the Initial Mass-Final Mass Relation (IMFMR), the transition time (t_tr), the envelope mass at the end of the envelope ejection (Me_r), the planetary nebula …
With a peak luminosity of ~10^47 erg/s, the December 27th 2004 giant flare from SGR1806-20 would have been visible by BATSE (the Burst and Transient Source Experiment) out to ~50 Mpc. It is thus plausible that some fraction of the short duration Gamma-Ray Bursts (sGRBs) in the BATSE catalogue were due to extragalactic magnetar giant flares. According to the most widely accepted current models, the remaining BATSE sGRBs were most likely produced by compact object (neutron star-neutron star or neutron star-black hole) mergers with intrinsically higher luminosities. Previously, by examining corre…
We examine the cosmic ray proton (CRp) population within the giant elliptical galaxy M 87 using the TeV gamma-ray detection of the HEGRA collaboration. In our scenario, the gamma-rays are produced by decaying pions which result from hadronic CRp interactions with thermal gas of the interstellar medium of M 87. By comparing the gamma-ray emission to upper limits of EGRET, we constrain the spectral index of the CRp population to alpha(GeV,TeV) < 2.275 within our scenario. Both the expected radial gamma-ray profile and the required amount of CRp support this hadronic scenario. The accompanying ra…
Near-infrared spectra of 21 giants in omega Centauri, spanning the whole range of metallicities observed in this cluster, are presented. This work is part of a coordinated photometric and spectroscopic campaign in the optical and in the infrared, aimed at studying the complex stellar population of omega Centauri and understanding its formation and chemical evolution. By analyzing the several CO and OH molecular bands and atomic lines in the spectra of the selected giants, metal abundances and abundance ratios have been obtained. The existence of three major metallicity regimes at [Fe/H]=-1.6, …
We report on the first in a series of studies of the Carina dwarf spheroidal galaxy, a nearby satellite of our Galaxy. Our two major results are: 1) precise BI photometry ($σ_{B-I} \simlt 0.05$ for $V \simlt 22$) for 11,489 stars in the Carina field, and 2) the detection of two, morphologically distinct, horizontal branches, which confirms that star formation in Carina occurred in two well-separated episodes. The old horizontal branch and RR Lyrae instability strip belong to a > 10 Gyr stellar population, while the populous red-clump horizontal branch belongs to an approximately 6 Gyr stellar …
The ionising stellar populations of eleven HII regions in the spiral galaxies: NGC628, NGC925, NGC1232 and NGC1637, all of them reported to have solar or oversolar abundances according to empirical calibrations, have been analysed using stellar population synthesis models. Four of the observed regions in the sample, show features which indicate the presence of a population of Wolf-Rayet (WR) stars with ages between 2.3 and 4.1 Myr. This population is sufficient to explain the emission line spectrum of the low metallicity region H13 in NGC628. This is not the case for the rest of the regions fo…
Stellar evolution tracks and isochrones are key inputs for a wide range of astrophysical studies; in particular, they are essential to the interpretation of photometric and spectroscopic observations of resolved and unresolved stellar populations. We have made available to the astrophysical community a large, homogenous database of up-to-date stellar tracks and isochrones, and a set of programs useful in population synthesis studies. In this paper we first summarize the main properties of our stellar model database (BaSTI) already introduced in Pietrinferni et al. (2004) and Pietrinferni et al…
We report the results of an extensive VLT high-resolution imaging campaign of the central region of the giant globular cluster omega Centauri. More than 100,000 stars have been measured in the inner 9'x 9' region of the cluster. On the basis of multiband Colour Magnitude Diagrams (CMD), we confirm the existence of multiple stellar populations along the Red Giant Branch (RGB). Moreover, thanks to the high-precision of this dataset, we conclude that the RGB does not present a smooth and continuous distribution, but shows a discrete structure: besides the metal-poor and the extreme metal-rich pop…
The near-infrared behavior of the red giant branch (RGB hereafter) as a function of abundance is examined with an unprecedented large sample of 27 Galactic globular clusters with Two Micron All Sky Survey photometry. We propose a new simplified analyses, involving the zero point of the RGB slope fit, and derive calibrations for the RGB slope, zero point, and tip. The weak metallicity sensitivity of the zero point leads to a ``fan''-like diagram for obtaining the abundance distributions in resolved stellar systems, and reddening estimates. Finally, we apply the new calibrations to the recently …
