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Galactic Cepheids with Spitzer. I. Leavitt Law and Colors
Classical Cepheid variable stars have been important indicators ofextragalactic distance and Galactic evolution for over a century. TheSpitzer Space Telescope has opened the possibility of extending thestudy of Cepheids into the mid- and far-infrared, where interstellarextinction is reduced. We have obtained photometry from images of asample of Galactic Cepheids with the Infrared Array Camera (IRAC) andMultiband Infrared Photometer for Spitzer instruments on Spitzer. Herewe present the first mid-infrared period-luminosity relations forClassical Cepheids in the Galaxy, and the first ever Cepheidperiod-luminosity relations at 24 and 70 ?m. We compare theserelations with theoretical predictions, and with period-luminosityrelations obtained in recent studies of the Large Magellanic Cloud. Wefind a significant period-color relation for the [3.6] - [8.0]IRAC color. Other mid-infrared colors for both Cepheids and non-variablesupergiants are strongly affected by variable molecular spectralfeatures, in particular deep CO absorption bands. We do not find strongevidence for mid-infrared excess caused by warm (~500 K) circumstellardust. We discuss the possibility that recent detections withnear-infrared interferometers of circumstellar shells around ?Cep, ell Car, Polaris, Y Oph, and RS Pup may be a signature of shockedgas emission in a dust-poor wind associated with pulsation-driven massloss.
| Characteristics of the Galaxy according to Cepheids
Classical and Type II Cepheids are used to reinvestigate specificproperties of the Galaxy. A new Type II reddening-free Cepheid distanceparametrization is formulated from Large Magellanic Cloud (LMC) Cepheids(OGLE), with uncertainties typically no larger than 5-15 per cent. Adistance to the Galactic centre of R0 = 7.8 +/- 0.6kpc isderived from the median distance to Type II Cepheids in the bulge(OGLE), R0 = 7.7 +/- 0.7kpc from a distance to the near sideof the bulge combined with an estimated bulge radius of 1.3 +/- 0.3kpcderived from planetary nebulae. The distance of the Sun from theGalactic plane inferred from classical Cepheid variables isZsolar = 26 +/- 3pc, a result dependent on the sample'sdistance and direction because of the complicating effects of Gould'sBelt and warping in the Galactic disc. Classical Cepheids and young openclusters delineate consistent and obvious spiral features, althoughtheir characteristics do not match conventional pictures of the Galaxy'sspiral pattern. The Sagittarius-Carina arm is confirmed as a majorspiral arm that appears to originate from a different Galactic regionthan suggested previously. Furthermore, a major feature is observed toemanate from Cygnus-Vulpecula and may continue locally near the Sun.Significant concerns related to the effects of metallicity on theVI-based reddening-free Cepheid distance relations used here are allayedby demonstrating that the computed distances to the Galactic centre, andto several globular clusters (M54, NGC 6441, M15 and M5) and galaxies(NGC 5128 and NGC 3198) which likely host Type II Cepheids: agree withliterature results to within the uncertainties. An additional empiricaltest is proposed to constrain any putative metallicity dependence ofCepheid distance determinations through forced matches of distanceestimates to a particular galaxy using both Type II and classicalCepheids.
| Observational studies of Cepheid amplitudes. I. Period-amplitude relationships for Galactic Cepheids and interrelation of amplitudes
Context: The dependence of amplitude on the pulsation period differsfrom other Cepheid-related relationships. Aims: We attempt torevise the period-amplitude (P-A) relationship of Galactic Cepheidsbased on multi-colour photometric and radial velocity data. Reliable P-Agraphs for Galactic Cepheids constructed for the U, B, V, R_C, andIC photometric bands and pulsational radial velocityvariations facilitate investigations of previously poorly studiedinterrelations between observable amplitudes. The effects of bothbinarity and metallicity on the observed amplitude, and the dichotomybetween short- and long-period Cepheids can both be studied. Methods: A homogeneous data set was created that contains basicphysical and phenomenological properties of 369 Galactic Cepheids.Pulsation periods were revised and amplitudes were determined by theFourier method. P-A graphs were constructed and an upper envelope to thedata points was determined in each graph. Correlations between variousamplitudes and amplitude-related parameters were searched for, usingCepheids without known companions. Results: Large amplitudeCepheids with companions exhibit smaller photometric amplitudes onaverage than solitary ones, as expected, while s-Cepheids pulsate withan arbitrary (although small) amplitude. The ratio of the observedradial velocity to blue photometric amplitudes, AV_RAD/A_B,is not as good an indicator of the pulsation mode as predictedtheoretically. This may be caused by an incorrect mode assignment to anumber of small amplitude Cepheids, which are not necessarily firstovertone pulsators. The dependence of the pulsation amplitudes onwavelength is used to identify duplicity of Cepheids. More than twentystars previously classified as solitary Cepheids are now suspected tohave a companion. The ratio of photometric amplitudes observed invarious bands confirms the existence of a dichotomy among normalamplitude Cepheids. The limiting period separating short- andlong-period Cepheids is 10.47 days. Conclusions:Interdependences of pulsational amplitudes, the period dependence of theamplitude parameters, and the dichotomy have to be taken into account asconstraints in modelling the structure and pulsation of Cepheids.Studies of the P-L relationship must comply with the break at 10.47°instead of the currently used “convenient” value of 10 days.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/504/959
