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# ζ Gem (Mekbuda)

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 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. Cepheid distances from interferometry .Long baseline interferometry is now able to resolve the pulsationalchange of the angular diameter of a significant number of Cepheids inthe solar neighborhood. This allows the application of a new version ofthe Baade-Wesselink (BW) method to measure their distance, for which wedo not need to estimate the star's temperature. Using angular diametermeasurements from the VLT Interferometer, we derived the distances tofour nearby Cepheids. For three additional stars, we obtained averagevalues of their angular diameters. Based on these new measurements andalready existing data, we derived calibrations of the Period-Luminosityand Period-Radius relations. We also obtained reliable surfacebrightness-color relations, that can be employed for the infraredsurface brightness version of the BW method. The metallicity dependence of the Cepheid period-luminosity relation .We have assessed the influence of the stellar iron content on theCepheid Period-Luminosity (PL) relation by relating the V band residualsfrom the \citet{fre01} PL relation to [Fe/H] for 68 Galactic andMagellanic Cloud Cepheids. The iron abundances were measured from FEROSand UVES high-resolution and high signal-to-noise optical spectra. Ourdata indicate that the stars become fainter as metallicity increases,until a plateau or turnover point is reached at about solar metallicity.This behavior appears at odds both with the PL relation beingindependent from iron abundance and with Cepheids becoming monotonicallybrighter as metallicity increases \citep[e.g.][]{ken98,sak04}. Period changes in cluster and association Cepheids.Cepheids belonging to open clusters and associations exhibit identicalcharacteristics to field Cepheids in terms of their period changes. Elemental Abundance Ratios in Stars of the Outer Galactic Disk. III. CepheidsWe 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. High resolution spectroscopy for Cepheids distance determination. I. Line asymmetryContext: .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. Extended envelopes around Galactic Cepheids. I. ℓ Carinae from near and mid-infrared interferometry with the VLTIWe present the results of long-baseline interferometric observations ofthe bright southern Cepheid ℓ Carinae in the infrared N (8-13 μm)and K (2.0-2.4 μm) bands, using the MIDI and VINCI instruments of theVLT Interferometer. We resolve in the N band a large circumstellarenvelope (CSE) that we model with a Gaussian of 3 Rstar(≈500 Rȯ ≈ 2-3 AU) half width at half maximum. Thesignature of this envelope is also detected in our K band data as adeviation from a single limb darkened disk visibility function. Thesuperimposition of a Gaussian CSE on the limb darkened disk model of theCepheid star results in a significantly better fit of our VINCI data.The extracted CSE parameters in the K band are a half width at halfmaximum of 2 Rstar, comparable to the N band model, and atotal brightness of 4% of the stellar photosphere. A possibility is thatthis CSE is linked to the relatively large mass loss rate of ℓ Car.Though its physical nature cannot be determined from our data, wediscuss an analogy with the molecular envelopes of RV Tauri, redsupergiants and Miras. Predicting accurate stellar angular diameters by the near-infrared surface brightness techniqueI report on the capabilities of the near-infrared (near-IR) surfacebrightness technique to predict reliable stellar angular diameters asaccurate as <~2 per cent using standard broad-band Johnson photometryin the colour range -0.1 <= (V-K)O<= 3.7 includingstars of A, F, G, K spectral type. This empirical approach is fast toapply and leads to estimated photometric diameters in very goodagreement with recent high-precision interferometric diametermeasurements available for non-variable dwarfs and giants, as well asfor Cepheid variables. Then I compare semi-empirical diameters predictedby model-dependent photometric and spectrophotometric (SP) methods withnear-IR surface brightness diameters adopted as empirical referencecalibrators. The overall agreement between all these methods is withinapproximately +/-5 per cent, confirming previous works. However, on thesame scale of accuracy, there is also evidence for systematic shiftspresumably as a result of an incorrect representation of the stellareffective temperature in the model-dependent results. I also comparemeasurements of spectroscopic radii with near-IR surface brightnessradii of Cepheids with known distances. Spectroscopic radii are found tobe affected by a scatter as