Optical and X-ray Transients from Planet-Star Mergers

Abstract

We evaluate the prompt observational signatures of the merger between a massive close-in planet (a `hot Jupiter') and its host star, events with an estimated Galactic rate of ~0.1-1/yr. Depending on the ratio of the mean density of the planet rho_p to that of the star rho_star, a merger results in three possible outcomes. If rho_p/rho_star > 5, then the planet directly plunges below the stellar atmosphere before being disrupted by tidal forces. Dissipation of orbital energy creates a hot wake behind the planet, producing a EUV/soft X-ray transient as the planet sinks below the stellar surface. The peak luminosity L_X ~ 1e36 erg/s is achieved weeks-months prior to merger, after which the stellar surface is enshrouded by an outflow. The final inspiral is accompanied by an optical transient powered by the recombination of hydrogen in the outflow, which peaks at L~1e37-38 erg/s on a timescale ~days. If instead rho_planet/rho_star < 5, then Roche Lobe overflow occurs above the stellar surface. For rho_p/rho_star < 1, mass transfer is stable, resulting the planet being accreted on a relatively slow timescale. However, for 1 < rho_p/rho_star < 5, mass transfer may instead be unstable, resulting in the planet being dynamically disrupted into an accretion disk around the star. Super-Eddington outflows from the disk power an optical transient with L~1e37-38 erg/s and characteristic duration ~week-months. The disk itself becomes visible once the accretion rate become sub-Eddington, resulting in a bolometric brightening and spectral shift to the UV. Optical transients from planet merger events may resemble classical novae, but are distinguished by lower ejecta mass and velocity ~100s km/s, and by hard pre- and post-cursor emission, respectively. Promising search strategies include combined optical, UV, and X-ray surveys of nearby massive galaxies with cadences from days to months.