Evolution of the ZTF SLRN-2020 star-planet merger

Abstract

We model the optical and infrared transient ZTF SLRN-2020, previously associated with a star-planet merger. We consider the scenario in which orbital decay via tidal dissipation led to the merger, and find that tidal heating within the star was likely unobservable in the archival image of the system taken $12\mathrm{yr}$ before the merger. The observed dust formation months before the merger is consistent with a planet of mass $M_\mathrm{p} \gtrsim 5M_\mathrm{J}$ ejecting material as it skims the stellar surface. This interaction gradually intensifies, leading to significant mass ejection on a dynamical timescale ($ \approx $ hours) as the planet plunges into the stellar interior. Part of the recombination transient associated with this dynamical mass ejection might be inaccessible to the optical observations because its duration ($ \approx $ hours) is comparable to the cadence. Correspondingly, the observed duration of the transient $\approx100\mathrm{d}$ is inconsistent with a single episode of dynamical mass ejection. Instead, the transient could be powered by the recombination of $ \approx 3.4\times10^{-5}M_\odot $ of hydrogen in an outflow, or the contraction of an inflated envelope of mass $ \approx 10^{-6}M_\odot $ that formed during the merger. The observed ejecta mass $320\mathrm{d}$ after the peak of the optical transient is $ \approx 1.3\times10^{-4}M_\odot$, consistent with the idea that a fraction of the ejecta might be unobservable in the light curve. Energetically, this post-merger ejecta mass suggests a planet at least as massive as Jupiter. Our results suggest that ZTF SLRN-2020 was the result of a merger between a star close to the main sequence and a planet with mass at least several times that of Jupiter.