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Random forest automated supervised classification of Hipparcos periodic variable stars
We present an evaluation of the performance of an automatedclassification of the Hipparcos periodic variable stars into 26 types.The sub-sample with the most reliable variability types available in theliterature is used to train supervised algorithms to characterize thetype dependencies on a number of attributes. The most useful attributesevaluated with the random forest methodology include, in decreasingorder of importance, the period, the amplitude, the V-I colour index,the absolute magnitude, the residual around the folded light-curvemodel, the magnitude distribution skewness and the amplitude of thesecond harmonic of the Fourier series model relative to that of thefundamental frequency. Random forests and a multi-stage scheme involvingBayesian network and Gaussian mixture methods lead to statisticallyequivalent results. In standard 10-fold cross-validation (CV)experiments, the rate of correct classification is between 90 and 100per cent, depending on the variability type. The main mis-classificationcases, up to a rate of about 10 per cent, arise due to confusion betweenSPB and ACV blue variables and between eclipsing binaries, ellipsoidalvariables and other variability types. Our training set and thepredicted types for the other Hipparcos periodic stars are availableonline.
| Near-infrared (JHK) Photometry of 131 Northern Galactic Classical Cepheids
Near-infrared photometric measurements for 131 Northern GalacticCepheids are presented. The Cepheid light curves are sampled with anaverage of 22 measurements per star fully covering the phase of eachCepheid. The J, H, and K light curves for each Cepheid were uniformlyinterpolated to find the intensity mean magnitudes within each band. Theresults are consistent within ±1% for 26 stars in common withprevious studies. This paper is the first in a projected series of twopapers which will provide additional fundamental data for Cepheids inthe Galaxy, namely, NIR photometry and line-of-sight extinction. In thecourse of this project, 93 additional variables were fortuitouslyobserved within the Cepheid program fields, 82 of which have previouslynot been identified.
| Observational studies of Cepheid amplitudes. I. Period-amplitude relationships for Galactic Cepheids and interrelation of amplitudes
Context: The dependence of amplitude on the pulsation period differsfrom other Cepheid-related relationships. Aims: We attempt torevise the period-amplitude (P-A) relationship of Galactic Cepheidsbased on multi-colour photometric and radial velocity data. Reliable P-Agraphs for Galactic Cepheids constructed for the U, B, V, R_C, andIC photometric bands and pulsational radial velocityvariations facilitate investigations of previously poorly studiedinterrelations between observable amplitudes. The effects of bothbinarity and metallicity on the observed amplitude, and the dichotomybetween short- and long-period Cepheids can both be studied. Methods: A homogeneous data set was created that contains basicphysical and phenomenological properties of 369 Galactic Cepheids.Pulsation periods were revised and amplitudes were determined by theFourier method. P-A graphs were constructed and an upper envelope to thedata points was determined in each graph. Correlations between variousamplitudes and amplitude-related parameters were searched for, usingCepheids without known companions. Results: Large amplitudeCepheids with companions exhibit smaller photometric amplitudes onaverage than solitary ones, as expected, while s-Cepheids pulsate withan arbitrary (although small) amplitude. The ratio of the observedradial velocity to blue photometric amplitudes, AV_RAD/A_B,is not as good an indicator of the pulsation mode as predictedtheoretically. This may be caused by an incorrect mode assignment to anumber of small amplitude Cepheids, which are not necessarily firstovertone pulsators. The dependence of the pulsation amplitudes onwavelength is used to identify duplicity of Cepheids. More than twentystars previously classified as solitary Cepheids are now suspected tohave a companion. The ratio of photometric amplitudes observed invarious bands confirms the existence of a dichotomy among normalamplitude Cepheids. The limiting period separating short- andlong-period Cepheids is 10.47 days. Conclusions:Interdependences of pulsational amplitudes, the period dependence of theamplitude parameters, and the dichotomy have to be taken into account asconstraints in modelling the structure and pulsation of Cepheids.Studies of the P-L relationship must comply with the break at 10.47°instead of the currently used “convenient” value of 10 days.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/504/959
| Galactic abundance gradients from Cepheids. On the iron abundance gradient around 10-12 kpc