significant as >~9 per cent, which is atleast three times greater than the formal error currently claimed by thespectroscopic technique. In contrast, pulsation radii predicted by theperiod-radius (PR) relation according to the Cepheid period result aresignificantly less dispersed, indicating a quite small scatter as aresult of the finite width of the Cepheid instability strip, as expectedfrom pulsation theory. The resulting low level of noise stronglyconfirms our previous claims that the pulsation parallaxes are the mostaccurate empirical distances presently available for Galactic andextragalactic Cepheids. Welchen Lichtwechsel kann ein Beobachter bei Cepheiden erwarten?Not Available Phase-dependent Variation of the Fundamental Parameters of Cepheids. III. Periods between 3 and 6 DaysWe present the results of a detailed multiphase spectroscopic analysisof six classical Cepheids with pulsation periods between 3 and 6 days.For each 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. Mean Angular Diameters and Angular Diameter Amplitudes of Bright CepheidsWe 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. CHARM2: An updated Catalog of High Angular Resolution MeasurementsWe present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773 The influence of chemical composition on the properties of Cepheid stars. I. Period-Luminosity relation vs. iron abundanceWe 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 Cepheid pulsations resolved by the VLTI.Not Available Period-luminosity relations for Galactic Cepheid variables with independent distance measurementsIn 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. A new Period-Radius relation for Galactic Classical CepheidsWe discuss a new Period-Radius (PR) relation for Galactic ClassicalCepheids, obtained by means of a new version of the CORS method whichhas been modified in order to be run with the Strömgren photometricsystem. The major change consists in the calibration of the SurfaceBrightness as a function of the two reddening free'' colourindexes [c1] and [m1], by means of the model atmospheres by Castelli etal. (1997). In this contribution we first briefly discuss somenumerical experiments performed on the basis of synthetic Cepheid lightcurves to test the accuracy of the method, and then report thePeriod-Radius relation for Classical Cepheids obtained by applying thethe new method to a sample of Galactic Cepheids. Cepheidenbeobachtung in der BAV: Ruckblick und Ausblick.Not Available Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.Not Available The Angular Size of the Cepheid l Carinae: A Comparison of the Interferometric and Surface Brightness TechniquesRecent interferometric observations of the brightest and angularlylargest classical Cepheid, l Carinae, with ESO's Very Large TelescopeInterferometer have resolved with high precision the variation of itsangular diameter with phase. We compare the measured angular diametercurve to the one that we derive by an application of theBaade-Wesselink-type infrared surface brightness technique and find anear-perfect agreement between the two curves. The mean angulardiameters of l Car from the two techniques agree very well within theirtotal error bars (1.5%), as do the derived distances (4%). This resultis an indication that the calibration of the surface brightnessrelations used in the distance determination of far-away Cepheids is notaffected by large biases. An Error Analysis of the Geometric Baade-Wesselink MethodWe derive an analytic solution for the minimization problem in thegeometric Baade-Wesselink method. This solution allows deriving thedistance and mean radius of a pulsating star by fitting its velocitycurve and angular diameter measured interferometrically. The method alsoprovides analytic solutions for the confidence levels of the best-fitparameters and accurate error estimates for the Baade-Wesselinksolution. Special care is taken in the analysis of the various errorsources in the final solution, among which are the uncertainties due tothe projection factor, the limb darkening, and the velocity curve. Wealso discuss the importance of the phase shift between the stellar lightcurve and the velocity curve as a potential error source in thegeometric Baade-Wesselink method. We finally discuss the case of theclassical Cepheid ζ Gem, applying our method to the measurementsderived with the Palomar Testbed Interferometer. We show how a carefultreatment of the measurement errors can be used to discriminate betweendifferent models of limb darkening by using interferometric techniques. Cepheid Brightness Distribution Detection by the Stellar ImagerA space based interferometer with a sub-milliarcsecond resolution couldbe used to make accurate measurements of