SMC Cepheids Research Articles

Peeping into recent star-formation history of Magellanic Clouds

Here, we study the distribution of Fundamental-mode Cepheids in the Magellanic Cloud as a function of their positions and ages using the data from the OGLE~IV survey. Age of the Cepheids are determined through well known period - age relations for the LMC and SMC Cepheids which are used to understand the star formation scenario in the Magellanic Cloud. The age distributions of the Cepheids in LMC and SMC show peak around $155^{+45}_{-35}$ Myr and $224^{+51}_{-42}$ Myr, respectively. This indicates that a major star formation event took place in the Magellanic Cloud at about 200\,Myr ago. It is believed that this event might have been triggered by a close encounter between the two components of the Magellanic Cloud or due to a possible tidal interaction between the Magellanic Cloud and Milky Way galaxy during one of its pericentric passages round the Milky Way. Cepheids are found to be asymmetrically distributed in both the LMC and SMC. A high-density clumpy structure is found to be located towards eastern side of the LMC and south-west direction of the SMC from their respective galactic centres.

Metallicity estimations of MW, SMC, and LMC classical Cepheids from the shape of the V- and I-band light curves

Context. Estimating the metallicity of classical Cepheids is of prime importance for studying metallicity effects on stellar evolution and the chemical evolution of galaxies, as well as on the period–luminosity relation used on the extragalactic distance scale. Aims. Our first aim is to establish new empirical relations for estimating the iron content of classical Cepheids for short and long periods based on Fourier parameters from the V- and I-band light curves. We go on to apply these relations to Cepheids from data on the Milky Way (MW) as well as the Small and Large Magellanic Clouds (SMC and LMC) from the literature. Methods. We retrieved the metallicities of 586 fundamental-mode Cepheids from spectroscopic determinations in the literature and we found well-sampled light curves for 545 of them in different V-band catalogs. We then described the shape of these light curves by applying a Fourier decomposition and we fit the empirical relations between the Fourier parameters and the spectroscopic metallicities individually, for short-period (2.5 < P < 6.3 days) and long-period Cepheids (12 < P < 40 days). We verified the accuracy of these relations by applying them to V-band light curves of Cepheids from the Small and Large Magellanic Clouds and comparing these derived metallicities to literature values. We calibrated new interrelations of Fourier parameters to convert these empirical relations into the I band. We then used these I-band relations to derive the metallicity of fundamental-mode Cepheids from OGLE-IV for MW, SMC, and LMC (486, 695, and 1697 stars, respectively). Finally, we mapped the metallicity distribution in these galaxies for the purpose of investigating potential applications in galactic archeology. Results. For short-period Cepheids, our best fit is given for a relation based on explicit amplitude terms A1 and A2 of the first and second harmonic, respectively. In the V and I bands, these empirical relations are found with an intrinsic scatter (rms) of 0.12 dex. This relation performs well for estimations of [Fe/H] between about −0.5 and 0.1 dex, but it remains uncertain outside this range because of the lack of a spectroscopic metallicity required for the calibration. For long-period Cepheids, we found a metallicity dependence on the Fourier parameters A1, ϕ21, and R41. We found an intrinsic scatter of 0.25 dex when using this relation. The empirical relations in the V and I bands allow us to derive the mean metallicity of a sample of MW, SMC, and LMC Cepheids that is in agreement with literature values within 1σ. We also show that these relations are precise enough to reconstruct the radial metallicity gradients within the MW from OGLE data. Conclusions. The empirical relations in the V and I bands that are calibrated in this work for short- and long-period Cepheids provide a useful new tool for estimating the metallicity of Cepheids that are not accessible via spectroscopy. The calibration can be improved with further high-resolution spectroscopic observations of metal-poor Cepheids and homogeneous photometries in the V and I bands.

