The discovery of a minuscule white dwarf, smaller than Jupiter, sheds light on the formation of stars less massive than the gas giant. Astronomers have found an exceptional binary system where a “dead star,” or white dwarf, orbits its extremely hot and diminutive stellar partner so rapidly that it completes nearly 72 years’ worth of orbit in just one Earth day.
Dubbed TMTS J0526, this system was detected by a team from Tsinghua University utilizing the Tsinghua University-Ma Huateng Telescope for Survey (TMTS). Situated approximately 2,760 light-years from Earth, TMTS J0526 consists of a carbon- and oxygen-rich white dwarf star, about 74% the mass of the sun. It revolves around a hot subdwarf star, roughly one-third the mass of our sun, and is only about 7 times wider than Earth, making it smaller than Jupiter and among the tiniest stars observed in terms of volume.
The duo of TMTS J0526 completes a full orbit approximately every 20.5 minutes, setting a record for this type of binary system, albeit slower than the HM Cancri system, which features two white dwarfs orbiting each other every 5.4 minutes.
Despite the white dwarf’s gravitational pull deforming its smaller companion into an ellipsoid shape, the latter remains more visible due to its thin hydrogen atmosphere. The significance of discovering TMTS J0526 extends beyond its remarkably short orbital period; it offers insights into the birth of such tiny subdwarf stars.
White dwarfs form when stars similar in size to the sun deplete their hydrogen fuel and can no longer resist gravitational collapse. This process transforms them into red giants before cooling down to become white dwarfs. These stars are prevented from further collapse by degeneracy pressure, a quantum effect. Some white dwarfs, including TMTS J0526, exist in binary systems, where they share a common envelope of gas with another star.
The ejection of this common envelope can occur due to phenomena such as a helium flash, resulting in the formation of a subdwarf star. Subsequent ejections may lead to the creation of even less massive subdwarfs. The resulting binary system emits gravitational waves, causing the white dwarf and subdwarf to spiral closer together and eventually form a compact binary with a short orbital period.
TMTS J0526’s extraordinarily brief orbital period provides observational evidence supporting the formation of tiny hot subdwarfs through the ejection of a secondary common envelope.