the disk brightnessdistributions for several nearby Cepheids. The direct imaging ofCepheids disk brightness distributions will allow a significantimprovement of our understanding of Cepheid atmospheres and pulsationprocesses and will yield a more accurate determination of thecosmological distance scale based on the Cepheids period-luminosity zeropoint. We present the results from our feasibility study of the diskbrightness distribution imaging of the nearby Cepheid zeta Gem using theStellar Imager (SI) (see http://hires.gsfc.nasa.gov./ si/). The SI, aNASA Vision Mission study project, is conceived as a large,multispacecraft sparse aperture (20-30 mirrors) imaging interferometerwith  500m maximum baseline operating at UV-optical wavelengths. Wecarried out simulations of the disk brightness distributions for thenearby Cepheid zeta Gem using time-dependent hydrodynamic atmosphericmodels and radiative transfer modeling in the MgII h&k lines and inthe nearby continuum. Images obtained at several pulsation phases wereused to produce simulated SI images. Our simulations, made with twodifferent configurations of the interferometer, show that SI will beable to measure changes in the zeta Gem brightness distribution atdifferent pulsational phases. We also discuss the potential andchallenges of a sparse aperture interferometry for imaging Cepheidsurfaces.Acknowledgement: This work was supported by the NSF, grant number9731923, the SAO Summer Intern Program, and by the SI Vision Missionstudy grant NNG04GM92G from NASA/GSFC to SAO. MK is a member of theChandra X-ray Center (NASA contract NAS8-39073). Cepheid distances from infrared long-baseline interferometry. III. Calibration of the surface brightness-color relationsThe recent VINCI/VLTI observations presented in Paper I have nearlydoubled the total number of available angular diameter measurements ofCepheids. Taking advantage of the significantly larger color rangecovered by these observations, we derive in the present paper highprecision calibrations of the surface brightness-color relations usingexclusively Cepheid observations. These empirical laws make it possibleto determine the distance to Cepheids through a Baade-Wesselink typetechnique. The least dispersed relations are based on visible-infraredcolors, for instance FV(V-K) = -0.1336 ± 0.0008 (V-K)+ 3.9530 ± 0.0006}. The convergence of the Cepheid (this work)and dwarf star (Kervella et al. \cite{kervella04c}) visible-infraredsurface brightness-color relations is strikingly good. The astrophysicaldispersion of these relations appears to be very small, and below thepresent detection sensitivity.Table \ref{table_measurements1} is only available in electronic form athttp://www.edpsciences.org Cepheid distances from infrared long-baseline interferometry. II. Calibration of the period-radius and period-luminosity relationsUsing our interferometric angular diameter measurements of sevenclassical Cepheids reported in Kervella et al. (\cite{kervella04},A&A, 416, 941 - Paper I), complemented by previously existingmeasurements, we derive new calibrations of the Cepheid period-radius(P-R) and period-luminosity (P-L) relations. We obtain a P-R relation oflog R = [0.767 ± 0.009] log P + [1.091 ± 0.011], only 1σ away from the relation obtained by Gieren et al.(\cite{gieren98}, ApJ, 496, 17). We therefore confirm their P-R relationat a level of Δ(log R) = ± 0.02. We also derive an originalcalibration of the P-L relation, assuming the slopes derived by Gierenet al. (\cite{gieren98}) from LMC Cepheids, αK = -3.267± 0.042 and αV = -2.769 ± 0.073. With aP-L relation of the form Mλ =αλ (log P - 1) + βλ, weobtain log P = 1 reference points of βK = -5.904± 0.063 and βV = -4.209 ± 0.075. Ourcalibration in the V band is statistically identical to the geometricalresult of Lanoix et al. (\cite{lanoix99}, MNRAS, 308, 969). Improvement of the CORS method for Cepheids radii determination based on Strömgren photometryIn 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-relationA 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. Cepheid distances from infrared long-baseline interferometry. I. VINCI/VLTI observations of seven Galactic CepheidsWe report the angular diameter measurements of seven classical Cepheids,X Sgr, η Aql, W Sgr, ζ Gem, β Dor, Y Oph and ℓ Carthat we have obtained with the VINCI instrument, installed at ESO's VLTInterferometer (VLTI). We also present reprocessed archive data obtainedwith the FLUOR/IOTA instrument on ζ Gem, in order to improve thephase coverage of our observations. We obtain average limb darkenedangular diameter values of /line{θLD}[X Sgr] = 1.471± 0.033 mas, /line{θLD[η Aql] = 1.839± 0.028 mas, /line{θLD}[W Sgr] = 1.312 ±0.029 mas, /line{θLD}[β Dor] = 1.891 ±0.024 mas, /line{θLD}[ζ Gem] =1.747 ±0.061 mas, /line{θLD}[Y Oph] = 1.437 ± 0.040mas, and /line{θLD}[ℓ Car] = 2.988 ± 0.012mas. For four of these stars, η Aql, W Sgr, β Dor, and ℓCar, we detect the pulsational variation of their angular diameter. Thisenables us to compute directly their distances, using a modified versionof the Baade-Wesselink method: d[η Aql] =276+55-38 pc, d[W Sgr] =379+216-130 pc, d[β Dor] =345+175-80 pc, d[ℓ Car] =603+24-19 pc. The stated error bars arestatistical in nature. Applying a hybrid method, that makes use of theGieren et al. (\cite{gieren98}) Period-Radius relation to estimate thelinear diameters, we obtain the following distances (statistical andsystematic error bars are mentioned): d[X Sgr] = 324 ± 7 ±17 pc, d[η Aql] = 264 ± 4 ± 14 pc, d[W Sgr] = 386± 9 ± 21 pc, d[β Dor] = 326 ± 4 ± 19pc, d[ζ Gem] = 360 ± 13 ± 22 pc, d[Y Oph] = 648± 17 ± 47 pc, d[ℓ Car] = 542 ± 2 ± 49pc.Tables 3 to 10 are only available in electronic form athttp://www.edpsciences.org Optical interferometry in astronomyHere I review the current state of the field of optical stellarinterferometry, concentrating on ground-based work although a briefreport of space interferometry missions is included. We pause both toreflect on decades of immense progress in the field as well as toprepare for a new generation of large interferometers just now beingcommissioned (most notably, the CHARA, Keck and VLT Interferometers).First, this review summarizes the basic principles behind stellarinterferometry needed by the lay-physicist and general astronomer tounderstand the scientific potential as well as technical challenges ofinterferometry. Next, the basic design principles of practicalinterferometers are discussed, using the experience of past and existingfacilities to illustrate important points. Here there is significantdiscussion of current trends in the field, including the new facilitiesunder construction and advanced technologies being debuted. This decadehas seen the influence of stellar interferometry extend beyond classicalregimes of stellar diameters and binary orbits to new areas such asmapping the accretion discs around young stars, novel calibration of thecepheid period-luminosity relation, and imaging of stellar surfaces. Thethird section is devoted to the major scientific results frominterferometry, grouped into natural categories reflecting these currentdevelopments. Lastly, I consider the future of interferometry,highlighting the kinds of new science promised by the interferometerscoming on-line in the next few years. I also discuss the longer-termfuture of optical interferometry, including the prospects for spaceinterferometry and the possibilities of large-scale ground-basedprojects. Critical technological developments are still needed to makethese projects attractive and affordable. A Revised Calibration of the MV-W(O I 7774) Relationship using Hipparcos Data: Its Application to Cepheids and Evolved StarsA new calibration of the MV-W(O I 7774) relationship hasbeen calculated using better reddening and distance estimates for asample of 27 calibrator stars of spectral types A to G, based onaccurate parallaxes and proper motions from the Hipparcos and Tychocatalogues. The present calibration predicts absolute magnitude withaccuracies of +/-0.38mag for a sample covering a large range ofMV, from -9.5 to +0.35 mag. The color term included in aprevious paper has been dropped since its inclusion does not lead to anysignificant improvement in the calibration. The variation of the O I7774 feature in the classical cepheid SS Sct has been studied. Wecalculated a phase-dependent correction to random phase OI featurestrengths in Cepheids, such that it predicts mean absolute magnitudesusing the above calibration. After applying such a correction, we couldincrease the list of calibrators to 58 by adding MV and O Itriplet strength data for 31 classical Cepheids. The standard error ofthe calibration using the composite sample was comparable to thatobtained from the primary 27 calibrators, showing that it is possible tocalculate mean Cepheid luminosities from random phase observations ofthe O I 7774 feature. We use our derived calibrations to estimateMV for a set of evolved objects to be able to locate theirpositions in the HR diagram. Annual report of the director for fiscal year 2001-2002.Not Available Cepheid Variables and the Circum/Interstellar MatterVarious aspects of the relation of classical Cepheids and inter- andcircumstellar matter are summarized. Emphasis is given to the questionof mass loss from Cepheids and to the role of these pulsating variablesin revealing the recent star formation history in their neighbourhood.
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