A magnetic white dwarf with five H α components

Abstract

G183−35 is an unusual white dwarf that shows an Hα 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 Hα 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.

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

“…Transformations in the emission profiles are consistent with a modest changes in the magnetic field across the surface as it rotates in and out of the view (more on this in Section 4.2). The broad-band photometric variability is clearly dominated by changes in the photospheric continuum (Table 3), and is almost certainly due to some type of magnetic field-dependent opacity, as seen in other high-magnetic field white dwarfs, such as Feige 7 or G183-35 (Achilleos et al 1992;Kilic et al 2019).…”

“…A combination of stellar rotation and variation in surface composition can produce continuum flux and polarimetric modulation via magnetic dichroism (Achilleos et al 1992), thus resulting in photometric variability. Moreover, spectroscopic changes in the Zeeman-split components may appear due to variations in the surface field strength through a rotation cycle (Kilic et al 2019).…”

The nearby magnetic cool DZ white dwarf PM J08186-3110

“…We have also shown that the abundance pattern changes between the FORS1 and X-shooter spectra taken 10 yr apart, suggesting that PM J08186−3110 has abundance spots due to the distribution of the magnetic field on the surface of the white dwarf. These abundance spots will reveal themselves as abundance variations over the rotational period, which could be a few hours, such as was observed in G183-35 (Kilic et al 2019). This hypothesis is supported by noted variations in the longitudinal field measured with FORS1 spectropolarimetry and, independently, with WHT/ISIS spectropolarimetry (Bagnulo & Landstreet 2019) .…”

“…The variations in these white dwarfs are a few percent, comparable to our upper limit. Finally, Kilic et al (2019) showed that the spectroscopic and photometric variations observed in G183−35 were caused by a variable magnetic field and a chemically inhomogeneous surface composition rotating on a period of about 4 hr. We compared the best-fitting model spectra to the observed photometric measurements.…”

Effects of Magnetic Fields in Hot White Dwarfs

“…While the core of most white dwarf stars are adequately modeled with one of the assumptions that the temperature or the magnetic field can be disregarded, some recent observations (Ref. [1][2][3][4]) have suggested that a few white dwarfs may require the inclusion of both temperature and magnetic field effects in the calculation of the matter equation of state. In that light, we examine for the first time the effects of including both temperature and magnetic field into the equation of state of white dwarfs.…”