With the recent find of a potentially habitable planet around the red dwarf Proxima Centaur,
astronomers have been studying this star with new passion.
Part of their attention focuses on the star’s behavior. M dwarfs are notorious for their flares,
and such stellar tantrums could be deadly for budding life on nearby planets.
But it’s only the young M dwarfs that are truly “flare stars.” Proxima Centaur,
on the other hand, is roughly 5 billion years old (according to scientists),
and although it certainly flares, we’re not sure what its
long-term activity is at such a respectable age. One thing astronomers once thought,
was that such stars wouldn’t have regular magnetic cycles like the one on our Sun.
Our star goes through an 11-year sunspot cycle as its global field changes directions,
but that shouldn’t happen on little stars like Proxima.
Or so they thought. Now both observations and computer simulations confirm that,
contrary to expectation, Proxima Centauri and stars like it do have regular magnetic cycles.
Previous observations have explored whether Proxima Centaur has an activity cycle,
with some suggesting that it does. Inspired by an article he saw about one of these studies in 2007 in Sky & Telescope,
Bradford Wargelin (Harvard-Smithsonian Center for Astrophysics) decided to investigate whether hints of a cycle were real.
Wargelin and his colleagues amassed optical, ultraviolet, and X-ray observations of the star from ground-
and space-based telescopes, spanning 22 years. The UV and X-ray photons generally come from flares,
and cyclic variations at these wavelengths are a lot more obvious than those in optical.
The team found that Proxima Centaur has a 7-year cycle,
with the X-rays and UV going up when the optical brightness goes down —
as would be expected if the star is plastered in starspots during times of heightened activity.
The 7-year cycle agrees well with previous studies that narrowed in on a 7- to 8-year range.
(Incidentally, the study mentioned in S&T argued for a 1.2-year cycle, which Wargelin’s team disproved.)
To understand what may be going on, scientists have turned to computer simulations.
Rakesh Yadav (also Harvard-Smithsonian Center for Astrophysics) and others have been working on what happens
inside M dwarfs and how they create their magnetic fields. The astronomers’
most recent work agrees that slow-spinning M dwarfs like Proxima Centaur,
which rotates around their axis every 83 days, do have differential rotation and magnetic cycles.
Fast rotators, on the other hand, have neither.
It all comes down to how well the magnetic field controls the star. Quickly spinning stars are generally young,
with very strong magnetic fields created by the boiling plasma.
These magnetic fields essentially hold the star’s plasma fixed in place as it rotates,
preventing differential rotation, Yadav explains. No differential rotation means no flip-flopping field:
the team does not see activity cycles in their simulations of fast-rotating, fully convective M dwarfs.
The stars do have very strong magnetic fields, but they’re frozen in place, never flipping.
Chaotic convection near the surface “shreds” the global field, creating small-scale activity that powers flares.
Conversely, the simulations show that as a star slows down, its global magnetic field weakens.
Such a field can’t hold the plasma hostage anymore, and the equator can break away from the poles, spinning faster.
The differential rotation creates widespread shears inside the convecting star and, thus,
a coherent field that flip-flops regularly.
Yadav’s team estimates in their latest paper, posted to the online preprint server arXiv.org on October 9th,
that a star with properties like Proxima’s would have a cycle of 9 years — very close to the observed 7-year cycle.
Although its field is weaker than a young M dwarf’s, it’s still a few hundred times stronger than the Sun’s.
If the simulations are right, we should not see activity cycles on young,
fast-rotating M dwarfs, but we should on older, slower ones.
It’ll take dedicated effort and years of observations to reveal whether that’s the case.
Astronomers are already making headway on that: recent work by Alejandro Suárez Mascareño
(Astrophysics Institute of the Canary Islands, Spain) and others has turned up cycles in several M dwarfs.