Variable Emission from a Gaseous Disc around a Metal-Polluted White Dwarf

TL;DR

This study reports on the discovery of variable gaseous emission lines from a debris disc around a white dwarf, revealing short-term dynamical changes likely caused by impacts from planetary material.

Contribution

First detection of short-term variability in gaseous debris around a white dwarf, providing new insights into planetary system evolution after stellar death.

Findings

Strong double-peaked Ca II emission lines observed in 2008.

Emission lines faded monotonically over years.

Variability possibly caused by impacts from planetary debris.

Abstract

We present the discovery of strongly variable emission lines from a gaseous disc around the DA white dwarf SDSS J1617+1620, a star previously found to have an infrared excess indicative of a dusty debris disc formed by the tidal disruption of a rocky planetary body. Time-series spectroscopy obtained during the period 2006-2014 has shown the appearance of strong double-peaked Ca II emission lines in 2008. The lines were weak, at best, during earlier observations, and monotonically faded through the remainder of our monitoring. Our observations represent unambiguous evidence for short-term variability in the debris environment of evolved planetary systems. Possible explanations for this extraordinary variability include the impact onto the dusty disc of either a single small rocky planetesimal, or of material from a highly eccentric debris tail. The increase in flux from the emission lines is sufficient that similar events could be detected in the broadband photometry of ongoing and future large-area time domain surveys.