2012MNRAS.425.2778M

We evaluate the prompt electromagnetic 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 ρ-_p to that of the star ρ^–_☉, a planet-star merger results in three possible outcomes. If ρ^–_p/ρ^–_☉≳5, then the planet directly plunges below the stellar atmosphere before being disrupted by tidal forces. The dissipation of orbital energy creates a hot wake behind the planet, producing a extreme ultraviolet (EUV)/soft X-ray transient that increases in brightness and temperature as the planet sinks below the stellar surface. The peak luminosity L_EUV/X ≲ 10³⁶ erg/s is achieved weeks to months prior to merger, after which the stellar surface is enshrouded by an outflow driven by the merger. The final stages of the inspiral are accompanied by an optical transient powered by the recombination of hydrogen in the outflow, which peaks at a luminosity of ☉10³⁷-10³⁸ erg/s on a time-scale ☉days.