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Astron. Astrophys. 338, 581-591 (1998)

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5. Summary

  • We have shown that PG3 SRVs are not the analogs to the field SRVs. The comparison of the P/(J-K)0 relation of the two SRV groups shows that they do not obey the same P/(J-K)0 relation. In addition their location in a colour-colour diagram differs slightly. All together this indicates a different nature between the two SRV groups.
  • The PG3 SRVs form a short period extension to the Miras PK0 and PC-relations. This indicates that the PG3 Miras and SRVs are both pulsating in the same mode, possibly the fundamental.
  • The field SRVs (the `blue' and the majority of the `red' group) are likely overtone pulsators.
  • The metallicities of the PG3 SRVs and Miras span the range from intermediate to approximately solar.
  • The age possibly covers a range from 4 - 10 Gyr. From the absence of LPVs in metal-poor globular clusters it is argued that the PG3 SRVs and Miras in the bulge are likely not older than 10 Gyr. From the upper mass limit of the bulge IRAS sources and the possible absence of bulge carbon stars one obtains a lower age limit of 4 Gyr.
  • Field and PG3 Miras follow the same P/(J-K)0 relation and cover the same region in the (J-H)0 vs (H-K)0 diagram. Therefore, the metallicity of the field and PG3 Miras should overlap each other. The Miras and SRVs in PG3 follow the Sgr I PK0-relation. This confirms independently the work of Whitelock et al. (1991) and Glass et al. (1995): they found no difference in the PL-relation for different galactic environments.

The following question arises: are there SRVs in PG3, similar to those found in the disc? The presence or absence of these stars might provide an indication of the age of the stars in PG3. The missing SRVs might be hidden among the irregular variables. According to Wess87 those are variable stars with little or no trace of periodicity for which the amplitudes do not exceed 1[FORMULA] @ For verification, a detailed study of these stars is desired.

Another question which arises concerns the nature of the large spread in metallicity. The large range of ages seems to indicate that both young and old stars can be present with intermediate up to solar metallicity. Even the presence of more massive, young stars, which can be metal-poorer than older stars, is possible. In a closed box model one expects an increasing metallicity towards younger ages. Is this an indication that a closed box model is not applicable? What is the origin of this behaviour?

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998