Oddball Pulsar Origin?

A few whirling neutron stars might get their start as very different objects, at least if this new analysis is correct.

White dwarfs usually only make a blazing stellar mark one way: by dying as a Type Ia supernova. But a new analysis suggests that white dwarfs might also be responsible for a particular type of stellar lighthouse: a small club of millisecond pulsars that don’t fit the mold.

Millisecond pulsars are neutron stars that spin thousands of times per second, their radiation beams flashing across our vision every time they whirl. They generally form in binary systems, in which the neutron star spins itself up by siphoning material off its companion star.

But astronomers have discovered a couple of millisecond pulsar systems that look different than they should if they formed this way. The tidal forces involved in the spin-up process should have circularized the stars’ orbits, yet the orbits of the recently detected PSR J2234+06 and PSR J1946+3417 are elongated.

Paulo Freire (Max Planck Institute for Radio Astronomy, Germany) and Thomas Tauris (Argelander Institute for Astronomy, Germany) suggest in an upcoming paper in the Monthly Notices of the Royal Astronomical Society that that’s because the pulsar didn’t start as a neutron star: instead, it was first an overweight white dwarf.

White dwarfs are famous for their strict mass limit of 1.4 solar masses, known as the Chandrasekhar limit. A white dwarf that exceeds this limit usually explodes (thanks to fusion, as a Type Ia supernova) or collapses (to form a neutron star). But a rapidly rotating white dwarf can stave off gravity, maintaining its high mass without collapsing in on itself.

In Freire and Tauris’s hypothesis, the pre-pulsar system still starts out as a binary. But here, a white dwarf is the body that spins itself up by siphoning material. Its rotation is so fast that, in lieu of collapsing, the object survives as a too-massive white dwarf — at least until accretion shuts off and the dwarf slows down a bit, the latter of which could take upwards of a billion years. Once the rotation slows, the white dwarf can’t delay its death any longer, and it collapses into a rapidly spinning pulsar. But because accretion ceases long before the neutron star forms, the binary members are not forced into the circular orbits astronomers usually see.

The white dwarf scenario is not the only one that could explain the elongated orbits: a triple system that ejected one of its members could also produce a millisecond pulsar system with a wide variety of orbits. But the white dwarf scenario only produces a tiny window of potential orbital eccentricities (0.09 to 0.12, where 0 is a perfect circle and 1 is a parabola) and, curiously enough, they’re akin to those of the two systems observed.

So, the new model isn’t a slam dunk. The key will now be to find more systems like these two and see whether they match all the scenario’s tight constraints.

Of course, the regular pathway still rules the roost: there are more than 100 millisecond pulsars that match the recycled neutron star scenario, and only two thus far that might have started as white dwarfs. But it’s still neat that these "lightweight” stellar cores might create some of their oddball, more massive brethren.

Reference: P. C. C. Freire and T. M. Tauris. "Direct formation of millisecond pulsars from rotationally delayed accretion-induced collapse of massive white dwarfs." Monthly Notices of the Royal Astronomical Society, in press.