We consider the procedure of isochrone fitting and its application to the study of red giant branch (RGB) photometry in old stellar populations. This is extended to consider the problems introduced by the inclusion of lower-magnitude regions of the Colour Magnitude Diagram (CMD). We refer especially to our previous paper Frayn & Gilmore (2002), where the details of our isochrone interpolation and fitting code are explained. We address the systematic errors inherent in the process of isochrone fitting, and investigate the extent to which simple stellar populations can be recovered from noisy ph…
The recent HST optical images, and the optical and ultraviolet spectra, of Magellanic planetary nebulae (PNe), together with the large data-base that has been collected in the past decade, allows unprecedented insight in the evolution of PNe and their central stars. In this paper we present a selection of recent results: The analysis of PN morphology, both in the optical and ultra-violet emission lines; the relation between nebular morphology and the chemistry produced by stellar evolution; the direct determination of the transition time from observations; and the study of the nature and evolu…
We present a statistically decontaminated Color Magnitude Diagram of a 1 deg X 1 deg field in the core of the Sagittarius dSph galaxy. Coupling this CMD with the most recent metallicity distributions obtained from high resolution spectroscopy we derive robust constraints on the mean age of the stellar population that dominates the galaxy (Pop A). Using three different sets of theoretical isochrones in the metallicity range -0.4<= [M/H]<= -0.7 and taking into consideration distance moduli in the range 16.90<= (m-M)_0<= 17.20 we find that the mean age of Pop A is larger than 5 Gyr, and the best-…
To test our new, improved Reimers-type mass-loss relation, given by Schroder & Cuntz in 2005 (ApJL 630, L73), we take a look at the best studied galactic giants and supergiants - particularly those with spatially resolved circumstellar shells and winds, obtained directly or by means of a companion acting as a probing light source. Together with well-known physical parameters, the selected stars provide the most powerful and critical observational venues for assessing the validity of parameterized mass-loss relations for cool winds not driven by molecules or dust. In this study, star by star, w…
We present Spitzer/IRAC observations at 3.6 and 4.5 microns along with optical data from the Local Group Galaxies Survey to investigate the evolved stellar population of the Local Group dwarf irregular galaxy WLM. These observations provide a nearly complete census of the asymptotic giant branch (AGB) stars. We find 39% of the infrared-detected AGB stars are not detected in the optical data, even though our 50% completeness limit is three magnitudes fainter than the red giant branch tip. An additional 4% of the infrared-detected AGBs are misidentified in the optical, presumably due to reddenin…
An unexpected characteristic of low surface brightness galaxies (LSBGs) is that a significant number are massive and possess substantial amounts of atomic gas. We present preliminary results of an ongoing program to obtain BVRIJHK imaging, along with some nuclear spectroscopy, of a well-defined sample of LSBGs which are gas-rich and of similar size to giant, high surface brightness spiral galaxies (HSBGs). These LSBGs span the entire range of Hubble disk morphologies. While their disks are bluer, on average, than comparable HSBGs, the optical morphology of massive LSBGs indicates that many of …
The DENIS/ISOGAL near/mid-IR survey of the Milky Way for the first time probes stellar populations in the innermost obscured regions of our galaxy. Ages, metallicities and extinction-corrected luminosities are derived for these stars individually by isochrone fitting to the preliminary IR photometry. A few hundred AGB stars with thick circumstellar dust envelopes are identified. An old metal-rich population dominates in the inner galactic Bulge, but there are indications for the presence of a younger population. The inner Bulge has a tri-axial shape, as traced by depth effects on the observed …
In this paper, we show that the optically bright stars above the tip of the first red giant branch (TRGB) in the color magnitude (CM) diagrams of nearby dwarf elliptical (dE) galaxies, commonly interpreted as indication of the existence of an intermediate-age population, may, in certain circumstances, be an artifact of observational effects on a pure old population system. For this purpose, model CM diagrams have been computed simulating the observational effects that can be found in different regions of a Local Group dE galaxy. The starting synthetic CM diagram represents a pure old system, w…
We investigate techniques that can be used to determine ages of recently star forming regions during the phase dominated in the near-IR by the asymptotic giant branch stars (10^8-10^9 yrs). In particular, we present selected near-IR spectroscopic features that identify the contribution of O-rich and C-rich AGB variable stars to the integrated spectra of post-starbursts. The observational strategy based on those features is presented. We discuss the robustness of our selected features in constraining the ages of the post-starburst population depending on its physical environment and on underlyi…
