On improved Cepheid distance estimators

The use of Cepheids as distance indicators on Galactic and extragalactic distance scales is based upon the Cepheid period - luminosity (PL) and period - luminosity - colour (PLC) relations. These relations are usually derived in terms of the properties of Cepheids at mean light - i.e. averaged over their pulsation cycle. In this paper, we derive a physical argument for the existence of PL and PLC relations at maximum light. We examine in detail a sample of Cepheids in the Large Magellanic Cloud, and compare the variance of some PL and PLC type distance indicators based on mean and maximum light. We show that for the LMC data considered, a PLC relation based on maximum light leads to a distance estimator with a dispersion about 10% smaller than its counterpart using mean light. We also show that for the LMC, a PLC type relation constructed using observations at both maximum and mean light has a significantly (> 50%) smaller dispersion than a PLC relation using either maximum or mean light alone. A comparable (> 30%) reduction in the dispersion of the corresponding distance estimator, however, in this case requires the relation be applied to a large (n > 30) group of equidistant Cepheids in, e.g., a distant galaxy. Recent HST observations of IC4182, M81 and M100 already provide suitable candidate data sets for this relation. The use of maximum light in constructing PLC type relations for galactic and extragalactic Cepheids is, therefore, shown to be an interesting topic for further study. These investigations are under way.