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High-resolution spectroscopy for Cepheids distance determination. IV. Time series of Hα line profiles
Context: In recent years, infrared interferometry has revealed thepresence of faint dusty circumstellar envelopes (CSE) around Cepheids.However the size, shape, chemical nature, and the interaction of the CSEwith the star itself are still under investigation. The presence of aCSE might have an effect on the angular diameter estimates used in theinterferometric Baade-Wesselink and surface-brightness methods ofdetermining the distance of Cepheids. Aims: By studying Hαprofiles as a function of the period, we investigate the permanent massloss and the CSE around Cepheids. Our high spectral- and time-resolutiondata, combined with a very good S/N, will be useful in constrainingfuture hydrodynamical models of Cepheids atmosphere and their closeenvironment. Methods: We present HARPS (High Accuracy Radial velocityPlanetary Search project developed by the European SouthernObservatory.) high-resolution spectroscopy (R = 120 000) of eightgalactic Cepheids: R Tra, S Cru, Y Sgr, β Dor, zeta Gem, RZ Vel,ell Car, and RS Pup, providing a good period sampling (P = 3.39 d to P =41.52 d). The Hα line profiles are described for all stars using a2D (wavelength versus pulsation phase) representation. For each star, anaverage spectral line profile is derived, together with its first moment(γ-velocity) and its asymmetry (γ-asymmetry). Results:Short-period Cepheids show Hα line profiles following thepulsating envelope of the star, while long-period Cepheids show verycomplex line profiles and, in particular, large asymmetries. We find anew relationship between the period of Cepheids and theirγ-velocities and -asymmetries. These results may be related to thedynamical structure of the atmosphere and to a permanent mass loss ofCepheids. In particular, we confirm for ell Car a dominant absorptioncomponent whose velocity is constant and nearly of zero kms-1 in the stellar rest frame. This component is attributedto the presence of circumstellar envelope. Conclusions: To understandthese very subtle γ effects, fully consistent hydrodynamicalmodels are required, including pulsating and evolutionary theories,convective energy transport, adaptive numerical meshes, and a refinedcalculation of the radiative transfer.Based on observations made with ESO telescopes at the Silla ParanalObservatory under program IDs 072.D-0419 and 073.D-0136.
| High-resolution spectroscopy for Cepheids distance determination. III. A relation between γ-velocities and γ-asymmetries
Context: Galactic Cepheids in the vicinity of the Sun have a residualline-of-sight velocity, or γ-velocity, which shows a systematicblueshift of about 2 km s-1 compared to an axisymmetricrotation model of the Milky Way. This term is either related to thespace motion of the star and, consequently, to the kinematic structureof our Galaxy, or it is the result of the dynamical structure of theCepheids' atmosphere. Aims: We aim to show that these residualγ-velocities are an intrinsic property of Cepheids. Methods: Weobserved eight galactic Cepheids with the HARPS (High Accuracy Radialvelocity Planetary Search project developed by the European SouthernObservatory.) spectroscope, focusing specifically on 17 spectral lines.For each spectral line of each star, we computed the γ-velocity(resp. γ-asymmetry) as an average value of the interpolated radialvelocity (resp. line asymmetry) curve. Results: For each Cepheid in oursample, a linear relation is found between the γ-velocities of thevarious spectral lines and their corresponding γ-asymmetries,showing that residual γ-velocities stem from the intrinsicproperties of Cepheids. We also provide a physical reference to thestellar γ-velocity: it should be zero when the γ-asymmetryis zero. Following this definition, we provide very precise andphysically calibrated estimates of the γ-velocities for all starsof our sample [ in km s-1] : -11.3 ± 0.3 [R TrA], -3.5± 0.4 [S Cru], -1.5 ± 0.2 [Y Sgr], 9.8 ± 0.1 [β Dor] , 7.1 ± 0.1 [ zeta Gem] , 24.6 ± 0.4 [RZ Vel],4.4 ± 0.1 [ ell Car] , 25.7 ± 0.2 [RS Pup]. Finally, weinvestigated several physical explanations for these γ-asymmetrieslike velocity gradients or the relative motion of the line-formingregion compared to the corresponding mass elements. However, none ofthese hypotheses seems to be entirely satisfactory to explain theobservations. Conclusions: To understand this very subtleγ-asymmetry effect, further numerical studies are needed.Cepheids' atmosphere are strongly affected by pulsational dynamics,convective flows, nonlinear physics, and complex radiative transport.Hence, all of these effects have to be incorporated simultaneously andconsistently into the numerical models to reproduce the observed lineprofiles in detail.Based on observations made with ESO telescopes at the Silla ParanalObservatory under programme IDs 072.D-0419 and 073.D-0136.Table 2 is only available in electronic form at http://www.aanda.org
| On the Enhancement of Mass Loss in Cepheids Due to Radial Pulsation
An analytical derivation is presented for computing mass-loss rates ofCepheids by using the method of Castor, Abbott, and Klein modified toinclude a term for momentum input from pulsation and shocks generated inthe atmosphere. Using this derivation, mass-loss rates of Cepheids aredetermined as a function of stellar parameters. When applied to a set ofknown Cepheids, the calculated mass-loss rates range from10-10 to 10-7 Msolar yr-1,larger than if the winds were driven by radiation alone. Infraredexcesses based on the predicted mass-loss rates are compared toobservations from optical interferometry and IRAS, and predictions aremade for Spitzer observations. The mass-loss rates are consistent withthe observations, within the uncertainties of each. The rate of periodchange of Cepheids is discussed and shown to relate to mass loss, albeitthe dependence is very weak. There is also a correlation between thelarge mass-loss rates and the Cepheids with slowest absolute rate ofperiod change due to evolution through the instability strip. Theenhanced mass loss helps illuminate the issue of infrared excess and themass discrepancy found in Cepheids.
