The First Mid-Infrared Spectra of Cool White Dwarfs

TL;DR

This study presents the first mid-infrared spectra of cool white dwarfs, demonstrating that current models fit most observations well, but revealing unexplained IR flux deficits in some cases suggesting unknown opacity sources.

Contribution

First mid-infrared spectra of cool white dwarfs are analyzed, highlighting model successes and discrepancies, especially the unexplained IR flux deficits in LHS 1126.

Findings

Models accurately reproduce optical and IR SEDs for most white dwarfs.

LHS 1126 shows significant IR flux deficits not explained by current CIA models.

Mid-IR spectra reveal potential unknown opacity sources in cool white dwarf atmospheres.

Abstract

We present the first mid-infrared spectra of two cool white dwarfs obtained with the Spitzer Space Telescope. We also present 3.5-8 micron photometry for 19 cool white dwarfs with 5000K < Teff < 9000K. We perform a detailed model atmosphere analysis of these white dwarfs by fitting their UBVRIJHK and Spitzer photometry with state-of-the-art model atmospheres, and demonstrate that the optical and infrared spectral energy distributions of cool white dwarfs are well reproduced by our grid of models. Our mid-IR photometry and 7.5-14.5 micron spectrum of WD0018-267 are consistent with a Teff = 5720K, pure hydrogen white dwarf model atmosphere. On the other hand, LHS 1126 remains peculiar with significant mid-IR flux deficits in all IRAC bands and a featureless spectrum in the 5.2-7.5 micron range. Even though this deficit is attributed to collision induced absorption (CIA) due to molecular hydrogen, the shape of the deficit cannot be explained with current CIA opacity calculations. The infrared portion of the LHS 1126 spectral energy distribution is best-fit with a power law index of -1.99; identical to a Rayleigh-Jeans spectrum. This argues that the deficit may be due to an unrecognized grey-like opacity source in the infrared.