Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1977
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977apj...218..633b&link_type=abstract
Astrophysical Journal, Part 1, vol. 218, Dec. 15, 1977, p. 633-653.
Astronomy and Astrophysics
Astronomy
260
Cepheid Variables, Galaxies, Local Group (Astronomy), Magellanic Clouds, Star Distribution, Stellar Evolution, Stellar Models, Andromeda Galaxy, Chemical Composition, Supernovae
Scientific paper
This paper investigates the effect that a variation in chemical composition has on the evolution of stars in the Cepheid domain as well as how this variation affects the resulting frequency-period distribution. A total of 12 evolutionary models are presented for five different chemical compositions, and pulsation theory is used in defining the instability strip and other relevant relationships. Theoretical composition-dependent frequency-period distributions are constructed and compared with the distributions observed in the Galaxy, the Large and Small Magellanic Clouds, M31, NGC 6822, M33, and IC 1613. The results show that: (1) either a decrease in Z or an increase in Y produces a brighter star for any mass; (2) the evolutionary track during core helium burning can reach higher surface temperatures as Z is decreased; (3) the temperature behavior of an evolutionary track is a parabolic function of Y; (4) all features of the frequency-period distribution are displaced to larger values of the period as either Z or Y is increased; and (5) the observed distribution in any galaxy cannot be matched exactly to a theoretical distribution constructed with models of one fixed composition. It is inferred that the character of the observed distribution for any galaxy depends on the existence of a composition spread and composition gradients, the form of the galactic birthrate function, and selection effects. A birthrate function with at least two components is postulated.
Becker S. A.
Iben Icko Jr.
Tuggle Roy Slaton
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