LMC Cepheids Research Articles

Hubble Space TelescopeFine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations

We present new absolute trigonometric parallaxes and relative proper motions for nine Galactic Cepheid variable stars: l Car, ζ Gem, β Dor, W Sgr, X Sgr, Y Sgr, FF Aql, T Vul, and RT Aur. We obtain these results with astrometric data from Fine Guidance Sensor 1r, a white-light interferometer on the Hubble Space Telescope. We find absolute parallaxes in milliarcseconds: l Car, 2.01 ± 0.20 mas; ζ Gem, 2.78 ± 0.18 mas; β Dor, 3.14 ± 0.16 mas; W Sgr, 2.28 ± 0.20 mas; X Sgr, 3.00 ± 0.18 mas; Y Sgr, 2.13 ± 0.29 mas; FF Aql, 2.81 ± 0.18 mas; T Vul, 1.90 ± 0.23 mas; and RT Aur, 2.40 ± 0.19 mas; average σπ/π = 8%. Two stars (FF Aql and W Sgr) required the inclusion of binary astrometric perturbations, providing Cepheid mass estimates. With these parallaxes we compute absolute magnitudes in V, I, K, and Wesenheit WVI bandpasses, corrected for interstellar extinction and Lutz-Kelker-Hanson bias. Adding our previous absolute magnitude determination for δ Cep, we construct period-luminosity relations (PLRs) for 10 Galactic Cepheids. We compare our new PLRs with those adopted by several recent investigations, including the Freedman and Sandage H0 projects. Adopting our PLR would tend to increase the Sandage H0 value, but leave the Freedman H0 unchanged. Comparing our Galactic Cepheid PLR with those derived from LMC Cepheids, we find the slopes for K and WVI to be identical in the two galaxies within their respective errors. Our data lead to a WVI distance modulus for the LMC m - M = 18.50 ± 0.03, uncorrected for any metallicity effects. Applying recently derived metallicity corrections yields a corrected LMC distance modulus of (m - M)0 = 18.40 ± 0.05. Comparing our PLR to solar-metallicity Cepheids in NGC 4258 results in a distance modulus 29.28 ± 0.08 that agrees with one derived from maser studies.

Seismology of triple-mode classical Cepheids of the Large Magellanic Cloud

We interpret the three periods detected in OGLE LMC Cepheids SC3-360128 and SC5-338399 as corresponding to the first three overtones of radial pulsations. This interpretation imposes stringent constraints on parameters of the stars and on their evolutionary status, which could only be the first crossing of the instability strip. Evolutionary models reproducing measured periods exist only in a restricted range of metallicities (Z=0.004-0.007). The models impose an upper limit on the extent of overshooting from the convective core. Absolute magnitude of each star is confined to a narrow interval. This allows to derive a new estimate of the distance to the LMC. We obtain m-M ranging from 18.34mag to 18.53mag, with a systematic difference between the two stars of about 0.13mag. The rates of period change predicted by the models are formally in conflict with the derived observational limits, though the uncertainities of measured dP/dt may be underestimated. If the discrepancy is confirmed, it would constitute a significant challenge to the stellar evolution theory.

Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period‐Luminosity Relation up to Solar Abundance

We have applied the infrared surface brightness (ISB) technique to derive distances to 13 Cepheids in the LMC which span a period range from 3 to 42 days. From the absolute magnitudes of the variables calculated from these distances, we find that the LMC Cepheids define tight period-luminosity relations in the V, I, W, J and K bands which agree exceedingly well with the corresponding Galactic PL relations derived from the same technique, and are significantly steeper than the LMC PL relations in these bands observed by the OGLE-II Project in V, I and W, and by Persson et al. in J and K. We find that the tilt-corrected true distance moduli of the LMC Cepheids show a significant dependence on period, which hints at a systematic error in the ISB technique related to the period of the stars. We identify as the most likely culprit the p-factor which converts the radial into pulsational velocities; our data imply a much steeper period dependence of the p-factor than previously thought, and we derive p=1.58 (+/-0.02) -0.15 (+/-0.05) logP as the best fit from our data, with a zero point tied to the Milky Way open cluster Cepheids. Using this revised p-factor law, the period dependence of the LMC Cepheid distance moduli disappears, and at the same time the Milky Way and LMC PL relations agree among themselves, and with the directly observed LMC PL relations, within the 1 sigma uncertainties. Our main conclusion is that the previous, steeper Galactic PL relations were caused by an erroneous calibration of the p-factor law, and that there is now evidence that the slope of the Cepheid PL relation is independent of metallicity up to solar metallicity, in both optical, and near-infrared bands.

