摘要：We identify a sub-Neptune exoplanet (Rp=2.5± 0.2 RÅ) transiting a solar twin in the Ruprecht 147 star cluster (3 Gyr, 300 pc, [Fe/H]=+0.1 dex). The ∼81 day light curve for EPIC 219800881 (V = 12.71) from K2 Campaign7 shows six transits with a period of 13.84days, a depth of ∼0.06%, and a duration of ∼4 hr. Based on our analysis of high-resolution MIKE spectra, broadband optical and NIR photometry, the cluster parallax and interstellar reddening, and isochrone models from PARSEC, Dartmouth, and MIST, we estimate the following properties for the host star: Må = 1.01±0.03M, Rå = 0.95±0.03R, and Teff =5695±50K. This star appears to be single based on our modeling of the photometry, the low radial velocity (RV) variability measured over nearly 10 yr, and Keck/NIRC2 adaptive optics imaging and aperture-masking interferometry. Applying a probabilistic mass–radius relation, we estimate that the mass of this planet is Mp=7+5–3 MÅ, which would cause an RV semi-amplitude of K=2±1ms-1that may be measurable with existing precise RV facilities. After statistically validating this planet with BLENDER, we now designate it K2-231b, making it the second substellar object to be discovered in Ruprecht 147 and the first planet; it joins the small but growing ranks of 22 other planetsand three candidates found in open clusters.
关键词：open clusters and associations: individual (Ruprecht 147, NGC 6774);planets and satellites: detection;planets and satellites: gaseous planets;stars: individual: (K2-231, EPIC 21980081, CWW 93, 2MASS J19162203–1546159)
