COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1645 Konkoly Observatory Budapest 1979 July 24 POSSIBLE HETEROGENEITY OF THE delta Sct COMPLEX The relation between the normal A, the metallic A_m and the variable stars of delta Sct type is theoretically examined by Vauclair (1976). In this work it is supposed that the normal A and the delta Sct stars come from the stable A_m phase. The time scale of the Am phenomenon is of the order of 1.5x10^6 years and it is possible to reach it on different stages of evolution, since it may be repeated many times at the same star (Vauclair, 1976). All this calls forth the question about the homogeneity of the delta Sct complex, in spite that the boundaries, in which their physical characteristics change, are narrow. Nevertheless, still on their setting apart as a separate group of variables by Eggen (1956), attempts are made to differenciate groups in the complex of delta Sct variables (Leung, 1970 - two groups with different masses, in different stages of evolution; Frolov, 1972 - two groups with different periods; Breger and Bregman, 1975 - two groups with different temperatures, different pulsation constants and different period-luminosity dependences; Dworak and Zieba, 1975 - two groups with different luminosities). On the basis of the systematized photometric data made in the Stromgren system published up to the middle of 1978 for about 120 variable stars of delta Sct type we decided to examine once more the question of the existence of sub-groups among them, which would allow the classification of some problems related to the evolution of these variables. The comparison of the effective temperatures T_eff and the absolute stellar magnitudes M_v for 65 stars of delta Sct type, for which there are calculated pulsation masses (Baglin et al., 1973; Breger, Bregman, 1975; McMillian et al., 1976; Kurtz, 1977), is shown in Fig.1. The analysis of the Figure using the [FIGURE 1] data of the pulsation masses of the stars shows that the masses increase in the direction of lower temperatures and higher luminosities. Therefore the stars of the "cooler" group have masses on the average by 0.5 M_Sun higher than the masses of the stars of the "hot" group (Breger and Bregman, 1975). Several stars which make an exception (tau Peg, delta Sct, HR 7501, 28 And, HR 8006, 26 Ari, HD 73798), have anomalies in their chemical composition, which are expressed in the increased abundance of metals from the group of iron. Besides, the errors on defining the pulsation masses reach up to 30 percent (Tsvetkov, 1977). The comparison of the amplitudes of the light change for the two groups after Breger and Bregman (1975) shows on the average a three-times higher average amplitude for the stars with T_eff <= 7 800d in comparison to the stars with T_eff >= 7 800d The position of the delta Sct stars on the diagram log g - T_eff (Fig.3) depends on their chemical composition (Lub, 1977). The stars with solar abundances of elements fall near the main sequence, built for the stars of the Hyades cluster (Stromgren, 1963). The stars with metallic deficit, such as the RR Lyr type stars,are grouped far from the main sequence, in the region of the giants with low log g. In this diagram the variable stars of delta Sct type [FIGURE 2] [FIGURE 3] are divided anew in two groups with a limit of about 7 800d, falling around the main sequence. In conclusion we may summarize confirming Leung's inferences with richer material: 1. The group of the "cool" delta Sct stars with T_eff <= 7 800d consists of stars with masses of about 2.1 M_Sun, which pulsate mainly in the basic or first harmonic frequency. They have, on the average, longer periods and larger amplitudes of light change. They are in their stage of evolution on the way to the giant branch crossing the instability strip for the first time. 2. The group of the "hot" delta Sct stars consists of stars with masses of about 1.6 M_Sun, which pulsate mainly in the second overtone or the first one. They are on the main sequence or descend from it. 3. The existence of two groups of delta Sct stars is a proof that the phenomenon "variability" of delta Sct type may come into being at different stages of stellar evolution. P. NIKOLOVA A. NIKOLOV G. MOMCHEV Department of Astronomy Astronomical Observatory University of Sofia Yambol References: Baglin, A., Breger, M., Chevalier, C., Hauck, B., Le Contel, J.M., Sareyan, J., Valtier, C., 1973, Astron.and Astroph., 23, 221 [BIBCODE 1973A&A....23..221B ] Breger, M., Bregman, J., 1975, preprint Chevalier, C., 1971, Astron. and Astroph., 14, 24 [BIBCODE 1971A&A....14...24C ] Dworak, T.Z., Zieba, S., 1975, I.B.V.S. No. 1005 Eggen, O.J., 1956, Publ.Astron.Soc.Pacific, 68, 235 [BIBCODE 1956PASP...68..238E ] Frolov, M.S., 1972, Astron.Zirk., No. 671, 3 [BIBCODE 1972ATsir.671....3F ] Kurtz, D., 1977, Publ.Astron.Soc.Pacific, 89, 50 [BIBCODE 1977PASP...89...50K ] Leung, K.C., 1970, Astron. J. 75, 643 [BIBCODE 1970AJ.....75..643L ] Lub, J., 1977, Dissertation, Leiden [BIBCODE 1977PhDT.......159L ] McMillian, R.S., Breger, M., Ferland, G.J., Loumos, G.L., Publ.Astron.Soc. Pacific 88, 495 [BIBCODE 1976PASP...88..495M ] Stromgren, B., 1963, in "Basic Astronomical Data", ed. by K.Aa. Strand, The University of Chicago Press, Chicago and London Tsvetkov, Ts., 1977,. Dissertation, Moscow Vauclair, G., 1976, Astron. and Astroph., 50, 435 [BIBCODE 1976A&A....50..435V ]