Readings from NASA's Chandra X-Ray Observatory are causing astronomers to question our understanding of exactly how the magnetic fields of stars are generated. The researchers discovered lower-than-expected X-ray emissions from low-mass stars, indicating that convection plays a slightly different role in magnetic field generation than previously thought.
Magnetic fields have a big impact on the behavior of stars. They're instrumental in creating sun spots, and can produce solar storms that damage satellites and power systems back on Earth. X-ray emissions, such as those measured by Chandra, are a good indicator of a star's magnetic field strength, and while larger stars tend to lose strength as they age, low-mass stars have a different internal structure, and haven't been thought to do so.
Numerous factors play a role in producing a star's magnetic field, from its rotation to the flow of gas inside it. The speed of rotation differs between the poles and equator of a star, and at different depths beneath its surface.
Besides rotation, the other big player in star magnetic field production is thought to be convection, wherein rising hot gas and descending cool gas form a circulating pattern. In larger stars, convection only occurs in the outer portion of the star – the outer third in the case of our own Sun. Meanwhile, the gas at the core of the body remains relatively still.
The interplay between those two areas – the circulating gas towards the surface and the still core – is thought to be central to magnetic field generation.
"In some ways you can think of the inside of a star as an incredibly complicated dance with many, many dancers," explained study co-author Jeremy Drake. "Some dancers move with each other while others move independently. This motion generates magnetic field, but how it works in detail is extremely challenging to determine."
For stars much smaller than the Sun, things work a little differently, with convection occurring all the way into their core. In the absence of the interaction along the convection zone/core boundary, scientists have theorized that the convection process alone is largely responsible for generating such stars' magnetic field. If that were indeed the case, then the field shouldn't weaken over time as is observed in larger stars, where interaction along that border decreases as the star slows its orbit over time.
That's where the Chandra observations step in. Studying four low-mass red dwarfs – stars that shouldn't, in theory, have their magnetic fields weaken as they age – they found the opposite to be true. Despite what's been theorized, the smaller stars' magnetic fields weakened just like their much bigger counterparts.
The findings turn what we thought we knew about Sun-like stars' magnetic fields upside down. Since low-mass stars, in which convection occurs down to the core, exhibit such similar properties as larger stars, activity at the zone/core boundary cannot play the prominent role in magnetic field generation that we thought it did.
Right now, astronomers don't have any concrete answers for exactly why that is.
"We found that these smaller stars have magnetic fields that decrease as they age, exactly as it does in stars like our Sun," said lead author Nicholas Wright. "This really goes against what we would have expected."
A paper detailing the findings was published online in the journal Nature.
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