A rotating white dwarf shows different compositions on its opposite faces

Abstract

White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterised by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink toward the centre and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen or helium. Several mechanisms compete with gravitational settling to change a white dwarf's surface composition as it cools, and the fraction of white dwarfs with helium atmospheres is known to increase by a factor ~2.5 below a temperature of about 30,000 K; therefore, some white dwarfs that appear to have hydrogen-dominated atmospheres above 30,000 K are bound to transition to be helium-dominated as they cool below it. Here we report observations of ZTF J203349.8+322901.1, a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. This peculiar nature is likely caused by the presence of a small magnetic field, which creates an inhomogeneity in temperature, pressure or mixing strength over the surface. ZTF J203349.8+322901.1 might be the most extreme member of a class of magnetic, transitioning white dwarfs -- together with GD 323, a white dwarf that shows similar but much more subtle variations. This new class could help shed light on the physical mechanisms behind white dwarf spectral evolution.

45 pages, 11 figures

References in corpus (25)

• The Gaia mission
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• The Zwicky Transient Facility: Data Processing, Products, and Archive
• The Zwicky Transient Facility: Science Objectives
• Magnetic White Dwarfs
• On the Spectral Evolution of Hot White Dwarf Stars. I. A Detailed Model-Atmosphere Analysis of Hot White Dwarfs from SDSS DR12
• The Zwicky Transient Facility: Observing System
• Fully-Automated Reduction of Longslit Spectroscopy with the Low Resolution Imaging Spectrometer at Keck Observatory
• White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8
• A highly magnetised and rapidly rotating white dwarf as small as the Moon
• HiPERCAM: a quintuple-beam, high-speed optical imager on the 10.4-m Gran Telescopio Canarias
• Hot DB White Dwarfs from the Sloan Digital Sky Survey
• $\text{Gaia}$ white dwarfs within 40 pc I: spectroscopic observations of new candidates
• Orbital Decay in a 20 Minute Orbital Period Detached Binary with a Hydrogen Poor Low Mass White Dwarf
• Polluted White Dwarfs: Mixing Regions and Diffusion Timescales
• Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts
• From Hydrogen to Helium: The Spectral Evolution of White Dwarfs as Evidence for Convective Mixing
• A deep test of radial differential rotation in a helium-atmosphere white dwarf: I. Discovery of pulsations in PG 0112+104
• Spectrophotometric analysis of magnetic white dwarf I: Hydrogen-rich compositions
• 3D spectroscopic analysis of helium-line white dwarfs
• Discovery of a hot ultramassive rapidly rotating DBA White Dwarf
• Calibration of the mixing length theory for structures of helium-dominated atmosphere white dwarfs
• Pulsation in the white dwarf HE 1017-1352: confirmation of the class of hot DAV stars
• Collision-induced satellite in the blue wing of the Balmer-beta line and consequences on the Balmer series

Cited by in corpus (3)

• The spectral evolution of white dwarfs: where do we stand?
• Discovery of magnetically guided metal accretion onto a polluted white dwarf
• A cool, magnetic white dwarf accreting planetary debris