5 R ratio

We have defined an R ratio to characterize the behavior of the Paschen series. We defined as such the ratio W(P17)/W(P12), because we cannot use P 18 because of its contamination with O I and P 11 because it is too near to the border of the observed region to be trustworthy.

  

Figure 1: The absolute magnitudes of the Ae and A type shell stars of our sample, based upon HIPARCOS parallaxes. The continuous line represents the ZAMS

Figure 1 is a plot of the R ratio in normal stars. The stars were taken from Andrillat et al. (1995). The ratios fall in three different regions for supergiants, giant and dwarfs, separated by the values 0.35 and 0.13. From Table 5 one can derive the ratios for the shell stars of our sample and verify that in the case of well visible shells, the ratios correspond to luminosity class III rather than V, where the spectral classifications would put the star. Since the Paschen lines are produced somewhere in the shell, this implies simply that the physical conditions there resemble more the atmosphere of a giant than that of a dwarf. A theoretical paper discussing the R ratio by Houziaux & Fremat is forthcoming.