The CFHT Open Star Cluster Survey. II. Deep CCD Photometry of the Old Open Star Cluster NGC 6819

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Abstract
We present analysis of deep CCD photometry for the very rich, old open star cluster NGC 6819. The science goals are to catalog the white dwarfs in the cluster and measure the cluster luminosity and mass functions. These CFH12K data results represent the first of nineteen open star clusters which were imaged as a part of the CFHT Open Star Cluster Survey. We find a tight, very rich, main-sequence and turnoff consisting of over 2900 cluster stars in the V, B-V color-magnitude diagram (CMD). Main-sequence fitting of the unevolved cluster stars with the Hyades star cluster yields a distance modulus of (m - M)V = 12.30 ± 0.12, for a reddening of E(B-V) = 0.10. These values are consistent with a newly calculated theoretical stellar isochrone of age 2.5 Gyr, which we take to be the age of the cluster. Both the depth gained in the photometry and the increased projected area of the CFH12K Mosaic CCD allow for detailed star counts in concentric annuli out to large angular radii. These indicate a much larger cluster extent (R = 95 ± 10), by a factor of ~2 over some previous estimates. Incompleteness tests confirm a slightly negatively sloped luminosity function extending to faint (V ~ 23) magnitudes which is indicative of a dynamically evolved cluster. Further luminosity function and mass segregation tests indicate that low-mass objects (M ≤ 0.65 M⊙) predominate in the outer regions of the cluster, 35 ≤ R ≤ 95. The estimation of the number of white dwarfs in NGC 6819, based on stellar evolution models, white dwarf cooling timescales, and conservation of star number arguments applied to the red giant stars of the cluster are in good agreement with the observed number. For those white dwarf candidates which pass both a statistical subtraction that removes background galaxies and field stars and a high star/galaxy confidence by using image classification, we show comparisons with white dwarf isochrones and cooling models which suggest the need for spectroscopy to confirm the white dwarf nature of the brighter objects. This is entirely feasible for all objects, before a statistical subtraction cut, with the current generation of 8 m–class telescopes and multiobject spectrometers.

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