The Cepheid variables and the stellar populations of the dwarf spheroidal galaxies

Journal Article · Mon Dec 01 04:00:00 EST 1975 · Astrophysical Journal

DOI:https://doi.org/10.1086/153981· OSTI ID:4050538

Norris, J.; Zinn, R.

A new hypothesis is presented concerning the stellar populations of the dwarf spheroidal galaxies. It is suggested that these systems have evolved separately from our Galaxy, with initial star formation occurring from an extremely metal-deficient protocloud some few billion years after the formation of the Galaxy. By far the large majority of the stars in the dwarf spheroidal galaxies formed at this time, leading to color-magnitude diagrams similar to those of the globular clusters of the Galaxy. The predominantly red horizontal branches of most of these galaxies result from their smaller ages. Little metal enrichment occurred during this initial phase; and later star formation at a substantially lower rate, continuing to about 10$sup 9$ years ago, has produced relatively young, quite metal-deficient objects. It is shown that these may be identified with the Cepheid variables in these systems to give a fully satisfactory explanation of the observed period-luminosity relation of the variable stars. It is argued that discrete differences in age between the dwarf spheroidal galaxies and our Galaxy, or helium abundance differences, are incapable of explaining the above-mentioned observations. The existence of Cepheid variables in the Small Magellanic Cloud similar to those in the dwarf spheroidal galaxies probably implies the existence of young, metal-deficient objects (Zapproximately-less-than10$sup -4$) in this system. Finally, it is suggested that age is the relevant ''second'' parameter necessary for the interpretation of the anomalous horizontal-branch morphology of the intergalactic globular clusters and those at the edge of the Galaxy. (AIP)

• Research Organization: Yale University Observatory and Mount Stromlo and Siding Spring Observatory, Research School of Physical Sciences, Australian National University
• Sponsoring Organization: USDOE
• NSA Number: NSA-33-024011
• OSTI ID: 4050538
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Vol. 202; ISSN 0004-637X
• Publisher: Institute of Physics (IOP)
• Country of Publication: United States
• Language: English

Related Subjects

*CEPHEIDS-- LUMINOSITY
*GALAXIES-- VARIABLE STARS
640102* --Physics Research--Astrophysics & Cosmology--Stars
AGE ESTIMATION
CHEMICAL COMPOSITION
COLOR
GALACTIC EVOLUTION
HELIUM
MAIN SEQUENCE STARS
METALS
MILKY WAY
N56200* --Physics (Astrophysics & Cosmology)--Stars
N56500 --Physics (Astrophysics & Cosmology)--Galaxies
STAR EVOLUTION