| 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.
| Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud
Context: For seven years, the EROS-2 project obtained a mass ofphotometric data on variable stars. We present a peculiar Cepheid-likestar, in the direction of the Small Magellanic Cloud, which demonstratesunusual photometric behaviour over a short time interval. Aims: Wereport on data of the photometry acquired by the MARLY telescope andspectroscopy from the EFOSC instrument for this star, calledEROS2 J005135-714459(sm0060n13842), which resemblesthe unusual Cepheid HR 7308. Methods: The lightcurve of our target is analysed using the Analysis of Variance method todetermine a pulsational period of 5.5675 days. A fit of time-dependentFourier coefficients is performed and a search for proper motion isconducted. Results: The light curve exhibits a previously unobservedand spectacular change in both mean magnitude and amplitude, which hasno clear theoretical explanation. Our analysis of the spectrum implies aradial velocity of 104 km s-1 and a metallicity of-0.4±0.2 dex. In the direction of right ascension, we measure aproper motion of 17.4±6.0 mas yr-1 using EROSastrometry, which is compatible with data from the NOMAD catalogue. Conclusions: The nature of EROS2 J005135-714459(sm0060n13842) remainsunclear. For this star, we may have detected a non-zero proper motion,which would imply that it is a foreground object. Its radial velocity,pulsational characteristics, and photometric data, however, suggest thatit is instead a Cepheid-like object located in the SMC. In such a case,it would present a challenge to conventional Cepheid models.Based on observations made by the EROS-2 collaboration with the MARLY,1.54 m Danish and 3.60 m telescopes at the European SouthernObservatory, La Silla, Chile. The photometric data is only available inelectronic form at the 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/486/891
| 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
| Cepheid parallaxes and the Hubble constant
Revised Hipparcos parallaxes for classical Cepheids are analysedtogether with 10 Hubble Space Telescope (HST)-based parallaxes. In areddening-free V, I relation we find that the coefficient of logP is thesame within the uncertainties in our Galaxy as in the Large MagellanicCloud (LMC), contrary to some previous suggestions. Cepheids in theinner region of NGC4258 with near solar metallicities confirm thisresult. We obtain a zero-point for the reddening-free relation and applyit to the Cepheids in galaxies used by Sandage et al. to calibrate theabsolute magnitudes of Type Ia supernova (SNIa) and to derive the Hubbleconstant. We revise their result for H0 from 62 to 70 +/-5kms-1Mpc-1. The Freedman et al. value is revisedfrom 72 to 76 +/- 8kms-1Mpc-1. These results areinsensitive to Cepheid metallicity corrections. The Cepheids in theinner region of NGC4258 yield a modulus of 29.22 +/- 0.03 (int.)compared with a maser-based modulus of 29.29 +/- 0.15. Distance modulifor the LMC, uncorrected for any metallicity effects, are 18.52 +/- 0.03from a reddening-free relation in V, I; 18.47 +/- 0.03 from aperiod-luminosity relation at K; 18.45 +/- 0.04 from aperiod-luminosity-colour relation in J, K. Adopting a metallicitycorrection in V, I from Macri et al. leads to a true LMC modulus of18.39 +/- 0.05.
