Stars may not be so unique

Stars may not be so unique
Stars may not be so unique

According to researchers from Rice University, the stars scattered across space look different, but more similar than previously thought.

New simulations by Rice scientists show that "cold" stars like the Sun have dynamic surface behavior that affects their energetic and magnetic environment. This stellar magnetic activity is the key to whether a given star has planets that can support life.

The work of Alison Farrish, David Alexander and Christopher Jones-Krull is published in The Astrophysical Journal. The study links the rotation of cool stars to the behavior of their surface magnetic flux, which determines the X-ray luminosity of a star in its corona, which could help predict how magnetic activity will affect any exoplanets in their systems.

The study follows other research by Farrish and Alexander, which showed that a star's cosmic "weather" could render planets in their habitable zone uninhabitable.

“All stars rotate downward during their lifetime as they lose angular momentum, and as a result they become less active,” Farrish said. “We think that the Sun was more active in the past, and this may have affected the early chemical composition of the Earth's atmosphere. Therefore, thinking about how the higher energetic emissions of stars change over time is very important for exoplanet research.”

“In a broader sense, we take the models developed for the Sun and see how well they adapt to the stars,” Jones-Krull said.

The researchers decided to model what distant stars look like based on the limited data available. The rotation and flux of some stars were determined, as well as their classification (types F, G, K and M), which gave information about their sizes and temperatures.

They compared the properties of the Sun, a G-type star, through its Rossby number, a measure of stellar activity that combines its rotational speed with subsurface fluid flows that affect the distribution of magnetic flux on the star's surface, with what they knew about other steep stars. Their models suggest that each star's "space weather" works in much the same way, affecting conditions on their respective planets.

"The study suggests that the stars, at least cold, are not too different from each other," said Alexander. “From our point of view, Alison's model can be applied without fear or preference when looking at exoplanets around stars M, F or K, and of course, as well as other G stars.

“It also suggests something much more interesting: the process by which the magnetic field is created can be very similar in all cold stars. It's a bit surprising,”he said. This may include stars that, unlike the Sun, are convective down to the core.

“All Sun-like stars combine hydrogen and helium in their cores, and this energy is first carried by the radiation of photons to the surface,” said Jones-Krull. “But it gets into 60% to 70% of the path, which is too dark, so it starts to be exposed to convection. Hot matter moves from below, energy is radiated away, and colder matter falls back down."

“Stars with a mass less than a third of the solar mass do not have a radiation zone; they are convective everywhere,”he said. “Many of the ideas about how stars generate magnetic fields are based on the existence of a boundary between radiation and convection zones, so stars that do not have this boundary can be expected to behave differently.This article shows that in many ways, they behave exactly like the Sun, if adjusted to their characteristics."

“The most magnetically active stars are the ones we call saturated,” Farrish said. “At some point, the increase in magnetic activity ceases to show the associated increase in high-energy X-rays. The reason why dumping more magnetism onto a star's surface does not produce more radiation is still a mystery. Conversely, the Sun is in an unsaturated mode where we actually see a correlation between magnetic activity and energy radiation,”she said. "This happens at a more moderate level of activity, and these stars are of interest because they can provide a more favorable environment for planets."

“The bottom line is that observations that cover four spectral classes, including both fully and partially convective stars, can be reasonably well represented by a model based on the Sun,” said Alexander. "It also supports the idea that even if a star 30 times more active than the Sun may not be a G-class star, this is still reflected in Alison's analysis."

“We need to be clear that we are not modeling any particular star or system,” he said. "We say that the statistically magnetic behavior of a typical M star with a typical Rossby number behaves similarly to that of the sun, which allows us to estimate its potential impact on planets."

Jones-Krull said this model would be useful in many ways. “One of my areas of interest is the study of very young stars, many of which, like low-mass stars, are fully convective,” he said. “Many of them are surrounded by disc material and they still form planets. We think that their interaction is determined by the stellar magnetic field."

"Alison's simulations can be used to study the large-scale structure of highly magnetically active stars, which could then test some ideas about how these young stars and their disks interact."

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