| Reddenings of FGK supergiants and classical Cepheids from spectroscopic data
Accurate and homogeneous atmospheric parameters(Teff,logg,Vt, [Fe/H]) are derived for 74 FGKnon-variable supergiants from high-resolution, high signal-to-noiseratio, echelle spectra. Extremely high precision for the inferredeffective temperatures (10-40K) is achieved by using the line-depthratio method. The new data are combined with atmospheric values for 164classical Cepheids, observed at 675 different pulsation phases, takenfrom our previously published studies. The derived values are correlatedwith unreddened B - V colours compiled from the literature for theinvestigated stars in order to obtain an empirical relationship of theform (B - V)0 = 57.984 -10.3587(logTeff)2 +1.67572(logTeff)3 - 3.356logg +0.0321Vt + 0.2615[Fe/H] + 0.8833(logg)(logTeff).The expression is used to estimate colour excesses E(B - V) forindividual supergiants and classical Cepheids, with a precision of+/-0.05 mag for supergiants and Cepheids with n = 1-2 spectra, reaching+/-0.025mag for Cepheids with n > 2 spectra, matching uncertaintiesfor the most sophisticated photometric techniques. The reddening scaleis also a close match to the system of space reddenings for Cepheids.The application range is for spectral types F0-K0 and luminosity classesI and II.
| The influence of chemical composition on the properties of Cepheid stars. II. The iron content
Context: The Cepheid period-luminosity (PL) relation is unquestionablyone of the most powerful tools at our disposal for determining theextragalactic distance scale. While significant progress has been madein the past few years towards its understanding and characterizationboth on the observational and theoretical sides, the debate on theinfluence that chemical composition may have on the PL relation is stillunsettled. Aims: With the aim to assess the influence of the stellariron content on the PL relation in the V and K bands, we have relatedthe V-band and the K-band residuals from the standard PL relations ofFreedman et al. (2001, ApJ, 553, 47) and Persson et al. (2004, AJ, 128,2239), respectively, to [Fe/H]. Methods: We used direct measurements ofthe iron abundances of 68 Galactic and Magellanic Cepheids from FEROSand UVES high-resolution and high signal-to-noise spectra. Results: Wefind a mean iron abundance ([Fe/H]) about solar (σ = 0.10) for ourGalactic sample (32 stars), ~-0.33 dex (σ = 0.13) for the LargeMagellanic Cloud (LMC) sample (22 stars) and ~-0.75 dex (σ = 0.08)for the Small Magellanic Cloud (SMC) sample (14 stars). Our abundancemeasurements of the Magellanic Cepheids double the number of starsstudied up to now at high resolution. The metallicity affects the V-bandCepheid PL relation and metal-rich Cepheids appear to be systematicallyfainter than metal-poor ones. These findings depend neither on theadopted distance scale for Galactic Cepheids nor on the adopted LMCdistance modulus. Current data do not allow us to reach a firmconclusion concerning the metallicity dependence of the K-band PLrelation. The new Galactic distances indicate a small effect, whereasthe old ones support a marginal effect. Conclusions: Recent robustestimates of the LMC distance and current results indicate that theCepheid PL relation is not Universal.Based on observations madewith ESO Telescopes at Paranal and La Silla Observatories underproposal ID 66.D-0571.Full Table [see full textsee full textsee full textsee full text] isonly available in electronic form at http://www.aanda.org
| Baade-Wesselink distances and the effect of metallicity in classical cepheids
Context: The metallicity dependence of the Cepheid PL-relation is ofimportance in establishing the extra-galactic distance scale. Aims: Theaim of this paper is to investigate the metallicity dependence of thePL-relation in V and K based on a sample of 68 Galactic Cepheids withindividual Baade-Wesselink distances (some of the stars also have anHST-based parallax) and individually determined metallicities fromhigh-resolution spectroscopy. Methods: Literature values of the V-band,K-band and radial velocity data have been collected for a sample of 68classical cepheids that have their metallicity determined in theliterature from high-resolution spectroscopy. Based on a (V-K)surface-brightness relation and a projection factor derived in aprevious paper, distances have been derived from a Baade-Wesselinkanalysis. PL- and PLZ-relations in V and K are derived. Results: Theeffect of the adopted dependence of the projection factor on period isinvestigated. The change from a constant p-factor to one recentlysuggested in the literature with a mild dependence on log P results in aless steep slope by 0.1 unit, which is about the 1-sigma error bar inthe slope itself. The observed slope in the PL-relation in V in the LMCagrees with both hypotheses. In K the difference between the Galacticand LMC slope is larger and would favour a mild period dependence of thep-factor. The dependence on metallicity in V and K is found to bemarginal, and independent of the choice of p-factor on period. Thisresult is severely limited by the small range in metallicity covered bythe Galactic Cepheids.
