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Register a new userTowards understanding dynamo action in M dwarfs
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Recent progress in observational studies of magnetic activity in M dwarfs urgently requires support from ideas of stellar dynamo theory. We propose a strategy to connect observational and theoretical studies. In particular, we suggest four magnetic configurations that appear relevant to dwarfs from the viewpoint of the most conservative version of dynamo theory, and discuss observational tests to identify the configurations observationally. As expected, any such identification contains substantial uncertainties. However the situation in general looks less pessimistic than might be expected. Several identifications between the phenomenology of individual stars and dynamo models are suggested. Remarkably, all models discussed predict substantial surface magnetic activity at rather high stellar latitudes. This prediction looks unexpected from the viewpoint of our experience observing the Sun (which of course differs in some fundamental ways from these late-type dwarfs). We stress that a fuller understanding of the topic requires a long-term (at least 15 years) monitoring of M dwarfs by Zeeman-Doppler imaging.
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We study the relation between stellar rotation and magnetic activity for a sample of 134 bright, nearby M dwarfs observed in the Kepler Two-Wheel (K2) mission during campaigns C0 to C4. The K2 lightcurves yield photometrically derived rotation periods for 97 stars (79 of which without previous period measurement), as well as various measures for activity related to cool spots and flares. We find a clear difference between fast and slow rotators with a dividing line at a period of ~10d at which the activity level changes abruptly. All photometric diagnostics of activity (spot cycle amplitude, flare peak amplitude and residual variability after subtraction of spot and flare variations) display the same dichotomy, pointing to a quick transition between a high-activity mode for fast rotators and a low-activity mode for slow rotators. This unexplained behavior is reminiscent of a dynamo mode-change seen in numerical simulations that separates a dipolar from a multipolar regime. A substantial number of the fast rotators are visual binaries. A tentative explanation is accelerated disk evolution in binaries leading to higher initial rotation rates on the main-sequence and associated longer spin-down and activity lifetimes. We combine the K2 rotation periods with archival X-ray and UV data. X-ray, FUV and NUV detections are found for 26, 41, and 11 stars from our sample, respectively. Separating the fast from the slow rotators, we determine for the first time the X-ray saturation level separately for early- and for mid-M stars.
Houdebine et al (2017: H17) combined CaII data with projected rotational velocities (v sin i) to construct rotation-activity correlations (RAC) in K-M dwarfs. The RAC slopes were used to argue that a transition between dynamo modes occurs at a spectral type between M2 and M3. H17 suggested that the dynamo transition corresponds to a transition to complete convection (TTCC). An independent study of GAIA data led Jao et al (2018) to suggest that the TTCC sets in near M3.0V, close to the H17 result. However, the changes in a star which cause TTCC signatures in GAIA data might not lead to changes in CaII emission at an identical spectral type: the latter are also affected by magnetic effects which depend on certain properties of convection in the core. Here, we use CaII emission fluxes in a sample of ~600 M dwarfs, and attempt to narrow down the transition from one dynamo mode to another: rather than relying on RAC slopes, we quantify how the CaII emission flux varies with spectral type to identify steps where the flux decreases significantly across a narrow range of spectral types. We suggest that the dynamo mode transition may be narrowed down to between M2.1 and M2.3. This is close to, but earlier than, the TTCC location identified by Jao et al (2018). We suggest that the transition in dynamo mode may be related to the existence of a small convective core which occurs for a finite time interval in certain low mass stars.
Helicity and alpha effect driven by the nonaxisymmetric Tayler instability of toroidal magnetic fields in stellar radiation zones are computed. In the linear approximation a purely toroidal field always excites pairs of modes with identical growth rates but with opposite helicity so that the net helicity vanishes. If the magnetic background field has a helical structure by an extra (weak) poloidal component then one of the modes dominates producing a net kinetic helicity anticorrelated to the current helicity of the background field. The mean electromotive force is computed with the result that the alpha effect by the most rapidly growing mode has the same sign as the current helicity of the background field. The alpha effect is found as too small to drive an alpha^{2} dynamo but the excitation conditions for an alphaOmega dynamo can be fulfilled for weak poloidal fields. Moreover, if the dynamo produces its own alpha effect by the magnetic instability then problems with its sign do not arise. For all cases, however, the alpha effect shows an extremely strong concentration to the poles so that a possible alphaOmega dynamo might only work at the polar regions. Hence, the results of our linear theory lead to a new topological problem for the existence of large-scale dynamos in stellar radiation zones on the basis of the current-driven instability of toroidal fields.
In the quiet Sun, magnetic fields are usually observed as small-scale magnetic elements, `salt and pepper, covering the entire solar surface. By using 3D radiative MHD numerical simulations we demonstrate that these fields are a result of local dynamo action in the top layers of the convection zone, where extremely weak `seed magnetic fields can locally grow above the mean equipartition field (e.g., from a $10^{-6}$ G `seed field to more than 1000 G magnetic structures). We find that the local dynamo action takes place only in a shallow, about 500 km deep, subsurface layer, from which the generated field is transported into deeper layers by convection downdrafts. We demonstrate that the observed dominance of vertical magnetic fields at the photosphere and the horizontal fields above the photosphere can be explained by multi-scale magnetic loops produced by the dynamo.
The phenomenon of solar torsional oscillations (TO) represents migratory zonal flows associated with the solar cycle. These flows are observed on the solar surface and, according to helioseismology, extend through the convection zone. We study the origin of the TO using results from a global MHD simulation of the solar interior that reproduces several of the observed characteristics of the mean-flows and magnetic fields. Our results indicate that the magnetic tension (MT) in the tachocline region is a key factor for the periodic changes in the angular momentum transport that causes the TO. The torque induced by the MT at the base of the convection zone is positive at the poles and negative at the equator. A rising MT torque at higher latitudes causes the poles to speed-up, whereas a declining negative MT torque at the lower latitudes causes the equator to slow-down. These changes in the zonal flows propagate through the convection zone up to the surface. Additionally, our results suggest that it is the magnetic field at the tachocline that modulates the amplitude of the surface meridional flow rather than the opposite as assumed by flux-transport dynamo models of the solar cycle.
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