# A Photometric and Spectroscopic Investigation of the DB White Dwarf Population using SDSS and Gaia Data

## TL;DR

This study analyzes the physical properties of DB white dwarfs using SDSS and Gaia data, comparing photometric and spectroscopic methods, and investigates their population characteristics, binary systems, and spectral evolution.

## Contribution

It provides a detailed comparison of atmospheric parameters derived from photometry and spectroscopy, and reveals the presence of unresolved double degenerate binaries among DB white dwarfs.

## Key findings

- Identification of a large population of unresolved DB+DB and DB+DA binaries.
- Comparison shows differences in parameters derived from photometric and spectroscopic methods.
- Insights into the spectral evolution and hydrogen origin in DB white dwarfs.

## Abstract

We present a comprehensive analysis of DB white dwarfs drawn from the Sloan Digital Sky Survey, based on model fits to $ugriz$ photometry and medium resolution spectroscopy from the SDSS. We also take advantage of the exquisite trigonometric parallax measurements recently obtained by the Gaia mission. Using the so-called photometric and spectroscopic techniques, we measure the atmospheric and physical parameters of each object in our sample ($T_{\rm eff}$, $\log g$, H/He, Ca/He, $R$, $M$), and compare the values obtained from both techniques in order to assess the precision and accuracy of each method. We then explore in great detail the surface gravity, stellar mass, and hydrogen abundance distributions of DB white dwarfs as a function of effective temperature. We present some clear evidence for a large population of unresolved double degenerate binaries composed of DB+DB and even DB+DA white dwarfs. In the light of our results, we finally discuss the spectral evolution of DB white dwarfs, in particular the evolution of the DB-to-DA ratio as a function of $T_{\rm eff}$, and we revisit the question of the origin of hydrogen in DBA white dwarfs.