A Spectro-photometric Analysis of Cool White Dwarfs in the Gaia and Pan-STARRS Footprint

TL;DR

This study analyzes 2880 cool white dwarfs within 100 parsecs using spectro-photometric data from Gaia and Pan-STARRS, revealing insights into their atmospheric compositions, spectral evolution, and the processes affecting their atmospheric transformations.

Contribution

It provides a comprehensive spectro-photometric analysis of cool white dwarfs, including detailed atmospheric modeling and new insights into their spectral evolution and atmospheric composition changes.

Findings

Little evidence for DA to He-atmosphere transformation between 10,000 K and 6,500 K.

At temperatures below 5200 K, most DC white dwarfs have H atmospheres.

He-atmosphere white dwarf fraction reaches 45% between 6,500 K and 5,500 K.

Abstract

We present a spectro-photometric analysis of 2880 cool white dwarfs within 100 pc of the Sun and cooler than Teff = 10,000 K, with grizy Pan-STARRS photometry and Gaia trigonometric parallaxes available. We also supplement our data sets with near-infrared JHK photometry, when available, which is shown to be essential for interpreting the coolest white dwarfs in our sample. We perform a detailed analysis of each individual object using state-of-the-art model atmospheres appropriate for each spectral type including DA, DC, DQ, DZ, He-rich DA, and the so-called IR-faint white dwarfs. We discuss the temperature and mass distributions of each subsample, as well as revisit the spectral evolution of cool white dwarfs. We find little evidence in our sample for the transformation of a significant fraction of DA stars into He-atmosphere white dwarfs through the process of convective mixing between Teff = 10,000 K and 6500 K, although the situation changes drastically in the range Teff = 6500 - 5500 K where the fraction of He-atmosphere white dwarfs reaches 45%. However, we also provide strong evidence that at even cooler temperatures (Teff < 5200 K), most DC white dwarfs have H atmospheres. We discuss a possible mechanism to account for this sudden transformation from He- to H-atmosphere white dwarfs involving the onset of crystallization and the occurrence of magnetism. Finally, we also argue that DQ, DZ, and DC white dwarfs may form a more homogeneous population than previously believed.