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Astron. Astrophys. 356, 127-133 (2000)

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Use of DPOSS data to study globular cluster halos: an application to M 92

V. Testa 1, S.R. Zaggia 2,3, S. Andreon 2, G. Longo 2, R. Scaramella 1, S.G. Djorgovski 4 and R. de Carvalho 5

1 Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio Catone, Italy
2 Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
3 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching, Germany
4 Department of Astronomy, California Institute of Technology, MS 105-24, Pasadena, CA 91125, USA
5 Observatorio Nacional de Rio de Janeiro, Rio de Janeiro, Brazil

Received 17 August 1999 / Accepted 25 January 2000

Abstract

We exploited the large areal coverage offered by the Digitized Palomar Observatory Sky Survey to analyze the outermost regions of the galactic globular cluster M 92 (NGC 6341). Two independent photometric reduction programs (SKICAT and DAOPHOT) were used to construct a color-magnitude diagram and a surface density profile for this cluster, based on J- and F-band DPOSS plates. A strong similarity has been found in the performance of the two programs in the low-crowded outermost cluster regions. After removing the background contribution, we obtained the cluster outer surface density profile down to a surface brightness magnitude of [FORMULA] 31 mag arcsec-2 and matched it with the inner profile of Trager et al. (1995). The profile shapes match very well: since our data are uncalibrated, the shift in magnitudes between the profiles has been also used to calibrate our profile. The analysis shows that the cluster has an extra tidal halo extending out to [FORMULA] from the cluster center at a [FORMULA] level over the background noise. This halo is revealed to be almost circular.

Key words: Galaxy: globular clusters: general – Galaxy: kinematics and dynamics – Galaxy: globular clusters: individual: M 92

Send offprint requests to: V. Testa (testa@coma.mporzio.astro.it)

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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