Astronomers discover helium-burning white dwarf

A white dwarf star can explode as a supernova when its mass exceeds the restrict of about 1.4 solar lots. A group led by the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching and involving the College of Bonn has now discovered a binary star system through which matter flows onto the white dwarf from its companion.

The system was discovered because of shiny, so-called super-soft X-rays, which originate within the nuclear fusion of the overflowed fuel close to the floor of the white dwarf. The bizarre factor about this supply is that it’s helium and never hydrogen that overflows and burns. The measured luminosity means that the mass of the white dwarf is rising extra slowly than beforehand thought potential, which can assist to grasp the variety of supernovae attributable to exploding white dwarfs. The outcomes have been printed within the journal Nature.

Exploding white dwarfs are usually not solely thought of the primary supply of iron within the universe, they’re additionally an essential device for cosmology. As so-called Sort Ia supernovae (SN Ia), all of them turn out to be roughly equally shiny, permitting astrophysics a exact dedication of the space of their host galaxies.

Nevertheless, even after a few years of intensive analysis, it stays unclear below what circumstances the mass of a white dwarf can develop to the so-called Chandrasekhar restrict. That is the theoretical higher restrict for the mass of a white dwarf, derived in 1930 by Indian-American astrophysicist and Nobel laureate Subrahmanyan Chandrasekhar.

Within the early Nineties, super-soft X-ray sources with secure hydrogen burning on their surfaces have been established as a brand new class of objects with ROSAT, and for a time these have been thought of potential candidates for SN Ia progenitors. The issue with these sources, nonetheless, is their hydrogen abundance: kind Ia supernovae present no hint of hydrogen.

For greater than 30 years, double star systems have been predicted, through which a white dwarf accretes and burns helium stably at its floor, however such sources have by no means been noticed. A world group led by the Max Planck Institute for Extraterrestrial Physics (MPE) has now discovered an X-ray supply whose optical spectrum is totally dominated by helium.

“The super-soft X-ray supply [HP99] 159 has been identified because the Nineties, when it was first noticed with ROSAT, extra just lately with XMM-Newton and now with eROSITA,” explains Jochen Greiner, who leads the evaluation of this supply at MPE. “Now, we have been in a position to determine it as an optical supply within the Massive Magellanic Cloud. In its spectrum we discovered primarily emission strains of helium originating from the accretion disk.”

Nevertheless, this doesn’t remedy the issue of SN Ia progenitors: theoretical fashions predict that about 2-5% of the matter of the helium companion star will likely be carried away by the SN Ia explosion and ejected into the surroundings. Nevertheless, this quantity of helium has not been present in most supernovae Ia noticed so far. There’s, nonetheless, a subclass with smaller luminosity, the SN Iax, through which the explosion is weaker, and subsequently much less helium is blown away.

The now found system [HP99] 159 may find yourself in such a SN Iax based on present data, because the measurements right here point out that steady helium burning in white dwarfs is feasible even at decrease accretion charges than theoretically predicted. The measured luminosity of [HP99] 159 is about ten instances smaller than anticipated on the canonical charge, whereas on the similar time the measured X-ray temperature is precisely within the anticipated vary for secure helium burning.

“The noticed X-ray brightness means that the burning of the inflowing helium within the white dwarf is stabilized by its fast rotation, making a closing supernova explosion of the system possible,” says Prof. Dr. Norbert Langer of the Argelander Institute for Astronomy, who can be a member of the Matter Transdisciplinary Analysis Space on the College of Bonn.

Since earlier measurements point out that the luminosity has remained the identical for about 50 years, a variety of accretion charges resulting in explosions ought to be potential.

“Stars with out hydrogen envelopes, such because the companion star present in [HP99] 159, are an essential intermediate step within the life cycle of binary stars that ought to happen in about 30% of such programs,” says Julia Bodensteiner of ESO, who has been learning huge stars since her grasp’s thesis at MPE. “There ought to be many such stars; however just a few have been noticed thus far.”

The group now hopes to search out dozens of comparable sources within the two Magellanic Clouds with eROSITA. This could permit them to additional constrain the circumstances for SN Ia progenitors.