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Search Results: 1 - 10 of 189898 matches for " G. Piotto "
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Hot Stellar Populations in Globular Clusters: a Photometrist's View
G. Piotto
Physics , 2003,
Abstract: We briefly review the recent results on hot horizontal-branch stars in globular clusters. Since the first Ivanfest, in 1992, there have been a number of new observational lines of evidence which have allowed significant progress in our understanding of blue-tail stars, though new, even more intriguing questions arise. Despite this progress, we still do not know the answer to the main question: why are there blue-HB-tail stars? The new photometric data bases collected in the last few years, and the forthcoming multi-fiber observational campaigns on 10m-class telescopes, might be the key to solve this puzzle. We will show an example of how these data bases can disclose important properties of blue HB stars.
Comparison between Observed and Theoretical Red Giant Branch Luminosity Functions of Galactic Globular Clusters
M. Zoccali,G. Piotto
Physics , 2000,
Abstract: V-band luminosity functions have been obtained for the upper main-sequence, sub-giant branch and red giant branch of 18 galactic globular clusters from HST data. A comparison with four sets of theoretical models has been performed. In contrast with what was found in several previous works, a Good general agreement has been found between the observed and theoretical LF at any metallicity [M/H]<-0.7. Possible discrepancies at higher metallicity, in the upper part of the RGB, need to be confirmed with further observational data and by extending all the models to the most metal rich regime. The SGB shape has been used to set an upper limit to the cluster age, and consequently a lower limit on the cluster distance. A discussion on the still open problem of the mismatch between the observed and theoretical RGB bump location is also presented.
HST Luminosity Functions of the Globular Clusters M10, M22, and M55. A comparison with other clusters
G. Piotto,M. Zoccali
Physics , 1999,
Abstract: From a combination of deep Hubble Space Telescope V and I images with groundbased images in the same bands, we have obtained color-magnitude diagrams of M10, M22, and M55, extending from just above the hydrogen burning limit to the tip of the red giant branch, down to the white dwarf cooling sequence. We have used the color-magnitude arrays to extract main sequence luminosity functions (LFs) from the turnoff to about 0.13 solar masses. The LFs of M10 is significantly steeper than that for the other two clusters. The difference cannot be due to a difference in metallicity. A comparison with the LFs from Piotto, Cool, and King (1997), shows a large spread in the LF slopes. This spread is also present in the local mass functions (MFs) obtained from the observed LFs using different theoretical mass--luminosity relations. The dispersion in the MF slopes remains also after removing the mass segregation effects by using multimass King-Michie models. The globular cluster MF slopes are also flatter than the MF slope of the field stars and of the Galactic clusters in the same mass interval. We interpret the MF slope dispersion and the MF flatness as an evidence of dynamical evolution which makes the present day globular cluster stellar MFs different from the intial MFs. The slopes of the present day MFs exclude that the low mass star can be dynamically relevant for the Galactic globular clusters.
HST multiband photometry of the globular cluster NGC 6388
G. Busso,G. Piotto,S. Cassisi
Physics , 2003,
Abstract: We present color-magnitude diagrams (CMD) of the globular cluster NGC 6388 based on HST multiband photometry (F255W, F336W, F439W, F555W). In this paper we focus our attention on the peculiar horizontal branch of this cluster. After a careful reddening correction, we fitted the observed CMD with theoretical models. For the first time we demonstrated that the HB of this very metal rich globular cluster extends beyond Te > 30.000 K, showing clear evidence of a population of blue hook (D'Cruz et al.2000, Brown et al. 2001) stars. Moreover, we could demonstrate that the HB tilt (slope) is not a reddening effect and is present in all the photometric bands.
The Helium content of Globular Clusters: light element abundance correlations and HB morphology. I. NGC6752
S. Villanova,G. Piotto,R. G. Gratton
Physics , 2009, DOI: 10.1051/0004-6361/200811493
Abstract: Helium has been proposed as the key element to interpret the observed multiple main sequences (MS), subgiant branches (SGB) and red giant branches (RGB), as well as the complex horizontal branch (HB) morphology in Globular Clusters (GC). However, up to now, He was never directly measured in suitable GC stars (8500
The Stellar Distribution of the Globular Cluster M55
S. Zaggia,G. Piotto,M. Capaccioli
Physics , 1997,
Abstract: We have used extensive V, I photometry (down to V=20.9) of 33615 stars in the direction of the globular cluster M55 to study the dynamical interaction of this cluster with the tidal fields of the Galaxy. An entire quadrant of the cluster has been covered, out to ~2.0 times the tidal radius. A CMD down to about 4 magnitudes below the turn-off is presented and analysed. A large population of BS has been identified. The BS are significantly more concentrated in the inner 300 arcsec, while they become less concentrated in the cluster envelope. We have obtained luminosity functions at various radial intervals from the center and their corresponding mass functions. Both clearly show the presence of mass segregation inside the cluster. A dynamical analysis shows that the observed mass segregation is compatible with what is predicted by multi-mass King-Michie models. The global mass function is very flat with a power-law slope of x=-1.0+/-0.4. This suggest that M55 might have suffered selective losses of stars, caused by tidal interactions with the Galactic disk and bulge. The radial density profile of M55 out to ~2.0 r_t suggests the presence of extra-tidal stars whose nature could be connected with the cluster.