Period-Luminosity-Shape of Light Curve Relation for Cepheids in LMC and SMC

In this paper we report our analysis of the period-luminosity-shape of light curve (P-L-S) relation. The data we used came from LMC and SMC Cepheid light curves produced by OGLE-II microlensing survey. We used Principal Components Analysis of covariance matrix of the sets for Cepheids in LMC and SMC separately. Our calibrating set was bounded to long period classical Cepheids pulsating in fundamental mode. The results are used to derive a method for estimation of distances from light curve shape.

Cepheid distances from infrared long-baseline interferometry

Using our interferometric observations of seven classical Cepheids reported in Kervella et al. (2003, Paper I), complemented by previously existing measurements, we derive new calibrations of the Cepheids Period-Radius (P-R) and Period-Luminosity (P-L) relations. We obtain a P-R relation of log R = [0.767 +/- 0.009] log P + [1.091 +/- 0.011], only 1 sigma away from the relation obtained by Gieren et al. (1998). We therefore confirm their P-R relation at a level of Delta(log R) = +/- 0.02. We also derive an original calibration of the P-L relation, assuming the slopes derived by Gieren et al. (1998) from LMC Cepheids, alpha_K = -3.267 +/- 0.042 and alpha_V = -2.769 +/- 0.073. With a P-L relation of the form M = alpha (log P - 1) + beta, we obtain log P = 1 reference points of beta(K) = -5.904 +/- 0.063 and beta(V) = -4.209 +/- 0.075. Our calibration in the V band is statistically identical to the geometrical result of Lanoix et al. (1999).

The Araucaria Project: An Improved Distance to the Sculptor Spiral Galaxy NGC 300 from Its Cepheid Variables

In a previous paper, we reported on the discovery of more than a hundred new Cepheid variables in the Sculptor Group spiral galaxy NGC 300 from wide-field images taken in the B and V photometric bands at ESO/La Silla. In this paper, we present additional VI data, derive improved periods and mean magnitudes for the variables, and construct period-luminosity relations in the V, I, and the reddening-independent V - I Wesenheit bands using 58 Cepheid variables with periods between 11 and 90 days. We obtain tightly defined relations, and by fitting the slopes determined for the LMC Cepheids by the OGLE II Project we obtain reddening-corrected distances to the galaxy in all bands, which show a slight offset to each other in the sense that the Wesenheit relation yields the smallest distance, whereas the I- and V-band distances are larger by 0.094 and 0.155 mag, respectively. We adopt as our best value the distance derived from the reddening-free Wesenheit magnitudes, which is 26.43 ± 0.04(random) ±0.05(systematic) mag. The distance moduli from both the V and I bands agree perfectly with the Wesenheit value if one assumes an additional reddening of E(B - V) = 0.05 mag intrinsic to NGC 300, in addition to the Galactic foreground reddening toward NGC 300 of 0.025 mag. Such a modest intrinsic reddening is supported by recent Hubble Space Telescope (HST) images of NGC 300, which show that this galaxy is relatively dustfree but also reveal that there must be some dust absorption in NGC 300. We argue that our current distance result for NGC 300 is the most accurate that has so far been obtained using Cepheid variables and that it is largely free from systematic effects due to metallicity, blending, and sample selection. It agrees very well with the recent distance determination from the tip of the red giant branch method obtained from HST data by Butler and coworkers, and it is consistent with the Cepheid distance to NGC 300 that was derived by Freedman and coworkers from CCD photometry of a smaller sample of stars.

Period-colour and amplitude-colour relations in classical Cepheid variables

In this paper we analyze the behavior of Galactic, LMC and SMC Cepheids in terms of period-color (PC) and amplitude-color (AC) diagrams at the phases of maximum, mean and minimum light. We find very different behavior between Galactic and Magellanic Cloud Cepheids. Motivated by the recent report of a break in LMC PC relations at 10 days (Tammann et al. 2002), we use the F-statistical test to examine the PC relations at mean light in these three galaxies. The results of the F-test support the existence of the a break in the LMC PC(mean) relation, but not in the Galactic or SMC PC(mean) relations. Furthermore, the LMC Cepheids also show a break at minimum light, which is not seen in the Galactic and SMC Cepheids. We further discuss the effect on the period-luminosity relations in the LMC due to the break in the PC(mean) relation.

