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 derive the radii and distances of 34 Galactic Cepheid variables. Radi us and distance results obtained from both versions of the technique a re in excellent agreement. The radii of 28 variables are used to deter mine the period-radius (PR) relation. This relation is found to have a smaller dispersion than in previous studies, and is identical to the PR relation found by Laney & Stobie from a completely independent meth od, a fact which provides persuasive evidence that the Cepheid PR rela tion is now determined at a very high confidence level. We use the acc urate infrared distances to determine period-luminosity (PL) relations in the V, I, J, H, and K passbands from the Galactic sample of Cephei ds. We derive improved slopes of these relations from updated LMC Ceph eid samples and adopt these slopes to obtain accurate absolute calibra tions of the PL relation. By comparing these relations to the ones def ined by the LMC Cepheids, we derive strikingly consistent and precise values for the LMC distance modulus in each of the passbands that yiel d a mean value of mu(0)(LMC) = 18.46 +/- 0.02. By analyzing the observ ed dispersions of the PL relations defined by the LMC and Galactic sam ples of Cepheids, we disentangle the contributions due to uncertaintie s in the reddenings, in distance measurement, and due to metallicity e ffects, and we estimate the intrinsic dispersion of the PL relation wi th the Wesenheit function. Assuming that the Galactic Cepheid distance s are typically accurate to +/-3% (as shown in a previous paper), and assuming an intrinsic spread in [Fe/H] of similar to 0.4 dex among the Cepheids of our sample as obtained by Fry & Carney, the observed disp ersion of the Galactic Cepheid PL relation suggests a metallicity depe ndence of Delta mu/Delta[Fe/H] approximate to 0.2, about half the valu e suggested by Sasselov et al. from EROS data. Since this estimate of the metallicity dependence of the PL (V) relation is rather uncertain, however, we prefer to retain mu(0)(LMC) = 18.46 as our best value, bu t with an increased uncertainty of +/-0.06, most of which is due to th e uncertainty in the appropriate metallicity correction. Our results s how that the infrared Barnes-Evans technique is very insensitive to bo th Cepheid metallicity and adopted reddening, and therefore is a very powerful tool to derive accurate distances to nearby galaxies by a dir ect application of the technique to their Cepheid variables, rather th an by comparing PL relations of different galaxies, which introduces m uch more sensitivity to metallicity and absorption corrections that ar e usually difficult to determine.