I am applying what we know about the Sun to other solar-type stars, to model surface patterns of activity on stars more active than the Sun. Here are some results below (backward in time).

How spots survive on active suns, amid facular cannibalism

Activity-related brightening (positive) or dimming (negative), as a function of the mean chromospheric activity level. Blue and orange lines show the numerical simulation results for axial inclinations of 90˚ and 0˚. The stellar sample is from Radick et al. (2018), based on observations made at Lowell and Fairborn observatories. Black stars have effective temperatures ±200 K around the solar value and with relative brightening uncertainty below 0.01. Grey stars are the rest of the sample. The grey-shaded band is the posterior distribution of a Bayesian linear regression to solar-like sample using Gaussian priors for a quadratic function.

Activity-related brightening (positive) or dimming (negative), as a function of the mean chromospheric activity level. Blue and orange lines show the numerical simulation results for axial inclinations of 90˚ and 0˚. The stellar sample is from Radick et al. (2018), based on observations made at Lowell and Fairborn observatories. Black stars have effective temperatures ±200 K around the solar value and with relative brightening uncertainty below 0.01. Grey stars are the rest of the sample. The grey-shaded band is the posterior distribution of a Bayesian linear regression to solar-like sample using Gaussian priors for a quadratic function.

Solar-type stars undergo a change in their patterns of variability, as they get older and become less active. Such a low-activity star is our Sun, becoming slightly brighter when more active (every decade in the course of its activity cycle), due to a slight overcompensation of bright faculae to dark spots at its visible surface. Departing from the solar activity level towards more active solar-like stars, spots start to dominate faculae when the star gets more active. In this study, led by Nina-Elisabeth Nèmec (MPS, now at Uni Göttingen), we explained for the first time why this happens. By carrying out surface flux transport simulations at various activity levels (bipolar region emergence frequencies), we showed that, as the star gets more and more active, magnetic flux cancellation gets more efficient among the active-region network (responsible for faculae to appear near the solar/stellar limb) than for the spots, which occupy a smaller area fraction of active regions. As the flux emergence rate increases, facular cancellation dominates over spot cancellation, making stellar variability dominated by spots. In short, spots survive, while faculae kill each other! Let me speculate a bit: this finding is also an indication that ‘large starspots’ inferred on active stars are not likely to be big, monolithic monsters, but rather, constellations of sunspot-size spots extended over large portions of the star. For otherwise, spots would also eat up each other.

Reference

Nèmec, N.-E., Shapiro, A.I., Işık, E., Sowmya, K., Solanki, S.K., Krivova, N.A., Cameron, R.H., Gizon, L. 2022, *Astrophys. J. Lett.* 934, L23

Predicting astrometric jitter for Sun-like stars: fast-rotating suns

Application #3 of the FEAT model

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Spot distribution from the FEAT simulation of a star that rotates 8 times faster than the Sun and also 8 times more active. Left: no nesting applied. Middle: active-longitude-type nesting. Right: Strong free nesting with a probability of 99%.

Spot distribution from the FEAT simulation of a star that rotates 8 times faster than the Sun and also 8 times more active. Left: no nesting applied. Middle: active-longitude-type nesting. Right: Strong free nesting with a probability of 99%.

Absolute displacements of the photocentre owing to combined action of stellar magnetic activity and an Earth-mass planet orbiting the star, using Gaia-G passband and at an inclination of 60˚.

Absolute displacements of the photocentre owing to combined action of stellar magnetic activity and an Earth-mass planet orbiting the star, using Gaia-G passband and at an inclination of 60˚.

Young suns that rotate much faster than our Sun are interesting objects. They are much more active than the Sun. Also, the surface distribution of activity is different: we expect much more magnetic flux accumulated around the polar regions. Hence, the astrometric jitter signal we expect from those stars somewhat differs from that of a solar-like star (see the story just below this one). Using the FEAT platform, our team, led by Sowmya Krishnamurty (MPS), simulated astrometric jitter of such rapidly rotating active stars, as would be observed by Gaia and JASMINE (planned launch in 2028 by JAXA). We show that one can actually sense the increase of activity level with the rotation rate, along with the distribution of activity over the surface (eg, whether it is highly clumped or not). We also find that the jitter is dominated by spots and that faculae become inefficient at these rotation rates, owing to the high activity levels.

Reference

Sowmya, K., Nèmec, N.-E., Shapiro, A.I., Işık, E., Krivova, N.A., Solanki, S.K. 2022, *Astrophys. J.* 934, 146

Predicting astrometric jitter for Sun-like stars: effects of inclination, metallicity, and active-region nesting

Application #2 of the FEAT model

FEAT-based simulations of astrometric jitter for solar-like nesting (left column), a free-nesting degree of 90% (middle column) and 99% (right column). From top to bottom, the axial inclination amounts to 90˚, 60˚, and 0˚.

FEAT-based simulations of astrometric jitter for solar-like nesting (left column), a free-nesting degree of 90% (middle column) and 99% (right column). From top to bottom, the axial inclination amounts to 90˚, 60˚, and 0˚.