Evidence for the disruption of a planetary system during the formation of the Helix Nebula

TL;DR

This study combines multi-wavelength observations and modeling to investigate dust around the Helix Nebula's white dwarf, providing evidence that the dust originates from a disrupted cometary cloud rather than a planetesimal belt.

Contribution

It offers the first detailed analysis linking dust excess around a planetary nebula's white dwarf to cometary disruption, challenging previous assumptions.

Findings

Dust likely from a cometary cloud, not a planetesimal belt.

Disruption of thousands of comets per year fuels the dust.

Infrared and millimeter data support a cometary origin.

Abstract

The persistence of planetary systems after their host stars evolve into their post-main sequence phase is poorly constrained by observations. Many young white dwarf systems exhibit infrared excess emission and/or spectral absorption lines associated with a reservoir of dust (or planetesimals) and its accretion. However, most white dwarfs are too cool to sufficiently heat any circumstellar dust to detectable levels of emission. The Helix Nebula (NGC 7293) is a young, nearby planetary nebula; observations at mid- and far-infrared wavelengths revealed excess emission associated with its central white dwarf (WD 2226-210). The origin of this excess is ambiguous. It could be a remnant planetesimal belt, a cloud of comets, or the remnants of material shed during the post-asymptotic giant branch phase. Here we combine infrared (SOFIA, Spitzer, Herschel ) and millimetre (ALMA) observations of the system to determine the origin of this excess using multi-wavelength imaging and radiative transfer modelling. We find the data are incompatible with a compact remnant planetesimal belt or post-asymptotic giant branch disc, and conclude the dust most likely originates from deposition by a cometary cloud. The measured dust mass, and lifetime of the constituent grains, implies disruption of several thousand Hale-Bopp equivalent comets per year to fuel the observed excess emission around the Helix Nebula's white dwarf.