| Color Excesses of Classical Cepheids in uvby Photometry
In order to determine color excess in the uvby color system forfundamental-mode classical Cepheids, 29 Cepheids whose reliable distancevalues were compiled by Ngeow and Kanbur were selected as calibrationstars. Then intrinsic photometric indices were calculated using givendistances to derive a calibrated empirical relation between(b-y)0 and period, [c1], and [m1]through a linear fit. This relation was used to determine color excessesof E(b-y) for 116 Cepheids, and the period-color relation was derived.
| Classical Cepheid pulsation models. XI. Effects of convection and chemical composition on the period-luminosity and period-Wesenheit relations
In spite of the relevance of classical Cepheids as primary distanceindicators, a general consensus on the dependence of theperiod-luminosity (PL) relation on the Cepheid chemical composition hasnot yet been achieved. From the theoretical point of view, our previousinvestigations were able to reproduce some empirical tests for suitableassumptions on the helium-to-metal relative enrichment, but theseresults relied on specific assumptions concerning the mass-luminosityrelation and the efficiency of the convective transfer in the pulsatingenvelopes. In this paper, we investigate the effects of the assumedvalue of the mixing-length parameter l/Hp on the pulsationproperties and we release the assumption of a fixed mass-luminosityrelation. To this purpose, new nonlinear convective fundamentalpulsation models have been computed for various chemical compositions(Z=0.004, 0.008, 0.01 and 0.02) and adopting l/H_p=1.7-1.8, which islarger than that (1.5) used in our previous papers. From the extendedmodel set, synthetic PL relations in the various photometric bands arederived using the predicted instability strip together with recentevolutionary tracks. We show that as the l/Hp value increasesthe pulsation region gets narrower, mostly due to the blueward shift ofthe red edge for fundamental pulsation, with the effect becomingstronger at the higher metal contents (Z≥ 0.01). However, thecomparison of the new models with previously computed models shows thatthe l/Hp variation has no consequence on the predictedperiod-Wesenheit (PW) relations, which instead are influenced by thepulsator metal content. On this basis, we present a straightforward wayto infer the distance and metal content of variables with observed BVIor BVK magnitudes. As for the PL relations, we show that either thezero-point and the slope are very slightly modified by thel/Hp variation, at constant chemical composition. We alsoconfirm that: (1) moving from visual to longer wavelengths, thepredicted period-magnitude distribution for a given metal contentbecomes narrower and its slope becomes steeper; (2) decreasing the metalcontent, the PL relations become steeper and brighter, with the amountof this metallicity effect decreasing from optical to near-infraredbands. Overall, we show that our pulsation relations appear fullyconsistent with the observed properties of Galactic and Magellanic CloudCepheids, supporting the predicted steepening and brightening of the PLrelations when moving from metal-rich to metal-poor variables. Moreover,we show that the distances inferred by the predicted PW relations agreewith recently measured trigonometric parallaxes, whereas they suggest acorrection to the values based on the Infrared Surface Brightnesstechnique, as already found from an independent method. Finally, alsothe pulsation metal contents suggested by the predicted PW relationsappear in statistical agreement with spectroscopic [Fe/H] measurements.
