Constraints on the Lifetimes of Disks Resulting from Tidally Destroyed Rocky Planetary Bodies

TL;DR

This study uses Spitzer IRAC data to analyze the infrared excesses of metal-polluted white dwarfs, estimating disk lifetimes and exploring differences between helium and hydrogen atmospheres, revealing insights into planetary debris evolution.

Contribution

It provides new estimates of disk lifetimes around white dwarfs and compares dust presence between different atmospheric types, expanding understanding of planetary debris around evolved stars.

Findings

Estimated disk lifetime of approximately 3*10^4 to 5*10^6 years.

Lower fraction of helium-rich white dwarfs with infrared excess compared to hydrogen-rich.

Higher average accretion rates onto helium-rich white dwarfs.

Abstract

Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared excesses in the spectral energy distributions of HE 0110-5630, GD 61, and HE 1349-2305. All three of these stars have helium-dominated atmospheres, and their infrared emissions are consistent with warm dust produced by the tidal destruction of (minor) planetary bodies. This study brings the number of metal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRAC to 53 and 38 respectively. It also nearly doubles the number of metal-polluted helium-rich white dwarfs found to have closely orbiting dust by Spitzer. From the increased statistics for both atmospheric types with circumstellar dust, we derive a typical disk lifetime of log[t_{disk} (yr)] = 5.6+-1.1 (ranging from 3*10^4 - 5*10^6 yr). This assumes a relatively constant rate of accretion over the timescale where dust persists, which is uncertain. We find that the fraction of highly metal-polluted helium-rich white dwarfs that have an infrared excess detected by Spitzer is only 23 per cent, compared to 48 per cent for metal-polluted hydrogen-rich white dwarfs, and we conclude from this difference that the typical lifetime of dusty disks is somewhat shorter than the diffusion time scales of helium-rich white dwarf. We also find evidence for higher time-averaged accretion rates onto helium-rich stars compared to the instantaneous accretion rates onto hydrogen-rich stars; this is an indication that our picture of evolved star-planetary system interactions is incomplete. We discuss some speculative scenarios that can explain the observations.