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ASTEP South: an Antarctic Search for Transiting ExoPlanets around the celestial south pole
Context. The Concordia base in Dome C, Antarctica, is an extremelypromising site for photometric astronomy due to the 3-month long nightduring the Antarctic winter, favorable weather conditions, and lowscintillation. Aims: The ASTEP project (Antarctic Search forTransiting ExoPlanets) is a pilot project to discover transiting planetsand understand the limits of visible photometry from the Concordia site. Methods: ASTEP South is the first phase of the ASTEP project. Theinstrument is a fixed 10 cm refractor with a 4k × 4k CCD camera ina thermalized box, pointing continuously a 3.88 ×3.88°2 field of view centered on the celestial southpole. We describe the project and report results of a preliminary dataanalysis. Results: ASTEP South became fully functional in June2008 and obtained 1592 hours of data during the 2008 Antarctic winter.The data are of good quality but the analysis has to account for changesin the PSF (point spread function) due to rapid ground seeing variationsand instrumental effects. The pointing direction is stable within 10arcsec on a daily timescale and drifts by only 34 arcsec in 50 days. Atruly continuous photometry of bright stars is possible in June (thenoon sky background peaks at a magnitude R ? 15 arcsec-2on June 22), but becomes challenging in July (the noon sky backgroundmagnitude is R ? 12.5 arcsec-2 on July 20). The weatherconditions are estimated from the number of stars detected in the field.For the 2008 winter, the statistics are between 56.3% and 68.4% ofexcellent weather, 17.9% to 30% of veiled weather (when the probablepresence of thin clouds implies a lower number of detected stars) and13.7% of bad weather. Using these results in a probabilistic analysis oftransit detection, we show that the detection efficiency of transitingexoplanets in one given field is improved at Dome C compared to atemperate site such as La Silla. For example we estimate that ayear-long campaign of 10 cm refractor could reach an efficiency of 69%at Dome C versus 45% at La Silla for detecting 2-day period giantplanets around target stars from magnitude 10 to 15. The detectionefficiency decreases for planets with longer orbital periods, but inrelative sense it is even more favorable to Dome C. Conclusions:This shows the high potential of Dome C for photometry and future planetdiscoveries.
| Helium lines in RR Lyrae spectra
Context: During a comparative study of atmospheric phenomena that occurduring the pulsation cycles of 10 RRab stars I unexpectedly found heliumlines in emission and/or absorption in all 10 stars during rising light.The progression of events in the time-evolution of the helium spectrumdiffers in detail from star to star, but 9 of the 10 stars share anumber of general characteristics, illustrated approximately by thebehavior of RV Oct described in this Note. Aims: Mylong-term aim is to provide a comprehensive empirical description ofatmospheric phenomena that take place during the pulsation cycles ofRRab stars. The short-term goal of this paper is to inform readers thatmeasurements of helium lines place new constraints on shock-wave modelsfor RRab stars. Methods: Results presented here are based onmeasurements and analysis of observations made with the echellespectrograph of the du Pont 2.5-m telescope at Las CampanasObservatory. Results: A general pattern of behavior emergesalbeit with significant star-to-star variation. He I ? 5376 (D3)appears first as a pure emission feature, unlike the Balmer lines forwhich the emission profile is always overlain initially by strongredward-displaced absorption. The phase of maximum D3 emission coincidesapproximately with that of H?, but the duration of conspicuous D3emission (in excess of the local continuum) is less than 15 mn,approximately half the duration of similarly defined H? emission.The emission phase is followed by the appearance, first, ofredward-displaced absorption, and shortly thereafter by a secondviolet-displaced absorption component that strengthens with advancingphase. Both absorption components gradually weaken and disappear afterradial-velocity minimum, which lags luminosity maximum by no more than0.01P as discussed in Sect. 3. However, bona fide He I ? 5876emission flux in excess of the local continuum flux reappears nearmaximum light for some stars in the sample and persists for about 45 mn.The radial velocities of the initial and final He I ? 5876emission features differ little from the time-average value of themetallic-line radial velocities for RV Octantis. However, the absorptionfeatures appear to distort the location of the emission centroid, whenthey are strong.