We present Spitzer Space Telescope IRAC photometry of the Local Group dwarf irregular galaxy IC 1613. We compare our 3.6, 4.5, 5.8, and 8.0 micron photometry with broadband optical photometry and find that the optical data do not detect 43% and misidentify an additional 11% of the total AGB population, likely because of extinction caused by circumstellar material. Further, we find that a narrowband optical carbon star study of IC 1613 detects 50% of the total AGB population and only considers 18% of this population in calculating the carbon to M-type AGB ratio. We derive an integrated mass-los…
We present new modelling of the spectrophotometric properties of intermediate age stellar populations in the near-infrared (NIR). We take into account the evolutionary and spectroscopic properties of the Asymptotic Giant Branch (AGB) stars which dominate the integrated NIR emission of those populations. The predictions of spectrophotometric narrow-band molecular indices require the use of an effective temperature -- colour (or spectrum) calibration for AGB stars; synthetic indices show a strong dependence upon this calibration. Preliminary results of NIR observations of a supermassive 500 Myr …
We present kinematics and stellar population properties of 17 dwarf early-type galaxies in the luminosity range -14> M_B> -19. Our sample fills the gap between the intensively studied giant elliptical and Local Group dwarf spheroidal galaxies. The dwarf ellipticals of the present sample have constant velocity dispersion profiles within their effective radii and do not show significant rotation, hence are clearly anisotropic. The dwarf lenticulars, instead, rotate faster and are, at least partially, supported by rotation. From optical Lick absorption indices, we derive metallicities and element…
Using deep high-resolution multi-band images taken with the Very Large Telescope and the Hubble Space Telescope, we discovered a new anomalous sequence in the Color Magnitude Diagram of omega Cen. This feature appears as a narrow, well-defined Sub Giant Branch (SGB-a), which merges into the Main Sequence of the dominant cluster population at a magnitude significantly fainter than the cluster Turn-Off (TO). The simplest hypothesis assumes that the new feature is the extension of the anomalous Red Giant Branch (RGB-a) metal-rich population discovered by Lee et al. (1999) and Pancino et al. (2000…
We present a review of the standard paradigm for giant planet formation, the core accretion theory. After an overview of the basic concepts of this model, results of the original implementation are discussed. Then, recent improvements and extensions, like the inclusion of planetary migration and the resulting effects are discussed. It is shown that these improvement solve the timescale problem. Finally, it is shown that by means of generating synthetic populations of (extrasolar) planets, core accretion models are able to reproduce in a statistically significant way the actually observed plane…
We present an empirical determination of the mass-loss rate as a function of stellar luminosity and effective temperature, for oxygen-rich dust-enshrouded Asymptotic Giant Branch stars and red supergiants. To this aim we obtained optical spectra of a sample of dust-enshrouded red giants in the Large Magellanic Cloud, which we complemented with spectroscopic and infrared photometric data from the literature. Two of these turned out to be hot emission-line stars, of which one is a definite B[e] star. The mass-loss rates were measured through modelling of the spectral energy distributions. We thu…
JHKs near-infrared photometry of stars in the Phoenix dwarf galaxy is presented and discussed. Combining these data with the optical photometry of Massey et al. allows a rather clean separation of field stars from Phoenix members. The discovery of a Mira variable (P = 425 days), which is almost certainly a carbon star, leads to an estimate of the distance modulus of 23.10+/-0.18 that is consistent with other estimates and indicates the existence of a significant population of age ~2 Gyr. The two carbon stars of Da Costa have M{bol} = -3.8 and are consistent with belonging to a population of si…
We present new empirical calibrations of the Red Giant Branch (RGB) Bump and Tip based on a homogeneous near-Infrared database of 24 Galactic Globular Clusters. The luminosities of the RGB Bump and Tip in the J, H and K bands and their dependence on the cluster metallicity have been studied, yielding empirical relationships. By using recent transformations between the observational and theoretical planes, we also derived similar calibrations in terms of bolometric luminosity. Direct comparison between updated theoretical models and observations show an excellent agreement. The empirical calibr…
We review current progress in the study of the stellar populations of early-type galaxies, both locally and at intermediate redshifts. In particular, we focus on the ages of these galaxies and their evolution in hopes of determining the star formation epochs of their stars. Due to serious remaining systematic uncertainties, we are unable to constrain these epochs precisely. We discuss our results on the evolution of stellar populations in the context of other observables, in particular the evolution of the Fundamental Plane of early-type galaxies.
