arXiv subjects

G. Piotto

Publications and source records attributed to G. Piotto.

365 records · Page 21Linked to original sources

arXiv1997

The metallicity of Palomar 1

Palomar 1 is a peculiar galactic globular cluster, suspected to be younger than the bulk of the Galactic halo objects. However, such a low age can be confirmed only after a reliable determination of the metallicity. In the present paper, we use the equivalent widths (W) of the Ca II triplet on medium resolution spectra in order to determine the metal content of Pal 1. From the comparison of the luminosity corrected W's in four stars of Palomar 1 with those of a sample of stars in each of three calibration clusters (M2, M15, and M71), we derive [Fe/H]=-0.6+/-0.2 on the Zinn & West (1984) scale or [Fe/H]=-0.7+/-0.2 on the Carretta & Gratton (1997) scale. We also obtain a radial velocity Vr=-82.8+/-3.3 Km/s for Pal 1.

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arXiv1997

The Stellar Distribution of the Globular Cluster M55

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.

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arXiv1997

Globular Clusters Color-Magnitude Diagrams with HST

In this paper we present the 13 B and V color-magnitude diagrams discussed by Piotto et al. in the same Procedings. The data have been collected with the WFPC2 camera on HST. We emphasize the discovery of blue horizontal branches in the two metal rich globular clusters NGC 6388 and NGC 6441, and the four-mode HB of NGC 2808. The significance of the gaps in the HB of NGC 2808 is also assesed.

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arXiv1997

HST Observations of High-Density Globular Clusters

In this paper we present preliminary results from our HST project aimed at exploring the connection between the stellar dynamics and stellar evolution in the cores of high-density globular clusters. We illustrate the discovery of horizontal branches (HB) with extended blue tails in the very metal rich globular clusters NGC 6388 ([Fe/H]=-0.6) and NGC 6441 ([Fe/H]=-0.5), and of a four-mode horizontal branch in NGC 2808. We briefly summarize the possible causes of these anomalies, which might be a signature of mass-loss processes,subtle composition variations, or dynamical effects. The existence of blue HBs in metal rich clusters and of significant narrow gaps between clumps on the HB are a challenge for the theoretical models. The composite color-magnitude diagram and luminosity function of 878 blue stragglers in our sample of 13 galactic globular clusters are also presented and briefly discussed.

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arXiv1997

A Comparison of Deep HST Luminosity Functions of Four Globular Clusters

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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