F. Fusi Pecci

A Helium Spread among the Main-Sequence Stars in NGC 2808

We have made a detailed study of the color distribution of the main sequence of the globular cluster (GC) NGC 2808, based on new deep HST WFPC2 photometry of a field in the uncrowded outskirts of the cluster. The observed color distribution of main-sequence stars is not Gaussian and is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. Using synthetic color-magnitude diagrams based on new stellar models, we estimate that the helium mass fraction of these stars is Y ~ 0.4. The newly found anomaly on the main sequence gives credence to the idea that GCs like NGC 2808 have undergone self-enrichment and that different stellar populations were born from the ejecta of the intermediate-mass asymptotic giant branch (AGB) stars of the first generation. The enhancement and spread of helium among the stars in NGC 2808 have recently been suggested as a simple way to explain the very peculiar morphology of its horizontal branch. We find that if in addition to the Y = 0.40 stars, roughly 30% of the stars have Y distributed between 0.26-0.29, while 50% have primordial Y, this leads to a horizontal-branch morphology similar to that observed. In this framework, three main stages of star formation are identified, the first with primordial helium content Y 0.24, the second born from the winds of the most massive AGBs of the first stellar generation (~6-7 M☉), with Y ~ 0.4, and a third born from the matter ejected from less massive AGBs (~3.5-4.5 M☉), with Y ~ 0.26-0.29. There could have been a long hiatus (several times 107 yr), between the second and third generation in which no star formed in the protocluster. We suggest that during this period, star formation has been inhibited by the explosion of late Type II supernovae deriving from binary evolution.

THE GIANT, HORIZONTAL, AND ASYMPTOTIC BRANCHES OF GALACTIC GLOBULAR CLUSTERS. I. THE CATALOG, PHOTOMETRIC OBSERVABLES, AND FEATURES

A catalog including a set of the most recent color-magnitude diagrams (CMDs) is presented for a sample of 61 Galactic globular clusters (GGCs). We used this database to perform a homogeneous systematic analysis of the evolved sequences (namely, the red giant branch [RGB], horizontal branch [HB], and asymptotic giant branch [AGB]). Based on this analysis, we present (1) a new procedure to measure the level of the zero-age horizontal branch (VZAHB) and a homogeneous set of distance moduli obtained by adopting the HB as standard candle; (2) an independent estimate for RGB metallicity indicators and new calibrations of these parameters in terms of both spectroscopic ([Fe/H]CG97) and global metallicity ([M/H], including also the ?-element enhancement), such that the set of equations presented can be used to simultaneously derive a photometric estimate of the metal abundance and the reddening from the morphology and the location of the RGB in the (V, B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and the AGB bump (in nine GGCs). The dependence of these features on metallicity is discussed. We find that by using the latest theoretical models and the new metallicity scales, the earlier discrepancy between theory and observations (~0.4 mag) completely disappears.

2MASS NIR photometry for 693 candidate globular clusters in M 31 and the Revised Bologna Catalogue

We have identified in the 2MASS database 693 known and candidate globular clusters in M 31. The 2MASS J, H, K magnitudes of these objects have been transformed to the same homogeneous photometric system of existing near infrared photometry of M 31 globulars, finally yielding J, H, K integrated photometry for 279 confirmed M 31 clusters, 406 unconfirmed candidates and 8 objects with controversial classification. Of these objects 529 lacked any previous estimate of their near infrared magnitudes. The newly assembled near infrared dataset has been implemented into a revised version of the Bologna Catalogue of M 31 globulars, with updated optical (UBVRI) photometry taken, when possible, from the most recent sources of CCD photometry available in the literature and transformed to a common photometric system. The final Revised Bologna Catalogue (available in electronic form) is the most comprehensive list presently available of confirmed and candidate M 31 globular clusters, with a total of 1164 entries. In particular, it includes 337 confirmed GCs, 688 GC candidates, 10 objects with controversial classification, 70 confirmed galaxies, 55 confirmed stars, and 4 H  regions lying within ∼3 ◦ from the center of the M 31 galaxy. Using the newly assembled database we show that the V − K color provides a powerful tool to discriminate between M 31 clusters and background galaxies, and we identify a sample of 83 globular cluster candidates, which is not likely to be contaminated by misclassified galaxies.

