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Astron. Astrophys. 363, 159-173 (2000)


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The anomalous Galactic globular cluster NGC 2808 *

Mosaic wide-field multi-band photometry

L.R. Bedin 1, G. Piotto 1, M. Zoccali 1, P.B. Stetson 2, I. Saviane 1, S. Cassisi 3 and G. Bono 4

1 Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
2 Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, BC, Canada
3 Osservatorio Astronomico di Collurania, Via M. Maggini, 64100 Teramo, Italy
4 Osservatorio Astronomico di Roma, Via Frascati 33, Monteporzio Catone, 00040 Roma, Italy

Received 12 April 2000 / Accepted 20 June 2000

Abstract

We present Johnson UBV, and Kron-Cousins I photometry for about 60,000 stars in a region of [FORMULA] arcmin2 centered on the Galactic globular cluster (GGC) NGC 2808. The central [FORMULA] arcsec region has been mapped also in the F218W, F439W, and F555W bands with the HST WFPC2 camera. Overall, we cover a field which extends from the center to about 1.7 tidal radii. Photometric calibration has been secured by more than 1,000 standards, and the HST B and V photometry has been linked to the groundbased system.

We confirm the anomalously elongated horizontal branch (HB) with gaps along it, and we show that the gaps are statistically significant. Most importantly, we find that:

  • the extended blue tail of HB (EBT) is present beyond 400 arcsec, corresponding to more than nine times the half mass radius [FORMULA], and extends to [FORMULA] also in these external regions;

  • also the gaps on the EBT are present at least out to 400 arcsec from the cluster center, and possibly beyond that. The location of the gaps in the HB seems to be the same all over the cluster.

  • there are no significant radial gradients in the distribution of the stars in the different HB stumps and some marginal evidence of an increase in the number of red giant branch (RGB) stars in the cluster core.

The observational facts presented in this paper seem to exclude the possibility that the EBTs originate from tidal stripping of the envelope of the RGB stars due to close encounters in high density environments or to mass transfer in close binaries. Also, it is not clear whether the presence of EBTs and gaps on them are the manifestation of the same physical phenomenon.

We have also shown that the jump recently identified in the HB using Strömgren-u photometry is clearly visible also in the U, [FORMULA] plane, at [FORMULA] K.

Finally, we present the luminosity function (LF) of the RGB. The LF clearly shows the presence of the usual RGB bump, but also a second feature, 1.4 magnitudes brighter in V, that we have named RGB heap . The RGB heap, visible also in other GGCs, is a new feature that, because of its position on the RGB, we have tentatively associated with the recently discovered K giant variables.

Key words: stars: Hertzsprung – Russel (HR) and C-M diagrams – stars: horizontal-branch – stars: luminosity function, mass function – stars: Population II – Galaxy: globular clusters: individual: NGC 2808

* Based on observations made at the European Southern Observatory, La Silla, Chile, and on observations with the NASA/ESA Hubble Space Telescope.

Send offprint requests to: G. Piotto (piotto@pd.astro.it)

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000

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