White dwarf systems: exoplanets and debris disks

TL;DR

White dwarf planetary systems offer a unique probe of exoplanetary composition and evolution as planetary bodies are tidally disrupted and accreted onto the WD. The paper reviews both observational techniques (infrared, gas, transits, variability, X-ray) and three main theoretical disk frameworks (geometrically thin optically-thick disks, eccentric disks, and pre-existing compact disks) to explain debris disk formation and metal pollution. It highlights the diversity and temporal variability of disks, the prevalence of pollution, and the rarity of direct detections of WD exoplanets, emphasizing how these systems function as astrophysical mass spectrometers for exo-Solar material. The discussion underscores ongoing challenges and the pivotal role of upcoming surveys and facilities in resolving mechanisms of delivery, disk evolution, and accretion, with significant implications for our understanding of planetary system lifecycles.

Abstract

Although there is abundant and diverse observational evidence in support of white dwarf stars hosting planets or debris disks which form in the catastrophic destruction of various planetary bodies, the key processes that explain these observations are still being intensely investigated. The study of white dwarf planetary systems offers a unique perspective on exo-solar composition, that cannot be obtained by any other means. This chapter describes the various observational techniques that are used in order to find and characterize exo-planets and debris disks around white dwarfs. In turn, it discusses how to theoretically interpret these observations by surveying an array of various research tools and models currently employed in this field.

Figures (4)

• Figure 1: Planetary architectures leading to delivery of planetesimals and the formation of debris disks, detailing the number of stars and planets which were investigated by different studies, and the class of affected planetesimals (Credit: Veras-2021, Figure 6).
• Figure 2: Debris disk formation and evolution following tidal disruptions: (a) small size (1 km) and semi-major axis (3 AU) results in a ring-like disk; (b) increasing size (50 km) results in some dispersion of the fragment orbital energies; (c) increasing size (500 km) and separation (30 AU) result in a bi-modal disk where half of the tidal fragments are unbound and the other half are tightly bound to the WD (Credit: BrouwersEtAl-2022, Figure 4); (d) A full hydrodynamical evolution of the tidal disruption of an Earth-sized planet, leading to a bimodal disk. For numerical reasons the planet apocenter is unrealistically placed at 0.2 au (top right). Each flyby of planetary remains near the WD generates new tidal disruptions (top to bottom sequence). Light and dark pixel tones indicate silicate mantle and iron core composition, respectively (Credit: MalamudPerets-2020, Figure 7).
• Figure 3: Listed are the seminal systems to be identified by each respective observational method. Credit: (a) NASA/JPL-Caltech/M. Kuchner (GSFC); (b) GansickeEtAl-2006; (c) VanderburgEtAl-2015; (d) CunninghamEtAl-2022.
• Figure 4: A schematic depiction (color coding - gray tones for debris consisting of fragments and green for gas) of different WD disk models, and their compliance with various constraints: (a) the fiducial WD disk model of Saturn-like rings is largely inapplicable; (b) the eccentric disk model probably applies to most polluted WDs, but is hard to reconcile with large accretion rates and no detected IR excess; (c) the pre-existing disk model probably complies with the largest number of constraints, however it is by design meant to address a limited fraction of polluted WDs.