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The Question

(Submitted November 01, 2005)

Do sunspots exist on bright stars, brown dwarfs, and non-solar type stars?

I have a difficulty in finding this information. In forums, they say they don't, while artworks show that they do.

It is known that G stars through M stars have sunspots.

Do W, O, B, A, F stars, Carbon Stars, and S stars have sunspots? Do Brown Dwarfs, or late - M, L, and T stars have sunspots?

The Answer

This is an interesting question that is still much debated among astronomers and one that generated a lot of interest around here (which is part of the reason why this reply took so long). Basically starspots are generated by the interaction of the stellar differential rotation (different parts rotating at different rates) with the surface magnetic field. Because the field is frozen into the photosphere, as the star differentially rotates the field becomes twisted and the magnetic pressure increases locally, which causes a decrease in the local temperature and an apparently dark spot. For this to happen you need a magnetic field, which requires an active stellar dynamo, which usually requires a convective stellar envelope around a radiative core.

Stars more massive than a few solar masses (O, B, and A) have convective cores and radiative envelopes, so that they should have very weak or non-existent magnetic fields and few if any spots. Similarly, very low mass stars (like brown dwarfs) are thought to be nearly entirely convective, so that the dynamo effect should be fairly weak, generating a weak field and few spots. Of course Jupiter has spots, so that probably brown dwarfs do have spots, but these spots are not the magnetically-generated variety. WR stars don't have observable spots since you can't see down to the photosphere because of their dense winds. Not sure whether C stars or S stars have been shown to have spots...

What happens in the hottest stars and in the coolest stars like brown dwarfs is particularly being debated right now.

In brown dwarfs, variability has been detected in a fraction of them (these observations are hard because brown dwarfs are so faint), and for various observational and theoretical reasons, this variability is generally believed to be due to 'weather', i.e., local variations in the atmospheric opacity (what you and I would call clouds), rather than starspots, although in a few cases (e.g., 2MASS J1155395-372735, Koen 2003 Monthly Notices of the Royal Astronomical Society, vol 346, page 473) the latter may be a viable explanation.

In hot stars, one of the big paradigm shifts in this field is the recognition that magnetic fields can play a role in their outer atmospheres. Since these star lack convective outer envelopes it's a bit of a puzzle how the magnetic fields are produced and persist. They are likely global in nature e.g., mostly dipolar and quadrupolar, and don't seem to produce localized cool starspots as are seen in the cooler stars with convective envelopes.

The Carbon and S stars are usually quite variable anyway, and their rotation periods would typically be years, so it would be hard to 'see' starpots on them (and because of their slow rotation their dynamos would be very weak anyway). They also likely have a pattern of large-scale hot and cool convective cells on their surfaces, such as have been claimed to exist on the M supergiant Betelgeuse which would also confuse things...


Hans Krimm, Mike Corcoran, and Steve Drake
for Ask an Astrophysicist

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