Spectroscopic observations of omega Centauri and the Sandage period-shift effect.
Abstract
This paper discusses the problems which affect a confident derivation of absolute magnitudes of horizontal branch (HB) stars of galactic globular clusters (GGC) and, as a consequence, of their distance moduli. Spectroscopic observations of RR Lyrae variables in ω Cen have been used to derive information on the evolutionary states of its HB stars. The effect of the HB evolution has been estimated also on the other galactic globular clusters in order to verify the Sandage effect in the absence of evolutionary effects. The authors selected a sample of clusters in which the RR Lyrae are representative of ZAHB properties. This unbiased sample shows a Sandage effect much smaller (ΔlogP/ΔlogZ ≡ -0.06) than that previously claimed by Sandage himself (≡ -0.116). If Baade's window variables are also taken into account an even smaller effect is found (≡ -0.04); the theoretical expected behaviour (-0.003±0.02) of the RR Lyrae stars of GGC is therefore not excluded on the basis of presently available data.
- Publication:
-
Memorie della Societa Astronomica Italiana
- Pub Date:
- 1986
- Bibcode:
- 1986MmSAI..57..361T
- Keywords:
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- Astronomical Spectroscopy;
- Centaurus Constellation;
- Horizontal Branch Stars;
- Galactic Clusters;
- Globular Clusters;
- Metallic Stars;
- Stellar Rotation;
- Variable Stars;
- Astrophysics