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J0658 Massive Hot Subdwarf Binary

Astronomers Discover Exceptionally Massive Hot Subdwarf Binary System LAMOST J0658

The phased radial-velocity curve and the associated phased, binned TESS light-curve observations of J0658. Credit: Mattig et al., 2025.

New Observations Reveal One of the Most Extreme Stellar Pairs Known

Researchers have uncovered a new binary star system known as LAMOST J065816.72+094343.1, consisting of a massive, high-temperature subdwarf and a companion that has yet to be directly observed. The discovery is reported in the January issue of Astronomy & Astrophysics.

More space and astronomy discoveries

What Do We Know About J0658?

LAMOST J0658 16.72+094343.1—commonly known as J0658—was first detected in 2018 by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and classified as a hot sdOB-type subdwarf.

Early observations revealed it to be helium-deficient, with an effective temperature of around 35,000K an a projected rotational velocity of 37km/s.

With much still unknown about J0658, a research team led by Fabian Mattig from the University of Potsdam, Germany, undertook a detailed analysis of archival LAMOST data. They also carried out follow-up observations using the Southern Astrophysical Research (SOAR) telescope and the Very Large Telescope (VLT) to better understand the star's true nature.

A Newly Confirmed Massive Binary System

To investigate the nature and future evolution of LAMOST J065816.72+094343.1, the researchers combined archival spectroscopic observations with new time-series data and high-resolution spectroscopy.

Through this detailed analysis, they identified J0658 as a newly discovered, exceptionally massive hot subdwarf binary with a very short orbital period.

Physical Properties of the J0658 System

  • Orbital period of approximately 0.32 days
  • Primary star radius of about 0.31 times that of the Sun
  • Primary star mass of roughly 0.82 solar masses
  • Unseen companion mass estimated at around 30% greater than the Sun

This places J0658 among the most massive and hottest known subdwarf binaries.

Unseen Companion Near a Critical Mass Limit

With the companion's mass lying close to the Chandrashekhar limit of around 1.4 solar masses, its true identity remains uncertain.

The researchers note that it could be:

  • An unusually heavy white dwarf, or
  • A low-mass neutron star

Its mass effectively bridges both regimes, making J0658 a particularly rare and scientifically valuable system.

Astrophysics and cosmic evolution insights

Possible Evolutionary Futures of the J0658 System

Astronomers suggest several potential evolutionary pathways, depending on the nature of the unseen companion.

If the Companion Is a Carbon-Oxygen White Dwarf

  • The system would experience stable helium accretion
  • The stars would merge within approximately 30 billion years

The outcome could be:

  • A Type Ia supernova, or
  • The formation of a detached double white dwarf system

If the Companion Is an Oxygen-Neon White Dwarf

  • The system would likely undergo accretion-induced collapse
  • This would lead to the formation of an intermediate-mass binary pulsar

If the Companion Is a Neutron Star

  • A similar outcome is expected
  • The system would first pass through a short-lived intermediate-mas X-ray binary phase

Why Further Observations Matter

The researchers therefore emphasize that further observations of J0658 are essential to determine which of these evolutionary scenarios accurately describes the system.

Clarifying the nature of the unseen companion could provide critical insight into stellar evolution, compact object formation and the origins of extreme cosmic events.

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