Cepheid Period‐Radius and Period‐Luminosity Relations and the Distance to the Large Magellanic Cloud

We have used the infrared Barnes-Evans surface brightness technique to derivethe radii and distances of 34 Galactic Cepheid variables. Radius and distanceresults obtained from both versions of the technique are in excellentagreement. The radii of 28 variables are used to determine the period-radiusrelation. This relation is found to have a smaller dispersion than in previousstudies, and is identical to the period-radius relation found by Laney & Stobiefrom a completely independent method, a fact which provides persuasive evidencethat the Cepheid period-radius relation is now determined at a very highconfidence level. We use the accurate infrared distances to determineperiod-luminosity relations in the V, I, J, H and K passbands from the Galacticsample of Cepheids. We derive improved slopes of these relations from updatedLMC Cepheid samples and adopt these slopes to obtain accurate absolutecalibrations of the PL relation. By comparing these relations to the onesdefined by the LMC Cepheids, we derive strikingly consistent and precise valuesfor the LMC distance modulus in each of the passbands which yield a mean valueof DM (LMC) = 18.46 +- 0.02. Our results show that the infrared Barnes-Evans technique is very insensitiveto both Cepheid metallicity and adopted reddening, and therefore a verypowerful tool to derive accurate distances to nearby galaxies by a directapplication of the technique to their Cepheid variables, rather than bycomparing PL relations of different galaxies, which introduces much moresensitivity to metallicity and absorption corrections which are usuallydifficult to determine.Comment: LaTeX, AASTeX style, 9 Figures, 10 Tables, The Astrophysical Journal in press (accepted Oct. 14, 1997). Fig. 3 replace