An international team led by astronomers from the Max Planck Institute for Extraterrestrial Physics has announced in the “Nature” journal the discovery of a unique binary star system in which matter flows onto the white dwarf from its companion. One of the paper’s authors presenting the results is Prof. Andrzej Udalski from the UW Astronomical Observatory.

The discovered system was found due to bright, so-called super-soft X-rays, which originate in the nuclear fusion of the overflowed gas near the surface of the white dwarf. The unusual thing about this source is that it is helium and not hydrogen that overflows and burns. Optical observations of the system collected by the OGLE project played an important role in its characterization. The project itself was led by the co-author of the paper, Prof. Andrzej Udalski from the Astronomical Observatory of the University of Warsaw.

A white dwarf star can explode as a supernova, so-called Type Ia (SN Ia), when its mass exceeds the limit of about 1.4 solar masses. SNe Ia are all roughly equally bright, allowing astrophysicists a very precise determination of the distance to their host galaxies. They are an important tool for cosmology. However, even after many years of intensive research, the circumstances under which the mass of a white dwarf can grow to the 1.4 solar masses limit remain unclear.

OGLE project observations

The binary system designated [HP99] 159 is located in the nearby galaxy – the Large Magellanic Cloud. It has been known since the 1990s, when it was first observed with the X-ray satellite ROSAT. Recent observations carried out by the eROSITA X-ray satellite allowed detection of the optical counterpart. It was observed over a decade by the OGLE sky survey. OGLE observations revealed that this is a binary system with the orbital period of 2.327 days. On the other hand, high resolution spectrum obtained by the SALT telescope in South Africa revealed mainly emission lines of helium originating from the accretion disk and no traces of hydrogen lines.

For over 30 years, double star systems have been predicted, in which a white dwarf accretes and burns helium stably at its surface, but such sources have never been observed. [HP99] 159 is the first such a system. However, the measurements indicate that the rate of the flow of the helium matter is considerably lower than theoretically predicted for the continuous helium burning. Since the previous measurements show that the X-ray luminosity of [HP99] 159 has remained the same for about 50 years, the discovered system may indicate that quiet helium burning in white dwarfs is possible even at lower accretion rates than theoretically predicted.

Implications of the discovery

Exploding white dwarfs with helium stars can be progenitors of a part of SNe Ia type and their subclass with smaller luminosity, SN Iax, in which the explosions are weaker. The discovery that the mass of the white dwarf may stably grow more slowly than previously thought possible, suggests that the systems with a white dwarf and less massive helium secondary components may also end-up as supernovae of type Iax. This finding may help to understand the number of supernovae caused by exploding white dwarfs. Detection of similar systems should further allow constraining the conditions for SN Ia progenitors.