Context: Classical Cepheids are excellent tracers of intermediate-massstars, since their distances can be estimated with very high accuracy.In particular, they can be adopted to trace the chemical evolution ofthe Galactic disk. Aims: Homogeneous iron abundance measurements for 33Galactic Cepheids located in the outer disk together with accuratedistance determinations based on near-infrared photometry are adopted toconstrain the Galactic iron gradient beyond 10 kpc. Methods: Ironabundances were determined using high resolution Cepheid spectracollected with three different observational instruments: ESPaDOnS@CFHT,Narval@TBL and FEROS@2.2m ESO/MPG telescope. Cepheid distances wereestimated using near-infrared (J,H,K-band) period-luminosity relationsand data from SAAO and the 2MASS catalog. Results: The least squaressolution over the entire data set indicates that the iron gradient inthe Galactic disk presents a slope of -0.052±0.003 textrm {dexkpc}-1 in the 5-17 kpc range. However, the change of the ironabundance across the disk seems to be better described by a linearregime inside the solar circle and a flattening of the gradient towardthe outer disk (beyond 10 kpc). In the latter region the iron gradientpresents a shallower slope, i.e. -0.012±0.014 textrm {dexkpc}-1. In the outer disk (10-12 kpc) we also found thatCepheids present an increase in the spread in iron abundance. Currentevidence indicates that the spread in metallicity depends on theGalactocentric longitude. Finally, current data do not support thehypothesis of a discontinuity in the iron gradient at Galactocentricdistances of 10-12 kpc. Conclusions: The occurrence of a spread in ironabundance as a function of the Galactocentric longitude indicates thatlinear radial gradients should be cautiously treated to constrain thechemical evolution across the disk.
| Photoelectric observations of Cepheids in UBV(RI)c (Berdnikov, 2008)
This catalog gathers the observation of 894 Cepheids made between 1986to 2004.Observations are listed in alphabetical order of the constellations. Thestandard deviation for every magnitude and color is 0.01mag.This version supersedes the 1997 edition (Cat. )(3 data files).
| Cepheid parallaxes and the Hubble constant
Revised Hipparcos parallaxes for classical Cepheids are analysedtogether with 10 Hubble Space Telescope (HST)-based parallaxes. In areddening-free V, I relation we find that the coefficient of logP is thesame within the uncertainties in our Galaxy as in the Large MagellanicCloud (LMC), contrary to some previous suggestions. Cepheids in theinner region of NGC4258 with near solar metallicities confirm thisresult. We obtain a zero-point for the reddening-free relation and applyit to the Cepheids in galaxies used by Sandage et al. to calibrate theabsolute magnitudes of Type Ia supernova (SNIa) and to derive the Hubbleconstant. We revise their result for H0 from 62 to 70 +/-5kms-1Mpc-1. The Freedman et al. value is revisedfrom 72 to 76 +/- 8kms-1Mpc-1. These results areinsensitive to Cepheid metallicity corrections. The Cepheids in theinner region of NGC4258 yield a modulus of 29.22 +/- 0.03 (int.)compared with a maser-based modulus of 29.29 +/- 0.15. Distance modulifor the LMC, uncorrected for any metallicity effects, are 18.52 +/- 0.03from a reddening-free relation in V, I; 18.47 +/- 0.03 from aperiod-luminosity relation at K; 18.45 +/- 0.04 from aperiod-luminosity-colour relation in J, K. Adopting a metallicitycorrection in V, I from Macri et al. leads to a true LMC modulus of18.39 +/- 0.05.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| New Period-Luminosity and Period-Color relations of classical Cepheids: I. Cepheids in the Galaxy
321 Galactic fundamental-mode Cepheids with good B, V, and (in mostcases) I photometry by Berdnikov et al. (\cite{Berdnikov:etal:00}) andwith homogenized color excesses E(B-V) based on Fernie et al.(\cite{Fernie:etal:95}) are used to determine their period-color (P-C)relation in the range 0.4~ 1.4). The latter effect is enhanced by asuggestive break of the P-L relation of LMC and SMC at log P = 1.0towards still shallower values as shown in a forthcoming paper.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/423
| Fundamental Parameters of Cepheids. V. Additional Photometry and Radial Velocity Data for Southern Cepheids
I present photometric and radial velocity data for Galactic Cepheids,most of them being in the southern hemisphere. There are 1250 Genevaseven-color photometric measurements for 62 Cepheids, the averageuncertainty per measurement is better than 0.01 mag. A total of 832velocity measurements have been obtained with the CORAVEL radialvelocity spectrograph for 46 Cepheids. The average accuracy of theradial velocity data is 0.38 km s-1. There are 33 stars withboth photometry and radial velocity data. I discuss the possiblebinarity or period change that these new data reveal. I also presentreddenings for all Cepheids with photometry. The data are availableelectronically. Based on observations obtained at the European SouthernObservatory, La Silla.
