A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan Digital Sky Survey

TL;DR

This study analyzes 126 cool white dwarfs from SDSS using optical and near-infrared data, revealing atmospheric compositions, ages, and potential halo members, and demonstrating effective selection methods for large samples.

Contribution

It provides a detailed model atmosphere analysis of cool white dwarfs, identifying atmospheric types and age estimates, and introduces a method for selecting large samples using proper motion and velocity cuts.

Findings

Over 80% of stars are consistent with pure hydrogen or helium atmospheres.

Eight stars show significant infrared absorption, best explained by mixed atmospheres.

Most stars are consistent with Galactic disk age of 8 Gyr; some may be 12-13 Gyr old.

Abstract

We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNO-B astrometry and a dozen previously known ultracool white dwarf candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool white dwarfs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope datasets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km/s. Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool white dwarf candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool white dwarfs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo white dwarf candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo.