Using HIPPARCOS data we obtain the following estimates of the zero-point of the RR Lyrae ,[Fe/H] relation, firstly, =0.77 0.17 at [Fe/H]=-1.66 using the Method of Statistical Parallax and HIPPARCOS proper motions for 84 Halo Field RR Lyraes and secondly, =0.78 0.29 at [Fe/H]=-1.39 from the HIPPARCOS trigonometric parallax of the single star RR Lyrae itself. These two estimates are in excellent agreement and give a final value of =0.77 0.15 at [Fe/H]=-1.53. This in turn is in very good agreement with the zero-point derived recently by several groups using Baade-Wesselink methods, namely =0.73 0.14 at [Fe/H]=-1.53. In our opinion this level of agreement, between three completely independent methods, as to the value of the zero-point of the RR Lyrae ,[Fe/H] relation is the main result of the paper.
Taking the HIPPARCOS zero-point and a value of 0.18 0.03 for the slope of the relation from the literature we find firstly, the distance modulus of the LMC is 18.26 0.15 and secondly, the mean age of the Globular Clusters is 17.4 3.0 GYrs. Comparing these values with recently published estimates, using other "standard candles" also calibrated with HIPPARCOS data, shows that there remain unresolved problems, both with the astrophysics and with the interpretation of HIPPARCOS data itself.
Interpretation of HIPPARCOS data. Firstly, in what cases should bias corrections be applied to the parallaxes ? This has a significant effect on the calibration of the subdwarfs (used to determine the distance and ages of the Globular Clusters - see Sect. 5.2). Secondly, for stars that are in binaries, what is the effect of the orbit on the observed parallax ? This has implications for the calibration of the zero-point of the Cepheid P-L relation (and hence the distance to the LMC - Sect. 5.1). Thirdly, for stars with very large angular diameters, what is the effect of surface inhomogeneities on the observed parallax ? This has relevance for the Miras (again Sect. 5.1).
Astrophysical problems. Firstly, what is the sensitivity of the zero-point of the Cepheid P-L relation to metallicity ? If, as suggested by recent work, this is significant then the metallicity differences between Cepheids in the Galaxy and the LMC will obviously lead to incorrect distance estimates (see discussion in Sect. 5.1). Secondly, what is the effect of evolution on the mean magnitude of RR Lyrae stars in metal-poor clusters ? If this effect is significant then the RR Lyraes in these clusters will be brighter than predicted, and hence the LMC distance would be increased and the Globular Cluster ages would be reduced (again see discussion in Sect. 5.1).
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998