WIYN Open Cluster Study : Wide-Field CCD Photometry of the Old Open Cluster NGC 6819

TL;DR

This study provides a detailed wide-field photometric analysis of the old open cluster NGC 6819, determining its fundamental parameters, structure, and variable blue straggler stars, revealing its larger size and dynamical evolution.

Contribution

It offers the first comprehensive wide-field CCD photometry of NGC 6819, refining its size, structure, age, distance, and identifying variable blue stragglers, advancing understanding of its evolution.

Findings

NGC 6819 is larger than previously thought with a radius of 13 arcminutes.

The cluster's age is approximately 2.6 billion years, with a distance modulus of 11.93.

Mass segregation is evident from the flattened luminosity and mass functions.

Abstract

We present a comprehensive photometric study of the old open cluster, NGC 6819 using 1x1 degree field VI MOSAIC CCD imaging taken with the WIYN 0.9m 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 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(sun)). 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.