Eating planets makes you younger: The magnetic dynamo rejuvenation of GJ 504 by planetary engulfment

Abstract

With the discovery of a few thousand exoplanets, questions have been raised regarding star-planet interactions and whether the presence of a companion may affect stellar properties. GJ 504 is an evolved (2 Gyr) Sun-like star with a short rotation period (3.4 d) and an intense magnetic activity, which is in stark contrast with what would be expected at such an evolutionary stage. One possible explanation is that a close-in, Jupiter-mass planet was pushed starwards by the action of stellar tides, inducing a stellar spin-up and ultimately a rejuvenation of the stellar magnetic dynamo. By characterising the large-scale magnetic field and magnetised wind of GJ 504, we aim to provide additional observational constraints to test such scenario. We analysed spectropolarimetric observations of GJ 504 collected with ESPaDOnS. Using Zeeman-Doppler imaging, we found a large-scale, dipolar, non-axisymmetric magnetic field with an average strength of 5.3 G, similar to that of evolved early-G type stars. We fed the magnetic field information into our 3D MHD simulation of the stellar wind and space environment of GJ 504, from which we constrained the wind-driven angular momentum loss ($\rm \dot{J}$). We then compared $\rm \dot{J}$ to rotational evolutionary tracks of GJ 504 for two scenarios: evolution with and without the engulfment of a close-in, Jupiter-mass companion. Between the two scenarios, only the planet engulfment can explain the observational constraints obtained previously in the literature, such as the stellar rotation and X-ray luminosity, and the $\rm \dot{J}$ we derived and rescaled to account for underestimated magnetic field strength. Although there are many other stars with similar masses and rotation periods whose rotation evolution does not require planet engulfment, we also identified HD 75332 as a candidate for planet engulfment, suggesting that GJ 504 may not be an isolated case.

Figures (6)

• Figure 1: Reconstructed large-scale magnetic field map in flattened polar view. From the left, the radial, azimuthal, and meridional components of the magnetic field vector are illustrated. Concentric circles represent different stellar latitudes: -30 $^{\circ}$, +30 $^{\circ}$, and +60 $^{\circ}$ (dashed lines), as well as the equator (solid line). The radial ticks are located at the rotational phases when the observations were collected (see Table \ref{['tab:log']}). The colour indicates the polarity and strength of the magnetic field.
• Figure 2: Simulated stellar wind of GJ 504. The star is at the centre and its rotation axis lies along the positive $\rm z_\star$. The $\rm x_\star-y_\star$ plane is coloured by the total wind velocity. The Alfvén surface is the region of space where the local wind speed matches the Alfvén wave speed, $\rm v_A B_w / \sqrt{4\pi\rho_w}$ (in cgs units), and is depicted as a translucent surface with two lobes, as expected for stars with dominant dipolar large-scale field configurations. The colour bar indicates the total wind velocity ($\rm u_{tot}$).
• Figure 3: Evolutionary tracks of GJ 504. The scenarios without (solid lines) and with engulfment (dashed lines) are shown. The spikes in the tracks correspond to the maximum transfer of angular momentum from the planetary orbit to the star, that occurs when the planet reaches the Roche limit. From the left: the stellar surface rotation rate, with the observational constraint taken from diMauro2022; stellar wind-driven angular momentum loss tracks with the unscaled $\rm \dot{J}_{wind}$ derived in Sect. \ref{['sec:wind']} (white marker), the $\rm \dot{J}_{wind, B\times5}$ obtained by scaling the magnetic field strength (orange marker), and $\rm \dot{J}_{wind,ZDI}$ rescaled by a factor of 20 (gray marker); X-ray luminosity, with the black dot indicating the observational constraint of Pezzotti2025; and global angular momentum, with the black dot showing the semi-empirical angular momentum, which is the model momentum of inertia multiplied by the surface rotation rate from diMauro2022.
• Figure 4: Journal of ESPaDOnS observations for GJ 504.
• Figure 5: Time series of Stokes $V$ LSD profiles and the ZDI models for GJ 504. The observations are shown in black and the models in red. The numbers on the right indicate the rotational cycle computed from Eq. \ref{['eq:ephemeris']}. The horizontal dashed line represents the zero point of the profiles, which is shifted vertically based on the rotational phase for visualisation purposes.