Stars rotate, but as time goes by the rotation periods become longer and longer. On the other hand astronomers have been using measurements of these stellar rotation periods to predict the ages of the stars. This method is called gyrochronology. To make this method work it is understood that the slowing down of the rotations is unchanged over millions of years, but this assumption has now been cast in doubt by an international group of researchers in a paper in the esteemed scientific magazine Nature.

One member of the group of researchers is Victor Silva Aguirre of the Stellar Astrophysics Centre, Aarhus University. He sums up the problem this way: "Earlier this year it was postulated in another paper in Nature that gyrochronology is a viable way of determining the ages of stars during their whole life. We have now shown that this does not hold for stars older than some 2.5 billion years. We have determined the ages of a sample of stars using asteroseismology, and we see, that for oldest stars gyrochronology does not hold up very well. In addition we have a suggestion that may explain what is wrong with the method".

In asteroseismology regular oscillations on the surface of stars are used to obtain knowledge of physical parameters deep inside the stars. The technique is similar to the one used by geologists to probe inside Earth with seismic methods. The research team has used data from 21 stars, observed over longer periods by the Kepler space telescope. With asteroseismology the astronomers obtain ages of stars which are independent of age determinations by other methods.

The basic explanation for the gradual spin down of stars is that magnetic fields from them is the cause. Stellar magnetic fields strech far out from the stars, and the fields catch hold of the gasses that escape from the stars as stellar winds. These gasses are forced to rotate with the same rate as the star does, but this again forces the star to loose rotational energy and rotate slower. A close metaphor for physics could be the ice scater doing a spin with a couple of weights or dumbbells in her hands. At first she spins fast holding the weights close to her body. When she extends her arms rotation slows down. Then she drops the weights and pulls her arms close again, but with the loss of the weights she can never regain enough rotational energy to enable her to rotate as fast as in the beginning.

It was previously thought that this braking process would continue over time and continously brake the rotation of the stars. The new observations make it obvious that this is not the case.

As this braking does not continue through the full life of the stars, it must be due to a relatively sudden change in the interaction between the hot gasses in the stellar winds and the magnetic fields inside them. The exact process is still unknown, but the research team suggests in the article that there is some weakening or change in the magnetic field, or a change in the way that the magnetic field interacts with the gas lost from the star.

Old stars will therefore keep their fast rotation rates much longer than previously thought. Using the rotation periods for age determinations in the future thus requires improved models of the interior of stars. At the same time more observations of real stars with asteroseismology are needed, enabling a better calibration of the various phases of the braking. At present we have these ages for only a few hundred stars. Even with the new TESS satellite due to be launched in 2017 we can ramp up this number to a few thousands, but that is still not enough!

A recalibration of the gyrochronological relations is needed for the method to work properly. If this succeeds there is a huge potential to determine the ages of thousands of stars using this relatively fast and easy method.

We need good determination of stellar ages. This is is at present especially difficult for stars similar to the Sun. With access to the ages of many thousands of stars, our efforts to do Galactic Archaeology will be much easier. The creation and evolution of our own Milky Way galaxy and the other galaxies still hide lots of unexplained riddles, and we hope to solve some of them by using gyrochronology to dig down into the galactic past.

Read the full article here.

For further information, please contact:

Assistant Professor Victor Silva Aguirre
Stellar Astrophysics Centre, Aarhus University
e-mail: victor@phys.au.dk