A test of the planet–star unipolar inductor for magnetic white dwarfs

N Walters,T R Marsh,J J Hermes,J D Landstreet,A Wallach,C J Manser,S Bagnulo,N Achilleos,M Hart,J Farihi

doi:10.1093/mnras/stab617

Abstract

ABSTRACT Despite thousands of spectroscopic detections, only four isolated white dwarfs exhibit Balmer emission lines. The temperature inversion mechanism is a puzzle over 30 years old that has defied conventional explanations. One hypothesis is a unipolar inductor that achieves surface heating via ohmic dissipation of a current loop between a conducting planet and a magnetic white dwarf. To investigate this model, new time-resolved spectroscopy, spectropolarimetry, and photometry of the prototype GD 356 are studied. The emission features vary in strength on the rotational period, but in antiphase with the light curve, consistent with a cool surface spot beneath an optically thin chromosphere. Possible changes in the line profiles are observed at the same photometric phase, potentially suggesting modest evolution of the emission region, while the magnetic field varies by 10 per cent over a full rotation. These comprehensive data reveal neither changes to the photometric period, nor additional signals such as might be expected from an orbiting body. A closer examination of the unipolar inductor model finds points of potential failure: the observed rapid stellar rotation will inhibit current carriers due to the centrifugal force, there may be no supply of magnetospheric ions, and no antiphase flux changes are expected from ohmic surface heating. Together with the highly similar properties of the four cool, emission-line white dwarfs, these facts indicate that the chromospheric emission is intrinsic. A tantalizing possibility is that intrinsic chromospheres may manifest in (magnetic) white dwarfs, and in distinct parts of the Hertzsprung–Russell diagram based on structure and composition.

Highlights

In recent years, there has been a rapid rate of white dwarf discovery, ensuing from the advent of modern large-scale surveys
To asses the origins of the broad-band photometric variability in GD 356, the semi-amplitude of the light curves in several bandpasses are compared with that expected from changes in the emission lines alone
This paper reports periodic variations in the photometry and spectropolarimetry of GD 356, which are linked to the rotational period

Summary

Introduction

There has been a rapid rate of white dwarf discovery, ensuing from the advent of modern large-scale surveys. From studies of the 20 pc sample (Holberg et al 2016; Hollands et al 2018), it is estimated that at least 12 per cent of white dwarfs are magnetic. This estimate is comparable to the conclusion of an earlier study based on spectropolarimetry of hydrogen-rich stars (Kawka et al 2007), but recent work taking advantage of metallined white dwarfs has increased this estimate to 20 ± 5 per cent (Landstreet & Bagnulo 2019; Bagnulo & Landstreet 2019, 2020).

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