| High-resolution spectroscopy for Cepheids distance determination. II. A period-projection factor relation
Context: The projection factor is a key quantity for the interferometricBaade-Wesselink (hereafter IBW) and surface-brightness (hereafter SB)methods of determining the distance of Cepheids. Indeed, it allows aconsistent combination of angular and linear diameters of the star. Aims: We aim to determine consistent projection factors that include thedynamical structure of the Cepheids' atmosphere. Methods:Hydrodynamical models of δ Cep and ℓ Car have been used tovalidate a spectroscopic method of determining the projection factor.This method, based on the amplitude of the radial velocity curve, isapplied to eight stars observed with the HARPS spectrometer. Theprojection factor is divided into three sub-concepts : (1) a geometricaleffect, (2) the velocity gradient within the atmosphere, and (3) therelative motion of the "optical" pulsating photosphere compared to thecorresponding mass elements (hereafter fo-g). Both, (1) and(3) are deduced from geometrical and hydrodynamical models,respectively, while (2) is derived directly from observations. Results:The Fe I 4896.439 Å line is found to be the best one to use in thecontext of IBW and SB methods. A coherent and consistentperiod-projection factor relation (hereafter Pp relation) is derived forthis specific spectral line: p = [-0.064 ± 0.020] log P + [1.376± 0.023]. This procedure is then extended to derive dynamicprojection factors for any spectral line of any Cepheid. Conclusions:This Pp relation is an important tool for removing bias in thecalibration of the period-luminosity relation of Cepheids. Moreover, itreveals a new physical quantity fo-g to investigate in thenear future.Table 1 is only available in electronic form at http://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.
| High resolution spectroscopy for Cepheids distance determination. I. Line asymmetry
Context: .The ratio of pulsation to radial velocity (the projectionfactor) is currently limiting the accuracy of the Baade-Wesselinkmethod, and in particular of its interferometric version recentlyapplied to several nearby Cepheids. Aims: .This work aims atestablishing a link between the line asymmetry evolution over theCepheids' pulsation cycles and their projection factor, with the finalobjective to improve the accuracy of the Baade-Wesselink method fordistance determinations. Methods: .We present HARPS high spectralresolution observations (R=120 000) of nine galactic Cepheids:R Tra, S Cru, YSgr, β Dor, ζGem, Y Oph, RZ Vel,ℓ Car and RS Pup, having agood period sampling (P=3.39d to P=41.52d). We fit spectral lineprofiles by an asymmetric bi-Gaussian to derive radial velocity,Full-Width at Half-Maximum in the line (FWHM) and line asymmetry for allstars. We then extract correlations curves between radial velocity andasymmetry. A geometric model providing synthetic spectral lines,including limb-darkening, a constant FWHM (hereafter σ_C) and therotation velocity is used to interpret these correlations curves.Results: .For all stars, comparison between observations and modellingis satisfactory, and we were able to determine the projected rotationvelocities and σC for all stars. We also find acorrelation between the rotation velocity (V_rot sin i) and the periodof the star: V_rot sin i= (-11.5 ± 0.9) log (P) + (19.8 ±1.0) [ km s-1] . Moreover, we observe a systematic shift inobservational asymmetry curves (noted γ_O), related to the periodof the star, which is not explained by our static model:γ_O=(-10.7 ± 0.1) log (P) + (9.7 ± 0.2) [in %]. Forlong-period Cepheids, in which velocity gradients, compression or shockwaves seem to be large compared to short- or medium-period Cepheids weobserve indeed a greater systematic shift in asymmetry curves.Conclusions: .This new way of studying line asymmetry seems to be verypromising for a better understanding of Cepheids atmosphere and todetermine, for each star, a dynamic projection factor.
| Angular diameter amplitudes of bright Cepheids.
Expected mean angular diameters and amplitudes of angular diametervariations are estimated for all monoperiodic Classical Cepheidsbrighter than < V > = 8.0 mag. The catalog is intended to helpselecting best Cepheid targets for interferometric observations.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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.
| Mean Angular Diameters and Angular Diameter Amplitudes of Bright Cepheids
We predict mean angular diameters and amplitudes of angular diametervariations for all monoperiodic PopulationI Cepheids brighter than=8.0 mag. The catalog is intended to aid selecting mostpromising Cepheid targets for future interferometric observations.
| 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
| Consistent distances from Baade-Wesselink analyses of Cepheids and RR Lyraes
By using the same algorithm in the Baade-Wesselink analyses of GalacticRR Lyrae and Cepheid variables, it is shown that, within 0.03-mag1σ statistical error, they yield the same distance modulus for theLarge Magellanic Cloud. By fixing the zero-point of thecolour-temperature calibration to those of the current infrared fluxmethods and using updated period-luminosity-colour relations, we get anaverage value of 18.55 for the true distance modulus of the LMC.
| 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
| 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).
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Dades d'Observació i Astrometria
| Constel·lació: | Vela |
| Ascensió Recta: | 08h37m01.30s |
| Declinació: | -44°06'52.9" |
| Magnitud Aparent: | 7.267 |
| Distancia: | 740.741 parsecs |
| Moviment propi RA: | -2.3 |
| Moviment propi Dec: | 3.7 |
| B-T magnitude: | 8.875 |
| V-T magnitude: | 7.4 |
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