By borrowing a technique used by seismologists to probe Earth's interior, astronomers have profiled for the first time the internal rotation of a white dwarf star, and find that it rotates at the same slow speed as its surface.

White dwarfs are the dense cores of low-mass stars that have exhausted their nuclear fuel, and are thought to represent the final evolutionary state of the majority of stars in our Galaxy. A typical white dwarf has about 60 percent the mass of the Sun crammed into a body the size of the Earth. Assuming that stars maintain their angular momentum throughout their evolution, a reduction in size at this evolutionary stage would be expected to spin up the star to a period of seconds to minutes. But the rotation periods of all white dwarfs observed to date rotate with periods of days or even years, leading astronomers to speculate that these stars may hide some of their rotation at deeper levels.

Generally only the outermost layers of a star can be observed, while the internal regions remain hidden from scrutiny. "However, during particular phases of their evolution, stars become unstable and pulsate," explains Gilles Fontaine of the University of Montreal, one of the scientists who made the new discovery. "During these phases, it is possible to measure the periods of the vibration modes present in pulsating stars."

The periods of the pulsation modes depend intimately on the global structure of a pulsating star and can be used to infer the internal structure of that star, a technique known as asteroseismology. "We devised an original method to decode this signature and, hence, infer the internal rotation profile of that star," says Fontaine. "This is the first time, to my knowledge, that this has been possible in a star (except for the case of our Sun)."

Using this technique on white dwarf star PG 1159-035 Fontaine and colleagues found that the star's interior rotates at the same slow speed as its surface. Moreover, they discovered that the star rotates with a uniform period of 34 hours through at least 90 percent of its depth. The findings support the idea that single white dwarfs have slow rotation periods thanks to the transferral of angular momentum to the outer reaches of the progenitor star in the stages preceding white-dwarf formation.

"Prior to our work, there was no other empirical evidence of that transfer since no one had probed the internal rotation state of evolved stars," Fontaine tells Astronomy Now. "Observations of the surface layers had established that white dwarfs rotate quite slowly, but this was limited only to these layers. It was certainly possible to imagine that the slowly rotating outer layers could hide a fast rotating core containing most of the angular momentum of the star. We found instead, at least in PG 1159-035, that this is not the case and that this star rotates quite slowly globally and 'rigidly', supporting theories that suggest a strong coupling between the core and the outer envelope in terms of rotation."

Fontaine adds that the timescales for this coupling are anywhere between 10,000 to 1,000,000 years, which is quite short compared to the age of PG 1159-035.

The team is already applying the novel technique to other types of pulsating stars, and in the meantime have analysed pulsation data already available for three other stars similar to PG 1159-035. "We were gratified to find out that they behave in exactly the same way as PG 1159-035: they rotate slowly and rigidly, that is, they also have lost essentially all of their angular momentum," says Fontaine.

The new research is presented in this week's edition of the journal Nature.