MagIcS
convective vs interface dynamos in low-mass stars

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Dynamo processes in cool stars

Low-mass stars like our Sun host extended convective envelopes in which dynamo processes continuously generate magnetic fields. Conventional dynamo theories - based mainly on solar observations - suggest that magnetic-field generation occurs mainly in an interface layer between the convective envelope and radiative core of a cool solar-like star; magnetic fields can however also be produced within the bulk of the convective zone itself. By concentrating on three specific stellar classes, namely solar twins, fully convective dwarfs and F dwarfs, we aim at finding out which stellar parameters mostly control the type of dynamo (interface vs distributed) that a cool star succeed at triggering.

Solar twins and magnetic cycles

Magnetic cycles in stars very similar to the Sun (regarding mass, rotation rate and age) can be investigated by the regular monitoring of their Zeeman signatures. By observing large-scale magnetic geometries of strict solar analogues, we can disclose the full range of magnetic configurations that solar twins (and therefore the Sun itself) are able to produce; comparing them with predictions of dynamo models yields new constraints that the Sun cannot provide. Monitoring a limited stellar sample over several years brings further information on potential magnetic cycles. For a Sun-like dynamo, we expect to observe polarity reversals of the large-scale magnetic field in at least some of stars in the sample. We can also estimate how standard the present activity level of the Sun is, and investigate how much time the Sun spends in low (ie Maunder minimum-like) activity states.

Fully-convective stars

Fully-convective M dwarfs clearly manage to produce strong fields without hosting a radiative/convective interface layer. ESPaDOnS observations of several late M dwarfs disclosed that fully-convective stars are able to produce long-lived, large-scale, highly-axisymmetric poloidal fields without differential rotation (Donati et al 2006, Science 311, 633). Such fields are fairly different from those of higher-mass solar-type stars and are at odds with predictions of the modern MHD models. By studying how convective zones are capable of producing large-scale mainly-axisymmetric poloidal fields, we can investigate the potential role of the solar convection zone per-se in producing the large-scale magnetic field of the Sun. Examining in details how large-scale magnetic topologies evolve as radiative cores start to build up (around spectral type M3) should in particular reveal key information on how interface and distributed dynamos respectively participate in producing stellar magnetic fields of cool stars.

F stars

F stars represent another extreme of dynamo generation. Their convection zones is very shallow and located just below the stellar photosphere, being an exact equivalent of the solar interface layer where the conventional solar dynamo is supposed to take place. F stars therefore represent an ideal laboratory to study directly magnetic topologies generated through interface dynamos without being filtered/modified by the additional presence of a thick upper convection zone. Observations have shown that F stars generally host very strong differential rotation and trigger mostly toroidal magnetic topologies. These observations can help us better understand the specific role of the solar interface layer in producing the large-scale magnetic field of the Sun.

Binary systems and giant stars

By comparing magnetic fields of binary and evolved stars with those of single dwarfs of similar properties (eg convective depths and rotation rate), we can investigate how tidal effects and evolution affect dynamo processes.

Related threads

The results obtained in this thread are also relevant to young low-mass protostars; they will help in determining the origin of magnetic fields in newly born low-mass stars and explain, eg the absence of a X-ray vs rotation rate relationship (Feigelson et al 2003, ApJ 584, 911). This research also shares obvious links with that on magnetic fields of planet-hosting stars by allowing a statistically significant comparison of magnetic topologies in stars with and without close-in giant planets. No Large Program with ESPaDOnS was submitted on this theme.

Core team and collaborators

Thread coordinators: S Marsden, P Petit, T Forveille, P Charbonneau

Agency/Country	Core team	Collaborators
F	Aurière, Ballot, Baraffe, Brun, Delfosse, Dintrans, Donati, Forveille, Lèbre, Morin, P Petit, Zahn	Aulanier, Chabrier, Josselin, deLaverny, Lopez-Ariste, Mathias, Palacios, Paletou, Ramirez, Semel
O	Barnes, Berdyugina, Cameron, Hussain, Jardine, Jeffers, Marsden, Reiners, Solanki	Brandenburg, Keller, Küker, Martin, Rüdiger, Schmitt
C	Charbonneau, Dobler, Milone, Mochnacki, Rucinski, Walker
T	Lim, Phan-Bao
H	Harrington	Kuhn
US	Browning, Johns-Krull, Saar, Toomre, Valenti	Basri, Feigelson, Mohanty