Using ZTF and followup with CHIMERA, a team of international astronomers discovered a highly magnetized and rapidly rotating white dwarf, “ZTF J190132.9+145808.7”, which is as small as the moon. Their study recently appeared in the Journal Nature.
White dwarfs represent the last stage of evolution of stars with mass less than about eight times that of the Sun and, like other stars, are often found in binaries. If the orbital period of the binary is short enough, energy losses from gravitational-wave radiation can shrink the orbit until the two white dwarfs come into contact and merge. Depending on the component masses, the merger can lead to a supernova of type Ia or result in a massive white dwarf. In the latter case, the white dwarf remnant is expected to be highly magnetised because of the strong magnetic dynamo that should arise during the merger, and be rapidly spinning from the conservation of the orbital angular momentum.
Now, a team of international astronomers using the Zwicky Transient Facility (ZTF) searched for short period objects that lie below the main white dwarf cooling sequence in the Gaia colour-magnitude diagram (see Fig. 1). They found, ZTF J190132.9+145808.7 (hereafter ZTF J1901+1458) showing promising small photometric variations. Followup with CHIMERA, a high-speed imaging photometer on the 200-inch Hale telescope, confirmed that this system has a period of just 6.94 minutes (see Fig. 2).
“The period of ZTF J1901+1458 is unusually short for a white dwarf, as white dwarf rotational periods typically are upwards of hours.”
In order to identify field strength they also considered all the allowed bound-bound hydrogen transitions and found that the white dwarf’s magnetic field ranging between 600 megagauss and 900 megagauss over its surface.
Additionally, using photometry, they found that the effective temperature, stellar radius and interstellar reddening of ZTF J1901+1458 to be Teff = 46,000 K, R∗ = 2,140 km and E(B–V) = 0.044, respectively. Moreover, it has mass between 1.327 and 1.365 solar masses. Based on this expected mass, astronomers think that it has an oxygen-neon internal composition and its central density is right at the threshold for electron capture on ²³Na.
“The radius is smaller than other white dwarfs and only slightly larger than that of the Moon”
Such a small radius implies that the star’s mass is close to the maximum white-dwarf mass and its rapid rotation, high mass and magnetism points towards the fact that it is likely a remnant of a white dwarf merger.
Finally, based on the luminosity of the white dwarf, they estimated the temperature in the core to be about 2–3 × 107 K. At such a high central temperature and density, the neutrino cooling of ZTF J1901+1458 will dominated by the ”Urca” process acting on ²³Na. This unusual neutrino cooling makes an age determination difficult.
“Over a few hundred million years, the heaviest elements, including Na, will gradually sink to the centre of ZTF J1901+1458. If the star lies at the small end of the radius constraint and if at least sixty percent of the ²³Na manages to sink and undergo beta decay before the core crystallises and sedimentation stops, electron-capture on ²⁴Mg would ensue. The star would shrink and the internal pressure would no longer be able to support the star, as the maximum allowed mass for the new composition would be lower than the mass of the white dwarf. The star would therefore collapse and heat up, leading to the onset of electron capture onto Ne and to the ignition of oxygen nuclear burning. The white dwarf would then undergo a disruptive thermonuclear supernova or implode to form a neutron star.”— they added.
Reference: Caiazzo, I., Burdge, K.B., Fuller, J. et al. A highly magnetized and rapidly rotating white dwarf as small as the Moon. Nature 595, 39–42 (2021). https://doi.org/10.1038/s41586-021-03615-y
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