2013ApJ...762....3Y

We present a comprehensive photometric study of the old open cluster, NGC 6819, using 1°x1° field VI MOSAIC CCD imaging taken with the WIYN 0.9 m telescope. The resultant color-magnitude diagram (CMD) shows a well-developed main sequence (MS) extending from V ∼ 14.5 mag down to our photometric limit of V ∼ 21 mag. Fitting theoretical isochrones with the adopted values of the reddening and metallicity (E(B - V) = 0.14, [Fe/H] = +0.09 dex) to the observed CMD yields a distance modulus of (m - M)₀= 11.93±0.10 and an age of ∼2.6 Gyr for NGC 6819. Our wide-field imaging reveals that NGC 6819 is larger in areal extent (R = 13') than previously thought. The wide field also benefits our estimate of the degree of field star contamination, and ultimately yields improved measurements of the structural parameters (r_c = 2.'80, r_t= 38.'2, and r_h= 7') and tidal mass of the cluster (M_tid = 3542.4 M_☉). The flattened luminosity and mass functions indicate that NGC 6819 has experienced mass segregation as a result of its dynamical evolution. Our variability study of the cluster blue straggler star (BSS) population using the Welch-Stetson variability index (I_WS) has revealed a number of variable BSS candidates.