THE AGE AND DISTANCE OF THE KEPLER OPEN CLUSTER NGC 6811 FROM AN ECLIPSING BINARY, TURNOFF STAR PULSATION, AND GIANT ASTEROSEISMOLOGY^∗

Abstract

We present the analysis of an eccentric, partially eclipsing long-period (P = 19.23 d) binary system KIC 9777062 that contains main sequence stars near the turnoff of the intermediate age open cluster NGC 6811. The primary is a metal-lined Am star with a possible convective blueshift to its radial velocities, and one star (probably the secondary) is likely to be a γ Dor pulsator. The component masses are 1.603 ± 0.006(stat.)±0.016(sys.) and 1.419 ± 0.003 ± 0.008M , and the radii are 1.744 ± 0.004 ± 0.002 and 1.544 ± 0.002 ± 0.002R . The isochrone ages of the stars are mildly inconsistent: the age from the mass-radius combination for the primary (1.05±0.05±0.09 Gyr, where the last quote was systematic uncertainty from models and metallicity) is smaller than that from the secondary (1.21 ± 0.05 ± 0.15 Gyr) and is consistent with the inference from the color-magnitude diagram (1.00 ± 0.05 Gyr). We have improved the measurements of the asteroseismic parameters ∆ν and ν max for helium-burning stars in the cluster. The masses of the stars appear to be larger (or alternately, the radii appear to be smaller) than predicted from isochrones using the ages derived from the eclipsing stars. The majority of stars near the cluster turnoff are pulsating stars: we identify a sample of 28 δ Sct, 15 γ Dor, and 5 hybrid types. We used the period-luminosity relation for high-amplitude δ Sct stars to fit the ensemble of the strongest frequencies for the cluster members, finding (m − M ) V = 10.37 ± 0.03. This is larger than most previous determinations, but smaller than values derived from the eclipsing binary (10.47 ± 0.05).

A Temporary Epoch of Stalled Spin-down for Low-mass Stars: Insights from NGC 6811 with Gaia and Kepler

“…While there is some tension between the age solutions for the two EB components, most of the data support an age for the cluster of 1 ± 0.05 Gyr (Sandquist et al 2016), in agreement with the Janes et al (2013) U BV RI photometric analysis. Sandquist et al (2016) also summarized the spectroscopic metallicity measurements in the literature from various sources, which agree on an approximately solar metallicity with values ranging from −0.02 to +0.04 dex. Finally, Sandquist et al (2016) found an interstellar reddening value of E(B−V ) = 0.07±0.02 (corresponding to A V = 0.22), while various literature values range from E(B − V ) = 0.05 to 0.14 (A V = 0.16 to 0.43).…”

The [Y/Mg] clock works for evolved solar metallicity stars

“…NGC 6811 is also a solar-metallicity cluster and it is the youngest in the sample. For this we have three targets, all confirmed members (Sandquist et al 2016). All targets, except the NGC 188 target, is confirmed by asteroseismology to be in the helium-core-burning phase (Corsaro et al 2012;Stello et al 2016;Arentoft et al 2017).…”

Variability in the Massive Open Cluster NGC 1817 from K2: A Rich Population of Asteroseismic Red Clump, Eclipsing Binary, and Main-sequence Pulsating Stars

“…Prior to Campaign 13, Frandsen & Arentoft (1998), Arentoft et al (2005) and Andersen et al (2009) identified 23 main sequence pulsating variables in the field of NGC 1817. The age of NGC 1817 is such that the cluster turnoff falls completely in the instability strip, similar to NGC 6811 in the main Kepler field (Sandquist et al 2016). As a result, pulsating variables are among the brightest stars in the cluster, and their oscillation spectra may reveal information about the internal changes that result from their evolution away from the main sequence.…”