However, despite years of diligent study, it is still unknown under what circumstances a white dwarf’s mass can increase to the Chandrasekhar limit. The Indian-American scientist and Nobel laureate Subrahmanyan Chandrasekhar established this in 1930 as the theoretical upper limit for the mass of a white dwarf.
In the early 1990s, supersoft X-ray sources with stable hydrogen burning on their surfaces were discovered as a new class of objects using ROSAT. For a time, those objects were thought to be candidates for SN Ia precursors. The problem with these sources is their amount of hydrogen, while Type Ia supernovae show no trace of hydrogen.
Binary star systems with white dwarfs steadily accreting and burning helium on their surfaces have been expected for more than 30 years, but these sources have never been seen. The Max Planck Institute for Extraterrestrial Physics (MPE) has led a global team in discovering an X-ray source whose optical spectra are completely dominated by helium.
The team has found a binary star system in which matter flows from its companion to the white dwarf. Bright, so-called supersoft X-rays resulting from the nuclear fusion of the spilled gas close to the surface of the white dwarf led to the discovery of the system. The peculiarity of this source is the overflow and combustion of helium instead of hydrogen.
It may be easier to understand the number of supernovae caused by exploding white dwarfs if the white dwarf’s mass is rising more slowly than previously thought possible, according to its measured luminosity.
Jochen Greiner, who leads the analysis of this source at MPE, said: “The super soft X-ray source [HP99] 159 has been known since the 1990s when it was first observed with ROSAT, more recently with XMM-Newton and now with eROSITA. Now we can identify it as an optical source in the Large Magellanic Cloud. In the spectrum, we mainly found emission lines of helium originating from the accretion disk.”
prof. Dr. Norbert Langer from the Argelander Institute for Astronomy, who is also a member of the Matter Transdsorption Research Area at the University of Bonn, said:The observed brightness of X-rays suggests that the burning of the inflowing helium in the white dwarf is stabilized by its rapid rotation, making a final supernova explosion of the system likely.”
Julia Bodensteiner from ESO, who has been studying massive stars since her master’s thesis at MPE, said: “Stars without hydrogen envelopes, such as the companion star we are in [HP99] 159, are an important intermediate step in the life cycle of binary stars that should occur in about 30% of such systems.”
“There should be many such stars, but only a few have been observed so far.”
- J. Greiner et al., A helium-burning white dwarf binary as a supersoft X-ray source, Nature (2023). DOI: 10.1038/s41586-023-05714-4