We investigate the evolution of initially metal-free, low-mass Red Giant stars through the He core flash at the tip of the Red Giant Branch. The low entropy barrier between the helium- and hydrogen-rich layers enables a penetration of the helium flash driven convective zone into the inner tail of the extinguishing H-burning shell. As a consequence, protons are mixed into high-temperature regions triggering a H-burning runaway. The subsequent dredge-up of matter processed by He and H burning enriches the stellar surface with large amounts of helium, carbon and nitrogen. Extending previous resul…
We have obtained high-resolution, high S/N spectra for six red giants in omega Cen: three belonging to the recently discovered, metal-rich Red Giant Branch (RGB-a, Pancino et al. 2000) and three to the metal intermediate population (RGB-MInt). Accurate Iron, Copper and alpha-elements (Ca and Si) abundances have been derived and discussed. In particular, we have obtained the first direct abundance determination based on high-resolution spectroscopy for the RGB-a population, <[Fe/H]>=-0.60+/-0.15. Although this value is lower than previous estimates based on Calcium triplet measurements, we conf…
I present the first results from a HST/NICMOS imaging study of the most metal-poor blue compact dwarf galaxy IZw18. The near infrared color-magnitude diagram (CMD) is dominated by two populations, one 10-20 Myr population of red super giants and one 0.5-5 Gyr population of asymptotic giant branch stars. Stars older than 1 Gyr are required to explain the observed CMD at the adopted distance of 12.6 Mpc, showing that IZw18 is not a young galaxy. The results hold also if the distance to IZw18 i s significantly larger. This rules out the possibility that IZw18 is a truly young galaxy formed recent…
We present an empirical calibration of the Mixing-Length free parameter $α$ based on a homogeneous Infrared database of 28 Galactic globular clusters spanning a wide metallicity range (-2.15$<$[Fe/H]$<$-0.2). Empirical estimates of the red giant effective temperatures have been obtained from infrared colors. Suitable relations linking these temperatures to the cluster metallicity have been obtained and compared to theoretical predictions. An appropriate set of models for the Sun and Population II giants have been computed by using both the standard solar metallicity $[Z/X]_{\odot}=0.0275$ and …
We present the evolutionary models of metal-free stars in the mass range from 0.8 to 1.2 Msun with up-to-date input physics. The evolution is followed to the onset of hydrogen mixing into a convection, driven by the helium flash at red giant or asymptotic giant branch phase. The models of mass M >= 0.9 Msun undergo the central hydrogen flash, triggered by the carbon production due to the 3-alpha reactions. We find that the border of the off-center and central ignition of helium core flash falls between 1.1 and 1.2 Msun; the models of mass M <= 1.1 Msun experience the hydrogen mixing at the tip…
We present an analysis of the many possible methods for simulating and analysing colour-magnitude diagrams, with application to studies of the field halo stellar populations of resolved galaxies. Special consideration is made to the analysis of stars on the Red Giant branch (RGB), and methods for obtaining metallicity distributions for old populations based on analysis of evolved sections of the Colour Magnitude Diagram (CMD). These tools are designed to provide a reliable and accurate method for the analysis of resolved Population II stars in the haloes of nearby galaxies. In particular, we i…
The time evolution of giant lobe-dominated radio galaxies (with projected linear size D>1 Mpc if H_{0}=50 km/s/Mpc and q_{0}=0.5 is analysed on the basis of dynamical evolution of the entire FRII-type population. Two basic physical parameters, namely the jet power Q_{0} and central density of the galaxy nucleus rho0 are derived for a sample of giants with synchrotron ages reliably determined, and compared with the relevant parameters in a comparison sample of normal-size sources consisting of 3C, B2, and other sources. Having the apparent radio luminosity P and linear size D of each sample sou…
The stellar population of the Pegasus dwarf irregular galaxy is investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with WFPC2. These and ground-based data are combined to produce color-magnitude diagrams which show the complex nature of the stellar population in this small galaxy. A young (< 0.5 Gyr) main sequence stellar component is present and clustered in two centrally-located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of A_V = 0.47 mag. The mean color of the well-populated re…