The effect of metallicity on Cepheid period-luminosity relations from a Baade-Wesselink analysis of Cepheids in the Milky Way and Magellanic Clouds

Context. The extragalactic distance scale builds on the Cepheid period-luminosity (PL) relation. Decades of work have not yet convincingly established the sensitivity of the PL relation to metallicity. This currently prevents a determination of the Hubble constant accurate to 1% from the classical Cepheid-SN Ia method. Aims. In this paper we carry out a strictly differential comparison of the absolute PL relations obeyed by classical Cepheids in the Milky Way (MW), LMC, and SMC galaxies. Taking advantage of the substantial metallicity difference among the Cepheid populations in these three galaxies, we want to establish a possible systematic trend of the PL relation absolute zero point as a function of metallicity, and to determine the size of such an effect in the optical and near-infrared photometric bands. Methods. We used a IRSB Baade-Wesselink-type method to determine individual distances to the Cepheids in our samples in the MW, LMC, and SMC. For our analysis, we used a greatly enhanced sample of Cepheids in the SMC (31 stars) compared to the small sample (5 stars) available in our previous work. We used the distances to determine absolute Cepheid PL relations in the optical and near-infrared bands in each of the three galaxies. Results. Our distance analysis of 31 SMC Cepheids with periods of 4–69 days yields tight PL relations in all studied bands, with slopes consistent with the corresponding LMC and MW relations. Adopting the very accurately determined LMC slopes for the optical and near-infrared bands, we determine the zero point offsets between the corresponding absolute PL relations in the three galaxies. Conclusions. We find that in all bands the metal-poor SMC Cepheids are intrinsically fainter than their more metal-rich counterparts in the LMC and MW. In the K band the metallicity effect is −0.23 ± 0.06 mag dex−1, while in the V, (V − I) Wesenheit index it is slightly stronger, −0.34 ± 0.06 mag dex−1. We find suggestive evidence that the metallicity sensitivity of the PL relation might be nonlinear, being small in the range between solar and LMC Cepheid metallicity, and becoming steeper towards the lower-metallicity regime.

Observational calibration of the projection factor of Cepheids

Context. The Baade-Wesselink (BW) method, which combines linear and angular diameter variations, is the most common method to determine the distances to pulsating stars. However, the projection factor, p-factor, used to convert radial velocities into pulsation velocities, is still poorly calibrated. This parameter is critical on the use of this technique, and often leads to 5–10% uncertainties on the derived distances. Aims. We focus on empirically measuring the p-factor of a homogeneous sample of 29 LMC and 10 SMC Cepheids for which an accurate average distances were estimated from eclipsing binary systems. Methods. We used the SPIPS algorithm, which is an implementation of the BW technique. Unlike other conventional methods, SPIPS combines all observables, i.e. radial velocities, multi-band photometry and interferometry into a consistent physical modelling to estimate the parameters of the stars. The large number and their redundancy insure its robustness and improves the statistical precision. Results. We successfully estimated the p-factor of several Magellanic Cloud Cepheids. Combined with our previous Galactic results, we find the following P−p relation: −0.08± 0.04(log P−1.18) + 1.24± 0.02. We find no evidence of a metallicity dependent p-factor. We also derive a new calibration of the period-radius relation, log R = 0.684± 0.007(log P−0.517) + 1.489± 0.002, with an intrinsic dispersion of 0.020. We detect an infrared excess for all stars at 3.6 μm and 4.5 μm, which might be the signature of circumstellar dust. We measure a mean offset of Δm3.6 = 0.057 ± 0.006 mag and Δm4.5 = 0.065 ± 0.008 mag. Conclusions. We provide a new P−p relation based on a multi-wavelength fit that can be used for the distance scale calibration from the BW method. The dispersion is due to the LMC and SMC width we took into account because individual Cepheids distances are unknown. The new P−R relation has a small intrinsic dispersion: 4.5% in radius. This precision will allow us to accurately apply the BW method to nearby galaxies. Finally, the infrared excesses we detect again raise the issue of using mid-IR wavelengths to derive period-luminosity relation and to calibrate the Hubble constant. These IR excesses might be the signature of circumstellar dust, and are never taken into account when applying the BW method at those wavelengths. Our measured offsets may give an average bias of ~ 2.8% on the distances derived through mid-IR P−L relations.

