Looking at flare activity in young M-dwarf stars, as we did in the last post, brings out a notable difference between these fast-spinning stars and stars like the Sun. Across stellar classifications from M- to F-, G- and K-class stars, there is commonality in the fusion of hydrogen into helium in the stellar cores. But the Sun has a zone at which energy carried toward the surface as radiative photons is absorbed or scattered by dense matter.
At this point, convection begins as colder matter moves downward and hot matter rises. This radiative zone giving way to convection is distinctive — stars in the M-class range, a third of the mass of the Sun and lower, do not possess a radiative core, but undergo convection throughout their interior.
Image: Interior structure of the Sun. Credit: kelvinsong / Wikimedia Commons CC BY-SA 3.0.
If we’re going to account for magnetic phenomena like starspots, flares and coronal mass ejections, we can come up with a model that fits stars with a radiative core, but fully convective stars might be expected to have a different kind of magnetic dynamo. What stands out in the data, however, is that the relationship between the star’s rotation and its magnetic activity appears the same for stars on both sides of what I might call the ‘convective divide.’ In both cases, the magnetic dynamo seems to be efficient despite the fact that M-dwarfs are fully convective.
Digging further into the subject is a new paper out of Rice University, where modeling of these phenomena examines the linkage between the rotation of stars and the behavior of their surface magnetic flux. The flux in turn governs the luminosity of the star at X-ray wavelengths, giving us a way to probe magnetic activity and its potential effects on planets in these systems. The paper explaining the new model has just run in The Astrophysical Journal. Lead author Alison Farrish comments on the implications over time as rotation periods change:
“All stars spin down over their lifetimes as they shed angular momentum, and they get less active as a result. 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.”
Image: Rice University scientists have shown that “cool” stars like the sun share dynamic surface behaviors that influence their energetic and magnetic environments. Stellar magnetic activity is key to whether a given star can host planets that support life. Credit: NASA.
Farrish and team used the Rossby number of our own star to model the behavior of other stars. This value measures stellar activity through the combination of rotational speed and subsurface liquid flows that influence how the magnetic flux is distributed on the stellar surface. Presumably the magnetic field in stars with a radiative zone is generated at the interface between the interior radiative region and the outer convective zone. The Rossby number relates the rotation of the star — determined through observation — to the internal convective activity of the star.
The results affirm that the mechanisms producing local ‘space weather’ are common across different stellar classes, meaning we can with some confidence examine planetary systems around M, F, G and K stars using the same model. The process generating a star’s magnetic field may thus turn out to be similar despite the presence, or lack, of a radiative core. Adds co-author Christopher Johns-Krull:
“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.”
Thus an M-dwarf with a Rossby number typical for its class shows magnetic behaviors close enough to the Sun for us to make predictions about their effect on its planets. The stellar magnetic field data are, in turn, affected by the activity cycle of individual stars, which the model does not include because this would demand lengthy observational study for each star. But from the perspective of magnetically active stars, the new model from Farrish and team can be applied to interactions within their systems.
We’re a long way from knowing whether M-dwarf systems like that at Proxima Centauri or the intriguing TRAPPIST-1 and L 98-59 could support living planets, but our models for their magnetic interactions can draw on what we see in our own Sun, despite its differences in age and stellar class. Refining that model for these systems will help us determine the most likely M-dwarf candidates for habitability.
The paper is Farrish et al., “Modeling Stellar Activity-rotation Relations in Unsaturated Cool Stars,” Astrophysical Journal Vol. 916, No. 2 (3 August 2021). Abstract.