Unstable low-mass planetary systems as drivers of white dwarf pollution

Abstract: At least 25% of white dwarfs show atmospheric pollution by metals, sometimes accompanied by detectable circumstellar dust/gas discs or (in the case of WD 1145+017) transiting disintegrating asteroids. Delivery of planetesimals to the white dwarf by orbiting planets is a leading candidate to explain these phenomena. Here, we study systems of planets and planetesimals undergoing planet-planet scattering triggered by the star's post-main sequence mass loss, and test whether this can maintain high rates of delivery over the several Gyr that they are observed. We find that low-mass planets (Earth to Neptune mass) are efficient deliverers of material and can maintain the delivery for Gyr. Unstable low-mass planetary systems reproduce the observed delayed onset of significant accretion, as well as the slow decay in accretion rates at late times. Higher-mass planets are less efficient, and the delivery only lasts a relatively brief time before the planetesimal populations are cleared. The orbital inclinations of bodies as they cross the white dwarf's Roche limit are roughly isotropic, implying that significant collisional interactions of asteroids, debris streams and discs can be expected. If planet-planet scattering is indeed responsible for the pollution of white dwarfs, many such objects, and their main-sequence progenitors, can be expected to host (currently undetectable) super-Earth planets on orbits of several au and beyond.

Unstable Low-Mass Planetary Systems as Drivers of White Dwarf Pollution

Planetary system dynamics during stellar evolution is a fascinating area of astrophysical research, particularly concerning the fate of planets orbiting post-main sequence stars. In this paper, Mustill et al. investigate a compelling dynamical mechanism potentially responsible for the observed metal pollution in white dwarf (WD) atmospheres—unstable planetary systems comprised of low-mass planets such as those with Earth to Neptune masses.

Key Findings

1. Prevalence of Pollution: Approximately 25% of WDs exhibit atmospheric metal pollution, often accompanied by circumstellar dust or gas discs, which suggests accretion of planetary material.
2. Scattering Mechanism: The research primarily focuses on planetesimal delivery driven by planet-planet scattering, which is triggered by stellar mass loss during the post-main sequence phase. This mechanism supports long-term (over several Gyr) material delivery from the planetary systems to the WD.
3. Mass Efficiency: The study demonstrates that low-mass planets—those comparable to Earth or Neptune—are efficacious in driving prolonged accretion onto WDs due to their ability to maintain scattering interactions over extended periods. In contrast, higher-mass planets, such as Jupiter-sized giants, produce a shorter-lived influence and clear out planetesimal populations more quickly.
4. Numerical Simulations: The integration of sophisticated N-body simulations reveals robust scenarios where planetary systems remain stable on the main sequence but become unstable due to increased planet-star mass ratios following stellar mass loss. Instabilities lead to scattering events that efficiently deliver planetesimals to WDs, supporting the observed accretion rates.
5. Orbital Dynamics: Planetesimals crossing the WD's Roche limit display roughly isotropic orbital inclinations, suggesting significant collisional interactions with debris streams and discs are expected.

Implications

1. Astrophysical Insights: This study enhances our understanding of planetary system evolution and the mechanisms by which mature stars contribute to WD pollution. It posits that many WD progenitors might host super-Earth planets on orbits of several AU, thus offering a plausible, yet currently undetectable, explanation for the widespread pollution phenomena.
2. Planetary Compositions: The analysis of WD pollution grants astronomers a unique window into the compositional insights of extrasolar planetary materials, akin to examining meteorite accretion on Earth.
3. Future Observations: While current observational constraints prevent the direct detection of these long-orbit planets, advancements in indirect techniques might uncover more about their prevalence and characteristics in these systems. The presence of such planets could significantly advance our understanding of planetary formation and evolution around intermediate-mass stars.

Future Directions

The findings in this paper open pathways for deeper inquiry into the specifics of other dynamical interactions, such as those influenced by additional forces (e.g., radiation pressure or stellar winds) that could alter the accretion process under different initial conditions. As computational capacities increase, simulations incorporating mutual gravitational interactions amongst planetesimals themselves might refine the understanding of disc dynamics and improve the prediction accuracy for WD pollution scenarios.

Moreover, further investigations into compositional anomalies might illuminate variations in accreted materials across different stellar progenitor masses, potentially shedding light on varying formation histories and the influence of stellar mass loss rates.

In summary, the paper by Mustill et al. presents significant insights into the complex dynamics of planetary systems during stellar evolution and outlines a credible mechanism for white dwarf pollution driven by planet-planet interactions in low-mass systems. Integrating these findings with ongoing and future planetary detection methods could reshape our understanding of planetary system demography and evolution in the universe.