A Trio of Metal-Rich Dust and Gas Disks Found Orbiting Candidate White Dwarfs with K-Band Excess

TL;DR

This study identifies metal-rich dust and gas disks around white dwarfs with K-band excess, revealing ongoing accretion and recent collision events, and establishes a lower limit on disk occurrence in young white dwarfs.

Contribution

It reports the discovery of multiple white dwarfs with circumstellar dust and gas disks, including the first gaseous disk around a cool star, expanding understanding of disk prevalence and composition.

Findings

At least 0.8% of young white dwarfs have dust disks.

All stars with dust excess are metal-rich.

No warm white dwarfs above 25,000 K show dust disks.

Abstract

This paper reports follow-up photometric and spectroscopic observations, including warm Spitzer IRAC photometry of seven white dwarfs from the SDSS with apparent excess flux in UKIDSS K-band observations. Six of the science targets were selected from 16,785 DA star candidates identified either spectroscopically or photometrically within SDSS DR7, spatially cross-correlated with HK detections in UKIDSS DR8. Thus the selection criteria are completely independent of stellar mass, effective temperature above 8000 K, and the presence (or absence) of atmospheric metals. The infrared fluxes of one target are compatible with a spatially-unresolved late M or early L-type companion, while three stars exhibit excess emissions consistent with warm circumstellar dust. These latter targets have spectral energy distributions similar to known dusty white dwarfs with high fractional infrared luminosities (thus the K-band excesses). Optical spectroscopy reveals the stars with disk-like excesses are polluted with heavy elements, denoting the ongoing accretion of circumstellar material. One of the disks exhibits a gaseous component - the fourth reported to date - and orbits a relatively cool star, indicating the gas is produced via collisions as opposed to sublimation, supporting the picture of a recent event. The resulting statistics yield a lower limit of 0.8% for the fraction dust disks at DA-type white dwarfs with cooling ages less than 1 Gyr. Two overall results are noteworthy: all stars whose excess infrared emission is consistent with dust are metal-rich; and no stars warmer than 25,000 K are found to have this type of excess, despite sufficient sensitivity.