We present a detailed analysis of the behaviour of the Red Clump K-band absolute magnitude (M(K,RC)) in simple and composite stellar populations, in light of its use as standard candle for distance determinations. The advantage of using M(K,RC), following recent empirical calibrations of its value for the solar neighbourhood, arises from its very low sensitivity to the extinction by interstellar dust. We provide data and equations which allow the determination of the K-band population correction Delta(M(K,RC)) (difference between the Red Clump brightness in the solar neighbourhood and in the p…
Deep V and I CCD images with sub-arcsec spatial resolution are used to investigate the stellar content of the central regions of the Local Group dwarf elliptical galaxy NGC147. Red giant branch (RGB) stars are resolved over the entire field, and the RGB-tip occurs at I ~ 20.5, suggesting that the distance modulus is 24.3. A comparison with globular cluster sequences indicates that the center of NGC147 is moderately metal-poor, with [Fe/H] ~ -1. This is not significantly different from what was found in the outer regions of the galaxy by Mould, Kristian & Da Costa (1983, ApJ, 270, 471). Moreove…
We apply the giant branch slope-[Fe/H] relation derived by Kuchinski et al. [AJ, 109, 1131 (1995)] to a sample of open clusters. We find that the slope of the giant branch in K vs. (J-K) color-magnitude diagrams correlates with [Fe/H] for open clusters as it does for metal-rich globular clusters but that the open cluster data are systematically shifted to less negative values of giant branch slope, at constant [Fe/H]. We use isochrone models to examine the theoretical basis for this relationship and find that for a given value of [Fe/H], the slope of the relationship remains constant with decr…
NGC5018, one of the weakest UV emitters among giant ellipticals (gE) observed with IUE, appears to consist of an optical stellar population very similar to that of the compact, dwarf elliptical M32, which is several magnitudes fainter in luminosity than NGC5018 and whose stellar population is know to be ~3 Gyr old. Here we show that the mid-UV spectra of these two galaxies are also very similar down to an angular scale hundreds times smaller than the IUE large aperture (as probed by HST/FOS UV spectra obtained through 0.86 arcsec apertures). This implies a reasonably close match of the populat…
We resolve stars of the nearest giant elliptical galaxy NGC 5128 using VLT with FORS1 and ISAAC. We construct deep U, V and Ks color-magnitude and color-color diagrams in two different halo fields (in the halo and in the north-eastern diffuse shell). In the outer, shell field, at ~14 kpc from the center of the galaxy, there is a significant recent star formation with stars as young as 10 Myr, approximately aligned with the prominent radio and x-ray jet from the nucleus of the host AGN. Ionized gas filaments are evident in ultraviolet images near the area where neutral HI and CO molecular gas w…
Theoretical predictions of Red Giant Branch stars' effective temperatures, colors, luminosities and surface chemical abundances are a necessary tool for the astrophysical interpretation of the visible--near infrared integrated light from unresolved stellar populations, the Color-Magnitude-Diagrams of resolved stellar clusters and galaxies, and spectroscopic determinations of red giant chemical abundances. On the other hand, the comparison with empirical constraints provides a stringent test for the accuracy of present generations of red giant models. We review the current status of red giant s…
The question of the existence of active tidal disruption around various dSph galaxies remains controversial. That debate often centers on the nature (bound vs. unbound) of extended populations of stars. However, the more fundamental issue of the very existence of the extended populations is still contentious. We present an evaluation of the debate centering on one particular dSph, Carina, for which claims both for and against the existence of stars beyond the King radius have been made. Our review includes an examination of all previous studies bearing on the Carina radial profile and shows th…
We show that collisions with stellar--mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al, 1996. We show that the missing objects are low--mass giants and AGB stars in the range 1-3 M$_{\odot}$. Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of Smoothed Particle Hydrodynamic calculati…