A Precision Determination of the Effect of Metallicity on Cepheid Absolute Magnitudes in VIJHK Bands from Magellanic Cloud Cepheids

Abstract Using high-quality observed period–luminosity (P–L) relations in both Magellanic Clouds in the VIJHK s bands and optical and near-infrared Wesenheit indices, we determine the effect of metallicity on Cepheid P–L relations by comparing the relative distance between the LMC and SMC as determined from the Cepheids to the difference in distance between the Clouds that has been derived with very high accuracy from late-type eclipsing binary systems. Within an uncertainty of 3%, which is dominated by the uncertainty on the mean difference in metallicity between the Cepheid populations in the LMC and SMC, we find metallicity effects smaller than 2% in all bands and in the Wesenheit indices, consistent with a zero metallicity effect. This result is valid for the metallicity range from −0.35 dex to −0.75 dex corresponding to the mean [Fe/H] values for classical Cepheids in the LMC and SMC, respectively. Yet most Cepheids in galaxies beyond the Local Group and located in the less crowded outer regions of these galaxies do fall into this metallicity regime, making our result important for applications to determine the distances to spiral galaxies well beyond the Local Group. Our result supports previous findings that indicated a very small metallicity effect on the near-infrared absolute magnitudes of classical Cepheids, and resolves the dispute about the size and sign of the metallicity effect in the optical spectral range. It also resolves one of the most pressing problems in the quest toward a measurement of the Hubble constant with an accuracy of 1% from the Cepheid–supernova Ia method.

PERIOD–LUMINOSITY RELATIONS DERIVED FROM THE OGLE-III FUNDAMENTAL MODE CEPHEIDS. II. THE SMALL MAGELLANIC CLOUD CEPHEIDS

In this paper we present multi-band period-luminosity (P-L) relations for fundamental mode Cepheids in the SMC. The optical VI-band mean magnitudes for these SMC Cepheids were taken from the third phase of the Optical Gravitational Lensing Experiment (OGLE-III) catalog. We also matched the OGLE-III SMC Cepheids to 2MASS and SAGE-SMC catalog to derive mean magnitudes in the JHK-bands and the four {\it Spitzer} IRAC bands, respectively. All photometry was corrected for extinction by adopting the Zaritsky's extinction map. Cepheids with periods smaller than $\sim2.5$ days were removed from the sample. In addition to the extinction corrected P-L relations in nine filters from optical to infrared, we also derived the extinction-free Wesenheit function for these Cepheids. We tested the nonlinearity of these SMC P-L relations (except the $8.0\mu\mathrm{m}$-band P-L relation) at 10 days: none of the P-L relations show statistically significant evidence of nonlinearity. When compared to the P-L relations in the LMC, the t-test results revealed that there is a difference between the SMC/LMC P-L slopes only in the V- and J-band. Further, we found excellent agreement between the SMC/LMC Wesenheit P-L slope. The difference in LMC and SMC Period-Wesenheit relation LMC and SMC zero points was found to be $\Delta \mu=0.483\pm0.015$ mag. This amounts to a difference in distance modulus between the LMC and SMC.

Radii of LMC and SMC Cepheids

AbstractWe collected long sets of precise radial velocity observations of radial velocities for 8 SMC and 14 LMC Cepheids. Based on these series of radial velocity measurements and on multicolor photoelectric observations, we calculated radii of the program extragalactic Cepheids using a modification of Balona's method. Our main goal was to determine the long‐period branch of the period‐radius diagram and reveal a dependence on metallicity. We derived the following period‐radius relation: log R = 1.06(±0.03)+0.74(±0.02) log P. We did not find any significant difference between the period‐radius relation of extragalactic Cepheids and that of fundamental‐mode galactic Cepheids. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Baade-Wesselink distances to Galactic and Magellanic Cloud Cepheids and the effect of metallicity