| A new calibration of Galactic Cepheid period-luminosity relations from B to K bands, and a comparison to LMC relations
Context: The universality of the Cepheid period-luminosity (PL)relations has been under discussion since metallicity effects wereassumed to play a role in the value of the intercept and, more recently,of the slope of these relations. Aims: The goal of the present study isto calibrate the Galactic PL relations in various photometric bands(from B to K) and to compare the results to the well-established PLrelations in the LMC. Methods: We use a set of 59 calibrating stars,the distances of which are measured using five different distanceindicators: Hubble Space Telescope and revised Hipparcos parallaxes,infrared surface brightness and interferometric Baade-Wesselinkparallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes forCepheids belonging to open clusters or OB stars associations. A detaileddiscussion of absorption corrections and projection factor to be used isgiven. Results: We find no significant difference in the slopes of thePL relations between LMC and our Galaxy. Conclusions: We conclude thatthe Cepheid PL relations have universal slopes in all photometric bands,not depending on the galaxy under study (at least for LMC and MilkyWay). The possible zero-point variation with metal content is notdiscussed in the present work, but an upper limit of 18.50 for the LMCdistance modulus can be deduced from our data.Tables 2, 6 and 7 are only available in electronic form athttp://www.aanda.org
| The reliability of Cepheid reddenings based on BVIC photometry
Externally determined values of E(B - V) (Espacered) for 40Galactic Cepheids are compared to reddenings determined using B - V andV - IC colour indices and the method of Dean, Warren &Cousins (EBVIC), updated to allow for metallicitycorrections. With three stars omitted on the grounds of uncertainty intheir space reddenings, we find thatThe two scales agree well in scale and zero-point, and there is nosignificant trend with period. Given the non-zero errors in the Cepheidspace reddenings, the estimated error in BVIC Cepheidreddenings is no more than 0.02.The above results are not significantly changed whether one corrects thereddenings for metallicity using older Bell models, or using more recentmodels by Sandage, Bell & Tripicco. Using the SBT models to correctthe reddenings of Cloud Cepheids for metallicity gives slightly smallerreddenings at a given metal deficiency, yielding `new' median reddeningsof 0.056 (Small Magellanic Cloud) and 0.076 (Large Magellanic Cloud) ifwe assume the same metal deficiencies as Caldwell and Coulson. Withmetal deficiencies of [M/H] = -0.7 and -0.25, the median reddenings are0.040 and 0.058.
| Detailed chemical composition of Galactic Cepheids. A determination of the Galactic abundance gradient in the 8-12 kpc region
Aims.The recent introduction of high-resolution/large spectral-rangespectrographs has provided the opportunity to investigate the chemicalcomposition of classical Cepheids in detail. This paper focusses on newabundance determinations for iron and 6 light metals (O, Na, Mg, Al, Si,Ca) in 30 Galactic Cepheids. We also give a new estimate of the Galacticradial abundance gradient. Methods: The stellar effective temperatureswere determined using the method of line depth ratios, and the surfacegravity and the microturbulent velocity vt by imposing theionization balance between Fe I and Fe II with the help of curves ofgrowth. Abundances were calculated with classical LTE atmosphere models. Results: Abundances were obtained with rms accuracies of about0.05-0.10 dex for Fe, and 0.05-0.20 dex for the other elements. Cepheidsin our sample have solar-like abundances, and current measurements agreequite well with previous determinations. We computed "single zone"Galactic radial abundance gradients for the 8-12 kpc region and found aslope for iron of -0.061 dex kpc-1.Based on observations made with the 1.52 m ESO Telescope at La Silla,Chile.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Period-colour and amplitude-colour relations in classical Cepheid variables - IV. The multiphase relations
The superb phase resolution and quality of the Optical GravitationalLensing Experiment (OGLE) data on the Large Magellanic Cloud (LMC) andSmall Magellanic Cloud (SMC) Cepheids, together with existing data onGalactic Cepheids, are combined to study the period-colour (PC) andamplitude-colour (AC) relations as a function of pulsation phase. Ourresults confirm earlier work that the LMC PC relation (at mean light) ismore consistent with two lines of differing slopes, separated at aperiod of 10 d. However, our multiphase PC relations reveal much newstructure which can potentially increase our understanding of Cepheidvariables. These multiphase PC relations provide insight into why theGalactic PC relation is linear but the LMC PC relation is non-linear.This is because the LMC PC relation is shallower for short (logP < 1)and steeper for long (logP > 1) period Cepheids than thecorresponding Galactic PC relation. Both of the short- and long-periodCepheids in all three galaxies exhibit the steepest and shallowestslopes at phases around 0.75-0.85, respectively. A consequence is thatthe PC relation at phase ~ 0.8 is highly non-linear. Further, theGalactic and LMC Cepheids with logP > 1 display a flat slope in thePC plane at phases close to the maximum light. When the LMCperiod-luminosity (PL) relation is studied as a function of phase, weconfirm that it changes with the PC relation. The LMC PL relation in Vand I band near the phase of 0.8 provides compelling evidence that thisrelation is also consistent with two lines of differing slopes joined ata period close to 10 d.
| Rate of Period Change as a Diagnostic of Cepheid Properties
The rate of period change P˙ for a Cepheid is shown to be aparameter that is capable of indicating the instability-strip crossingmode for individual objects and, in conjunction with light amplitude,the likely location of the object within the instability strip. Theobserved rates of period change in over 200 Milky Way Cepheids aredemonstrated to be in general agreement with predictions from stellarevolutionary models, although the sample also displays features that areinconsistent with some published models and indicative of the importanceof additional factors not fully incorporated in models to date.
| Infrared surface brightness distances to Cepheids: a comparison of Bayesian and linear-bisector calculations.
