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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.
| Elemental Abundance Ratios in Stars of the Outer Galactic Disk. III. Cepheids
We present metallicities, [Fe/H], and elemental abundance ratios,[X/Fe], for a sample of 24 Cepheids in the outer Galactic disk based onhigh-resolution echelle spectra. The sample members have galactocentricdistances covering 12 kpc<=RGC<=17.2 kpc, making themthe most distant Galactic Cepheids upon which detailed abundanceanalyses have been performed. We find subsolar ratios of [Fe/H] andoverabundances of [α/Fe], [La/Fe], and [Eu/Fe] in the programstars. All abundance ratios exhibit a dispersion that exceeds themeasurement uncertainties. As seen in our previous studies of old openclusters and field giants, enhanced ratios of [α/Fe] and [Eu/Fe]reveal that recent star formation has taken place in the outer disk withType II supernovae preferentially contributing ejecta to theinterstellar medium and with Type Ia supernovae playing only a minorrole. The enhancements for La suggest that asymptotic giant branch starshave contributed to the chemical evolution of the outer Galactic disk.Some of the young Cepheids are more metal-poor than the older openclusters and field stars at comparable galactocentric distances. Thisdemonstrates that the outer disk is not the end result of the isolatedevolution of an ensemble of gas and stars. We showed previously that theolder open clusters and field stars reached a basement metallicity atabout 10-11 kpc. The younger Cepheids reach the same metallicity but atlarger galactocentric distances, roughly 14 kpc. This suggests that theGalactic disk has been growing with time, as predicted from numericalsimulations. The outer disk Cepheids appear to exhibit a bimodaldistribution for [Fe/H] and [α/Fe]. Most of the Cepheids continuethe trends with galactocentric distance exhibited by S. M. Andrievsky'slarger Cepheid sample, and we refer to these stars as the ``GalacticCepheids.'' A minority of the Cepheids show considerably lower [Fe/H]and higher [α/Fe], and we refer to these stars as the ``MergerCepheids.'' One signature of a merger event would be compositiondifferences between the Galactic and Merger Cepheids. The Cepheidssatisfy this requirement, and we speculate that the distinctcompositions suggest that the Merger Cepheids may have formed under theinfluence of significant merger or accretion events. The short lifetimesof the Cepheids reveal that the merger event may be ongoing, with theMonoceros Ring and Canis Major galaxy being possible merger candidates.This paper makes use of observations obtained at the National OpticalAstronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA), Inc., under contract fromthe National Science Foundation. We also employ data products from theTwo Micron All Sky Survey, which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center,California Institute of Technology, funded by the National Aeronauticsand Space Administration and the National Science Foundation.
| 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.
| 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.
| Galactic Cepheids. II. Lithium
We report on the discovery of two lithium Cepheids in the Galaxy, basedon observations made with the echelle spectrograph of the Apache PointObservatory. We have used high-resolution, high signal-to-noise ratiospectra to determine abundances of chemical elements in 16 classicalCepheids. Only two of our program stars show a lithium line, RX Aur andYZ Aur (RX Aur has been also classified by us as a new nonradialpulsator). For the others, including the stars with [N/C]<0.2, Li isdepleted up to logN(Li)<1.0. Hence, it appears that mixing depletesLi before stars enter the instability strip. According to stellarmodels, the main mixing event takes place when Teff dropsbelow 4000 K, which is outside the red edge of the instability strip;i.e., after stars have crossed the instability strip for the first time.
| 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.
| Mean JHK Magnitudes of Fundamental-Mode Cepheids from Single-Epoch Observations
We present an empirical method for converting single-point near-infraredJ, H, and K measurements of fundamental-mode Cepheids to meanmagnitudes, using complete light curves in V or I bands. The algorithmis based on the template light curves in the near-infrared bandpasses.The mean uncertainty of the method is estimated to about 0.03 mag, whichis smaller than the uncertainties obtained in other approaches to theproblem in the literature.
| 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.
| Radial Velocities of Galactic Cepheids
We report 490 radial velocities for 16 Galactic Cepheid variables. Thetypical uncertainty of a single velocity is +/-0.40 km s-1.Comparison with published velocities shows excellent agreement. Two ofthe Cepheids (Z Lac, S Sge) are known binaries and exhibit orbitalvelocity changes in our observing interval.
| Phase-dependent Variation of the Fundamental Parameters of Cepheids. II. Periods Longer than 10 Days
We present the results of a detailed multiphase spectroscopic analysisof 14 classical Cepheids with pulsation periods longer than 10 days. Foreach star, we have derived phased values of effective temperature,surface gravity, microturbulent velocity, and elemental abundances. Weshow that the elemental abundance results for these Cepheids areconsistent for all pulsational phases.
| Zur Beobachtung und Auswertung von Cepheiden-Lichtkurven.
Not Available
| Welchen Lichtwechsel kann ein Beobachter bei Cepheiden erwarten?
