Dead Star Comes Back to Life While Devouring Companion

Astronomers have captured the first evidence of a white dwarf, the stellar remnant left over when smaller stars die, coming back to life while feeding on a companion star.

The team, led by Durham University researchers, observed the white dwarf binary TW Pictoris, where a dead star is feeding on material from its stellar companion.

In the system, which is 1,400 light-years from Earth, material from the companion star forms a disc around the white dwarf and falls to its surface in a process that astronomers call accretion. As the material, which is mainly hydrogen and helium, reaches the surface of the dead star it briefly becomes brighter.

Astronomers have observed this behavior in feeding white dwarfs in binary systems— stars that form together and then go on to orbit each other—before. What scientists had not seen are the abrupt rises and falls in brightness as observed in this system.

Because the flow of material to the surface of the white dwarf from the accretion disc is steady and constant, abrupt changes in luminosity, defined by astronomers as the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical objects, are unexpected.

Dr. Simone Scaringi from Durham University's Centre for Extragalactic Astronomy said: "The brightness variations seen in accreting white dwarfs are generally relatively slow, occurring on timescales of days to months.

"To see the brightness of TW Pictoris plummet in 30 minutes is in itself extraordinary as it has never been seen in other accreting white dwarfs and is totally unexpected from our understanding of how these systems are supposed to feed through the accretion disc. It appears to be switching on and off."

Scargini is the lead author of a paper describing the discovery published in the journal Nature Astronomy. The team made their observations of the white dwarf with Transiting Exoplanet Survey Satellite (TESS). The role of TESS is usually to observe planets outside the solar system, more commonly known as exoplanets.

The team believes that in the case of TW Pictoris these abrupt changes in luminosity are being driven by reconfigurations of the stellar remnant's magnetic field.

When the white dwarf springs back to life as its luminosity increases, it is feeding on material from the accretion disc as usual. But when the luminosity is lowered abruptly they believe that the magnetic field of the white dwarf is spinning so rapidly that it blocks the flow of the material to the surface.

The regulating of this material from the accretion disc, essentially fuel powering the white dwarf's luminosity, causes the increase and decrease of its electromagnetic output.

The unexpected finding helps astronomers draw the connection between white dwarfs and another form of stellar remnants, neutron stars, which are more massive and much much smaller because of how dense they are.

White dwarfs are formed when stars around the size of the sun run out of hydrogen to burn and collapse. This collapse triggers the burning of heavier elements in larger stars.

Eventually, this fuel is exhausted, and the process repeats for these more massive stars until they form neutron stars, stellar remnants that are less abundant than white dwarfs.

The team believes that the magnetic phenomenon they observed switching this white dwarf on and off again could also be at play when material is accreted onto the surface of a neutron star. This means that they could study the accretion process occurring around neutron stars by closely examining how it proceeds around white dwarfs.

"This really is a previously unrecognized phenomenon and because we can draw comparisons with similar behavior in the much smaller neutron stars," Scaringi concluded. "It could be an important step in helping us to better understand the process of how other accreting objects feed on the material that surrounds them and the important role of magnetic fields in this process."

White dwarf
A artist's illustration of the white dwarf MV Lyrae accreting as it draws in material from a companion star. New research has revealed a white dwarf that springs back to life when feeding on material from a stellar companion. Helena Uthas