On the nature of the planet-powered transient event ZTF SLRN-2020

Abstract

The Red Nova ZTF SLRN-2020 is the third transient event with properties that are compatible with the merger of a planet with a main sequence (or close to) star on a dynamical timescale. While the two first transient events occurred in young stellar objects, ZTF SLRN-2020 occurred in an old system. Nonetheless, I show that the three star-planet intermediate luminosity optical transients (ILOTs, also termed Red Novae) occupy the same area in the energy-time diagram of ILOTs. Based on models for ILOTs that are power by stellar binary interaction I suggest that the planet in ZTF SLRN-2020 launched jets at about its escape speed before it was engulfed by the star. Interestingly, the escape speed from the planet is similar to the orbital speed of the planet. This leads to an outflow with a very low terminal velocity, much below the escape velocity from the star, and in concentration around ~45 degrees to the equatorial plane. As well, the planet might have lost back some of the accreted mass just before engulfment, forming an accretion disk around the star. This disk might have launched jets during the main outburst of the event. The jets form a bipolar expanding nebula.