The Geometry of the G29-38 White Dwarf Dust Disk from Radiative Transfer Modeling

TL;DR

This study models the G29-38 white dwarf dust disk using radiative transfer calculations, revealing a narrow, vertically structured disk with an inner edge farther from the star than previously thought, challenging flat-disk assumptions.

Contribution

First radiative transfer model of G29-38's dust disk accounting for temperature and optical depth gradients, providing new insights into its geometry and vertical structure.

Findings

Inner disk edge at 92-100 R_WD, farther than previous estimates

Disk width is narrow, ≤10 R_WD, suggesting limited spreading

Disk has a half-opening angle ≥1.4°, indicating vertical structure

Abstract

Many white dwarfs host disks of dust produced by disintegrating planetesimals and revealed by infrared excesses. The disk around G29-38 was the first to be discovered and is now well-observed, yet we lack a cohesive picture of its geometry and dust properties. Here we model the G29-38 disk for the first time using radiative transfer calculations that account for radial and vertical temperature and optical depth gradients. We arrive at a set of models that can match the available infrared measurements well, although they overpredict the width of the 10 $\mu m$ silicate feature. The resulting set of models has a disk inner edge located at 92-100 $R_\text{WD}$ (where $R_\text{WD}$ is the white dwarf radius). This is farther from the star than inferred by previous modeling efforts due to the presence of a directly illuminated front edge to the disk. The radial width of the disk is narrow ($\leq$10 $R_\text{WD}$); such a feature could be explained by inefficient spreading or the proximity of the tidal disruption radius to the sublimation radius. The models have a half-opening angle of $\geq$1.4$^\circ$. Such structure would be in strong contradiction with the commonly employed flat-disk model analogous to the rings of Saturn, and in line with the vertical structure of main-sequence debris disks. Our results are consistent with the idea that disks are collisionally active and continuously fed with new material, rather than evolving passively after the disintegration of a single planetesimal.