| Hypersonic shock wave evidence in RR Lyrae stars. First detection of neutral line disappearance in S Arae
Context: We carried out a new high resolution spectroscopic analysis ofS Arae, one of the largest amplitude RR Lyrae star, to constrain thedynamical structure of the outermost layers of RR Lyrae stars. Aims: New high precision spectroscopic observations of S Arae wereobtained using UVES at VLT-UT2 and FEROS at ESO-1.52 telescopes, duringthe rise of the S Arae light curve, over an entire pulsation period andcovering a large wavelength range, from 376 to 1000 nm. Methods:We used the first line moment method to measure the radial velocity andto establish the radial velocity curves. The atmosphere stratificationwas done by the line identification method derived from the Kuruczatmosphere models. Results: For the first time, we show theexistence of hypersonic shock waves in the photosphere of RR Lyraestars. Our spectroscopic results reveal a new mechanism: the neutralline disappearance phenomenon. All neutral metallic absorption lines,over the whole spectrum, undergo a total line disappearance during thehump, while the ionized lines show line doubling connected by thetwo-step Schwarzschild's mechanism. Thus, a hypersonic shock wavepropagating through the photosphere around the maximum light phaseresults in the line disappearance. The shock energy is high enough toionize the atoms, forming the layer crossed by the shock wave. The shockat first recedes, then becomes stationary and finally advances with avery high Mach number crossing the high atmosphere of the star. Westress the need for additional theoretical efforts to come to a betterunderstanding of hypersonic shock wave and dynamic pulsational motionrelationship.Based on observations obtained at the European Southern Observatory inChile in the programs P67.D-0321(B) andP67.D-0321(A).Tables 1 and 2 are only available in electronic form athttp://www.aanda.org
| The luminosities and distance scales of type II Cepheid and RR Lyrae variables
Infrared and optical absolute magnitudes are derived for the type IICepheids κ Pav and VY Pyx using revised Hipparcos parallaxes andfor κ Pav, V553 Cen and SW Tau from pulsational parallaxes.Revised Hipparcos and HST parallaxes for RR Lyrae agree satisfactorilyand are combined in deriving absolute magnitudes. Phase-corrected J, Hand Ks mags are given for 142 Hipparcos RR Lyraes based onTwo-Micron All-Sky Survey observations. Pulsation and trigonometricalparallaxes for classical Cepheids are compared to establish the bestvalue for the projection factor (p) used in pulsational analyses.The MV of RR Lyrae itself is 0.16 +/- 0.12 mag brighter thanpredicted from an MV-[Fe/H] relation based on RR Lyrae starsin the Large Magellanic Cloud (LMC) at a modulus of 18.39 +/- 0.05 asfound from classical Cepheids. This is consistent with the prediction ofCatelan & Cortés that it is overluminous for its metallicity.The results for the metal- and carbon-rich Galactic disc stars, V553 Cenand SW Tau, each with small internal errors (+/-0.08 mag) have a meandeviation of only 0.02 mag from the period-luminosity (PL) relationestablished by Matsunaga et al. for type II Cepheids in globularclusters and with a zero-point based on the same LMC-scale. Comparingdirectly the luminosities of these two stars with published data on typeII Cepheids in the LMC and in the Galactic bulge leads to an LMC modulusof 18.37 +/- 0.09 and a distance to the Galactic Centre of R0= 7.64 +/- 0.21kpc. The data for VY Pyx agree with these results withinthe uncertainties set by its parallax. Evidence is presented thatκ Pav may have a close companion and possible implications of thisare discussed. If the pulsational parallax of this star is incorporatedin the analyses, the distance scales just discussed will be increased by~0.15 +/- 0.15 mag. V553 Cen and SW Tau show that at optical wavelengthsPL relations are wider for field stars than for those in globularclusters. This is probably due to a narrower range of masses in thelatter case.
