THE K2-ESPRINT PROJECT III: A CLOSE-IN SUPER-EARTH AROUND A METAL-RICH MID-M DWARF

Abstract

We validate a R[subscript p]=2.32 ± 0.24R[subscript ⊕] planet on a close-in orbit (P = 2.260455 ± 0.000041 days) around K2-28 (EPIC 206318379), a metal-rich M4-type dwarf in the Campaign 3 field of the K2 mission. Our follow-up observations included multi-band transit observations from the optical to the near-infrared, low-resolution spectroscopy, and high-resolution adaptive optics (AO) imaging. We perform a global fit to all of the observed transits using a Gaussian process-based method and show that the transit depths in all of the passbands adopted for the ground-based transit follow-ups (r₂′,z [subscript s,]₂,J,H,K[subscript s]) are within ~2σ of the K2 value. Based on a model of the background stellar population and the absence of nearby sources in our AO imaging, we estimate the probability that a background eclipsing binary could cause a false positive to be <2 × 10⁻⁵. We also show that K2-28 cannot have a physically associated companion of stellar type later than M4, based on the measurement of almost identical transit depths in multiple passbands. There is a low probability for an M4 dwarf companion (≈0.072 [subscript -0.04] [superscript +0.02]), but even if this were the case, the size of K2-28b falls within the planetary regime. K2-28b has the same radius (within 1σ) and experiences irradiation from its host star similar to the well-studied GJ 1214b. Given the relative brightness of K2-28 in the near-infrared (m[subscript Kep]= 14.85 mag and mH = 11.03 mag) and relatively deep transit (0.6%–0.7%), a comparison between the atmospheric properties of these two planets with future observations would be especially interesting.