Bayesian statistical calculations and linear-bisector calculations forobtaining Cepheid distances and radii by the infrared surface brightnessmethod have been compared for a set of 38 Cepheids. The distancesobtained by the two techniques agree to 1.5% ± 0.6% and the radiiagree to 1.1% ± 0.7%. Thus the two methods yield the samedistances and radii at the 2sigma level. This implies that the shortdistance to the LMC found in recent linear-bisector studies of Cepheidsis not a result of simplifications in the mathematical approach. Thecomputed uncertainties in distance and radius are larger in the Bayesiancalculation typically by a factor of three.
| Infrared Surface Brightness Distances to Cepheids: A Comparison of Bayesian and Linear-Bisector Calculations
We have compared the results of Bayesian statistical calculations andlinear-bisector calculations for obtaining Cepheid distances and radiiby the infrared surface brightness method. We analyzed a set of 38Cepheids using a Bayesian Markov Chain Monte Carlo method that had beenrecently studied with a linear-bisector method. The distances obtainedby the two techniques agree to 1.5%+/-0.6%, with the Bayesian distancesbeing larger. The radii agree to 1.1%+/-0.7%, with the Bayesiandeterminations again being larger. We interpret this result asdemonstrating that the two methods yield the same distances and radii.This implies that the short distance to the Large Magellanic Cloud foundin recent linear-bisector studies of Cepheids is not caused bydeficiencies in the mathematical treatment. However, the computeduncertainties in distance and radius for our data set are larger in theBayesian calculation by factors of 1.4-6.7. We give reasons to favor theBayesian computations of the uncertainties. The larger uncertainties canhave a significant impact on interpretation of Cepheid distances andradii obtained from the infrared surface brightness method.
| Pulsation and Evolutionary Masses of Classical Cepheids. I. Milky Way Variables
We investigate a selected sample of Galactic classical Cepheids withavailable distance and reddening estimates in the framework of thetheoretical scenario provided by pulsation models, computed with metalabundance Z=0.02, helium content in the range of Y=0.25-0.31, andvarious choices of the stellar mass and luminosity. After transformingthe bolometric light curve of the fundamental models into BVRIJKmagnitudes, we derived analytical relations connecting the pulsationperiod with the stellar mass, the mean (intensity averaged) absolutemagnitude, and the color of the pulsators. These relations are usedtogether with the Cepheid observed absolute magnitudes in order todetermine the ``pulsation'' mass, Mp, of each individualvariable. The comparison with the ``evolutionary'' masses,Me,can, given by canonical (no convective core overshooting,no mass loss) models of central He-burning stellar structures revealsthat the Mp/Me,can ratio is correlated with theCepheid period, ranging from ~0.8 at logP=0.5 to ~1 at logP=1.5. Wediscuss the effects of different input physics and/or assumptions on theevolutionary computations, as well as of uncertainties in the adoptedCepheid metal content, distance, and reddening. Eventually, we find thatthe pulsational results can be interpreted in terms of mass loss duringor before the Cepheid phase, whose amount increases as the Cepheidoriginal mass decreases. It vanishes around 13 Msolar andincreases up to ~20% at 4 Msolar.
| 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 toderive distances to 13 Cepheid variables in the LMC that span a periodrange from 3 to 42 days. From the absolute magnitudes of the variablescalculated from these distances, we find that the LMC Cepheids definetight period-luminosity (PL) relations in the V, I, W, J, and K bandsthat agree exceedingly well with the corresponding Galactic PL relationsderived from the same technique and are significantly steeper than theLMC PL relations in these bands observed by the OGLE-II Project in V, I,and W and by Persson and coworkers in J and K. We find that the LMCCepheid distance moduli we derive, after correcting them for the tilt ofthe LMC bar, depend significantly on the period of the stars, in thesense that the shortest period Cepheids have distance moduli near 18.3,whereas the longest period Cepheids are found to lie near 18.6. Sincesuch a period dependence of the tilt-corrected LMC distance modulishould not exist, there must be a systematic, period-dependent error inthe ISB technique not discovered in previous work. We identify as themost likely culprit the p-factor, which is used to convert the observedCepheid radial velocities into their pulsational velocities. Bydemanding (1) a zero slope on the distance modulus versus period diagramand (2) a zero mean difference between the ISB and ZAMS fitting distancemoduli of a sample of well-established Galactic cluster Cepheids, wefind that p=1.58(+/-0.02)-0.15(+/-0.05)logP, with the p-factor dependingmore strongly on Cepheid period (and thus luminosity) than indicated bypast theoretical calculations. When we recalculate the distances of theLMC Cepheids with the revised p-factor law suggested by our data, we notonly obtain consistent distance moduli for all stars but also decreasethe slopes in the various LMC PL relations (and particularly in thereddening-independent K and W bands) to values that are consistent withthe values observed by OGLE-II and Persson and coworkers. From our 13Cepheids, we determine the LMC distance modulus to be 18.56+/-0.04 mag,with an additional estimated systematic uncertainty of ~0.1 mag. Usingthe same corrected p-factor law to redetermine the distances of theGalactic Cepheids, the new Galactic PL relations are also foundconsistent with the observed optical and near-infrared PL relations inthe LMC. Our main conclusion from the ISB analysis of the LMC Cepheidsample is that, within current uncertainties, there seems to be nosignificant difference between the slopes of the PL relations in theMilky Way and LMC. With literature data on more metal-poor systems, itseems now possible to conclude that the slope of the Cepheid PL relationis independent of metallicity in the broad range in [Fe/H] from -1.0 dexto solar abundance, within a small uncertainty. The new evidence fromthe first ISB analysis of a sizable sample of LMC Cepheids suggests thatthe previous, steeper Galactic PL relations obtained from this techniquewere caused by an underestimation of the period dependence in themodel-based p-factor law used in the previous work. We emphasize,however, that our current results must be substantiated by newtheoretical models capable of explaining the steeper period dependenceof the p-factor law, and we will also need data on more LMC fieldCepheids to rule out remaining concerns about the validity of ourcurrent interpretation.