| Photometry and radial velocities of cepheids and other variable stars in the Galaxy and the LMC
UBVRIc and radial velocity measurements are presented for Galactic andLMC Cepheids, and for several variables of other type. The photometrycomprises 168 objects with 1790 phases, and the speedometry 15 objectswith 97 phases.
| 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
| 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.
| Galactic Cepheids. Catalogue of light-curve parameters and distances
We report a new version of the catalogue of distances and light-curveparameters for Galactic classical Cepheids. The catalogue listsamplitudes, magnitudes at maximum light, and intensity means for 455stars in BVRI filters of the Johnson system and (RI)_C filters of theCron-Cousins system. The distances are based on our new multicolour setof PL relations and on our Cepheid-based solution for interstellarextinction law parameters and are referred to an LMC distance modulus of18.25. The catalogue is only available in electronic form at the CDS viaanonymous ftp (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Multi-colour PL-relations of Cepheids in the bt HIPPARCOS catalogue and the distance to the LMC
We analyse a sample of 236 Cepheids from the hipparcos catalog, usingthe method of ``reduced parallaxes'' in V, I, K and the reddening-free``Wesenheit-index''. We compare our sample to those considered by Feast& Catchpole (1997) and Lanoix et al. (1999), and argue that oursample is the most carefully selected one with respect to completeness,the flagging of overtone pulsators, and the removal of Cepheids that mayinfluence the analyses for various reasons (double-mode Cepheids,unreliable hipparcos solutions, possible contaminated photometry due tobinary companions). From numerical simulations, and confirmed by theobserved parallax distribution, we derive a (vertical) scale height ofCepheids of 70 pc, as expected for a population of 3-10 Msunstars. This has consequences for Malmquist- and Lutz-Kelker (Lutz &Kelker 1973, Oudmaijer et al. 1998) type corrections which are smallerfor a disk population than for a spherical population. The V and I datasuggest that the slope of the Galactic PL-relations may be shallowerthan that observed for LMC Cepheids, either for the whole period range,or that there is a break at short periods (near log P_0 ~ 0.7-0.8). Westress the importance of two systematic effects which influence thedistance to the LMC: the slopes of the Galactic PL-relations andmetallicity corrections. In order to assess the influence of thesevarious effects, we present 27 distance moduli (DM) to the LMC. Theseare based on three different colours (V,I,K), three different slopes(the slope observed for Cepheids in the LMC, a shallower slope predictedfrom one set of theoretical models, and a steeper slope as derived forGalactic Cepheids from the surface-brightness technique), and threedifferent metallicity corrections (no correction as predicted by one setof theoretical models, one implying larger DM as predicted by anotherset of theoretical models, and one implying shorter DM based onempirical evidence). We derive DM between 18.45 +/- 0.18 and 18.86 +/-0.12. The DM based on K are shorter than those based on V and I andrange from 18.45 +/- 0.18 to 18.62 +/- 0.19, but the DM in K could besystematically too low by about 0.1 magnitude because of a bias due tothe fact that NIR photometry is available only for a limited number ofstars. From the Wesenheit-index we derive a DM of 18.60 +/- 0.11,assuming the observed slope of LMC Cepheids and no metallicitycorrection, for want of more information. The DM to the LMC based on theparallax data can be summarised as follows. Based on the PL-relation inV and I, and the Wesenheit-index, the DM is 18.60 ± 0.11(± 0.08 slope)(^{+0.08}_{-0.15} ;metallicity), which is ourcurrent best estimate. Based on the PL-relation in K the DM is ;;;;18.52 +/- 0.18 (± 0.03 ;slope) (± 0.06 ;metallicity)(^{+0.10}_{-0} ;sampling ;bias). The random error is mostly due to thegiven accuracy of the hipparcos parallaxes and the number of Cepheids inthe respective samples. The terms between parentheses indicate thepossible systematic uncertainties due to the slope of the GalacticPL-relations, the metallicity corrections, and in the K-band, due to thelimited number of stars. Recent work by Sandage et al. (1999) indicatesthat the effect of metallicity towards shorter distances may be smallerin V and I than indicated here. From this, we point out the importanceof obtaining NIR photometry for more (closeby) Cepheids, as for themoment NIR photometry is only available for 27% of the total sample.This would eliminate the possible bias due to the limited number ofstars, and would reduce the random error estimate from 0.18 to about0.10 mag. Furthermore, the sensitivity of the DM to reddening,metallicity correction and slope are smallest in the K-band. Based ondata from the ESA HP astrometry satellite.
