A white dwarf bound to the transiting planetary system WASP-98

TL;DR

This study investigates a white dwarf near the WASP-98 planetary system to determine its properties and age, highlighting the challenges with featureless spectra and suggesting that white dwarf analysis can provide independent system ages.

Contribution

The paper presents a method to estimate the age of a planetary system using a nearby white dwarf with featureless spectra, emphasizing the potential and limitations of this approach.

Findings

The white dwarf is at least 3.6 billion years old.

Its kinematics suggest thick disc membership.

Spectral analysis of DC white dwarfs is challenging for precise age determination.

Abstract

WASP-98 is a planetary system containing a hot Jupiter transiting a late-G dwarf. A fainter star 12 arcsec distant has previously been identified as a white dwarf, with a distance and proper motion consistent with a physical association with the planetary system. We present spectroscopy of the white dwarf, with the aim of determining its mass, radius and temperature and hence the age of the system. However, the spectra show the featureless continuum and lack of spectral lines characteristic of the DC class of white dwarfs. We therefore fitted theoretical white dwarf spectra to the ugriz apparent magnitudes and Gaia DR2 parallax of this object in order to determine its physical properties and the age of the system. We find that the system is old, with a lower limit of 3.6 Gyr, but theoretical uncertainties preclude a precise determination of its age. Its kinematics are consistent with membership of the thick disc, but do not allow us to rule out the thin-disc alternative. The old age and low metallicity of the system suggest it is subject to an age-metallicity relation, but analysis of the most metal-rich and metal-poor transiting planetary systems yields only insubstantial evidence of this. We conclude that the study of bound white dwarfs can yield independent ages to planetary systems, but such analysis may be better-suited to DA and DB rather than DC white dwarfs.