Volume 390, Number 3, August II 2002
|Page(s)||961 - 965|
|Section||Galactic structure, stellar clusters, and populations|
|Published online||14 August 2002|
The white dwarf cooling sequence in the old open cluster NGC 188*
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
Corresponding author: G. Andreuzzi, firstname.lastname@example.org
Accepted: 6 June 2002
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 ) 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 4 Gyr old, an age that is set solely by the photometric limit of our data. Classical methods provide an estimate of 7 Gyr (Sarajedini et al. 1999).
Key words: open clusters and associations: individual: NGC 188 / stars: white dwarfs / stars: luminosity function
© ESO, 2002
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