A period-luminosity-colour relation for Mira variables

Abstract

Time-averaged J, H, K and mbol magnitudes are given for 29 O-rich and 20 C-rich Mira variables in the Large Magellanic Cloud. PL and PC relations are derived. The PL relations for O-Miras and C-Miras are very similar at K. For the O-Miras the residuals from the PL and PC relations are correlated showing the existence of PLC relations. Such relations are derived and compared with the theoretical pulsation equation. Data are also given for six variables with periods greater than 420 d. These are brighter by 0.7 mag than expected from extrapolations of the PL or PLC relations. Using Miras in galactic globular clusters to estimate the cluster distances (from the PLC relation) gives a mean horizontal branch luminosity of + 0.71 ± 0.09 (internal standard error) at [Fe/H] = − 0.58, in good agreement with that derived in other ways. For the C-Miras, a significant color term is estimated for the PLC relation at J. There is, however, little evidence for a significant color term at K and the evidence for one in mbol is marginal.