Geometry of the Large Magellanic Cloud Disk: Results from MACHO and the Two Micron All Sky Survey

We present a detailed study of the viewing angles and geometry of the inner LMC (ρ 4°) based on a sample of more than 2000 MACHO Cepheids with complete {VR}KC light curves and single-phase Two Micron All Sky Survey (2MASS) JHKs observations. The sample is considerably larger than any previously studied subset of LMC Cepheids and has an improved areal coverage. Single-epoch random-phase 2MASS photometry is corrected using MACHO V light curves to derive mean JHKs magnitudes. We analyze the resulting period-luminosity relations in VRJHKs to recover statistical reddening and distance to each individual Cepheid, with respect to the mean distance modulus and reddening of the LMC. By fitting a plane solution to the derived individual distance moduli, the values of LMC viewing angles are obtained: position angle θ = 1510 ± 24, inclination i = 307 ± 11. In the so-called ring analysis, we find a strong dependence of the derived viewing angles on the adopted center of the LMC, which we interpret as being due to deviations from planar geometry. Analysis of residuals from the plane fit indicates the presence of a symmetric warp in the LMC disk and the bar elevated above the disk plane. Nonplanar geometry of the inner LMC explains a broad range for values of i and θ in the literature and suggests caution when deriving viewing angles from inner LMC data.

Nonradial Pulsations in Classical Cepheids of The Magellanic Clouds

AbstractWe have performed systematic frequency analysis of the LMC Cepheids observed by OGLE project. Several new types of pulsation behaviour are identified, including triple-mode and amplitude-modulated double-mode pulsations. In ~ 10% of the first overtone Cepheids we find low amplitude secondary periodicities corresponding to nonradial modes. This is the first evidence for excitation of nonradial oscillations in Classical Cepheid variables.

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. ([CITE]) and with homogenized color excesses based on Fernie et al. ([CITE]) are used to determine their period-color (P-C) relation in the range . The agreement with colors from different model calculations is good to poor. – Distances of 25 Cepheids in open clusters and associations (Feast [CITE]) and of 28 Cepheids with Baade-Becker-Wesselink (BBW) distances (Gieren et al. [CITE]) are used in a first step to determine the absorption coefficients , , and 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 . They are therefore combined into a single mean Galactic P-L relation. The analysis of HIPPARCOS parallaxes by Groenewegen & Oudmaijer ([CITE]) gives absolute magnitudes which are brighter by in V and in I at . Agreement with P-L relations from different model calculations for the case [Fe/H] = 0 is impressive to poor. Galactic Cepheids are redder in 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. ([CITE],c); the effect is less pronounced in . Also the , 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 at , 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 at short periods () and fainter at long periods (). The latter effect is enhanced by a suggestive break of the P-L relation of LMC and SMC at towards still shallower values as shown in a forthcoming paper.

Calibration of the distance scale from galactic Cepheids

New estimates of the distances of 36 nearby galaxies is presented. These are based on the calibration of the V- and I-band Period-Luminosity relations for galactic Cepheids measured by the HIPPARCOS mission. The distance moduli are obtained in a classical way. The statistical bias due to the incompleteness of the sample is corrected according to the precepts introduced by Teerikorpi ([CITE]). We adopt a constant slope (the one obtained with LMC Cepheids). The correction for incompleteness bias introduces an uncertainty that depends on each galaxy. On average, this uncertainty is small (0.04 mag) but it may reach 0.3 mag. We show that the uncertainty due to the correction of the extinction is small (propably less than 0.05 mag). The correlation between the metallicity and the morphological type of the host galaxy suggests that we should reduce the application to spiral galaxies in order to bypass the problem of metallicity. We suspect that the adopted PL slopes are not valid for all morphological types of galaxies. This may induce a mean systematic shift of 0.1 mag on distance moduli. A comparison with the distance moduli recently published by Freedman et al. ([CITE]) shows there is a reasonably good agreement with our distance moduli.

