A Magnetic White Dwarf with Five H$\alpha$ Components

TL;DR

This study presents time-resolved spectroscopy of the unusual magnetic white dwarf G183-35, revealing surface spots and magnetic field variability, and suggests complex magnetic and surface composition effects cause its unique five-component Hα line.

Contribution

It provides the first detailed time-resolved spectroscopic and photometric analysis of G183-35, demonstrating the role of spots and magnetic field changes in its spectral variability.

Findings

Detected two sets of absorption lines with 4-hour periodic variability.

Observed low-level photometric variability at the same period.

Proposed surface spots and magnetic field changes as the cause of variability.

Abstract

G183$-$35 is an unusual white dwarf that shows an H$\alpha$ line split into five components, instead of the usual three components seen in strongly magnetic white dwarfs. Potential explanations for the unusual set of lines includes a double degenerate system containing two magnetic white dwarfs and/or rotational modulation of a complex magnetic field structure. Here we present time-resolved spectroscopy of G183$-$35 obtained at the Gemini Observatory. These data reveal two sets of absorption lines that appear and disappear over a period of about 4 hours. We also detect low-level (0.2%) variability in optical photometry at the same period. We demonstrate that the spectroscopic and photometric variability can be explained by the presence of spots on the surface of the white dwarf and a change in the average field strength from about 4.6 MG to 6.2 MG. The observed variability is clearly due to G183$-$35's relatively short spin period. However, rotational modulation of a complex magnetic field by itself cannot explain the changes seen in the central H$\alpha$ component. An additional source of variability in the line profiles, most likely due to a chemically inhomogeneous surface composition, is also needed. We propose further observations of similar objects to test this scenario.