(abridged) The aim of this paper is to investigate the metallicity dependence of the PL relation in V and K, based on a sample of 128 Galactic, 36 LMC, and 6 SMC Cepheids with individual Baade-Wesselink (BW) distances and individually determined metallicities from high-resolution spectroscopy. The p-relation finally adopted is 1.50 -0.24log P. The slope of this relation is based on the condition that the distance to the LMC does not depend on period or (V-K) colour and that the slope of the PL relation based on the BW distances agrees with that based on apparent magnitude. The zero point of the relation is tight to the Cepheids with HST and revised Hipparcos parallaxes as well as to Cepheids in clusters. The slope of the Galactic and LMC K-band relation formally agrees within the errors, and combining all Cepheids (including the SMC) results in a negligible metallicity dependence. A similar conclusion is found for the reddening-free Wesenheit relation. In the V-band the situation is more complex. The slope of the LMC and the Galactic PL relation differ at the 3sigma level. Combining the sample nevertheless results in a metallicity term significant at the 2sigma level. The details of the comparison of BW-based distances and Cepheids with HST and revised Hipparcos parallaxes also play a role. The method used by Storm et al. would lead to larger DM of 18.37 and 18.81 for the LMC and SMC, respectively. The LMC DM is shorter than the currently accepted value, which is in the range 18.42 to 18.55 (Walker 2012), and it is speculated that the p-factor may depend on metallicity.

A GLOBAL PHYSICAL MODEL FOR CEPHEIDS

We perform a global fit to ~5,000 radial velocity and ~177,000 magnitude measurements in 29 photometric bands covering 0.3 to 8.0 microns distributed among 287 Galactic, LMC, and SMC Cepheids with P > 10 days. We assume that the Cepheid light curves and radial velocities are fully characterized by distance, reddening, and time-dependent radius and temperature variations. We construct phase curves of radius and temperature for periods between 10 and 100 days, which yield light curve templates for all our photometric bands and can be easily generalized to any additional band. With only 4 to 6 parameters per Cepheid, depending on the existence of velocity data and the amount of freedom in the distance, the models have typical rms light and velocity curve residuals of 0.05 mag and 3.5 km/s. The model derives the mean Cepheid spectral energy distribution and its derivative with respect to temperature, which deviate from a black body in agreement with metal-line and molecular opacity effects. We determine a mean reddening law towards the Cepheids in our sample, which is not consistent with standard assumptions in either the optical or near-IR. Based on stellar atmosphere models we predict the biases in distance, reddening, and temperature determinations due to the metallicity and we quantify the metallicity signature expected for our fit residuals. The observed residuals as a function of wavelength show clear differences between the individual galaxies, which are compatible with these predictions. In particular, we find that metal-poor Cepheids appear hotter. Finally, we provide a framework for optimally selecting filters that yield the smallest overall errors in Cepheid parameter determination, or filter combinations for suppressing or enhancing the metallicity effects on distance determinations. We make our templates publicly available.

Calibrating the Cepheid period-luminosity relation from the infrared surface brightness technique

The extragalactic distance scale builds directly on the Cepheid Period-Luminosity (PL) relation as delineated by the sample of Cepheids in the Large Magellanic Cloud (LMC). However, the LMC is a dwarf irregular galaxy, quite different from the massive spiral galaxies used for calibrating the extragalactic distance scale. Recent investigations suggest that not only the zero-point but also the slope of the Milky Way PL relation differ significantly from that of the LMC, casting doubts on the universality of the Cepheid PL relation. We want to make a differential comparison of the PL relations in the two galaxies by delineating the PL relations using the same method, the infrared surface brightness method (IRSB), and the same precepts. The IRSB method is a Baade-Wesselink type method to determine individual distances to Cepheids. We apply a newly revised calibration of the method as described in an accompanying paper (Paper I) to 36 LMC and five SMC Cepheids and delineate new PL relations in the V,I,J, & K bands as well as in the Wesenheit indices in the optical and near-IR. We present 509 new and accurate radial velocity measurements for a sample of 22 LMC Cepheids, enlarging our earlier sample of 14 stars to include 36 LMC Cepheids. The new calibration of the IRSB method is directly tied to the recent HST parallax measurements to ten Milky Way Cepheids, and we find a LMC barycenter distance modulus of 18.45+-0.04 (random error only) from the 36 individual LMC Cepheid distances. We find a significant metallicity effect on the Wvi index gamma(Wvi)=-0.23+-0.10 mag/dex as well as an effect on the slope. The K-band PL relation on the other hand is found to be an excellent extragalactic standard candle being metallicity insensitive in both slope and zero-point and at the same time being reddening insensitive and showing the least internal dispersion.

