DETERMINING THE AGE OF THE KEPLER OPEN CLUSTER NGC 6819 WITH A NEW TRIPLE SYSTEM AND OTHER ECLIPSING BINARY STARS*

Abstract

ABSTRACT As part of our study of the old (∼2.5 Gyr) open cluster NGC 6819 in the Kepler field, we present photometric (Kepler and ground-based BVR C I C ) and spectroscopic observations of the detached eclipsing binary WOCS 24009 (Auner 665; KIC 5023948) with a short orbital period of 3.6 days. WOCS 24009 is a triple-lined system, and we verify that the brightest star is physically orbiting the eclipsing binary using radial velocities and eclipse timing variations. The eclipsing binary components have masses M B = 1.090 ± 0.010 M ⊙ and M C = 1.075 ± 0.013 M ⊙, and radii R B = 1.099 ± 0.006 ± 0.005 R ⊙ and R C = 1.069 ± 0.006 ± 0.013 R ⊙. The bright non-eclipsing star resides at the cluster turnoff, and ultimately its mass will directly constrain the turnoff mass: our preliminary determination is M A = 1.251 ± 0.057 M ⊙. A careful examination of the light curves indicates that the fainter star in the eclipsing binary undergoes a very brief period of total eclipse, which enables us to precisely decompose the light of the three stars and place them in the color–magnitude diagram (CMD). We also present improved analysis of two previously discussed detached eclipsing stars in NGC 6819 (WOCS 40007 and WOCS 23009) en route to a combined determination of the cluster’s distance modulus (m − M) V = 12.38 ± 0.04. Because this paper significantly increases the number of measured stars in the cluster, we can better constrain the age of the CMD to be 2.21 ± 0.10 ± 0.20 Gyr. Additionally, using all measured eclipsing binary star masses and radii, we constrain the age to 2.38 ± 0.05 ± 0.22 Gyr. The quoted uncertainties are estimates of measurement and systematic uncertainties (due to model physics differences and metal content), respectively.