(Abridged) We have investigated the reliability of the widely used I-band Tip of the RGB relative distances for a sample of Local Group galaxies with complex Star Formation Histories (SFR) and Age Metallicity Relationships (AMR) namely the LMC, SMC and LGS3. The use of the K-band is also discussed. By employing theoretical stellar population synthesis techniques, we find that using actual determinations of SFR and AMR of the LMC and SMC, their RGB is populated by stars much younger (by 9 Gyr) than the Galactic globular cluster counterparts, on which the I-band (and K-band) TRGB absolute magnit…
We have used high spatial resolution data from the Hubble Space Telescope and wide-field ground-based observations to search for blue straggler stars (BSS) over the entire radial extent of the large stellar system omega Centauri. We have detected the largest population of BSS ever observed in any stellar system. Even though the sample is restricted to the brightest portion of the BSS sequence, more than 300 candidates have been identified. BSS are thought to be produced by the evolution of binary systems (either formed by stellar collisions or mass exchange in binary stars). Since systems like…
We present V and I photometry of a 9.4' X 9.4' field centered on the dwarf spheroidal galaxy Leo II. The Tip of the Red Giant Branch is identified at I^{TRGB}=17.83 +/- 0.03 and adopting <[M/H]> = -1.53 +/- 0.2 from the comparison of RGB stars with Galactic templates, we obtain a distance modulus (m-M)_0=21.84 +/- 0.13, corresponding to a distance D=233 +/- 15 Kpc. Two significant bumps have been detected in the Luminosity Function of the Red Giant Branch. The fainter bump (B1, at V=21.76 +/- 0.05) is the RGB bump of the dominant stellar population, while the actual nature of the brightest one…
We present deep V,I photometry of two $9.4' x 9.4' field in the outer regions of the M33 galaxy. We obtain a robust detection of the luminosity of the Red Giant Branch Tip (I{TRGB}=20.72 +- 0.08) from which we derived a new estimate of the distance modulus of M33, (m-M)_0=24.64 +- 0.15, corresponding to a distance D=847 +- 60 Kpc. By comparison of the color and magnitude of the observed Red Giant Branch stars with ridge lines of template globular clusters we obtained the photometric metallicity distribution of the considered fields in three different metallicity scales. The derived metallicity…
Deep near infrared photometry of the newly discovered Galactic globular Cluster GLIMPSE-C01 is reported. We derived for the first time the metal abundance of this object from the slope of the RGB: [Fe/H]=-1.61+/-0.14 in the scale of Zinn (as implemented in Harris 1996), [Fe/H]=-1.44+/-0.12 in the scale of Caretta & Gratton (1997), and [Fe/H]=-1.12+/-0.12 in the scale of Ferraro et al. (1999). The tip and the clump of the red giant branch were used to confirm the estimates of Kobulnicky et al. (2005), placing the cluster at D~3.7+/-0.8 kpc, behind A_v~15 mag of visual extinction. The best fit t…
It has been suspected for nearly 50 years that clusters of galaxies contain a population of intergalactic stars, ripped from galaxies during cluster formation or when the galaxies' orbits take them through the cluster center. Support for the existence of such a population of free-floating stars comes from measurements of the diffuse light in clusters, and from recent detections of planetary nebulae with positions and/or velocities far removed from any observed cluster galaxy. But estimates for the mass of the diffuse population and its distribution relative to the galaxies are still highly unc…
A stellar population synthesis model, suitable for comparison with Giant Extragalactic HII Regions (GEHRs), is constructed incorporating the recent developments in modelling stellar evolution by Maeder and co-workers and stellar atmospheres by Kurucz. A number of quantities suitable for comparison with broad band data of GEHRs in visible and near infrared parts of the spectrum are synthesized in addition to the hydrogen and helium ionizing photon production rates at solar metallicities, for three scenarios of star formation --- (i) Instantaneous burst (IB) (ii) Continuous star formation (CSF) …
We study the red giant populations of two dE galaxies, AM 1339-445 and AM 1343-452, with the aim of investigating the number and luminosity of any upper asymptotic giant branch (AGB) stars present. The galaxies are members of the Centaurus A group (D~3.8 Mpc) and are classified as outlying (R~350 kpc) satellites of Cen A. The analysis is based on near-IR photometry for individual red giant stars, derived from images obtained with ISAAC on the VLT. The photometry, along with optical data derived from WFPC2 images retrieved from the HST science archive, enable us to investigate the stellar popul…