On the effects of cluster density and concentration on the horizontal branch morphology - The origin of the blue tails

Possible relationships between horizontal branch (HB) morphology in Galactic globular clusters and the cluster structure and dynamical evolution are investigated. New HB observables are defined and determined using a theoretical framework deduced from HB models. Data for 53 Galactic globular clusters are used to obtain correlations between the observables. It is found that the net length of the HB and the presence and extent of blue tails in particular are correlated with the cluster density and concentrations, in the sense of more concentrated or denser clusters having bluer and longer HB morphologies. This effect is especially strong for the intermediate metallicity clusters. Thus, the cluster environment can affect the stellar evolution leading to the HB and therefore the HB morphology. This result is interpreted in terms of an enhanced mass removal from the HB progenitors.

Two distinct sequences of blue straggler stars in the globular cluster M 30

Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy’s history. ‘Blue stragglers’ are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M 30. This globular cluster is thought to have undergone ‘core collapse’, during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer.

The fraction of binary systems in the core of 13 low-density Galactic globular clusters

We used deep observations collected with Advanced Camera for Surveys (ACS) at Hubble Space Telescope (HST) to derive the fraction of binary systems in a sample of 13 low-density Galactic globular clusters. By analysing the colour distribution of main-sequence stars we derived the minimum fraction of binary systems required to reproduce the observed colour–magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6 per cent within the core radius. The estimated global fractions of binary systems range from 10 to 50 per cent depending on the cluster. A dependence of the relative fraction of binary systems on the cluster age has been detected, suggesting that the binary disruption process within the cluster core is active and can significantly reduce the binary content in time.

Low-mass X-ray binaries in globular clusters: A new metallicity effect

Galactic Globular Clusters (GCs) containing bright X sources (L_x > 10^{36} erg/s), commonly associated with Low Mass X-Ray Binaries (LMXBs), are found to be significantly denser and more metal-rich than normal non-X-ray clusters both in the Galaxy and in M31. Within a framework where LMXBs in GCs are generated via tidal captures in high-density clusters and (2+1) encounters in low-density globulars, the higher incidence of LMXBs with increasing metallicity is shown to be {\it intrinsic} and not just a by-product of other effects. Two possible mechanisms are examined: the first one assumes a dependence of the cluster IMF on metallicity as recently published in the literature. The observed LMBXs, more frequently occurring in metal rich clusters, agrees with the predicted number of NS only if metallicity accounts for a minor contribution to the observed variation of the IMF slope. Other alternatives explored, such as the total variation of the observed IMF slopes is due to (a) just metallicity and (b) the combination of metallicity and position in the Galaxy lead to a clear-cut disagreement with the data. The second mechanism assumes that, at fixed cluster density, the rate of tidal captures depends on radius and mass of the capturing star. Based on standard stellar models, stars with higher metal content have wider radii and higher masses, hence the rate of tidal captures increases with increasing metallicity. From the order of magnitude computations made, this new effect by itself could explain the observed ratio of 4 between the frequencies of X-ray clusters in the metal-rich and metal-poor groups we observationally determined. However, there is no reason to exclude that both mechanisms can be at work.

The peculiar horizontal branch of NGC 2808

We present an accurate analysis of the peculiar horizontal branch (HB) of the massive Galactic globular cluster NGC 2808, based on high-resolution far-UV and optical images of the central region of the cluster obtained with the Hubble Space Telescope. We confirm the multimodal distribution of stars along the HB: four sub-populations separated by gaps are distinguishable. The detailed comparison with suitable theoretical models showed that (i) it is not possible to reproduce the luminosity of the entire HB with a single helium abundance, while an appropriate modelling is possible for three HB groups by assuming different helium abundances in the range 0.24 <� Y < 0.4 that are consistent with the multiple populations observed in the main sequence; and (ii) canonical HB models are not able to properly match the observational properties of the stars populating the hottest end of the observed HB distribution, the so-called blue hook region. These objects are probably ‘hot flashers’, stars that peel off the red giant branch before reaching the tip and ignite helium at high T eff. Both of these conclusions are based on the luminosity of the HB in the optical and ultraviolet bands and do not depend on specific assumptions about mass loss.

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.

Discovery of Carbon/Oxygen-depleted Blue Straggler Stars in 47 Tucanae: The Chemical Signature of a Mass Transfer Formation Process*

We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 blue straggler stars (BSSs) in 47 Tuc. We discovered that a subpopulation of BSSs shows a significant depletion of carbon and oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of a mass transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.