| The influence of chemical composition on the properties of Cepheid stars. I. Period-Luminosity relation vs. iron abundance
We have assessed the influence of the stellar iron content on theCepheid Period-Luminosity (PL) relation by relating the V band residualsfrom the Freedman et al. (\cite{fre01}) PL relation to [Fe/H] for 37Galactic and Magellanic Clouds Cepheids. The iron abundances weremeasured from FEROS and UVES high-resolution and high-signal to noiseoptical spectra. Our data indicate that the stars become fainter asmetallicity increases, until a plateau or turnover point is reached atabout solar metallicity. Our data are incompatible with both nodependence of the PL relation on iron abundance, and with the linearlydecreasing behavior often found in the literature (e.g. Kennicutt et al.\cite{ken98}; Sakai et al. \cite{sak04}). On the other hand, non-lineartheoretical models of Fiorentino et al. (\cite{fio02}) provide a fairlygood description of the data.Based on observations made with ESO Telescopes at La Silla and ParanalObservatories under proposal ID 66.D-0571.Table \ref{tab:log} is only available in electronic form athttp://www.edpsciences.org
| The metallicity dependence of the Cepheid PL-relation
A sample of 37 Galactic, 10 LMC and 6 SMC cepheids is compiled for whichindividual metallicity estimates exist and BVIK photometry in almost allcases. The Galactic cepheids all have an individual distance estimateavailable. For the MC objects different sources of photometry arecombined to obtain improved periods and mean magnitudes. Amulti-parameter Period-Luminosity relation is fitted to the data whichalso solves for the distance to the LMC and SMC. When all three galaxiesare considered, without metallicity effect, a significant quadratic termin log P is found, as previously observed and also predicted in sometheoretical calculations. For the present sample it is empiricallydetermined 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 thezero point in the VIWK PL-relation (-0.6 ± 0.4 or -0.8 ±0.3 mag/dex depending on the in- or exclusion of one object), in thesense that metal-rich cepheids are brighter. The small significance ismostly due to the fact that the Galactic sample spans a narrowmetallicity range. The error is to a significant part due to the errorin the metallicity determinations and not to the error in the fit.Including the 5 MC cepheids broadens the observed metallicity range anda metallity effect of about -0.27 ± 0.08 mag/dex in the zeropoint is found in VIWK, in agreement with some previous empiricalestimates, but now derived using direct metallicity determinations forthe cepheids themselves.
| Period-luminosity relations for Galactic Cepheid variables with independent distance measurements
In this paper, we derive the period-luminosity (PL) relation forGalactic Cepheids with recent independent distance measurements fromopen cluster, Barnes-Evans surface brightness, interferometry and HubbleSpace Telescope astrometry techniques. Our PL relation confirms theresults from recent works, which showed that the Galactic Cepheidsfollow a different PL relation to their Large Magellanic Cloud (LMC)counterparts. Our results also show that the slope of the Galactic PLrelation is inconsistent with the LMC slope with more than 95 per centconfidence level. We apply this Galactic PL relation to find thedistance to NGC 4258. Our result of μo= 29.49 +/- 0.06 mag(random error) agrees at the ~1.4σ level with the geometricaldistance of μgeo= 29.28 +/- 0.15 mag from water masermeasurements.