Variable Stars in Leo A: RR Lyrae Stars, Short-Period Cepheids, and Implications for Stellar Content

We present the results of a search for short-period variable stars in Leo A. We have found 92 candidate variables, including eight candidate RR Lyrae stars. From the RR Lyrae stars, we measure a distance modulus of (m - M)(0) = 24.51 +/- 0.12, or 0.80 +/- 0.04 Mpc. This discovery of RR Lyrae stars confirms for the first time the presence of an ancient ( older than similar to11 Gyr) population in Leo A, accounting for at least 0.1% of the galaxy's V luminosity. We have also discovered a halo of old (more than similar to2 Gyr) stars surrounding Leo A, with a scale length roughly 50% larger than that of the dominant young population. We also report the discovery of a large population of Cepheids in Leo A. The median absolute magnitude of our Cepheid sample is M-V = -1.1, fainter than 96% of SMC and 99% of LMC Cepheids. Their periods are also unusual, with three Cepheids that are deduced to be pulsating in the fundamental mode having periods of under 1 day. Upon examination, these characteristics of the Leo A Cepheid population appear to be a natural extension of the classical Cepheid period-luminosity relations to low metallicity, rather than being indicative of a large population of anomalous Cepheids. We demonstrate that the periods and luminosities are consistent with the expected values of low-metallicity blue helium-burning stars (BHeB's), which populate the instability strip at lower luminosities than do higher metallicity BHeB's.

Search for second overtone mode Cepheids in Magellanic Clouds

The analysis of CCD observations of ten new suspected second overtone mode Cepheids in the SMC does not confirm the nature of such stars; most of them appear to be first overtone mode Cepheids. A careful analysis of the OGLE best second overtone mode candidates and of the second overtone component of double mode Cepheids in SMC and LMC has been performed in order to clarify the possible effects of a resonance between pulsation modes. There are some indications that such a resonance is indeed operating in these pulsators. A brief discussion of the effects of blending on the light curve shape is also reported, and the usefulness of the Fourier parameters as mode discriminators is discussed.

Metallicity effects on classical Cepheids: Velocity curve morphology of outer disc Cepheids

We present new radial velocity data for eleven classical Cepheids situated in the outer parts of the Galactic disc. The resultant velocity curves for these metal-deficient Cepheids are decomposed in Fourier parameters, and compared to solar-metallicity Cepheid data, in order to study the effect of metallicity on the pulsation properties. Up to 8 days, the phase shift is found to follow very closely the corresponding sequence for solar-metallicity Cepheids, indicating the absence of metallicity effects on for low periods. However, metal-deficient Cepheids show a slightly larger A 1 amplitude and R 21 amplitude ratio for days. At days, there is some evidence that the phase shift is significantly larger for metal-deficient Cepheids than at solar metallicity. A posteriori, this effect is also detected in photometric data for outer disc and LMC Cepheids in the 12-20 day period range. In good qualitative agreement with the predictions of theoretical pulsation models, we therefore see some indication of a metallicity dependence of the resonance properties. The largest effects on velocity curve shape are expected in the crucial 8-12 day period range -around the resonance for fundamental-mode Cepheids -that is not covered by our data. Possible targets for future measurements are suggested to fill this gap, and to improve the quantitative determination of metallicity effects on the structural properties of Cepheid velocity curves.

Final Results from theHubble Space TelescopeKey Project to Measure the Hubble Constant

We present here the final results of the Hubble Space Telescope Key Project to measure the Hubble constant. We summarize our method, the results and the uncertainties, tabulate our revised distances, and give the implications of these results for cosmology. The analysis presented here benefits from a number of recent improvements and refinements, including (1) a larger LMC Cepheid sample to define the fiducial period-luminosity (PL) relations, (2) a more recent HST Wide Field and Planetary Camera 2 (WFPC2) photometric calibration, (3) a correction for Cepheid metallicity, and (4) a correction for incompleteness bias in the observed Cepheid PL samples. New, revised distances are given for the 18 spiral galaxies for which Cepheids have been discovered as part of the Key Project, as well as for 13 additional galaxies with published Cepheid data. The new calibration results in a Cepheid distance to NGC 4258 in better agreement with the maser distance to this galaxy. Based on these revised Cepheid distances, we find values (in km/sec/Mpc) of H0 = 71 +/- 2 (random) +/- 6 (systematic) (type Ia supernovae), 71 +/- 2 +/- 7 (Tully-Fisher relation), 70 +/- 5 +/- 6 (surface brightness fluctuations), 72 +/- 9 +/- 7 (type II supernovae), and 82 +/- 6 +/- 9 (fundamental plane). We combine these results for the different methods with 3 different weighting schemes, and find good agreement and consistency with H0 = 72 +/- 8. Finally, we compare these results with other, global methods for measuring the Hubble constant.

