South African Telescopes Plays Significant Role in Discovering a Rare Spinning White Dwarf Star

In a press release by the South African Astronomical Observatory (SAAO), an international team of astronomers, including teams from South African Astronomical Observatory (SAAO), the University of Cape Town (UCT) and the South African Radio Astronomy Observatory (SARAO) have discovered a rare type of spinning white dwarf star in a binary star system – only the second one known. This discovery provides a new understanding of the role of magnetic fields in stellar evolution.

The discovery published in Nature Astronomy shows the newly detected white dwarf pulsar, J191213.72-441045.1 (J1912-4410 for short), in a binary system with an orbital period of just over 4 hours but a rotation period a little over 5 minutes long, meaning it’s spinning about 270 times faster than the Earth does.

Some researchers involved in discovering the spinning white dwarf star were also instrumental in discovering the nature of the first such system, AR Scorpii, in 2016. This system, a rapidly rotating and strongly magnetic (about a billion times the Earth’s magnetic field) white dwarf pulsars, lash their stellar companion – a red dwarf – with powerful beams of charged particles and radiation, causing the entire system to brighten and fade dramatically over the minutes-long rotation period of the white dwarf. In addition, the bulk of the energy of these systems is powered by the slowing down of the spinning white dwarf due to the drag exerted by its strong magnetic field.

White dwarfs are small dense stars,  which means that a teaspoon of white dwarf material would weigh around 15 tons. They form when a low-mass star, like the Sun, or even less massive, has burnt all its fuel and lost its outer layers. Sometimes referred to as “stellar fossils”, they offer insight into different aspects of star formation and evolution. White dwarfs begin their lives at scorching temperatures before cooling down over billions of years. In addition, they are usually the size of Earth, although with a 200,000 times greater mass and nearer to the mass of the Sun.

A radio image from MeerKAT showing the position of J1912-4410, plus the radio light curve showing the occasional 5 min spin pulses (credit: Ian Heywood). Source: SAAO

Furthermore, a critical theory, the “dynamo model”, explains the reason behind the strong magnetic fields detected in the binary system. The model depicts that white dwarfs, like the Earth, have dynamos (electromagnetic generators) in their cores but are much more powerful. To test the validity of this hypothesis, scientists needed to search for other white dwarf pulsars to see if their predictions held.

According to Dr Ingrid Pelisoli, Research Fellow at the University of Warwick’s Department of Physics and lead author, “The origin of magnetic fields is a big open question in many fields of astronomy, and this is particularly true for white dwarf stars. The magnetic fields in white dwarfs can be more than a million times stronger than the magnetic field of the Sun, and the dynamo model helps to explain why. The discovery of J1912−4410 provided a critical step forward in this field”. We used data from a few different survey telescopes to find potential white dwarf pulsar candidates, focusing on those with similar characteristics to AR Sco. After observing a few dozen candidates, we found one that showed identical light variations to AR Sco. We followed these up with several other telescopes, both from the ground and space”.

Some of the telescopes instrumental in the discovery included the optical telescopes hosted at the Sutherland site of the SAAO and the 10-m diameter Southern African Large Telescope (SALT).

However, observations were also done using the MeerKAT radio telescope array, following a request for Director’s discretionary time by Professor Patrick Woudt (UCT) and Professor Buckley. These observations revealed sharp radio pulses at the same five-minute spin period, but only at particular times, coinciding at the same orbital phase. For Professor Woudt, “These beautiful MeerKAT observations show how short repeat snapshot radio observations can reveal the strongly pulsed radio emission, as is demonstrated in these observations of J1912-4410”. 

“The radio light curve of J1912-4410 is remarkably different to AR Sco, with only a brief period when the very narrow pulses are seen. In fact, in the first ~40 min observation, we were lucky to catch them at all, and if we’d not, the second longer 8-hour observation may not have happened.” Professor Buckley added.

The significance of this finding lies in its conformation to the dynamo model’s predictions. Given the old age of the white dwarfs within the pulsar system, they are expected to exhibit a cool temperature. This discovery is crucial as it supports the predictions made by the dynamo model. Due to their old age, the white dwarfs in the pulsar system should be cool. Their companions must also be close enough for the gravitational pull of the white dwarf.  It ensures the force is strong enough to steal mass from the closely orbiting companions, causing them to spin fast. 

For more information, contact Dr David Buckley.

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