Thick Disks around White Dwarfs viewed 'Edge-off': Effects on Transit Properties and Infrared Excess

TL;DR

This paper models how geometrically thick dust disks around white dwarfs affect observable transit properties and infrared excess, providing explanations for recent observations and emphasizing the importance of considering disk thickness.

Contribution

It introduces a model for thick, geometrically extended disks around white dwarfs, analyzing their impact on transits and infrared emission, which was not thoroughly explored before.

Findings

Thick disks can cause significant transit reddening and depth variations.

Inner rim heating can explain infrared excess in certain white dwarf systems.

Optically thin outer layers produce silicate emission features.

Abstract

A significant fraction of white dwarfs (WDs) host dust/debris disks formed from the tidal disruption of asteroids and planetesimals. Several studies indicate that the disks can attain significant vertical heights through collisional cascade. In this work I model the effects of geometrically thick disks on two primary observables: photometric transits by the disk when viewed at high inclinations and infrared dust emission. Specifically, I consider disks with a Gaussian vertical profile with scale heights comparable to or larger than the WD radius. I primarily focus on inclinations $\gtrsim$$87$ degrees (`edge-off'), which can produce significant transits with moderate disk thickness. Both the transit depth and color become strong functions of inclination, and I explore their dependence on the disk parameters. I show that such a setup can produce the recently discovered reddening in the transit of WD J1013$-$0427. Moving to infrared emission, I show that the contribution from the heated inner rim can be substantial even at high inclinations. It can potentially explain the infrared excess observed in two transiting debris systems, WD 1145$+$017 and WD 1232$+$563, consistently with the transits. The other two important radiation components are the optically thin dust emission from the disk's outer layers and the optically thick emission from the backwarmed disk interior. Extending my analysis to G29-38 shows that the former can adequately produce the silicate emission feature with optically thin dust mass of $>$$10^{17}$ grams. The inner dense layers, on the other hand, allow the disk to contain orders of magnitude larger net dust mass. Overall, I show that thick disk effects can be significant and should be taken into account. I motivate detailed studies to quantify the effects accurately.