Cepheid Variables in the Large Magellanic Cloud Cluster NGC 1866. I. New BVRI CCD Photometry

We report BV(RI)C CCD photometric data for a group of seven Cepheid variables in the young, rich cluster NGC 1866 in the Large Magellanic Cloud. The photometry was obtained as part of a program to determine accurate distances to these Cepheids by means of the infrared surface brightness technique and to improve the LMC Cepheid database for constructing Cepheid P-L and P-L-C relations. Using the new data together with data from the literature, we have determined improved periods for all variables. For five fundamental mode pulsators, the light curves are now of excellent quality and will lead to accurate distance and radius determinations once complete infrared light curves and radial velocity curves for these variables become available.

Cepheid Models Based on Self-Consistent Stellar Evolution and Pulsation Calculations: The Right Answer?

We have computed stellar evolutionary models for stars in a mass range characteristic of Cepheid variables (3<m/M<12) for different metallicities representative of the Galaxy and the Magellanic Clouds populations. The stellar evolution calculations are coupled to a linear nonadiabatic stability analysis in order to get self-consistent mass-period-luminosity relations. The period-luminosity (PL) relation as a function of metallicity is analyzed and compared with the recent EROS observations in the Magellanic Clouds. The models reproduce the observed width of the instability strips for the SMC and LMC. We determine a statistical PL relationship, taking into account the evolutionary timescales and a mass distribution given by a Salpeter mass function. Excellent agreement is found with the SMC PL relationship determined by Sasselov et al. The models reproduce the change of slope in the PL relationship near P~2.5 days discovered recently by the EROS collaboration (Bauer; Bauer et al.) and thus explain this feature in terms of stellar evolution. Some discrepancy, however, remains for the LMC Cepheids. The models are also in good agreement with beat Cepheids observed by the MACHO and EROS collaborations. We show that most of the first-overtone/second-overtone beat Cepheids have not yet ignited central helium burning; they are just evolving off the main sequence toward the red giant branch.

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. Radius and distance results obtained from both versions of the technique are in excellent agreement. The radii of 28 variables are used to determine the period-radius relation. This relation is found to have a smaller dispersion than in previous studies, and is identical to the period-radius relation found by Laney & Stobie from a completely independent method, a fact which provides persuasive evidence that the Cepheid period-radius relation is now determined at a very high confidence level. We use the accurate infrared distances to determine period-luminosity relations in the V, I, J, H and K passbands from the Galactic sample of Cepheids. We derive improved slopes of these relations from updated LMC Cepheid samples and adopt these slopes to obtain accurate absolute calibrations of the PL relation. By comparing these relations to the ones defined by the LMC Cepheids, we derive strikingly consistent and precise values for the LMC distance modulus in each of the passbands which yield a mean value of DM (LMC) = 18.46 +- 0.02. Our results show that the infrared Barnes-Evans technique is very insensitive to both Cepheid metallicity and adopted reddening, and therefore a very powerful tool to derive accurate distances to nearby galaxies by a direct application of the technique to their Cepheid variables, rather than by comparing PL relations of different galaxies, which introduces much more sensitivity to metallicity and absorption corrections which are usually difficult to determine.

Magellanic Cloud Cepheids: Abundances

We have determined parameters and abundances for three SMC and nine LMC Cepheids that have not been previously studied spectroscopically for metallicity, as well as three SMC and one LMC Cepheids observed previously by Luck & Lambert. This work nearly doubles the number of Cepheids with spectroscopic metallicities in the SMC and triples the number in the LMC. For Galactic reference stars we have determined the abundances of 11 supergiants of spectral type F0 to K1. These abundances are used herein to examine the dispersion in cosmic [Fe/H] values found in LMC Cepheids by Luck & Lambert, and in later papers will be used to correct the distance moduli obtained by us for these Cepheids using the visual surface brightness technique. The Magellanic Cloud Cepheids tend to be longer-period Cepheids (the period range examined was from 37 to 133 days), and thus have physical gravities (derived from the period-luminosity relation and an inferred mass) that are 1.0 dex and lower. Spectroscopic gravities (derived by forcing the total Fe content, as determined from Fe I and Fe II, to be equal) are even lower. Fortunately, the [Fe/H] content as determined from Fe I is not strongly dependent on gravity and is thus well determined. We combine our abundances with other published high-resolution spectroscopic results, deriving a mean [Fe/H] for the SMC of -0.68 (σ = 0.13, n = 25). The total range seen in the [Fe/H] ratios derived herein is -0.84 to -0.65. This result is in agreement with previous studies showing a small intrinsic range in the metallicities in the SMC. For the LMC, from the Cepheids studied here we obtain -0.30 (σ = 0.12, n = 10), with a range of -0.55 to -0.19. Combining all available data, we obtain a mean [Fe/H] of -0.34 (σ = 0.15, n = 32). From this analysis, there is little evidence for a significant metallicity dispersion in the LMC.