Not Available
| The secret lives of Cepheids: evolutionary changes as found from historic and modern observations.
Not Available
| Spectroscopy on a Shoestring: Worthwhile Science for Undergraduates
We present the results of an ongoing pilot program utilizing acommercially available SBIG Self-Guided Spectrograph (SGS) and arelatively small Celestron 14" on-campus telescope. Low resolutionspectroscopy (2.4Å) is being carried out at red wavelengths(6400-7600Å). We chose this wavelength range to take advantage ofthe high quantum efficiency of the detector and because of the presenceof important spectra features such as Hα and Titanium Oxide (TiO).We are testing the scientific viability of the spectroscopic system byinitially observing a sample of Be stars as well as bright pulsating redgiants,supergiants, and cepheids.Be stars are noteworthy because they have strong and often variableHα 6563Å emission. We have photometric observations of thePleiades star Pleione (BU Tau), ω Ori, and X Per (HD 24534). Thisallows the comparison of Hα emission changes on both short timescales as well as long ones, particularly for both ω Ori andPleione. For Pleione, dramatic night to night changes were seen in boththe strength and profile of the Hα emission feature. Photometricobservations of ω Ori have been carried out at Villanova since1982 to the present. Having simultaneous spectroscopy and photometry isallowing insights into the nature of its variability.Spectroscopy of pulsating red giants/supergiants as well as cepheids areof interest. Some representative stars include Mira, α Ori, and TVGem, as well as the cepheids δ Ceph, Polaris, and SV Vul. Thesestars undergo spectral changes as they pulsate. For the M stars inparticular, the TiO bands are sensitive to temperature changes in thestar. Complementary V and Wing near-IR TiO band photometry is also beingcarried out for the cooler stars. The initial results of this study willbe presented, showing worthwhile results using a small spectrograph anddedicated telescopes. We acknowledge support of this research by theNASA Delaware Space Grant College and Fellowship Program NGT5-40024.
| On the crossing mode of the long-period Cepheid SV Vulpeculae
The Cepheid SV Vul is demonstrated to be in the second crossing of theinstability strip on the basis of its well-defined period decrease of-214.3 ± 5.5 s yr-1. Recent arguments from itsatmospheric abundance pattern that it is in the first crossing are notsupported by the historical photometric data. Furthermore, anexamination of atmospheric abundance patterns for other Cepheids inadvanced strip crossing modes suggests that the presence ofCNO-processed material in the atmospheres of intermediate-mass starsdoes not arise from red supergiant dredge-up phases but from meridionalmixing during previous evolution as rapidly-rotating main-sequenceB-type stars.
| Improvement of the CORS method for Cepheids radii determination based on Strömgren photometry
In this paper we present a modified version of the CORS method based ona new calibration of the Surface Brightness function in theStrömgren photometric system. The method has been tested by meansof synthetic light and radial velocity curves derived from nonlinearpulsation models. Detailed simulations have been performed to take intoaccount the quality of real observed curves as well as possible shiftsbetween photometric and radial velocity data. The method has been thenapplied to a sample of Galactic Cepheids with Strömgren photometryand radial velocity data to derive the radii and a new PR relation. As aresult we find log R = (1.19 ± 0.09) + (0.74 ± 0.11) logP (rms = 0.07). The comparison between our result and previous estimatesin the literature is satisfactory. Better results are expected from theadoption of improved model atmosphere grids.
| 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
| The Galactic abundance gradient from Cepheids. V. Transition zone between 10 and 11 kpc
This paper reports on the spectroscopic investigation of 12 Cepheidswhich are situated in the crucial region of galactocentric distancesfrom 9 kpc to 12 kpc, where according to our previous results(Andrievsky et al. \cite{andret02c}; Luck et al. \cite{lucket03}) theradial metallicity distribution experiences an obvious change. Inparticular, the wriggle in the iron abundance distribution is found tofall approximately at galactocentric distances 10-11 kpc (assuminggalactocentric distance of the Sun RG, ȯ = 7.9 kpc).Within the transition zone from 10 to 11 kpc the relative-to-solar ironabundance decreases approximately to -0.2 dex. The new sample of stars,analyzed in present paper, gives results supporting the previousconclusion about the multimodal character of the metallicitydistribution in galactic disc. Using a quite simple consideration ofgalactic chemical evolution we show that the observed distribution canbe explained in the framework of a model which includes the spiral arms.In particular, the wriggle feature associated with RG ≈ 11kpc can be interpreted as a change of metallicity level in the vicinityof the galactic corotation resonance.Based on spectra collected at the 3.6-m Canada-France-Hawaii Telescope.Table A1 (Appendix) is 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/413/159
| Cepheidenbeobachtung in der BAV: Ruckblick und Ausblick.
