PERIOD-LUMINOSITY AND PERIOD-LUMINOSITY-COLOR RELATIONS FOR MIRA VARIABLES AT MAXIMUM LIGHT

In this paper we discuss the evidence for a period-luminosity (PL) rel ation and a period-luminosity-colour (PLC) relation at maximum light f or Mira variables. We confirm the existence of such relations in the J , H and K bands, and also based on bolometric magnitudes, for oxygen-r ich (O) and carbon-rich (C) Miras in the Large Magellanic Cloud. We de monstrate that in the J and H bands the maximum light PL relations hav e a significantly smaller dispersion than their counterparts at mean l ight, while the K-band and bolometric PL relations have a dispersion c omparable to that at mean light, In the J, H and K bands the fitted PL relations for the O Miras are found to have smaller dispersion than t hose for the C Miras, at both mean and maximum light, while the conver se is true for the relations based on bolometric magnitudes. The inclu sion of a non-zero log period term in the regression fits is found to be highly significant in all cases except that of the C Miras in the J band, for which the data are found to be consistent with having const ant absolute magnitude, Moreover, if we consider only C Miras with per iods in excess of 250 d. the data are also substantially more consiste nt with constant absolute magnitude at mean and maximum light in the H , K and bolometric wavebands. This suggests the possibility of employi ng C Miras as standard candles. We suggest both a theoretical justific ation for the existence of Mira PL relations at maximum light and an e xplanation of why these relations should have a smaller dispersion tha n at mean light. The existence of such maximum light relations offers the possibility of extending the range and improving the accuracy of t he Mira distance scale to Galactic globular clusters and to other gala xies.