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Astron. Astrophys. 327, 1004-1016

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The stellar distribution of the globular cluster M 55*

Simone R. Zaggia1,2, Giampaolo Piotto1, and Massimo Capaccioli2,3

1Dipartimento di Astronomia, Università di Padova, Vicolo dell' Osservatorio 5, I-35122 Padova, Italy
2Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131 Napoli, Italy
3Dipartimento di Scienze Fisiche, Università Federico II, Mostra d'Oltremare, Padiglione 19, I-80125 Napoli, Italy

Received 29 May 1997 / Accepted 11 July 1997


We have used extensive V, I photometry (down to V = 20.9) of 33615 stars in the direction of the globular cluster M 55 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 $\simeq1.5$ 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 than the other cluster stars 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 $x=-1.0\pm0.4$. This suggest that M 55 might have suffered selective losses of stars, caused by tidal interactions with the Galactic disk and bulge.

The radial density profile of M 55 out to $\sim 2\times r_t$ suggests the presence of extra-tidal stars whose nature could be connected with the cluster.

Key words: stars: luminosity function, mass function; HR-diagramm - globular clusters: general - M 55 - Galaxy: kinematics and dynamics

*Based on observations made at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: S.R. Zaggia. e-mail: zaggia@na.astro.it

© European Southern Observatory (ESO) 1997

Online publication: October 20, 1997
Last change: April 6, 1998