October 14, 2010

White Dwarfs as Pulsars

A little more than two years ago, in early 2008, there was a shocking discovery from the Suzaku team. Suzaku is an orbiting X-ray observatory, operated jointly by JAXA and NASA. It had detected a white dwarf star, AE Aquarii, emit high-energy X-ray pulses. What’s more was that these pulses lined up exactly with the white dwarf’s spin period of 33 seconds. The star was behaving nearly like a pulsar.

Before I go into further discussion about white dwarfs acting like pulsars, I want to explain what the two types of stars are.

White Dwarfs

White dwarfs are essentially remains of stars that initially had a mass of less than about 8 solar masses (like the Sun). As these stars finish undergoing their fusion reactions, they form a carbon-oxygen core that cannot undergo fusion reactions, which is enveloped by a layer of helium fusion and a layer of hydrogen fusion. Ultimately, the outer layers are expelled, and the carbon-oxygen core remains. Therefore, the composition of most white dwarfs is carbon and oxygen.

These stars have a mass similar to that of the Sun, but a volume which is near the size of the Earth. Subsequently, white dwarfs have a very high density. White dwarfs also can have strong magnetic fields.

Furthermore, since their radius is greatly reduced from their original states, white dwarfs also have a very high angular velocity. This is a result of the conservation of most of the angular momentum of the original star. Conservation of angular momentum is often demonstrated by a spinning ice skater. As a spinning ice skater brings his/her arms in (comparable to the radius of the star shrinking), he/she starts spinning much faster. Therefore, as the star shrinks dramatically, its rotation increases greatly.


Larger stars, those with a mass greater than 8 solar masses, go a different route. In these stars, a growing iron core in the center of the star becomes so massive that it cannot support its own mass. The core collapses, creating neutrons and neutrinos as electrons are forced into the protons. This collapse results in an immense shockwave, which in turn causes the star itself to explode in what is called a supernova. The outer layers of the star are expelled in the supernova, leaving behind the remnants of the core of the star. This remaining star is very dense and very compressed, and is made up of mostly neutrons. These are called neutron stars.

The mass of a neutron star is typically not much greater than the mass of white dwarfs, usually between 1.35 and 2.1 solar masses. Their radius, on the other hand, is very small, just around 12 km. (In the case of very massive initial stars, the resulting neutron star is so massive that it further collapses into a black hole.) Therefore, their densities are extremely high. As Wikipedia states, it is comparable to the mass of the entire human population being squeezed into the size of a sugar cube.

In addition to their great density, neutron stars can also have a very large magnetic field. Plus, like white dwarfs, they conserve most of the angular momentum of their stars, so that when neutron stars form, they start spinning fast. They retains a very small fraction of the radius of the original star, so this results in an angular velocity which is much greater than that of a white dwarf. These two characteristics combine to create a pulsar.

The rotating magnetic field results in a powerful electric field forming on the star. This field accelerates protons and electrons on the surface of the star, and creates an electromagnetic beam which travels out from the magnetic poles of the star. The magnetic pole do not necessarily line up with the rotational axis of the star (just like the Earth’s magnetic poles do not correspond to the rotational axis of our planet), and as the star spins rapidly, the electromagnetic beam also spins around very rapidly. Sometimes, these beams can point in the direction of the Earth during their rotation, and when that happens, we can detect them as a pulsating source of emissions, resulting in the name pulsars (from pulsating stars). The period of the pulsation is the period of the rotation of the pulsar (ranging from 1.4 milliseconds to 8.5 seconds), and is very regular. In fact, the first pulsar discovered was temporarily dubbed LGM-1 for “Little Green Men,” since the discoverers considered extraterrestrial origins for the highly regular signal.

Eventually as energy is lost through the beam, the star slows down, and the pulsar mechanism turns off. This is expected to take place after around 10 to 100 million years.

White Dwarfs Acting Like Pulsars

White dwarfs and pulsars are often regarded as completely separate types of stars. That is why the discovery from the Suzaku team was so surprising. There were hard X-ray pulses which were being detected, and lining up exactly with the rotational period of the white dwarf star. The team had discovered a white dwarf star which was acting almost exactly like a pulsar.

This discovery has recently been brought back into focus because a recently published paper, led by Kazumi Kashiyama at Kyoto University, proposes a way for white dwarfs to act like pulsars.

This method could help explain the source of the high abundance of positrons in the range of 10 - 100 GeV, and of electrons and positrons in the range of 100 GeV - 1 TeV detected by the PAMELA spacecraft. Previously, a large number of objects were offered as the source of these particles, including pulsars. These, though, are expected not to result in the amount that has been detected by PAMELA. White dwarfs may however result in the amount of particles detected.

Typically, a white dwarf simply cannot reach a sufficient angular velocity to act like a pulsar. Their size during formation simply does not reduce as dramatically as it does for a neutron star. Kashiyama and others propose that in some cases, during a merger or accreting a large amount of mass from another star, can result in a greater angular velocity. Plus, about 10% of white dwarfs already are expected to have sufficiently powerful magnetic fields. Combining this magnetic field and sufficiently large angular velocity can result in a white dwarf behaving like a pulsar.

The proposition seems plausible since the idea of a white dwarf gaining mass is not very strange at all. This happens during the formation of a Type Ia supernova in binary star systems when a white dwarf gains mass from an accompanying star, or when two white dwarfs collide together. Furthermore, since white dwarfs have weaker magnetic fields, they would lose their energy slower, and keep spinning for a longer time. Subsequently, they can power the pulsar for a longer time, possibly accounting for the amount of particles detected by PAMELA. It also suggests that many of the pulsars that we observe in our own galaxy could in fact be white dwarf pulsars.

Next, it is necessary to provide conclusive evidence for the existence of such stars. AE Aquarii seems like a great candidate since it is a binary system, which could account for the white dwarf star accreting mass. In the meantime, this type of research, at least for me, is extremely exciting since it continues to show how the universe continues to surprise and amaze us.

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