Astronomers unearthed an extraordinary binary system featuring a white dwarf orbiting its scorching stellar companion rapidly that it completes nearly 72 years of movement in a single Earth day.
The discovery is detailed in a paper, titled "A seven-Earth-radius helium-burning star inside a 20.5-min detached binary," published in the journal Nature Astronomy.
Discovery of Tiny Subdwarfs Challenges Previous Star Size Theories
New findings challenge prior ideas about star sizes, identifying hot subdwarfs, and burning helium at their cores, as the smallest-known stars. The Tsinghua University team, utilizing the Tsinghua University-Ma Huateng Telescope for Survey (TMTS), located an extraordinary binary system named TMTS J0526, situated approximately 2,760 light-years away.
Within TMTS J0526, a carbon- and oxygen-rich white dwarf, constituting 74% of the sun's mass, orbits a hot subdwarf star, a third of our sun's mass, and is only about 7 times wider than Earth, making it smaller than Jupiter and one of the tiniest stars observed in terms of volume.
Remarkably, the components of TMTS J0526 complete one orbit in just 20.5 minutes, setting a record for this binary type, albeit slower than the HM Cancri system, featuring two white dwarfs completing an orbit in 5.4 minutes.
The larger white dwarf in TMTS J0526 remains invisible, but researchers confirmed its presence by observing distortions in the orbit of the smaller star. Beyond its brief orbital period, the significance of TMTS J0526 lies in its potential to unravel the mysteries of how such diminutive subdwarf stars come into existence.
This finding aligns with a theory proposed by a Chinese team over two decades ago, suggesting the possibility of small stars existing in binary systems.
The initial observation of the J0526 system occurred at the Tsinghua University-Ma Huateng Telescope for Survey, with additional analysis conducted using data from larger telescopes worldwide. The discovery challenges established theories about star size and nature, prompting speculation about even smaller stars with unknown properties.
TMTS J0526 Sheds Light on Tiny Hot Subdwarf Star Formation Insights
White dwarfs emerge from sun-sized stars depleting their hydrogen cores, and transforming into red giants. Surrounded by gas and dust, some white dwarfs, like our future sun, exist in binary systems with shared envelopes. Quantum degeneracy pressure prevents further collapse, a trait shared with neutron stars, known collectively as "degenerate stars."
The Binary Population Synthesis theory proposes that during a common envelope phase, a thermonuclear explosion called a helium flash can eject the common envelope, turning the companion star into a subdwarf with about 45% of the sun's mass.
A second common envelope ejection, triggered by the ignition of a helium-rich core in the companion, could create an even less massive subdwarf with around 32-36% of the sun's mass. Following the ejection, the resulting hot subdwarf and white dwarf form a binary, emitting gravitational waves as they orbit each other.
The TMTS J0526 binary system provides observational evidence for the formation of a tiny hot subdwarf through the ejection of a secondary common envelope. The system's extraordinarily short orbital period of around 20 minutes suggests a rapid, intense process leading to the creation of this compact hot subdwarf and white dwarf binary.
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