1201.2662 (S. Rappaport et al.)

Possible Disintegrating Short-Period Super-Mercury Orbiting KIC 12557548    [PDF]

S. Rappaport, A. Levine, E. Chiang, I. El Mellah, J. Jenkins, B. Kalomeni, M. Kotson, L. Nelson, L. Rousseau-Nepton, K. Tran

We report here on the discovery of stellar occultations, observed with Kepler, that recur periodically at 15.685 hour intervals, but which vary in depth from a maximum of 1.2% to a minimum that can be less than 0.2%. The star that is apparently being occulted is KIC 12557548, a V = 16 magnitude K dwarf with T_eff = 4400 K. The out-of-occultation behavior shows no evidence for ellipsoidal light variations, indicating that the mass of the orbiting object is less than ~3 M_J. Because the eclipse depths are highly variable, they cannot be due solely to transits of a single planet with a fixed size. We discuss but dismiss a scenario involving a binary giant planet whose mutual orbit plane precesses, bringing one of the planets into and out of a grazing transit. This scenario seems ruled out by the dynamical instability that would result from such a configuration. The much more likely explanation involves macroscopic particles - e.g., dust, possibly in the form of micron-sized pyroxene grains - escaping the atmosphere of a slowly disintegrating planet not much larger than Mercury in size. The planetary surface is hot enough to sublimate; the resultant silicate vapor accelerates off the planet via a Parker-type thermal wind, dragging dust grains with it. We infer a mass loss rate from the observations of order ~1 M_earth/Gyr, with a dust-to-gas ratio possibly of order unity. For our fiducial 0.1 M_earth planet (twice the mass of Mercury), the evaporation timescale may be ~0.2 Gyr. Smaller mass planets are disfavored because they evaporate still more quickly, as are larger mass planets because they have surface gravities too strong to sustain outflows with the requisite mass-loss rates. The occultation profile evinces an ingress-egress asymmetry that could reflect a comet-like dust tail trailing the planet; we present simulations of such a tail.

View original: http://arxiv.org/abs/1201.2662