Improved Calibration of Cosmic Distance Scale by Cepheid Pulsation Parallaxes

I report on a modern empirical realization of the geometric Baade-Wesselink (BW) method aimed at providing accurate distances to Cepheid variable stars. The method is carefully calibrated using high-precision observational data now available from spectroscopic and interferometric techniques. The distance-dependent linear radius R of a Cepheid, derived from the period P according to the period-radius (PR) relation, is matched to the distance-independent angular diameter ? inferred from the surface brightness-color (SC) correlation using the infrared color (V - K) as brightness indicator. The resulting pulsation parallax is found to be largely insensitive to most of known biasing effects, notably the reddening and metallicity, because of the application of the magnitude-color combination (V, V - K) in deriving stellar angular sizes. The reliability of the actual BW realization is critically checked using 25 galactic Cepheids in clusters with available high-precision photometry, reddenings and distances by the zero-age main-sequence (ZAMS) fitting approach. I find that 20 of these stars show an average residual (BW - ZAMS) = -(0.01 ? 0.03) mag with a scatter ? = ?0.12 mag, whereas the remaining five stars deviate by up to 5 ?. The fairly good agreement with 20 primary calibrators gives high confidence on the reliability of the method which can provide now an independent route to the problem of cosmic distance scale calibration. According to the overall residual, the actual uncertainty on the absolute galactic distance scale is suggested to be ?0.04 mag, i.e., a factor of 2 smaller than that currently quoted for the Pleiades distance modulus which limits the ZAMS calibration. The BW approach is then applied to the extragalactic range by including the Magellanic Clouds Cepheids (LMC and SMC) with available high-precision photometry in V and K passbands. An accurate PR relation is recalibrated by a composite fit to galactic and extragalactic Cepheids. The relation shows a dispersion as low as ? = ?0.11 mag nearly close to the scatter affecting BW distances to calibrating galactic Cepheids. It is argued that this scatter is likely due to the asymptotic spread set by the finite width of the instability strip. The BW distances to the LMC and SMC are also determined to be ?0(LMC) = (18.58 ? 0.024) mag and ?0(SMC) = (19.00 ? 0.025) mag or d(LMC) = (52.0 ? 0.6) kpc and d(SMC) = (63.2 ? 0.7) kpc with uncertainties not including the contribution due to the absolute distance scale calibration. The Cepheid-based distance to the LMC shows close agreement with the geometric expansion parallax of the SN 1987A in LMC given by d(SN 1987A) = (51.1 ? 1.5) kpc (Panagia et al. 1996). The BW method is also calibrated by including the Johnson-Cousins color (V - I) relevant in the Hubble Space Telescope (HST) observations of Cepheids. By using sets of SMC, LMC, and galactic Cepheids with high-precision V, I, and K photometry, it is shown that the angular sizes predictable by the magnitude-color combination (V, V - I) can be affected by the metallicity. However, the induced effects on BW distances are found to be as small as 0.03 mag up to the metal content of the SMC. Furthermore, it is demonstrated that the current HST approach to the extragalactic distances using a linear combination of period-luminosity (PL) relations in V and I passbands yields the same observational relation as that of the BW realization in the magnitude-color combination (V, V - I). This achievement plays a major role in improving the extragalactic distance scale set by Cepheid data from HST observations. First, all distances spanning from the galactic to extragalactic range can be now sampled by the same BW relation strongly supported by the fundamental results of several observational techniques. Second, all distances will result to be affected by the metallicity as the BW data, i.e., as the SC correlation applied for inferring the Cepheid angular sizes. Third, all distances will suffer from random errors as the almost reddening-free BW data which cancel out the extinction on a star-by-star basis, notably the differential reddening internal to the parent galaxies. In order to show this relevant improvement, the BW distance to the Virgo galaxy M100 is determined to be ?0(M100) = (31.03 ? 0.06) mag or d(M100) = (16.1 ? 0.5) Mpc with a random error lowered by about a factor of 3 with respect to that derived according to the PL relations by the HST procedure.