Rotation of low-mass stars stops slowing down as they get older

When Dr Guy Davies began work on his most recent paper, he expected his research to confirm the accepted method of estimating the ages of low-mass stars like the Sun from their mass and rotation rates, called gyrochronology. But it didn’t. What he and a group of fellow astrophysicists found, in fact, was that this method does not work for stars the same age or older than the Sun.

The discovery, revealed in the recently published paper entitled 'Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars', has opened up a new, exciting area in the field of stellar physics that could shed fresh light on how the Sun’s activity affects what happens here on Earth, particularly in terms of our weather and so called space weather, as well as elsewhere in our galaxy.

Stars, like planets, spin around an axis. As they age, this rotation slows or ‘spins down’, due to their magnetic field interacting with a wind of particles flowing away from their surface. What Guy and his colleagues have found is that at a critical point in the life of Sun-like stars, this rotation stops slowing down. They don’t yet understand why, but know that whatever has changed is connected to the stars’ magnetic field, which acts like a brake.

‘The observations of all the Sun-like stars that we’ve done show that they don’t do what we used to think they did: at some point in their lives, they stop spinning down,’ says Guy, a Research Fellow in helioseismology and asteroseismology in the School of Physics and Astronomy.

‘The astrophysics community thought that stellar rotation carried on slowing down throughout the lives of older stars, but we’ve discovered that this slowing ceases when stars reach their middle age'.

As well as challenging existing theories about how stars evolve, the discovery is particularly exciting because our star, the Sun, is now at the age – 4.5 billion years old – when this cessation of the slowdown occurs.

‘We know the Sun is approaching this critical epoch, so we think it will stop slowing down in the next few hundreds of millions of years,’ says Guy, the only UK author of the paper, written by scholars from the US, France and Denmark, which was published in the journal Nature earlier this year.

‘There are a lot of things that will change when this happens that we will feel and see – or not see – here on Earth. One of the big things of the 21^st century is space weather. The Sun makes space a dangerous place by throwing off large amounts of radiation and charged particles as it spins down. If that spin-down process stops, then the space weather will lessen and the Sun will become noticeably easier to live with.’

So how did Guy and co-authors, led by Jennifer L van Saders from Carnegie Observatories in California, make their discovery?

‘The reason we started writing this paper is that in 2003 someone called Sydney Barnes coined the term gyrochronology to describe the method for working out the age of a Sun-like star, but gyrochronology had never been tested on older stars.’

Other than the Sun, the oldest stars tested by this method were only 2.5 billion years old.

‘So what we did was to use a couple of neat methods to test the theory,’ says Guy, who spent at least six months undertaking the necessary research, which was funded by the Science and Technology Facilities Council. ‘We made a selection of stars – about 35 – whose ages we had determined using asteroseismology, which is the study of the natural music of the stars: sound that gets trapped in their interiors, just like the sound in a musical instrument, and resonates outward; the frequencies telling us what’s going on inside the star, allowing its age to be calculated. Then we measured the rotation rates and, together with other observations, were able to ask if gyrochronology worked. And the answer was “no, not for stars more evolved than the Sun”. No one was expecting that. We were anticipating that everything would carry on performing as before and that we would have a nice paper saying that gyrochronology works.’

As is often the case, the study’s findings pose more questions than they answer – but they are questions Guy is itching to try to address.

‘How it works is different to how we thought it worked, and what we were expecting, and it’s probably more complicated than we imagined. This is the first time we have seen this phenomenon and it opens up a new area of study. Even better than that, explaining the solar dynamo (the physical process that generates the Sun’s magnetic field) has been a long-standing problem in solar physics, and it’s now apparent that we can’t explain it just by looking at the Sun; we need to look at other stars too.

‘In terms of my own research, I want to follow up on this by looking for clues in the Sun for what’s happening and see whether we can build a much more complete theory of what’s driving it.’