The K2-ESPRINT Project III: A Close-in Super-Earth around a Metal-rich Mid-M Dwarf

We validate a $R_p=2.32\pm 0.24R_\oplus$ planet on a close-in orbit ($P=2.260455\pm 0.000041$ 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 the observed transits using a Gaussian process-based method and show that the transit depths in all passbands adopted for the ground-based transit follow-ups ($r'_2, z_\mathrm{s,2}, J, H, K_\mathrm{s}$) are within $\sim 2\sigma$ 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 $< 2\times 10^{-5}$. 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 a M4 dwarf companion ($\approx 0.072_{-0.04}^{+0.02}$), but even if this were the case, the size of K2-28b falls within the planetary regime. K2-28b has the same radius (within $1\sigma$) and experiences a similar irradiation from its host star as the well-studied GJ~1214b. Given the relative brightness of K2-28 in the near infrared ($m_\mathrm{Kep}=14.85$ mag and $m_H=11.03$ 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.