Evolution of the ZTF SLRN-2020 star-planet merger

TL;DR

This paper models the ZTF SLRN-2020 transient as a star-planet merger, analyzing the dynamics, ejecta, and observational signatures to conclude a massive planet merged with a main-sequence star.

Contribution

It provides a detailed physical model of the merger process, explaining the observed transient features and ejecta in terms of tidal interactions and mass ejection mechanisms.

Findings

The transient's duration suggests multiple ejection episodes.

Ejecta mass is consistent with a planet several times Jupiter's mass.

Tidal heating was likely unobservable before the 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.