| Direct calibration of the Cepheid period-luminosity relation
After the first release of Hipparcos data, Feast & Catchpole gave anew value for the zero-point of the visual Cepheid period-luminosityrelation, based on trigonometric parallaxes. Because of the largeuncertainties on these parallaxes, the way in which individualmeasurements are weighted is of crucial importance. We thereforeconclude that the choice of the best weighting system can be aided by aMonte Carlo simulation. On the basis of such a simulation, it is shownthat (i) a cut-off in π or in σ_ππ introduces a strongbias; (ii) the zero-point is more stable when only the brightestCepheids are used; and (iii) the Feast & Catchpole weighting givesthe best zero-point and the lowest dispersion. After correction, theadopted visual period-luminosity relation is=-2.77logP-1.44+/-0.05. Moreover, we extend this study to thephotometric I band (Cousins) and obtain=-3.05logP-1.81+/-0.09.
| I- and JHK-band photometry of classical Cepheids in the HIPPARCOS catalog
By correlating the \cite[Fernie et al. (1995)]{F95} electronic databaseon Cepheids with the ``resolved variable catalog'' of the hipparcosmission and the simbad catalog one finds that there are 280 Cepheids inthe hipparcos catalog. By removing W Vir stars (Type ii Cepheids),double-mode Cepheids, Cepheids with an unreliable solution in thehipparcos catalog, and stars without photometry, it turns out that thereare 248 classical Cepheids left, of which 32 are classified asfirst-overtone pulsators. For these stars the literature was searchedfor I-band and near-infrared data. Intensity-mean I-band photometry onthe Cousins system is derived for 189 stars, and intensity-mean JHK dataon the Carter system is presented for 69 stars.
| The effect of artificial viscosity on a hydrodynamic model of Cepheids
The effect of artificial viscosity on a hydrodynamic simulation ofstellar radial pulsation is examined for the purpose of studying thedependence of the modal coupling on the sharpness of the shock front.The model used in our study is a 2.5-d first-overtone pulsator in theCepheid instability strip. By increasing the parameters in the vonNeumann-Richtmyer formula of artificial viscosity, we obtained a lowgrowth rate together with a small pulsation amplitude. The time scalerelated to pulsation mode-switching is also increased.
| The shape and scale of Galactic rotation from Cepheid kinematics
A catalog of Cepheid variables is used to probe the kinematics of theGalactic disk. Radial velocities are measured for eight distant Cepheidstoward l = 300 deg; these new Cepheids provide a particularly goodconstraint on the distance to the Galactic center, R0. We model the diskwith both an axisymmetric rotation curve and one with a weak ellipticalcomponent, and find evidence for an ellipticity of 0.043 +/- 0.016 nearthe sun. Using these models, we derive R0 = 7.66 +/- 0.32 kpc andv(circ) = 237 +/- 12 km/s. The distance to the Galactic center agreeswell with recent determinations from the distribution of RR Lyraevariables and disfavors most models with large ellipticities at thesolar orbit.
| Monitoring the Evolution of Cepheid Variables
Described here are preliminary results of a pilot project to monitorchanges in the ephemerides of northern hemisphere Cepheid's using anSBIG camera attached to the 0.4-m telescope of the campus obversatory atSaint Mary's University. Epochs of maximum light for fifteen Cepheid'shave been derived using published light curves for each variable astemplates, and the results are being used to update the O-C ephemeridesfor the program stars. Results for BB Her are presented here. Periodchanges for Cepheid variables are demonstrated to be an excellent meansof pinpointing their evolutionary status, as well as for investigatingother peculiarities of the class.
