Rice models infer small stars share similar dynamics to our sun, key to
planet habitability.
Stars scattered throughout the cosmos look different, but they may be more
alike than once thought, according to Rice University researchers.
New modeling work by Rice scientists shows that “cool” stars like the sun
share the dynamic surface behaviors that influence their energetic and
magnetic environments. This stellar magnetic activity is key to whether a
given star hosts planets that could support life.
The work by Rice postdoctoral researcher Alison Farrish and astrophysicists
David Alexander and Christopher Johns-Krull appears in a published study in
The Astrophysical Journal. The research links the rotation of cool stars
with the behavior of their surface magnetic flux, which in turn drives the
star’s coronal X-ray luminosity, in a way that could help predict how
magnetic activity affects any exoplanets in their systems.
The study follows another led by Farrish and Alexander that showed a star’s
space “weather” may make planets in their “Goldilocks zone” uninhabitable.
“All stars spin down over their lifetimes as they shed angular momentum, and
they get less active as a result,” Farrish said. “We think the sun in the
past was more active and that might have affected the early atmospheric
chemistry of Earth. So thinking about how the higher energy emissions from
stars change over long timescales is pretty important to exoplanet studies.”
“More broadly, we’re taking models that were developed for the sun and
seeing how well they adapt to stars,” said Johns-Krull.
The researchers set out to model what far-flung stars are like based on the
limited data available. The spin and flux of some stars have been
determined, along with 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 speed of rotation
with its subsurface fluid flows that influence the distribution of magnetic
flux on a star’s surface, with what they knew of 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 cool stars — are not too
dissimilar 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 or F or K stars, as well, of course, as other G stars.
“It also suggests something much more interesting for established stellar
physics, that the process by which a magnetic field is generated may be
quite similar in all cool stars. That’s a bit of a surprise,” he said. This
could include stars that, unlike the sun, are convective down to their
cores.
“All stars like the sun fuse hydrogen and helium in their cores and that
energy is first carried in the radiation of photons toward the surface,”
Johns-Krull said. “But it hits a zone about 60% to 70% of the way that’s
just too opaque, so it starts to undergo convection. Hot matter moves from
below, the energy radiates away, and the cooler matter falls back down.
“But stars with less than a third of the mass of the sun don’t have a
radiative zone; they’re convective everywhere,” he said. “A lot of ideas
about how stars generate a magnetic field rely on there being a boundary
between the radiative and the convection zones, so you would expect stars
that don’t have that boundary to behave differently. This paper shows that
in many ways, they behave just like the sun, once you adjust for their own
peculiarities.”
Farrish, who recently earned her doctorate at Rice and begins a postdoctoral
research assignment at NASA’s Goddard Space Flight Center soon, noted the
model applies only to unsaturated stars.
“Conversely, the sun is in the unsaturated regime, where we do see a
correlation between magnetic activity and energetic emission,” she said.
“That happens at a more moderate activity level, and those stars are of
interest because they might provide more hospitable environments for
planets.”
“The bottom line is 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 even though a star that is 30 times more active
than the sun may not be a G-class star, it’s still captured by the analysis
that Alison has done”.
“We do have to be clear that we’re 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 in a similar
fashion to that of the sun which allows us to assess its potential impact on
its planets.”
A critical wild card is a star’s activity cycle, which can’t be incorporated
into the models without years of observation. (The sun’s cycle is 11 years,
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 studying very young stars, many of which are, like
low-mass stars, fully convective,” he said. “Many of these have disc
material around them and are still forming planets. How they interact is
mediated, we think, by the stellar magnetic field.
“So, Alison’s modeling work can be used to learn about the large-scale
structure of very magnetically active stars, and that can then allow us to
test some ideas about how these young stars and their disks interact.”
Reference:
Modeling Stellar Activity-rotation Relations in Unsaturated Cool Stars by
Alison O. Farrish, David Alexander, Christopher M. Johns-Krull and Minjing
Li, 3 August 2021, The Astrophysical Journal.
Tags:
Space & Astrophysics