New discovery finds starving white dwarfs are binge eaters

14 December 2017

University of Canterbury astrophysicist Dr Simone Scaringi has made an unexpected and exciting new discovery related to the way white dwarfs grow in space.

  • whitedwarf1_NWS_block

    An artistic representation showing the system the researchers observed during its "binge eating" phase. Image: Helena Uthas

whitedwarf2_NWS_block

An artistic representation showing the system the researchers observed when it is "quiet". Image: Helena Uthas

University of Canterbury astrophysicist Dr Simone Scaringi has made an unexpected and exciting new discovery related to the way white dwarfs grow in space.

The New Zealand-based researcher and astrophysics lecturer’s co-authored paper, titled “Magnetically gated accretion in an accreting ‘non-magnetic’ white dwarf” has been published in the latest issue of Nature (14 December).

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.

Most 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, Dr Scaringi says.

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. This sporadic activity is best explained by the presence of a strong magnetic field comparable to that of 1000 fridge magnets,” Dr Scaringi says.

“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.”

The paper’s primary author, Dr Scaringi says this is fundamental research for the field. There have been hints that accretion disks essentially behave in the same way 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.

“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,” he says.

“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.”

Nature 552, 210–213, 14 December 2017 (doi:10.1038/nature24653)

Spokesperson and primary author: Dr Simone Scaringi, Lecturer in Astrophysics, College of Science, University of Canterbury, New Zealand, Email: simone.scaringi@canterbury.ac.nz, Phone: +6433694650

Co-authors: Prof Tom J. Maccarone, Department of Physics and Astronomy, Texas Tech University, United States. Dr Caroline D’Angelo, Leiden Observatory, Leiden University, The Netherlands. Prof Christian Knigge, School of Physics and Astronomy, University of Southampton, United Kingdom. Prof Paul J. Groot, Department of Astrophysics/IMAPP, Radboud University, The Netherlands.

For further information please contact:

Margaret Agnew, Senior External Relations Advisor, University of Canterbury
Phone: +64 3 369 3631 | Mobile: +64 275 030 168margaret.agnew@canterbury.ac.nz
Tweet UC @UCNZ and follow UC on Facebook

JosiahTuala_NWS_block

UC students vie for 2018 Young New Zealander of the Year award

Two University of Canterbury (UC) students are among the final 10 nominees shortlisted for 2018 Young New Zealander of the Year award, while UC alumni ...

CantyRoll_NWS_block

Researchers and students to bring 600-year-old treasure to the world

University of Canterbury staff and students are working to translate and digitise a unique medieval manuscript to make it accessible to the world, and ...