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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.
| Evidence for a universal slope of the period-luminosity relation from direct distances to Cepheids in the LMC.
We have applied the infrared surface brightness (ISB) technique toderive distances to 13 Cepheid variables in the LMC which have periodsfrom 3-42 days. The corresponding absolute magnitudes define PLrelations in VIWJK bands which agree exceedingly well with thecorresponding Milky Way relations obtained from the same technique, andare in significant disagreement with the observed LMC Cepheid PLrelations, by OGLE-II and Persson et al., in these bands. Our datauncover a systematic error in the p-factor law which transforms Cepheidradial velocities into pulsational velocities. We correct the p-factorlaw by requiring that all LMC Cepheids share the same distance.Re-calculating all Milky Way and LMC Cepheid distances with the revisedp-factor law, we find that the PL relations from the ISB technique bothin LMC and in the Milky Way agree with the OGLE-II and Persson et al.LMC PL relations, supporting the conclusion of no metallicity effect onthe slope of the Cepheid PL relation in optical/near infrared bands.
| How good are RR Lyrae and Cepheids really as distance indicators? . The observational approach
A number of recent technical developments, including the Hipparcossatellite, the Hubble Space Telescope fine guidance sensors and longbase line near-IR interferometry has made it possible to employ severallargely geometrical methods to determine direct distances to RR Lyraestars and Cepheids. The distance scale now rests on a much firmer basisand the significant differences between the distances based on RR Lyraestars (short) and Cepheids (long) to the LMC have been largelyeliminated. The effects of metallicity on the RR Lyrae period-luminosity(PL) relation in the K-band as well as on the Cepheid PL relationappears to be the main remaining issues but even here empirical resultsare beginning to show convergence. I review here some of these recentdevelopments seen from the perspective of the near-IR surface brightnessmethod.
| 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.
| Extended envelopes around Galactic Cepheids. II. Polaris and δ Cephei from near-infrared interferometry with CHARA/FLUOR
We present the results of long-baseline interferometric observations ofthe classical Cepheids Polaris and δ Cep in the near infrared K'band (1.9-2.3 μm), using the FLUOR instrument of the CHARA Array.Following our previous detection of a circumstellar envelope (CSE)around ℓ Car (Kervella et al. 2006), we report similar detectionsaround Polaris and δ Cep. Owing to the large data set acquired onPolaris, in both the first and second lobes of visibility function, wehave detected the presence of a circum-stellar envelope (CSE), locatedat 2.4±0.1 stellar radii, accounting for 1.5±0.4% of thestellar flux in the K band. A similar model is applied to the δCep data, which shows improved agreement compared to a model withoutCSE. Finally, we find that the bias in estimating the angular diameterof δ Cep in the framework of the Baade-Wesselink method(Mérand et al 2005b) is of the order of 1% or less in the K band.A complete study of the influence of the CSE is proposed in thiscontext, showing that at the optimum baseline for angular diametervariation detection, the bias is of the order of the formal precision inthe determination of the δ Cep pulsation amplitude (1.6%).
| Extended envelopes around Galactic Cepheids. I. ℓ Carinae from near and mid-infrared interferometry with the VLTI
We 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.
| Dust scattering in the Miras R Car and RR Sco resolved by optical interferometric polarimetry
We present optical interferometric polarimetry measurements of theMira-like variables R Car and RR Sco, using the Sydney UniversityStellar Interferometer. By making visibility measurements in twoperpendicular polarizations, the relatively low-surface brightness lightscattered by atmospheric dust could be spatially separated from thebright Mira photospheric flux. This is the first reported successful useof long-baseline optical interferometric polarimetry. Observations wereable to place constraints on the distribution of circumstellar materialin R Car and RR Sco. The inner radius of dust formation for both starswas found to be less than 3 stellar radii: much closer than the expectedinnermost stable location for commonly assumed astrophysical `dirtysilicate' dust in these systems (silicate dust with a significant ironcontent). A model with the dust distributed over a shell which isgeometrically thin compared to the stellar radius was preferred over anoutflow. We propose dust components whose chemistry and opacityproperties enable survival at these extreme inner radii.
| Predicting accurate stellar angular diameters by the near-infrared surface brightness technique
I 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.
| Cepheid Pulsation Models at Varying Metallicity and ΔY/ΔZ
In this paper we present an extended set of nonlinear convectivepulsation models at varying metallicity and ΔY/ΔZ ratio. Thepredicted instability strip and bolometric light curves are discussed bycomparing the new models with our previous ones. In particular, thedependence on both metal and helium abundances is investigated. Bytransforming the bolometric light curves into the observational bands,we are able to derive both period-color-luminosity and Wesenheitrelations for each selected chemical composition. Syntheticperiod-luminosity relations are obtained by populating the instabilitystrip according to specific assumptions on the number of pulsators andthe mass distribution. These theoretical results are compared withrecent accurate data by Sandage et al. and Kervella et al. in order totest the predictive capabilities of the models. We confirm our previousresults that the theoretical metallicity correction to the Key ProjectCepheid distance scale depends on both the period range andΔY/ΔZ ratio, becoming important for periods longer than 20days and ΔY/ΔZ>1.5.
| 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.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We 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 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
| Cepheid pulsations resolved by the VLTI.
Not Available
| Cepheidenbeobachtung in der BAV: Ruckblick und Ausblick.
Not Available
| The Angular Size of the Cepheid l Carinae: A Comparison of the Interferometric and Surface Brightness Techniques
Recent 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.
| Cepheid distances from infrared long-baseline interferometry. III. Calibration of the surface brightness-color relations
The 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 relations
Using 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).
| 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.
| Cepheid distances from infrared long-baseline interferometry. I. VINCI/VLTI observations of seven Galactic Cepheids
We 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
| 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
| 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.
| Cepheid Variables and the Circum/Interstellar Matter
Various 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.
| From Amateur Astronomer to Observatory Director: The Curious Case of R. T. A. Innes
Robert Innes was one of a select band of amateur astronomers who madethe transition to professional ranks towards the end of the nineteenthcentury. Initially he had a passion for mathematical astronomy, butafter settling in Sydney he developed a taste for observationalastronomy, specialising in the search for new double stars. He quicklybecame known for his success in this field and for his publications onsolar system perturbations, and with John Tebbutt's patronage managed tosecure a clerical position at the Royal Observatory, Cape of Good Hope.Once there he continued to observe in his spare time and to publish,and, with strong support from Sir David Gill, was appointed foundingDirector of the Transvaal Observatory. By the time he died in 1933,Innes had received an honorary D.Sc. from Leiden University, and hadestablished an international reputation as a positional astronomer. Thispaper provides an interesting case study of a well-known`amateur-turned-professional', and an example of the ways in whichpatronage played a key role in nineteenth and early twentieth centuryAustralian and South African astronomy.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
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