USING ULTRA LONG PERIOD CEPHEIDS TO EXTEND THE COSMIC DISTANCE LADDER TO 100 Mpc AND BEYOND

We examine the properties of 17 long period (80-180 days) and very luminous (median absolute magnitude of M_I= -7.93 and M_V= -7.03) Cepheids to see if they can serve as an useful distance indicator. We find that these Ultra Long Period (ULP) Cepheids have a relatively shallow Period-Luminosity (PL) relation, so in fact they are more "standard candle"-like than classical Cepheids. In the reddening-free Wesenheit index, the slope of the ULP PL relation is ~10 times less steep than the standard PL relation for the SMC Cepheids. The scatter of our sample about the W_I PL relation is 0.22 mag, approaching that of classical Cepheids and Type Ia Supernovae. We expect this scatter to decrease as bigger and more uniform samples of ULP Cepheids are obtained. We also measure a non-zero period derivative for one ULP Cepheid (SMC HV829) and use the result to probe evolutionary models and mass loss of massive stars. ULP Cepheids main advantage over classical Cepheids is that they are more luminous, and as such show great potential as stellar distance indicators to galaxies up to 100 Mpc and beyond.

New period-luminosity and period-color relations of classical Cepheids

The photometric data for 460 classical, fundamental-mode Cepheids in the SMC with measured by Udalski et al. have been analyzed for their period-color (P-C) and period-luminosity (P-L) relations, and for the variation in amplitude across the instability strip in a similar way to what was done in Papers I and II of this series. The SMC Cepheids are bluer in at a given period than for both the Galaxy and the LMC. Their P-C relation in is best fit by two lines intersecting at days. Their break must necessarily exist also in the P-L relations in B and/or V, but it remains hidden in the magnitude scatter. An additional pronounced break of the P-L relations in B, V, and I occurs at days. The observed slope of the lines of constant period in the HR diagram agrees with the theoretical expectation from the pulsation equation. The largest amplitude Cepheids for periods less than 13 days occur near the blue edge of the instability strip. The sense is reversed in the period interval from 13 to 20 days, as in the Galaxy and the LMC. The SMC P-L relation is significantly flatter than that for the Galaxy, NGC 3351, NGC 4321, M31, all of which have nearly the same steep slope. The SMC P-L slope is intermediate between that of these steep slope cases and the very shallow slope of Cepheids in the lower metallicity galaxies of NGC 3109 and Sextans A/B, consistent with the premise that the Cepheid P-L relation varies from galaxy-to-galaxy as a function of metallicity. Failure to take the slope differences in the P-L relation into account as a function of metallicity using Cepheids as distance indicators results in incorrect Cepheid distances. Part of the 15% difference between our long distance scale – now independently supported by tip of the red-giant branch (TRGB) distances – and that of the HST Key Project short scale is due to the effect of using an inappropriate P-L relation.

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.

The metallicity dependence of the CepheidPL-relation

A sample of 37 Galactic, 10 LMC and 6 SMC cepheids is compiled for which individual metallicity estimates exist and BVIK photometry in almost all cases. The Galactic cepheids all have an individual distance estimate available. For the MC objects different sources of photometry are combined to obtain improved periods and mean magnitudes. A multi-parameter Period-Luminosity relation is fitted to the data which also solves for the distance to the LMC and SMC. When all three galaxies are considered, without metallicity effect, a significant quadratic term in log P is found, as previously observed and also predicted in some theoretical calculations. For the present sample it is empirically determined that for log P < 1.65 linear PL-relations may be adopted, but this restricts the sample to only 4 LMC and 1 SMC cepheid. Considering the Galactic sample a metallicity effect is found in the zero point in the VIWK PL-relation (-0.6 \pm 0.4 or -0.8 \pm 0.3 mag/dex depending on the in- or exclusion of one object), in the sense that metal-rich cepheids are brighter. The small significance is mostly due to the fact that the Galactic sample spans a narrow metallicity range. The error is to a significant part due to the error in the metallicity determinations and not to the error in the fit. Including the 5 MC cepheids broadens the observed metallicity range and a metallity effect of about -0.27 \pm 0.08 mag/dex in the zero point is found in VIWK, in agreement with some previous empirical estimates, but now derived using direct metallicity determinations for the cepheids themselves.

