The K2-ESPRINT Project III: A Close-in Super-Earth around a Metal-rich Mid-M Dwarf
Hirano et al
We validate a candidate super-Earth (Rp=2.38±0.25R⊕) on a close-in orbit (P=2.26 days) around 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. The phase-folded K2 transit light curve has a V-shape because the transit duration around this small star is comparable to the 30-minute K2 cadence. However, the light curves from our follow-up observations exhibit a sharp ingress and/or egress and flat bottom, ruling out a grazing eclipse of a binary system. We perform a global fit to all ground-based observations using a Gaussian process-based method and show that the transit depths in all passbands (r′2,zs,2,J,H,Ks) are within 2.2σ 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 less than 2×10−5. We also show that given the almost constant transit depths in the five passbands, EPIC 206318379 cannot have a physically associated companion later than M4, and the probability that it has another M4 dwarf is low as well (≈0.0721+0.023−0.036), even in which case the size of EPIC 206318379b falls on the planetary regime. EPIC 206318379b has the same radius (within 1σ) and experiences a similar irradiation from its host star as the well-studied GJ 1214b. A comparison between the atmospheric properties of these two objects with future observations would be especially interesting.