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Astron. Astrophys. 358, 575-582 (2000)

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4. Comparison with the observations - the case of CR Dra

On May 21 1997, we obtained high resolution spectra of the active dM0e star Gl 616.2 (CR Dra) using the 1.9m telescope at the Observatoire de Haute Provence (OHP) and the fibre fed spectrograph ELODIE (Baranne et al. 1996). This spectrograph provides a spectral resolution of [FORMULA] throughout the visible part of the spectrum (between 3900 and 6700 Å). The OHP is situated at a relatively low altitude, and seeing is very rarely better than 2 arc sec (which is the diameter of the fibres).

We show in Fig. 5 the latest solar results based on measurements of both a `quiet' Sun region and an active region, superimposed with curves for [FORMULA] and [FORMULA]. These results would suggest that perhaps [FORMULA] from [FORMULA] to [FORMULA] is an appropriate approximation while from 100,000 K to 300,000 K it is constant at [FORMULA]30 km s-1. For the lower photospheric region, we have used the results as derived in Sect. 2.2 which implies a level of [FORMULA] 5 km s-1 for the photospheric layers. As shown in Fig. 2, changes are only apparent in the line profiles for values of greater than or equal to 5 km s-1.

[FIGURE] Fig. 5. The non-thermal velocity as determined by Teriaca et al. (1999) for an active region (squares) plus a `quiet' sun region (circles). The lines corresponds to [FORMULA] and [FORMULA].

To compare our modelling results to the observations, we build a grid of models with the non-thermal velocity equal [FORMULA]. In this grid of models we change the position of the transition region from log mTR= -4.8 to -3.8 and the position of the temperature minimum from log m0=-2.5 to -1.0. The comparison for H[FORMULA] and Na I D lines are given in Fig. 6. We have normalized the fluxes to the continuum level at 5845 Å and 6540 Å for Na I D and H[FORMULA] lines respectively. One can see that the Sodium doublet is not fitted perfectly, especially in the wings. The main reason for this discrepancy is in the treatment of background opacities (see Andretta et al. 1997). Additional opacities would lower the continuum level and consequently increase the relative fluxes.

[FIGURE] Fig. 6. The comparison between the observed and calculated fluxes in H[FORMULA] (left panel) and Na I D lines (right panel) for CR Dra. The top panels show the case of log m0=-2.0, middle panel for log m0=-1.5 and bottom panel log m0=-1.0. The position of the transition region is as follows: log TRm=-4.8 (solid line), -4.6 (dotted line), -4.4 (dashed line), -4.2 (dot-dashed line), -4.0 (dot-dot-dashed line) and -3.8 (long dashed line).

The Sodium fits however suggest that the position of the temperature minimum is around log m0=-1. Using that value we estimate from the H[FORMULA] fits, that the transition region is at log mTR= -4.3. These values have to be confirmed with a better treatment of molecular opacities.

From the various fits one can see that the chosen value of 0.7 cs, which influences the width of the emission cores, agrees quite well with the observed ones for both H[FORMULA] and Na I D.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000