| New GCVS Versions for Three Southern Constellations
We are currently working on a version of the General Catalogue ofVariable Stars (GCVS) revised taking into account the new dataaccumulated since the 4th GCVS edition. A draft new version will bereleased for each constellation as soon as the work for theconstellation is finished. It will contain all stars of the 4th GCVSedition plus a complete catalogue of the stars added to the GCVS in theName Lists of Variable Stars Nos. 67 - 78. Now we are ready for thefirst release, containing more than 1300 variable stars in theconstellations of Antlia, Ara, and Telescopium. When preparing therelease, we actively used modern data-mining possibilities to improvevariability types and light elements. This paper introduces the firstrelease of the new GCVS version and presents new results (types, lightelements), based mainly on data mining, for 213 stars.
| The GEOS RR Lyr Survey
Not Available
| Multiperiodic Galactic field RR Lyrae stars in the ASAS catalogue
The All Sky Automated Survey (ASAS) monitors bright stars (8 < V <14 mag) south of declination +28°. The ASAS Catalogue of VariableStars (ACVS) presently contains 50099 objects; among them are 2212objects classified as RR Lyrae pulsating variables. We use ASASphotometric V-band data to search for multiperiodicity in those stars.We find that 73 of 1435 RRab stars and 49 of 756 RRc stars exhibit theBlazhko effect. We observe a deficiency of RRab Blazhko variables withmain pulsation periods greater than 0.65 d. The Blazhko periods of RRcstars exhibit a strongly bimodal distribution. During our study wediscovered the Blazhko effect with multiple periods in object ASAS050747-3351.9 = SU Col. Blazhko periods of 89.3 and 65.8 d and acandidate of 29.5 d were identified with periodogram peaks near thefirst three harmonics of the main pulsation. These observations mayinspire new models of the Blazhko effect, which has eluded a consistenttheory since its discovery about one hundred years ago. Long-term lightcurve changes were found in 29 stars. We also found 19 Galactic doublemode pulsators (RRd), of which four are new discoveries, raising thenumber of ASAS discoveries of such objects to 16, out of 27 known in thefield of our Galaxy.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Southern RR Lyrae Stars Exhibiting the Blazhko Effect
Blazhko periods are given for 43 RRab stars based on data from ASAS3.
| Proper identification of RR Lyrae stars brighter than 12.5 mag
RR Lyrae stars are of great importance for investigations of Galacticstructure. However, a complete compendium of all RR-Lyraes in the solarneighbourhood with accurate classifications and coordinates does notexist to this day. Here we present a catalogue of 561 local RR-Lyraestars (V_max ≤ 12.5 mag) according to the magnitudes given in theCombined General Catalogue of Variable Stars (GCVS) and 16 fainter ones.The Tycho2 catalogue contains ≃100 RR Lyr stars. However, manyobjects have inaccurate coordinates in the GCVS, the primary source ofvariable star information, so that a reliable cross-identification isdifficult. We identified RR Lyrae from both catalogues based on anintensive literature search. In dubious cases we carried out photometryof fields to identify the variable. Mennessier & Colome (2002,A&A, 390, 173) have published a paper with Tyc2-GCVSidentifications, but we found that many of their identifications arewrong.