| Radii and Distances of Cepheids. I. Method and Measurement Errors
We develop a formulation of the Baade-Wesselink method which uses theFourier coefficients of the observables. We derive an explicit, analyticexpression to determine the mean radius from each Fourier order. Thesimplicity of this method allows us to derive the uncertainty in themean radius due to measurement errors. Using simulations and a recentdata set we demonstrate that the precision of the radius measurementwith optical magnitudes is in most cases limited by the accuracy of themeasurement of the phase difference between the light and the colorindex curve. In this case it is advantageous to determine the inverseradius, because it has normal errors.
| Fundamental parameters of Cepheids. IV. Radii and luminosities.
A temperature scale for Cepheids is presented, based on Genevaphotometry. It uses new atmosphere models computed at various values ofmicroturbulent velocity. The scale so-obtained is compared to other onesand the importance of microturbulence effects is shown. This calibrationis applied to 20 Cepheids for which the variation of microturbulence isknown. The detailed variations of temperature and gravity are derived.The behavior of the photometric gravity is in very good agreement withthe effective gravity (sum of GM/R^2^ and of the derivative of theradial velocity). Thus static atmosphere models can be used to describethe temperature and gravity variations in Cepheids under the followingconditions: i) the microturbulence has to be taken into account, ii) theeffective gravity must be considered instead of the static gravity, iii)in some cases, a small phase interval around minimum radius does notgive reliable results. The temperature and bolometric corrections arethen used to derive radii and distances via the Baade-Wesselinktechnique. The resulting Period-Radius and Period-Luminosity relationsare discussed. Our results compare very well with similar analysis basedon infra-red (JHK) photometry.
| Cepheid radii and the CORS method revisited.
We have refined the CORS method, introduced in 1980 for the computationof the cepheid radii, in order to extend its applicability to recent andextensive sets of observations. The refinement is based on thecomputation, from observational data only, of one of the terms of thesolving equation, previously based only on precise calibrations ofphotometric colors. A limited number of assumptions, generally acceptedin the literature, is used. New radii are computed for about 70cepheids, and the resulting P-R relation is discussed.
| The Behlen Observatory Variable Star Survey.IV.
Finding charts, accurate coordinates, and light curves are presented for106 variable stars including 6 which are newly discovered. Parametersdescriptive of the light curves are tabulated including periods for 16stars which lacked them. The periods from the General Catalog ofVariable Stars for ten stars were found to be seriously in error. Theclassification of the stars is discussed. Revisions or refinements ofthe classifications from the General Catalogue of Variable Stars aresuggested for 20 stars and classifications are given for 7 which werepreviously unclassified. 15%-23% of the Bailey type ab RR Lyrae starsshow scatter in their light curves which is suggestive of the Blazhkoeffect.
| Colour excesses of F-G supergiants and Cepheids from Geneva photometry.
A reddening scale for F-G supergiants and Cepheids is presented.Supergiants with low reddenings or in clusters form the basis of thecalibration. In this sense, it is entirely empirical. The data have beenobtained in the Geneva photometric system. Comparisons with otherreddening scales show no disagreement. The only problem is with Fernie'sscale for Cepheids (1990), where a systematic trend exists. Its originis not clear. It is suggested to extend the number of supergiants withindependently obtained colour excesses in order to test the existence ofa possible luminosity dependence of the calibration. A period-colourrelation for Cepheids is deduced, on the basis of the present reddeningcorrections. It gives strong support for V473 Lyr being a secondovertone pulsator.
| Fundamental parameters of Cepheids. III. Turbulence variations.