| The effect of metallicity on the Cepheid Period-Luminosity relation from a Baade-Wesselink analysis of Cepheids in the Galaxy and in the Small Magellanic Cloud
We have applied the near-IR Barnes-Evans realization of theBaade-Wesselink method as calibrated by Fouqué & Gieren(\cite{FG97}) to five metal-poor Cepheids with periods between 13 and 17days in the Small Magellanic Cloud as well as to a sample of 34 GalacticCepheids to determine the effect of metallicity on the period-luminosity(P-L) relation. For ten of the Galactic Cepheids we present new accurateand well sampled radial-velocity curves. The Baade-Wesselink analysisprovides accurate individual distances and luminosities for the Cepheidsin the two samples, allowing us to constrain directly, in a purelydifferential way, the metallicity effect on the Cepheid P-L relation.For the Galactic Cepheids we provide a new set of P-L relations whichhave zero-points in excellent agreement with astrometric andinterferometric determinations. These relations can be used directly forthe determination of distances to solar-metallicity samples of Cepheidsin distant galaxies, circumventing any corrections for metallicityeffects on the zero-point and slope of the P-L relation. We findevidence for both such metallicity effects in our data. Comparing ourtwo samples of Cepheids at a mean period of about 15 days, we find aweak effect of metallicity on the luminosity similar to that adopted bythe HST Key Project on the Extragalactic Distance Scale. The effect issmaller for the V band, where we find Δ MV/Δ[Fe/H] = -0.21±0.19, and larger for the Wesenheit index W, wherewe find Δ MW/Δ [Fe/H] = -0.29±0.19. Forthe I and K bands we find Δ MI/Δ [Fe/H] =-0.23± 0.19 and Δ MK/Δ [Fe/H] =-0.21± 0.19, respectively. The error estimates are 1 σstatistical errors. It seems now well established that metal-poorCepheids with periods longer than about 10 days are intrinsicallyfainter in all these bands than their metal-rich counterparts ofidentical period. Correcting the LMC distance estimate of Fouquéet al. (\cite{FSG03}) for this metallicity effect leads to a revised LMCdistance modulus of (m-M)_0 = 18.48± 0.07, which is also inexcellent agreement with the value of (m-M)_0 = 18.50± 0.10adopted by the Key Project. From our SMC Cepheid distances we determinethe SMC distance to be 18.88±0.13 magirrespective of metallicity.Some of the observations reported here were obtained with the MultipleMirror Telescope, operated jointly by the Smithsonian Institution andthe University of Arizona.Tables A.2-A.11 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/531
| Interstellar Extinction and the Intrinsic Colors of Classical Cepheids in the Galaxy, the LMC, and the SMC
New methods are applied to samples of classical cepheids in the galaxy,the Large Magellanic Cloud, and the Small Magellanic Cloud to determinethe interstellar extinction law for the classical cepheids, R B:R V:RI:R J:R H:R K= 4.190:3.190:1.884:0.851:0.501:0.303, the color excessesfor classical cepheids in the galaxy,E(B-V)=-0.382-0.168logP+0.766(V-I), and the color excesses for classicalcepheids in the LMC and SMC, E(B-V)=-0.374-0.166logP+0.766(V-I). Thedependence of the intrinsic color (B-V)0 on the metallicity of classicalcepheids is discussed. The intrinsic color (V-I)0 is found to beabsolutely independent of the metallicity of classical cepheids. A highprecision formula is obtained for calculating the intrinsic colors ofclassical cepheids in the galaxy:(-)0=0.365(±0.011)+0.328(±0.012)logP.
| 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 mostcases) I photometry by Berdnikov et al. (\cite{Berdnikov:etal:00}) andwith homogenized color excesses E(B-V) based on Fernie et al.(\cite{Fernie:etal:95}) are used to determine their period-color (P-C)relation in the range 0.4~ 1.4). The latter effect is enhanced by asuggestive break of the P-L relation of LMC and SMC at log P = 1.0towards still shallower values as shown in a forthcoming paper.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/423
| The galactic abundance gradient from Cepheids. IV. New results for the outer disc
As a continuation of our previous work on the abundance gradient in theouter part of the galactic disc, this paper presents results on themetallicicty distribution over galactocentric distances up to 15 kpc.The outer disc is clearly separated from the middle part by theexistence of a step in the metallicity distribution at about 10 kpc.Taking the region of galactocentric distances from 10 kpc to 15 kpc, onecan derive an iron gradient -0.03 +/- 0.01 dex kpc-1 (25stars). The existence of a discontinuity can be caused by the effectivesuppression of mixing processes near the corotation circle where theradial component of the gas velocity should be very small.Based on spectra collected at the New Technology Telescope at ESO - LaSilla, Chile, under programme 68.D-0201(B).Table A.1 is only available in electronic form athttp://www.edpsciences.org
| Period Changes in Galactic Classical Cepheids. Slow Evolution of Long-Period Cepheids
We compared period changes derived from O-C diagrams for 63 classicalCepheids from our Galaxy with model calculations. We found that forCepheids with log P > 1.0 the observed changes are smaller thanpredicted values, except variable SZ Cas. However some of the firstovertone Cepheids, particularly EU Tau and Polaris, change its periodmuch faster than it follows from theory. Summary of the known data onperiod changes in Cepheids from the Galaxy and from the MagellanicClouds (previous papers) leads to conclusion that none of the 999Cepheids is undergoing the first crossing of the instability strip. Alsothe observed period changes for long-period Cepheids are a few timesslower than predicted by the models. These results imply that muchlarger fraction of helium is burned in the Cepheid stage than it ispredicted by models.
