New period-luminosity and period-color relations of classical Cepheids: I. Cepheids in the Galaxy

321 Galactic fundamental-mode Cepheids with good B, V, and (in most cases) I photometry by Berdnikov et al. (2000) and with homogenized color excesses E(B-V) based on Fernie et al. (1995) are used to determine their period-color (P-C) relation in the range 0.4 < log P less than or similar to 1.6. The agreement with colors from different model calculations is good to poor. - Distances of 25 Cepheids in open clusters and associations (Feast 1999) and of 28 Cepheids with Baade-Becker-Wesselink (BBW) distances (Gieren et al. 1998) are used in a first step to determine the absorption coefficients R-B = 4.17, R-V = 3.17, and R-I = 1.89 appropriate for Cepheids of intermediate color. The two sets of Galactic Cepheids with known distances define two independent P-L relations which agree very well in slope; their zero points agree to within 0.(m)12 +/- 0.(m)04. They are therefore combined into a single mean Galactic P-L-B,L-V,L-I relation. The analysis of HIPPARCOS parallaxes by Groenewegen & Oudmaijer (2000) gives absolute magnitudes which are brighter by 0.(m)21 +/- 0.(m)11 in V and 0.(m)18 +/- 0.(m)12 in I at log P = 0.85. Agreement with P-L relations from different model calculations for the case [Fe/H] = 0 is impressive to poor. Galactic Cepheids are redder in (B - V)(0) than those in LMC and SMC as shown by the over 1000 Cloud Cepheids with good standard B, V, I photometry by Udalski et al. (1999b,c); the effect is less pronounced in (V-I)(0). Also the (B-V)(0), (V-I)(0) two-color diagrams differ between Cepheids in the Galaxy and the Clouds, attributed both to the effects of metallicity differences on the spectral energy distributions of the Cepheids and to a shift in the effective temperature of the middle of the instability strip for LMC and SMC relative to the Galaxy by about Delta log T-e similar to 0.02 at M-V = - 4.(m)0, hotter for both LMC and SMC. Differences in the period-color relations between the Galaxy and the Clouds show that there cannot be a universal P-L relation from galaxy-to-galaxy in any given color. The inferred non-uniqueness of the slope of the P-L relations in the Galaxy, LMC, and SMC is born out by the observations. The Cloud Cepheids follow a shallower overall slope of the P-L relations in B, V, and I than the Galactic ones. LMC and SMC Cepheids are brighter in V than in the Galaxy by up to 0.(m)5 at short periods (log P = 0.4) and fainter at long periods (log P greater than or similar to 1.4). The latter effect is enhanced by a suggestive break of the P-L relation of LMC and SMC at log P = 1.0 towards still shallower values as shown in a forthcoming paper.