Some earlier modelling, e.g. Houdebine et al. (1995) suggested that the level of micro-turbulence would not have an influence on the Hydrogen line profiles. This is clearly not the case as shown in the previous sets of figures. Of course one could argue that small scale motions with velocities close or even higher than the sound velocity are physically doubtful because of the generation of shocks. We think that in the light of new solar observations (SOHO) and the richness of small scale motions, we have to examine even these high levels of non-thermal velocities.
From the modelling it is clear that there is a lowering of the electron density with increasing turbulent velocity. Also, with increasing turbulent velocity, H shows a deeper self-reversal, while H changes from emission to absorption. In Na I , there are changes in the shape of the line wings with the lines becoming narrower with increasing turbulent velocity, particularly for values greater than 5 km s-1 in the photosphere.
Fairly good results were achieved for fitting the observed profiles of CR Dra. From our modelling we conclude that the model with transition region at log mTR=-4.3 and a temperature minimum at log m0=-1.0 gives the best results for H , where Na I D is used to estimate the position of the temperature minimum.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000