4.2 Magnetism and Hα activity

The possibility to measure the surface magnetic fields in M stars from molecular FeH lines opens the opportunity to study the generation and consequences of magnetic fields in a large sample (see Table 2). The most frequently used FeH lines around 1 µm can be observed with near-infrared instrumentation, but also with spectrographs like HIRES or UVES that operate at optical wavelengths. With optical spectrographs, it is often possible to observe molecular FeH and simultaneously cover the most frequently used activity indicator in cool stars, the emission line of Hα. Chromospheric emission is known to be variable on timescales of minutes and strictly simultaneous observation of Hα together with the magnetic field is, therefore, particularly useful. Figure 20View Image shows the relation between normalized Hα luminosity log LHα∕Lbol and the average magnetic field Bf for the M dwarf measurements given in Table 2. A deeper discussion of this relation can be found in Reiners and Basri (2007, 2010Jump To The Next Citation Point).
View Image

Figure 20: Normalized Hα luminosity as a function of Bf . Filled black circles: Early-/mid-M type stars of spectral type M0 – M6; blue triangles: spectral type M7; green stars: spectral type M8; red upside down triangles: spectral type M9.

One can draw two interesting conclusions from the data shown in Figure 20View Image. First, in early-M dwarfs (≤ M6), the relation between magnetic field and chromospheric activity follows a curve similar to the rotation-activity relation; chromospheric activity grows with average field strength in the low-field regime (Bf ≤ 2000 G) but saturates at a critical field strength. The critical field strength in early M stars seems to be close to 2000 G. At this point, however, the data sample is rather sparse and uncertainties are high so that such conclusions can only be preliminary. This saturation – if existent – is different from the saturation of the field itself (at Ro ∼ 0.1). The rotation-activity relation implies that fields cannot be stronger than 3 – 4 kG in general. A saturation of chromospheric activity at 2000 G would mean that additionally, Hα emission saturates at even lower rotation rates when the field is sufficiently strong.

Figure 20View Image also contains data for cooler stars of spectral type M7 – M9. In these stars, the level of log LHα∕Lbol is lower on average than in hotter stars, a reason may be the growing atmospheric neutrality that weakens the coupling between the ionized atmosphere and magnetic fields hence rendering magnetic heating ineffective. Interestingly, there is a hint in Figure 20View Image that the field strength at which saturation occurs may grow to larger fields in cooler stars. In other words, cooler stars need stronger field strengths to generate the same level of activity than hotter stars, and the saturation is not limited by a fixed field strength but by a maximum level of chromospheric emission (logLH α∕Lbol).

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