December 14, 2017
Artistic representation of the MV Lyrae system during phases where the accretion disk
penetrates the magnetospheric boundary and bursts of accretion are observed.
Credit: Helena Uthas
University of Canterbury astrophysicist Simone Scaringi led the study, “Magnetically gated accretion in an accreting ‘non-magnetic’ white dwarf,” published today (Dec. 14) in the latest issue of the journal Nature. Texas Tech’s own Tom Maccarone is one of the study’s co-authors.
A white dwarf is what stars like the Sun become after they have exhausted their nuclear fuel. White dwarfs are dense objects roughly the same size as Earth but with as much mass as the Sun. They accrete, or grow, by sucking in mass from the outer layers of their companion stars.
White dwarfs have long been considered “non-magnetic.” When white dwarfs grow at very low rates, they gain mass in distinct and sudden bursts where they “binge eat” for a short period of time, said Scaringi, a New Zealand-based researcher and astrophysics lecturer.
By examining several years of data from the Kepler space-based observatory, a team of international researchers found one of these non-magnetic white dwarfs behaving as if it had a strong magnetic field.
“We have seen episodes of strong flares of accretion interrupted by periods with no evidence of accretion,” Scaringi said. “This sporadic activity is best explained by the presence of a strong magnetic field comparable to that of 1,000 fridge magnets.
“This magnetic field ‘gates’ the accretion, causing the matter to pile up until it has a gravitational attraction stronger than the magnetic forces holding it back, indicating for the first time that even ‘non-magnetic’ white dwarfs can have very strong magnetic fields.”
Scaringi, the paper’s primary author, said this is fundamental research for the field. There have been hints that accretion disks essentially behave similarly independent of the accretor – whether that is a white dwarf, black hole, neutron star or young proto-star.
“Now we have further evidence that magnetic accretors like the one in our paper also behave in the same way, irrespective of their origin,” Scaringi said. “Similar bursts have been observed in accreting neutron stars – which are much smaller and have magnetic fields much higher than our white dwarf – and in young stellar objects, which are on the other end, being much larger and owning much weaker magnetic fields.
“Our result closes the gap in that our new observations of accretion bursts in MV Lyrae – a peculiar nova-like star consisting of a red dwarf and a white dwarf in Lyra constellation – show the magnetic field strength distribution of systems displaying magnetic gating and underscores the universality of magnetospheric accretion across an enormous range of stellar parameters.”
The other co-authors of the study are Caroline D’Angelo of the Leiden Observatory at Leiden University in the Netherlands; Christian Knigge, a professor in the University of Southampton’s School of Physics and Astronomy, and Paul J. Groot, a professor in the Department of Astrophysics/IMAPP at Radboud University in the Netherlands.
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