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Appendix A
Line-strength indices of a sample of 17 hot stars (T > 10000 K). Toward an extension of the Lick Fitting Functions

The aim of the present work (cf. also Papers II and III) is that of looking for the presence of recent star formation events connected to interaction, that can have involved only a few percentages of the total mass of the galaxies. Both observations and numerical/hydrodynamical simulations (see for references Barnes & Hernquist 1992) indicate that induced star formation is a possible effect of the interaction.

Young stellar populations, if present, are made recognizable in a galaxy by the presence of very hot stars (T > 10000 K). These kinds of stars are under-represented in the sample from which W92 derived his fitting functions. Consequently, the knowledge of the indices behaviour in the range of stellar temperatures greater than 10000 K is a necessary step in the construction of suitable fitting functions.

  
\begin{figure}
\includegraphics [width=8cm]{1429f8.eps}\end{figure} Figure 8: Fitting functions of W92 for gravity g=4.5 and different metallicities (lines: solid line [Fe/H]=-2, short dash [Fe/H]=-1, medium dash [Fe/H]=0, long dash [Fe/H]=1). Full squares represent our measures on a sample of 17 hot stars, listed in Table 14. Note that in the case of the H$\beta$index, fitting functions from W92 do not match our observations
We have observed 17 hot stars (whose parameters are listed in Table 14) in order to test the Lick standard fitting functions in the high temperature regime. Figure 8 shows a comparison between Lick fitting functions for some indices and stellar data collected in the present work. In the case of the H$\beta$ index, the only classic age indicator in the "red'' part of the spectrum, it is quite evident that the standard fitting functions do not match our observations. This suggests that the extrapolation starting from lower values of stellar temperatures towards higher ones need to be better investigated on a larger sample. We have used our new data to construct a tentative extension of the Lick Group fitting functions for the 16 common "red'' indices. Table 13 lists the coefficients of these new extensions, obtained through a linear interpolation of the values starting from log $T_{\rm e}=4$ for the different metallicities and gravities.

Acknowledgements

ML acknowledges the kind hospitality of the Brera Observatory and of the Padova Observatory during her PhD Thesis. We thank Ginevra Trinchieri for the careful reading of the manuscript. ML, AB and CC acknowledge the support by the European Community under TMR grant ERBFMRX-CT96-0086.


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