TESS Asteroseismology of Red Giants in the Old Supersolar-metallicity Open Clusters NGC 188 and NGC 6791

Abstract

Open clusters are fundamental laboratories for investigating stellar and Galactic evolution, and serve as important benchmarks for asteroseismic analyses. Using a “boutique” method to analyze TESS photometry, we study red giants in two old supersolar-metallicity open clusters: NGC 188 and NGC 6791. By comparing Kepler Space Telescope (Kepler) and TESS observations for NGC 6791, similar oscillation mode frequencies are recovered, however we find a systematic offset of 2.2% with a scatter of 9% in the νmax measurements. We attribute this discrepancy to the lower signal-to-noise ratio of the TESS data for these relatively faint stars. For the brighter cluster NGC 188, we present new seismic measurements in 17 red giants. We estimate average seismic masses for the red giant branch (RGB) of MRGB,NGC 188 = 1.13 ± 0.04(rand.) −0.19+0.12 (sys.) M⊙ and red clump of MRC,NGC 188 = 1.11 ± 0.01(rand.) −0.19+0.11 (sys.) M⊙, consistent with independent mass estimates for this cluster and with similar precision to previous Kepler studies. From the difference between the average evolutionary phase masses, we estimate an integrated RGB mass loss of ΔM = 0.02 ± 0.04(rand.) ± 0.01(sys.) M⊙, supporting the evidence for lower mass loss at higher metallicities. Using asteroseismology and chemical abundances, we identify three binary interaction candidates: two undermassive stars and one overmassive star potentially exhibiting dipole-mode suppression. Finally, we derive an average seismic cluster age of 7.0 ± 0.9 Gyr, in good agreement with previous literature ages. Our analysis demonstrates the strong potential of TESS asteroseismology for open clusters, and motivates extending this investigation to other TESS clusters that span a wider range of ages and metallicities.