# A New Generation of Cool White Dwarf Atmosphere Models. IV. Revisiting the Spectral Evolution of Cool White Dwarfs

## TL;DR

This paper revisits the spectral evolution of cool white dwarfs using advanced atmosphere models and a large observational sample, revealing new insights into their compositional changes and challenging existing theories.

## Contribution

It introduces a new generation of white dwarf atmosphere models and applies them to a large sample to clarify spectral evolution mechanisms.

## Key findings

- Identified a decrease in hydrogen to helium ratio between 7500 K and 6250 K, indicating convective mixing.
- Observed an increase in hydrogen-rich white dwarfs down to 5000 K, then a decline below that temperature.
- Challenged the idea that interstellar medium accretion dominates spectral evolution at low temperatures.

## Abstract

As a result of competing physical mechanisms, the atmospheric composition of white dwarfs changes throughout their evolution, a process known as spectral evolution. Because of the ambiguity of their atmospheric compositions and the difficulties inherent to the modeling of their dense atmospheres, no consensus exists regarding the spectral evolution of cool white dwarfs ($T_{\rm eff}<6000$ K). In the previous papers of this series, we presented and observationally validated a new generation of cool white dwarf atmosphere models that include all the necessary constitutive physics to accurately model those objects. Using these new models and a homogeneous sample of 501 cool white dwarfs, we revisit the spectral evolution of cool white dwarfs. Our sample includes all spectroscopically identified white dwarfs cooler than 8300 K for which a parallax is available in Gaia DR2 and photometric observations are available in Pan-STARRS1 and 2MASS. Except for a few cool carbon-polluted objects, our models allow an excellent fit to the spectroscopic and photometric observations of all objects included in our sample. We identify a decrease of the ratio of hydrogen to helium-rich objects between 7500 K and 6250 K, which we interpret as the signature of convective mixing. After this decrease, hydrogen-rich objects become more abundant up to 5000 K. This puzzling increase, reminiscent of the non-DA gap, has yet to be explained. At lower temperatures, below 5000 K, hydrogen-rich white dwarfs become rarer, which rules out the scenario according to which accretion of hydrogen from the interstellar medium dominates the spectral evolution of cool white dwarfs.