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| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
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| Cepheid Variables in the AAVSO International Database
There are 205,500 visual observations for 148 Cepheids as well as 182photoelectric observations for 7 Cepheids in the AAVSO InternationalDatabase. These data were reduced with Hertzprung's method and 2,010times of maximum brightness were obtained. O-C diagrams for 21well-observed Cepheids are presented and results obtained are comparedwith existing data.
| Sodium enrichment of the stellar atmospheres. II. Galactic Cepheids
The present paper is a continuation of our study of the sodium abundancein supergiant atmospheres (Andrievsky et al. 2002a). We present theresults on the NLTE abundance determination in Cepheids, and the derivedrelation between the sodium overabundance and their masses.
| Distances to Cepheid open clusters via optical and K-band imaging
We investigate the reddening and main-sequence-fitted distances to 11young, Galactic open clusters that contain Cepheids. Each clustercontains or is associated with at least one Cepheid variable star.Reddening to the clusters is estimated using the U-B:B-V colours of theOB stars and the distance modulus to the cluster is estimated via B-V:Vand V-K:V colour-magnitude diagrams. Our main-sequence fitting assumesthat the solar-metallicity zero-age main sequence of Allen appliesuniversally to all the open clusters, although this point iscontroversial at present. In this way we proceed to calibrate theCepheid period-luminosity (PL) relation and find MV=-2.87× logP- 1.243 +/- 0.09, MK=-3.44 × logP- 2.21 +/-0.10 and absolute distance moduli to the Large Magellanic Cloud (LMC) of18.54 +/- 0.10 from the V-band and 18.48 +/- 0.10 from the K-band givingan overall distance modulus to the LMC of μ0= 18.51 +/-0.10. This is in good agreement with the previous Cepheid PL-K result ofLaney & Stobie at μ0= 18.51 +/- 0.09 and with theHipparcos parallax-calibrated Cepheid PL-K estimate of Feast &Catchpole at μ0= 18.66 +/- 0.10 when no account is takenof the LMC metallicity.We also find that the two-colour U-B:B-V diagrams of two importantclusters are not well fitted by the standard main-sequence line. In onecase, NGC 7790, we find that the F stars show a UV excess and in thesecond case, NGC 6664, they are too red in U-B. Previous spectroscopicestimates of the metallicity of the Cepheids in these clusters appear tosuggest that the effects are not due to metallicity variations. Otherpossible explanations for these anomalies are positional variations inthe dust reddening law and contamination by foreground or backgroundstars.
| Period-Luminosity Relations for Classical Cepheids
BVIJHK period-luminosity relations for classical cepheids in the galaxyand magellanic clouds which pulsate in the fundamental mode are obtainedon the basis of four samples. The period-luminosity relations for theBVI bands obtained here are more accurate than the analogous relationsexisting up to the present. Period-luminosity relations in terms of Wfunctions are also obtained for these samples and have the same accuracyas the currently existing analogous relations.
| The nature of the Cepheid T Antliae
The variable star T Antliae, sometimes suspected to be a type IIpulsator, is demonstrated to be a classical Cepheid in the thirdcrossing of the instability strip. It exhibits a positive period changeof +0.514 +/- 0.016 s yr-1, consistent with a star on theblue side of the instability strip and evolving through it for the thirdtime. The Cepheid exhibits no random fluctuations in pulsation period,although superimposed upon its evolutionary O-C trend are very subtlevariations that may be indicative of orbital motion about an unseencompanion. Archival spectroscopic data also indicate that the star sitson the edge of a putative cluster of B-type stars that may be physicallyassociated with the Cepheid. Additional photometric and spectroscopicdata for the Cepheid and cluster are needed to strengthen the case. Thefield reddening of T Ant established from nearby early-type stars isEB-V=0.30 +/- 0.01 (EB-V(B0)=0.316 +/- 0.014).
| A Bayesian Analysis of the Cepheid Distance Scale
We develop and describe a Bayesian statistical analysis to solve thesurface brightness equations for Cepheid distances and stellarproperties. Our analysis provides a mathematically rigorous andobjective solution to the problem, including immunity from Lutz-Kelkerbias. We discuss the choice of priors, show the construction of thelikelihood distribution, and give sampling algorithms in a Markov chainMonte Carlo approach for efficiently and completely sampling theposterior probability distribution. Our analysis averages over theprobabilities associated with several models rather than attempting topick the ``best model'' from several possible models. Using a sample of13 Cepheids we demonstrate the method. We discuss diagnostics of theanalysis and the effects of the astrophysical choices going into themodel. We show that we can objectively model the order of Fourierpolynomial fits to the light and velocity data. By comparison withtheoretical models of Bono et al. we find that EU Tau and SZ Tau areovertone pulsators, most likely without convective overshoot. Theperiod-radius and period-luminosity relations we obtain are shown to becompatible with those in the recent literature. Specifically, we findlog()=(0.693+/-0.037)[log(P)-1.2]+(2.042+/-0.047) andv>=-(2.690+/-0.169)[log(P)-1.2]-(4.699+/-0.216).
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