Evolution of the ZTF SLRN-2020 star-planet merger

TL;DR

This work models the ZTF SLRN-2020 transient as a star–planet merger, combining pre-merger tidal decay, near-surface drag during surface interaction, and post-merger ejecta dynamics to reproduce the optical/IR light curve and remnant signatures. The analysis constrains the planetary companion to be at least a few Jupiter masses, consistent with the pre-merger dust masses and the total radiated energy, while highlighting that the observed 100-day light curve cannot originate from a single dynamical ejection event. Two viable powering channels emerge: hydrogen recombination in an outflow and the contraction of an inflated envelope around the merger remnant, with both likely contributing alongside some dynamical ejecta not captured in the light curve. The results illustrate how multi-epoch photometry, dust evolution, and remnant spectroscopy can jointly illuminate the physics of star–planet mergers and set quantitative bounds on companion masses.

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.

Figures (5)

• Figure 1: Tidal evolution tracks for a sunlike star with a $\qty{10}{\jupitermass}$ companion. The abscissa shows the ratio of the orbital decay time to the thermal time at the location of energy deposition (see equations \ref{['eq:tdecay_tthermal']} and \ref{['eq:tau_tide_merge']}); the ratio must be greater than unity for the deposited energy to be observable before the merger. The ordinate shows the ratio of the tidal luminosity to the intrinsic stellar luminosity (see equation \ref{['eq:ltide']}); the ratio must be greater than unity for the tidal energy deposition to be significant. Each line corresponds to a different tidal quality factor. The squares mark the $\qty{-12}{}$ epoch at which the archival images of the progenitor were taken. When computing the thermal time, we assume energy is deposited in the outermost $\qty{2e-2}{\solarmass}$ of the star. None of these tidal quality factors yield observable tidal heating before the merger (upper right region of the figure), suggesting that the archival image was unaffected by the star-planet interaction.
• Figure 2: Important quantities during the merger between a sunlike star and a Neptune ($\qty{15}{\earthmass}, \qty{3.5}{\earthradius}$, left panel) or a giant planet ($\qty{10}{\jupitermass}$, $\qty{1}{\jupiterradius}$, right panel), as a function of the timescale of orbital decay as a result of drag. Vertical dashed lines show the two epochs at which pre-merger constraints exist, as well as the orbital period at the surface of the star. The plots show the Mach number of the planet $\mathcal{M} \equiv v_\mathrm{orb} / c_\mathrm{s}$, $\varepsilon_\rho\equiv$ number of density scale heights across the planet, the depth $z \equiv R_\star - r$ (with a circle indicating the location where the depth equals twice the radius of the planet), the stellar density $\rho$, and the density scale height $H_\rho$. The motion of the planet is always supersonic, leading to shocks. During the surface interaction, the flow is strongly stratified at the scale of the planet ($\varepsilon_\rho > 1$).
• Figure 3: Mass shocked by the planet as a function of the drag decay time, for different planet masses. Dots show the point at which the planet is fully immersed. A dashed vertical line shows the orbital period of the planet when the orbital separation equals the stellar radius. Black error bars show constraints for the ejecta mass at two pre-merger epochs. These constraints suggest a planet with $M_\mathrm{p} \gtrsim \qty{5}{\jupitermass}$ satisfy these constraints.
• Figure 4: Comparison between the time needed for ejecta at speed $v_\mathrm{ej}$ to reach the observed photosphere ($R_\mathrm{phot}/v_\mathrm{ej}$) and the duration of the transient $t_{90}$ (defined as the time since peak at which 90% of the total radiated energy has been radiated). Scatter points show the observed properties of several stellar mergers. For most of them, these two timescales are within a factor of a few from each other. ZTF SLRN-2020 lies somewhere in the shaded region, depending on the definition of the duration of the transient (only the plateau $\approx \qty{25}{}$ or the full light curve $\approx\qty{100}{}$) and on the assumed speed of the ejecta (ranging from the speed of the expanding inner dust shell $\approx \qty{35}{\kilo\meter\per\second}$ to the escape velocity from a sunlike star $\approx \qty{618}{\kilo\meter\per\second}$). The duration of ZTF SLRN-2020 is much longer than the time it would take ejecta to reach the observed photosphere, so it is likely not powered by hydrogen recombination from a single episode of mass ejection.
• Figure 5: Bolometric properties of ZTF SLRN-2020 as a function of time. Hollow points show the observations De2023. Solid opaque lines show a model of a contracting envelope of mass $\qty{1.1e-6}{\solarmass}$ around the merger remnant; semi-transparent lines show models with masses three times as small or as large. The horizontal dashed line is the constant effective temperature ($\qty{5850}{\kelvin}$) assumed in the envelope model. A dotted line shows the approximate required mass loss rate if the light curve were powered by the recombination of hydrogen in an outflow.