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Issue A&A
Volume 390, Number 3, August II 2002
Page(s) 961 - 965
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20020858



A&A 390, 961-965 (2002)
DOI: 10.1051/0004-6361:20020858

The white dwarf cooling sequence in the old open cluster NGC 188

G. Andreuzzi1, H. B. Richer2, M. Limongi1 and M. Bolte3

1  INAF-Astronomical Observatory of Rome, via di Frascati I-33, 00040 Monteporzio Catone, Roma, Italy
2  Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
3  University of California, UCO/Lick Observatory, Santa Cruz, CA 95064, USA

(Received 17 January 2002 / Accepted 6 June 2002)

Abstract
We develop the white dwarf luminosity function (LF) of the old open cluster NGC 188 in order to determine a lower limit to the age of the cluster by using the faint end of the cooling sequence.

To produce an extensive sequence of the cooling white dwarfs we imaged four contiguous HST-WFPC2 fields in the center of the cluster in the F555W and F814W filters. After imposing selection criteria on the detected objects we found a white dwarf cooling sequence (down to $V \simeq 26.5$ ) including 28 candidate white dwarfs in the cluster. The exposures are not deep enough to reach the end of this sequence, but the results of our analysis allow us to establish a lower limit to the age of the cluster independently of the isochrone fit to the cluster turnoff.

The most ancient white dwarfs found are $\simeq$4 Gyr old, an age that is set solely by the photometric limit of our data. Classical methods provide an estimate of $\simeq$7 Gyr (Sarajedini et al. 1999).


Key words: open clusters and associations: individual: NGC 188 -- stars: white dwarfs -- stars: luminosity function

Offprint request: G. Andreuzzi, gloria@coma.mporzio.astro.it

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