The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156

Abstract

The distribution of white dwarf rotation periods provides a means for constraining angular momentum evolution during the late stages of stellar evolution, as well as insight into the physics and remnants of double degenerate mergers. Although the rotational distribution of low mass white dwarfs is relatively well constrained via asteroseismology, that of high mass white dwarfs, which can arise from either intermediate mass stellar evolution or white dwarf mergers, is not. Photometric variability in white dwarfs due to rotation of a spotted star is rapidly increasing the sample size of high mass white dwarfs with measured rotation periods. We present the discovery of 22.4 minute photometric variability in the lightcurve of EGGR 156, a strongly magnetic, ultramassive white dwarf. We interpret this variability as rapid rotation, and our data suggest that EGGR 156 is the remnant of a double degenerate merger. Finally, we calculate the rate of period change in rapidly rotating, massive, magnetic WDs due to magnetic dipole radiation. In many cases, including EGGR 156, the period change is not currently detectable over reasonable timescales, indicating that these WDs could be very precise clocks. For the most highly magnetic, rapidly rotating massive WDs, such as ZTF J1901+1450 and RE J0317−853, the period change should be detectable and may help constrain the structure and evolution of these exotic white dwarfs.

Catalog of magnetic white dwarfs with hydrogen dominated atmospheres

“…We found a good fit to the observed spectra except for the lateral components of the H β line, which is expected to change as shown for SDSS J030407.40-002541.74. Another interesting case is the star SDSS J225726.05+075541.6 featured in Williams et al (2022). They stated that the observed Balmer lines are significantly weaker than predicted by the atmospheric models, which could be explained if the star were in an unresolved binary system, but the allowable parameter space for such a binary is minuscule.…”