“Cool” stars might not be so unique

Stars scattered around the cosmos look different, but they may look more alike than previously thought, according to researchers at Rice University.

New modeling work by scientists at Rice shows that “cold” stars like the sun share dynamic surface behaviors that influence their energetic and magnetic environments. This stellar magnetic activity is essential to know if a given star harbors planets that could harbor life.

The work of postdoctoral researcher Rice Alison Farrish and astrophysicists David Alexander and Christopher Johns-Krull are featured in a study published in The Journal of Astrophysics. The research links the rotation of cold stars to the behavior of their surface magnetic flux, which in turn drives the star’s coronary x-ray brightness, in a way that could help predict how magnetic activity affects exoplanets. of their systems.

The study follows another by Farrish and Alexander which showed that a star’s spatial “weather” can make planets in their “Goldilocks zone” uninhabitable.

“All stars slow down over the course of their lives as they lose their angular momentum and become less active as a result,” Farrish said. “We believe the sun in the past was more active and this could have affected the early atmospheric chemistry of Earth. So it’s quite important to think about how the higher energy emissions from stars change over long periods of time. periods of time. “

“More broadly, we take models that were developed for the sun and see how well they adapt to the stars,” Johns-Krull said.

Researchers set out to model what distant stars look like based on the limited data available. The spin and flux of some stars were determined, as well as their classification – the F, G, K and M types – 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 its fluxes of subterranean fluids that influence the distribution of magnetic flux at the surface. of a star, along with what they knew from other cool stars. Their models suggest that each star’s “space weather” works in much the same way, influencing conditions on their respective planets.

“The study suggests that stars, at least cold stars, are not too different from each other,” Alexander said. “From our perspective, Alison’s model can be applied without fear or favor when we look at exoplanets around M, F or K stars, as well, of course, as other G stars.

“It also suggests something much more interesting to established stellar physics, which is that the process by which a magnetic field is generated can be quite similar in all cold stars. It’s a bit of a surprise,” he said. -he declares. This could include stars which, unlike the sun, are convective to the core.

“All stars like the sun merge hydrogen and helium in their nucleus and this energy is first carried by photon radiation to the surface,” Johns-Krull said. “But it gets to an area about 60% to 70% of the distance that is just too opaque, so it starts to convect. Hot matter moves down, energy radiates, and matter more. cold falls.

“But stars with less than a third of the sun’s mass don’t have a radiative zone; they’re convective everywhere,” he said. “Many ideas of how stars generate a magnetic field rely on the existence of a boundary between the radiative and convection zones, so one would expect stars that do not have this boundary to Behave differently. This article shows that in many ways, they behave like the sun, once you adjust to their own peculiarities. “

Farrish, who recently received his PhD from Rice and is soon starting a postdoctoral research mission at NASA’s Goddard Space Flight Center, noted that the model only applies to unsaturated stars.

“The most magnetically active stars are the ones we call ‘saturated’,” Farrish said. “At some point, an increase in magnetic activity ceases to show the associated increase in high-energy X-ray emissions. The reason why dumping more magnetism on the star’s surface doesn’t give you more d ’emissions remain a mystery.

“Conversely, the sun is in the unsaturated regime, where we see a correlation between magnetic activity and energy emission,” she said. “It happens at a more moderate level of activity, and these stars are interesting because they could provide more hospitable environments for the planets.”

“The bottom line is that the observations, which span four spectral types including both fully and partially convective stars, can be reasonably well represented by a model generated from the sun,” Alexander said. “It also reinforces the idea that while a star 30 times more active than the sun may not be a G-class star, it is still captured by the analysis performed by Alison.”

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

A critical wildcard is the activity cycle of a star, which cannot be incorporated into models without years of observation. (The sun’s cycle is 11 years, as evidenced by sunspot activity when its magnetic field lines are most distorted.)

Johns-Krull said the model will still be useful in many ways. “One of my areas of interest is the study of very young stars, many of which are, like low-mass stars, fully convective,” he said. “Many of them are surrounded by disks and still form planets. The way they interact is mediated, we believe, by the stellar magnetic field.

“So Alison’s modeling work can be used to learn more about the large-scale structure of very magnetically active stars, and this can then allow us to test some ideas about how these young stars and their disks interact. “

Minjing Li, a visiting undergraduate student at China University of Science and Technology, is co-author of the article. Alexander is professor of physics and astronomy and director of the Rice Space Institute. Johns-Krull is a professor of physics and astronomy.