Spitzer Observations of the Oldest White Dwarfs in the Solar Neighborhood

TL;DR

This study uses Spitzer infrared observations to analyze the spectral energy distributions of cool white dwarfs, confirming model accuracy and constraining the presence of debris disks around these ancient stellar remnants.

Contribution

It provides detailed infrared spectral analysis of cool white dwarfs, validating model atmospheres and setting limits on debris disk occurrence in old white dwarfs.

Findings

Models accurately reproduce observed spectra within 5%.

No significant mid-infrared excesses detected in most targets.

Debris disks are present in less than 1% of the studied white dwarfs.

Abstract

We present Spitzer 5-15 micron spectroscopy of one cool white dwarf and 3.6-8 micron photometry of 51 cool white dwarfs with T_eff < 6000 K. The majority of our targets have accurate BVRIJHK photometry and trigonometric parallax measurements available, which enables us to perform a detailed model atmosphere analysis using their optical, near- and mid-infrared photometry with state- of-the-art model atmospheres. We demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our best fit models are consistent with the observations within 5% in all filters except the IRAC 8 micron band, which has the lowest signal- to-noise ratio photometry. Excluding the ultracool white dwarfs, none of the stars in our sample show significant mid-infrared flux deficits or excesses. The non-detection of mid-infrared excess flux around our 2-9 Gyr old targets constrain the fraction of cool white dwarfs with warm debris disks to 0.8% (+1.5% -0.8%).