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Astron. Astrophys. 326, 249-256 (1997)

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6. Discussion

In this paper we presented chromospheric models for two M dwarf stars considered as "basal" stars due to the very low level of chromospheric activity, to be compared with the model for a very active, flare star, computed in Paper I. These models are not based on a single spectral feature, but on many lines corresponding to three different atoms, the continuum in a broad wavelength range, from the ultraviolet to the infrared, and the Mg II h and k flux. All in all, the agreement between models and observations is remarkably good.

As can be seen in Fig. 1, the main difference between the two dM stars can be found in the structure of the initial chromospheric rise, just above the temperature minimum. This explains the higher intensity of the K1 minimum for Gl 628. However, this difference is not reflected in a larger cooling rate at these heights, and is not important from an energetic point of view.

There is a slight difference (about 100 K at [FORMULA]) between the high photosphere of Gl 628 and the others, to fit the narrower K1 of Gl 628. On the other hand, this modification made the fit to the Na D lines worse for Gl 628 than for Gl 588. In general, the overall fit for Gl 628 is not as good as for Gl 588, suggesting that there might be some problems with the modelling of this star. However the difference in the high photosphere of Gl 628 does not affect the continuum emission that originates in lower layers, and we can conclude that the observations are compatible with the assumption of a similar photosphere for the three stars.

Considering that both Gl 588 and Gl 628 were classified as "basal" stars, it should be noted that even in these cases there is a marked chromosphere, well above a radiative equilibrium atmosphere. This confirms the conclusions of other authors (e.g. Cram & Mullan 1979; Giampapa & Liebert 1986).

Comparing the dM and the dMe stars, we can see that, as expected, the presence of the Balmer lines in emission (and the much larger Ca II and Mg II fluxes) is determined by the structure of the chromosphere, much hotter for AD Leo than for the other two stars. This, in turn, implies a much larger electron density, and a closer coupling of the source function with the Planck function. In fact, the model for AD Leo is 2000 K hotter than the ones for the dM stars, at [FORMULA].

From an energetic point of view, it can be seen comparing the results presented here with the ones in Paper I, that the additional energy required to transform a dM star into an active star is needed in the high chromosphere, just below the transition region. The mid-chromosphere, in turn, is automatically heated by the energy emitted above it, and reabsorbed in this region, in particular, in the Mg II and Balmer lines.

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998