They discover a white dwarf that burns helium

March 24 () –

Astronomers in Germany have found a binary star system in which helium flows, and not hydrogentoward the white dwarf from its companion.

The system was found due to bright X-rays, called super soft, originating from the nuclear fusion of the overflowing gas near the surface of the white dwarf.

The measured luminosity suggests that the mass of the white dwarf is growing more slowly than previously thought possible, which may help to understand the number of supernovae caused by exploding white dwarfs. The results have been published in the journal Nature.

Exploding white dwarfs are not only considered the main source of iron in the universe, but are also an important tool for cosmology. Like so-called Type Ia (SN Ia) supernovae, they all become approximately equally bright, allowing astrophysicists to accurately determine the distance of their host galaxies.

However, even after many years of intensive research, it is not clear under what circumstances the mass of a white dwarf can grow up to the so-called Chandrasekhar limit. This is the theoretical upper limit for the mass of a white dwarf, obtained in 1930 by the Indian-American astrophysicist and Nobel laureate Subrahmanyan Chandrasekhar.

In the early 1990s, supersoft X-ray sources with stable hydrogen burning on their surfaces became established as a new class of ROSAT objects, and were for a time considered potential candidates for SN Ia progenitors. The problem with these sources, however, is their abundance of hydrogen: Type Ia supernovae show no trace of hydrogen.

Double star systems, in which a white dwarf star stably accumulates and burns helium on its surface, have been predicted for more than 30 years, but such sources have never been observed. An international team led by the Max Planck Institute for Extraterrestrial Physics (MPE) has now found an X-ray source whose optical spectrum is completely dominated by helium.

“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,” he explains. it’s a statement Jochen Greiner, who leads the analysis of this source at MPE. “Now, we were able to identify it as an optical source in the Large Magellanic Cloud. In its spectrum we find mainly helium emission lines originating from the accretion disk.”

However, this does not solve the problem of SN Ia’s progenitors: theoretical models predict that around 2-5% of the matter from the helium companion star will be swept up by SN Ia’s explosion and ejected into the environment. However, this amount of helium has not been found in most supernovae observed to date. There is, however, a subclass with less luminosity, the SN Iax, in which the explosion is weaker and, therefore, less helium is ejected.

The system now discovered [HP99] 159 could end up in such SN Iax based on current knowledge, as measurements here indicate that continued helium burning in white dwarfs is possible even at lower than theoretically predicted accretion rates. The measured brightness of [HP99] 159 is about ten times less than expected at the canonical rate, while at the same time the measured X-ray temperature is exactly in the range expected for stable helium combustion.

“The observed X-ray brightness suggests that the burning of the incoming helium in the white dwarf is stabilized by its rapid rotation, making a final supernova explosion of the system likely,” says Professor Norbert Langer of the Argelander Institute for Astronomy, who he is also a member of the Transdisciplinary Matter Research Area at the University of Bonn.

Since the above measurements indicate that the luminosity has remained the same for about 50 years, a wide range of accretion rates leading to explosions should be possible.

“Stars without a hydrogen envelope, such as the companion star in [HP99] 159, are an important intermediate step in the life cycle of binary stars that should occur in about 30% of such systems,” says Julia Bodensteiner of ESO (European Southern Onbservatory), who has been studying massive stars since her thesis on Master’s in MEP. “There should be a lot of such stars, but only a few have been observed so far.”

The team now hopes to find dozens of similar sources in the two Magellanic Clouds with eROSITA. This should allow them to further restrict the conditions for SN Ia parents.