Considerations arising from the faint absolute magnitude of halo RR Lyrae variables and an error in the Cepheid PLC relation

Details Considerations arising from the faint absolute magnitude of halo RR Lyrae variables and an error in the Cepheid PLC relation Considerations arising from the faint absolute magnitude of halo RR Lyrae variables and an error in the Cepheid PLC relation

Jun 1983

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983a%26a...122..255c&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 122, no. 1-2, June 1983, p. 255-260.

Astronomy and Astrophysics

Astrophysics

10

Halos, Magellanic Clouds, Stellar Evolution, Stellar Magnitude, Variable Stars, Cepheid Variables, Distance, Mathematical Models, Stellar Color

Scientific paper

A recent discussion of Galactic and LMC Cepheids by Martin, Warren, and Feast, deriving the PLC relation colour term β0=2.70 in agreement with earlier theoretical predictions by Sandage and others, gave a modulus muC(LMC)=18.69±0.15. This result was consistent with the previous investigation by Sandage and Tammann which gave 18.59 and possibly also with that of de Vaucouleurs who, when reddening differences were allowed for, gave 18.31. The modulus of the LMC based on the statistical parallax of field RR Lyrae stars (i. e. assuming v=1.0) gives an appreciably smaller figure however, μR(LMC) =17.9±0.3 whilst μR(SMC)=18.3±0.3 is less by 0.m7 or so than the value given by Sandage and Taminann for this system. The discrepancies, taken together, are significant at the ∼3 σ level and appear to justify a review of possible systematic errors affecting μC and μR. Clube and Dawe concluded earlier that the determination of β0 may be an important source of error; metallicity differences between Cloud and Galactic Cepheids have also been suggested by Pel, van Genderen and Lub. Each of these separately may diminish μC by up to around 0.m.5, especially in the case of the SMC but neither correction has yet been settled. Numerical simulations of the effects of uncorrected absorption by Brodie and Madore have however confirmed the possibility of an error in β0. Although the need for any such correction was subsequently denied by Feast and Balona it is shown in this paper that their discussion is based on an incorrect formulation of the problem coupled with the adoption of an extreme value for the observed range of Cepheid colours at constant period, and that there are sound theoretical and observational reasons for preferring p0 0. The RR Lyrae moduli for the Magellanic Clouds are thus the more likely to be correct and if combined with suitably adjusted Cepheid moduli imply a revision of the extragalactic distance scale leading to H0 ≧ 90 km s-1.

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