THE M1-INDEX CURVE IN PULSATING STARS

We have carried out a systematic study of the behaviour of the delta-m1 metallicity parameter for SX Phe and high amplitude delta-Sct (pop. I) stars. It is shown that it has a different behaviour for the two samples of stars. The delta-m1 parameter increases (SX Phe) or decreases (delta-Sct) towards minimum light for these two types of variables. Moreover, the m1-index variation is larger when the metal abundance of the star is smaller. An analogous effect is found when we analyse other photometric indices such as DELTA-S of Preston, (k-b)2 of Jones or DELTA[B-L] of Walraven applied to RR Lyr, SX Phe and high amplitude delta-Sct stars. This behaviour, in uvby-beta photometry, can be explained by means of the theoretical indices calculated for different chemical compositions. The m1-index variation is related to the atmospheric parameters T(e) and log g and with the chemical composition. Fixing T(e) and log g, the amplitude of the m1-index curve is larger when the metal abundance is smaller, although an inversion around [Me/H] = 0.5 is found. It is also shown that the m1-index variation is larger as temperature or gravity increases. This dependence is larger when the metallicity is larger, while for very low metal abundances the m1-index variation is nearly independent from both temperature and gravity.