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