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.

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.

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.

Nonlinear Convective Pulsation Models of Second Overtone SMC Cepheids

AbstractNonlinear convective pulsation models representative of SMC Cepheids pulsating in the second overtone mode are presented. Model predictions are compared with observations and implications are derived for second overtone selection criteria. The role of overtone Cepheids as distance indicators is finally investigated and the derived distance scale is at odds with the so called short distance scale.

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.

Empirical Approach to Cepheid Period-Luminosity-Color and Period-Luminosity Relations by Surface Brightness

view Abstract Citations (16) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Empirical Approach to Cepheid Period-Luminosity-Color and Period-Luminosity Relations by Surface Brightness di Benedetto, G. P. Abstract Empirical period-luminosity-color and period-luminosity (PLC [V, B-V] and PL[V] relations according to Stefan's law are derived for Cepheid variables by applying thermal scales of nonvariable giant and supergiant stars established by modern observational interferometry. Intrinsic surface brightnesses are predicted by the apparent color index V - K as fiducial reference unbiased by metallicity and gravity effects, along with current optical absorption data. Linear radii are inferred from periods through the period-radius relation, assumed to be insensitive to abundance and brightness variations. The dependence of PLC and PL zero points to abundance effects is further investigated. I compare current Cepheid data from our metal- normal Galaxy (GAL) with those in the Magellanic Clouds (LMC and SMC) of poorer metal content. The major achievement is that the PL(V) relations are found to be severely biased by metallicity, whereas the PLC(V, B- V) relations turn out to be much less affected by abundances than previously expected. The new PLC and PL relations calibrated on fundamental quantities are compared with those from the classical zero-age main-sequence (ZAMS) fitting empirical approach. It is shown that all metal-normal equations lead to remarkably consistent luminosities of GAL Cepheids, to within +/- 0.05 mag (PL) and +/- 0.02 mag (PLC), whereas metal-poor LMC and SMC results appear to be significantly discrepant. It is emphasized that theoretical thermal scales applied for the derivation of ZAMS metal- corrected relations, either explicit or implicit, are mainly responsible for such observed discrepancies. The actual empirical approach appears to overcome most of the previous difficulties closely related to the poor knowledge of Cepheid thermal properties. Theoretical Cepheid thermal scales are critically reviewed. It is found that recent metal-normal color-brightness relations lead to results remarkably consistent with those from observational scales, whereas metal-poor predictions appear to be overestimated, at least for Cepheid periods log P (days) > 1 currently observed in extragalactic work. Theoretical PLC(V, B-V) and PL(V) relations revised according to unbiased conversion scales now support the above finding that PLC relations are less affected by metallicity than PL relations. This also enables the reconciliation of evolution, pulsation, and stellar atmosphere theories since the ratio of evolution to pulsation mass is found to be very close to 1 for all GAL, LMC, and SMC Cepheids. Because of the poorer metal content of the Magellanic Clouds, current PL( V) distance to the LMC and SMC are found to be underestimated, being referred to metal-normal GAL Cepheids as primary calibrators. Distance moduli through either infrared photometry (V, V - K) (pulsation parallaxes) or optical photometry (V, B- V) now become (m - M)_0_(LMC) = 18.64 +/- 0.02, (m - M)_0_(SMC) = 19.09 +/- 0.02. Publication: The Astrophysical Journal Pub Date: October 1995 DOI: 10.1086/176291 Bibcode: 1995ApJ...452..195D Keywords: GALAXIES: DISTANCES AND REDSHIFTS; GALAXIES: MAGELLANIC CLOUDS; STARS: ABUNDANCES; STARS: FUNDAMENTAL PARAMETERS; STARS: OSCILLATIONS; STARS: VARIABLES: CEPHEIDS full text sources ADS | data products SIMBAD (3) NED (2)