| The Distance Scale for Classical Cepheid Variables
New radii, derived from a modified version of the Baade-Wesselink (BW)method that is tied to published KHG narrowband spectrophotometry, arepresented for 13 bright Cepheids. The data yield a best-fittingperiod-radius relation given bylog=1.071(+/-0.025)+0.747(+/-0.028)logP0. In combination with other high-quality radiusestimates recently published by Laney & Stobie, the new data yield aperiod-radius relation described bylog=1.064(+/-0.0006)+0.750(+/-0.006)logP0, which simplifies to ~P3/4.The relationship is used to test the scale of Cepheid luminositiesinferred from cluster zero-age main-sequence (ZAMS) fitting, for whichwe present an updated list of calibrating Cepheids located in stellargroups. The cluster ZAMS-fitting distance scale tied to a Pleiadesdistance modulus of 5.56 is found to agree closely with the distancescale defined by Hipparcos parallaxes of cluster Cepheids and alsoyields Cepheid luminosities that are a good match to those inferred fromthe period-radius relation. The mean difference between absolute visualmagnitudes based on cluster ZAMS fitting,C, and those inferred for 23 clusterCepheids from radius and effective temperature estimates,BW, in the sense of C-BW is+0.019+/-0.029 s.e. There is no evidence to indicate the need for amajor revision to the Cepheid cluster distance scale. The absolutemagnitude differences are examined using available [Fe/H] data for thecluster Cepheid sample to test the metallicity dependence of theperiod-luminosity relation. Large scatter and a small range ofmetallicities hinder a reliable estimate of the exact relationship,although the data are fairly consistent with predictions from stellarevolutionary models. The derived dependence isΔMV(C-BW)=+0.06(+/-0.03)-0.43(+/-0.54)[ Fe/H].
| Fundamental Parameters of Cepheids. V. Additional Photometry and Radial Velocity Data for Southern Cepheids
I present photometric and radial velocity data for Galactic Cepheids,most of them being in the southern hemisphere. There are 1250 Genevaseven-color photometric measurements for 62 Cepheids, the averageuncertainty per measurement is better than 0.01 mag. A total of 832velocity measurements have been obtained with the CORAVEL radialvelocity spectrograph for 46 Cepheids. The average accuracy of theradial velocity data is 0.38 km s-1. There are 33 stars withboth photometry and radial velocity data. I discuss the possiblebinarity or period change that these new data reveal. I also presentreddenings for all Cepheids with photometry. The data are availableelectronically. Based on observations obtained at the European SouthernObservatory, La Silla.
| Calibration of the distance scale from galactic Cepheids. I. Calibration based on the GFG sample
New estimates of the distances of 36 nearby galaxies are presented basedon accurate distances of galactic Cepheids obtained by Gieren et al.(1998) from the geometrical Barnes-Evans method. The concept of``sosie'' is applied to extend the distance determination toextragalactic Cepheids without assuming the linearity of the PLrelation. Doing so, the distance moduli are obtained in astraightforward way. The correction for extinction is made using twophotometric bands (V and I) according to the principles introduced byFreedman & Madore (1990). Finally, the statistical bias due to theincompleteness of the sample is corrected according to the preceptsintroduced by Teerikorpi (1987) without introducing any free parameters(except the distance modulus itself in an iterative scheme). The finaldistance moduli depend on the adopted extinction ratioRV/RI and on the limiting apparent magnitude ofthe sample. A comparison with the distance moduli recently published bythe Hubble Space Telescope Key Project (HSTKP) team reveals a fairagreement when the same ratio RV/RI is used butshows a small discrepancy at large distance. In order to bypass theuncertainty due to the metallicity effect it is suggested to consideronly galaxies having nearly the same metallicity as the calibratingCepheids (i.e. Solar metallicity). The internal uncertainty of thedistances is about 0.1 mag but the total uncertainty may reach 0.3 mag.The table of the Appendix and Table 3 are available in electronic format CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/383/398, and on ouranonymous ftp-server www-obs.univ-lyon1.fr (pub/base/CEPHEIDES.tar.gz).
| Photoelectric Observations of Southern Cepheids in 2001
A total of 2097 photometric observations in the BVIc systemare presented for 117 Cepheids located in the southern hemisphere. Themain purpose of the photometry is to provide new epochs of maximumbrightness for studying Cepheid period changes, as well as to establishcurrent light elements for the Cepheids.
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