Observations of the line broadening during the cycle of pulsation areanalysed for 41 Cepheids. The main components of this line broadeningare the stellar atmospheric turbulence and the velocity fields due topulsation and to axial rotation. The variation of the broadening due topulsation is calculated for each star by using the observed radialvelocity curve. The residual broadening at maximum radius allows to showthat the value of the equatorial rotational velocity is smaller than ~10km/s for the large majority of classical Cepheids. It will be shown thatturbulence is the major effect that can explain the variation of thelines widths during the cycle of pulsation. We show that the behavioursof classical and s-Cepheids are different with respect to turbulence anda new method is proposed to separate these classes. This leads to thediscovery of a few new s-Cepheids. The second overtone Cepheid V473 Lyrdoes not follow the sequence of s-Cepheids, that are highly suspected tobe overtone pulsators. It is also shown that the turbulence is in phasewith the acceleration and, particularly, that the maximum turbulence isattained near minimum radius, where the acceleration is also at itsmaximum.
| Rotation curve of the system of classical Cepheids and the distance to the galactic center
Not Available
| Fundamental parameters of Cepheids. II. Radial velocity data.
Radial velocity data for 40 Cepheid stars obtained with the CORAVELspectrometer are presented. They represent 1203 individual observations.The mean number of measurements per star is 41, ranging from 10 to 133.For each star, a Fourier analysis has been made and we searched theperiod that gave the best fit for the velocity curve. When the datacovered a sufficiently long time interval, it has been looked for periodchanges or phase jumps. The results have been compared to those found inthe literature; the choice of the best periods is discussed, taking intoconsideration the uncertainty on the period, the smoothness of the fitand several other parameters. New orbital elements, based on CORAVELmeasurements only, are given for DL Cas and W Sgr.
| Fundamental parameters of Cepheids. I. Photometric data in the Geneva system.
Photometric data in the Geneva system for 26 Cepheid stars are given,representing 869 individual measurements. After a short explanation onthe analysis, the choice of the period and the quality of the fit of thelight curve are discussed. The data and the fitted light- and colourcurves are given, as well as the Fourier coefficients of the best fitfor each star.
| New radial velocities for classical cepheids. Local galactic rotation revisited
New centre-of-mass radial velocities are calculated for 107 classicalcepheids from CORAVEL observations. We generally determine thesevelocities from four to six measurements carefully spaced in phase, byfitting a "typical" radial velocity curve or the mirror image of thelight curve. A decomposition in Fourier series is used for stars withmore than 10 measurements. Distances are then computed through aperiod-luminosity-colour relation for 278 classical cepheids with knownradial velocity, and an axisymmetric galactic rotation model is appliedto the sample, using a generalised non-linear least square method withuncertainties on both the velocities and the distances. The bestresults, with a rotation curve modelled as a third order polynomial,are: Rsun_=8.09 +/-0.30 kpc, A=15.92 +/-0.34 km/s/kpc, 2ARsun_=257 +/-7 km/s, A2=d^2theta(R)/d R^2^=-3.38+/-0.38 km/s/kpc^2^, A3=d^3theta(R)/d R^3^=1.99 +/-0.62km/s/kpc^3^, u_0_=9.32 +/-0.80 km/s, v_0_=11.18 +/-0.65 km/s. The effectof modifying the distance scale of cepheids, the absorption coefficientor the fitting procedure algorithm are examined. It appears that theproduct 2 A Rsun_ is very robust towards these changes. Theextended sample of classical cepheids with known radial velocitypresented in this paper seems to imply a higher value for A thananterior studies. The radial velocity residuals show a systematic k-termof about 2 km/s. New evidence from cluster cepheids excludes anintrinsic cause for this shift, and a dynamical cause is proposed from acomparison with a N-body simulation of the Galaxy. The simulation showsthat a systematic bias of this magnitude is typical. The structure ofthe local residual velocity field is examined in some detail.
| The Fourier decomposition as a mode discriminator - New first overtone pulsators among Cepheids with P less than 5.5 D
New very accurate light curves of 12 Cepheids with P less than 5.5 d arepresented. Their Fourier decomposition enabled us to individuate eightother stars which do not follow the Hertzsprung progression. Theproperties of the Fourier parameters of this subclass, now composed by28 objects, are consequently better defined. In the phi(21)-P plane 2stars located on the lower sequence and 2 other stars located on thediscontinuity at 3 d were found. All these 28 stars occupy the sameregion in the R(21)-P plane and the phi(31) values, when available,originate a unique sequence in the phi(31)-P plane. All these loci arewell separated from those of the classical Cepheids, which arefundamental mode pulsators. The hypotheses of a first overtone pulsationand of a resonance between this mode and a higher overtone explain theobservational facts well.
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