Comparison between predicted and empirical DV(Bump -HB) in Galactic Globular Clusters
M. Zoccali,S. Cassisi,G. Piotto,G. Bono,M. Salaris
Physics , 1999, DOI: 10.1086/312064
Abstract: We present observational estimates of DV(Bump-HB) in a sample of 28 Galactic Globular Clusters (GGCs) observed by HST. The photometric accuracy and the sizable number of stars measured in each cluster allowed us to single out the RGB Bump both in metal-poor and in metal-rich GGCs. Empirical values are compared with homogeneous theoretical predictions which account for both H and He burning phases over a wide range of metal abundances (0.0001 < Z < 0.02). We found that, within current observational uncertainties on both iron and $\alpha$-element abundances, theory and observations are in very good agreement, provided that the metallicity scale by Carretta & Gratton (1997) as extended by Cohen et al. (1999) is adopted. Interesting enough, we also found that both theoretical and observed values show a change in the slope of the DV(Bump-HB)-[M/H] relation toward higher metal contents.
On the observational properties of He-burning stars: some clues on the tilt of the HB in metal rich clusters
G. Raimondo,V. Castellani,S. Cassisi,E. Brocato,G. Piotto
Physics , 2002, DOI: 10.1086/339428
Abstract: We investigate the predicted Color-Magnitude distribution of metal-rich Horizontal Branch (HB) stars, discussing selected theoretical models computed under various assumptions about the star metallicity and the efficiency of super-adiabatic convection. We find that canonical Zero Age Horizontal Branches with metallicity larger or of the order of Z=0.002 should be all affected by a tilt, by an amount which increases when the metallicity is increased and/or the mixing length is decreased, reaching a tilt of $\Delta V \sim$0.2 mag in the case of solar metallicity when a mixing length value $\alpha$=1.6 is assumed ($\Delta V$ is the magnitude difference between the top of the blue HB and the fainter magnitude reached by the red HB). Uncertainties in the luminosity of the red HB due to uncertainty in the mixing length value are discussed. We finally discuss the much larger tilt observed in the clusters NGC 6441 and NGC 6388, reporting additional evidence against suggested non-canonical evolutionary scenarios. Numerical experiments show that differential reddening could produce such sloped HBs. Further, HST-PC imaging of NGC 6441 gives clear indications about the occurrence of differential reddening across the cluster. However, the same imaging shows that the observed slope of the red HB {\em is not} an artifact of differential reddening. We finally show that sloping red HBs in metal rich clusters are a common occurrence not necessarily correlated with the appearance of extended blue HB.
Some clues on the tilt of the Horizontal Branches in metal-rich clusters
G. Raimondo,V. Castellani,S. Cassisi,E. Brocato,G. Piotto
Physics , 2002,
Abstract: We discuss some clues on the large tilt observed in the Horizontal Branch of the metal-rich galactic globular clusters NGC 6388 and NGC 6441. This not yet understood feature is investigated from theoretical and observational sides.
A Comparison of Deep HST Luminosity Functions of Four Globular Clusters
G. Piotto,A. M. Cool,I. R. King
Physics , 1997, DOI: 10.1086/118347
Abstract: From deep color-magnitude arrays made from V and I images taken with the Hubble Space Telescope's WFPC2 camera we have determined luminosity functions (LFs) down to a level that corresponds to about 0.13 solar masses, for the low-metal-abundance globular clusters M15, M30, M92, and NGC 6397. Because of the similarity of the metallicities of these clusters, differences in their luminosity functions directly trace differences in their mass functions. The LFs of M15, M30, and M92 agree closely over the entire observed range, whereas that of NGC 6397 drops away sharply at the faintest magnitudes. We suggest that the deficiency of low-mass stars in NGC 6397 is due to tidal shocks, to ejection through internal relaxation, or to a combination of the two. With the presently available mass-luminosity relations, we find that even in M15, M30, and M92 the mass functions probably do not rise so fast as to make the low-mass end dominant.
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