| RR Lyrae stars: kinematics, orbits and z-distribution
RR Lyrae stars in the Milky Way are good tracers to study the kinematicbehaviour and spatial distribution of older stellar populations. Arecently established well documented sample of 217 RR Lyr stars withV<12.5 mag, for which accurate distances and radial velocities aswell as proper motions from the Hipparcos and Tycho-2 catalogues areavailable, has been used to reinvestigate these structural parameters.The kinematic parameters allowed to calculate the orbits of the stars.Nearly 1/3 of the stars of our sample have orbits staying near the MilkyWay plane. Of the 217 stars, 163 have halo-like orbits fulfilling one ofthe following criteria: Θ < 100 km s-1, orbiteccentricity >0.4, and normalized maximum orbital z-distance>0.45. Of these stars roughly half have retrograde orbits. Thez-distance probability distribution of this sample shows scale heightsof 1.3±0.1 kpc for the disk component and 4.6±0.3 kpc forthe halo component. With our orbit statistics method we found a(vertical) spatial distribution which, out to z=20 kpc, is similar tothat found with other methods. This distribution is also compatible withthe ones found for blue (HBA and sdB) halo stars. The circular velocityΘ, the orbit eccentricity, orbit z-extent and [Fe/H] are employedto look for possible correlations. If any, it is that the metal poorstars with [Fe/H] <1.0 have a wide symmetric distribution aboutΘ=0, thus for this subsample on average a motion independent ofdisk rotation. We conclude that the Milky Way possesses a halo componentof old and metal poor stars with a scale height of 4-5 kpc having randomorbits. The presence in our sample of a few metal poor stars (thus partof the halo population) with thin disk-like orbits is statistically notsurprising. The midplane density ratio of halo to disk stars is found tobe 0.16, a value very dependent on proper sample statistics.
| Subsystems of RR Lyrae Variable Stars in Our Galaxy
We have used published, high-accuracy, ground-based and satelliteproper-motion measurements, a compilation of radial velocities, andphotometric distances to compute the spatial velocities and Galacticorbital elements for 174 RR Lyrae (ab) variable stars in the solarneighborhood. The computed orbital elements and published heavy-elementabundances are used to study relationships between the chemical,spatial, and kinematic characteristics of nearby RR Lyrae variables. Weobserve abrupt changes of the spatial and kinematic characteristics atthe metallicity [Fe/H]≈-0.95 and also when the residual spatialvelocities relative to the LSR cross the critical value V res≈290km/s. This provides evidence that the general population of RR Lyraestars is not uniform and includes at least three subsystems occupyingdifferent volumes in the Galaxy. Based on the agreement between typicalparameters for corresponding subsystems of RR Lyrae stars and globularclusters, we conclude that metal-rich stars and globular clusters belongto a rapidly rotating and fairly flat, thick-disk subsystem with a largenegative vertical metallicity gradient. Objects with larger metaldeficiencies can, in turn, be subdivided into two populations, but usingdifferent criteria for stars and clusters. We suggest that field starswith velocities below the critical value and clusters with extremelyblue horizontal branches form a spherical, slowly rotating subsystem ofthe protodisk halo, which has a common origin with the thick disk; thissubsystem has small but nonzero radial and vertical metallicitygradients. The dimensions of this subsystem, estimated from theapogalactic radii of orbits of field stars, are approximately the same.Field stars displaying more rapid motion and clusters with redderhorizontal branches constitute the spheroidal subsystem of the accretedouter halo, which is approximately a factor of three larger in size thanthe first two subsystems. It has no metallicity gradients; most of itsstars have eccentric orbits, many display retrograde motion in theGalaxy, and their ages are comparatively low, supporting the hypothesisthat the objects in this subsystem had an extragalactic origin.
| Empirical relations for cluster RR Lyrae stars revisited
Our former study on the empirical relations between the Fourierparameters of the light curves of the fundamental mode RR Lyrae starsand their basic stellar parameters has been extended to considerablylarger data sets. The most significant contribution to the absolutemagnitude MV comes from the period P and from the firstFourier amplitude A1, but there are statistically significantcontributions also from additional higher order components, mostimportantly from A3 and in a lesser degree from the Fourierphase varphi51. When different colors are combined inreddening-free quantities, we obtain basically period-luminosity-colorrelations. Due to the log Teff(B-V,log g,[Fe/H]) relationfrom stellar atmosphere models, we would expect some dependence also onvarphi 31. Unfortunately, the data are still not extensiveand accurate enough to decipher clearly the small effect of this Fourierphase. However, with the aid of more accurate multicolor data on fieldvariables, we show that this Fourier phase should be present either inV-I or in B-V or in both. From the standard deviations of the variousregressions, an upper limit can be obtained on the overall inhomogeneityof the reddening in the individual clusters. This yields sigmaE(B-V)<~ 0.012 mag, which also implies an average minimumobservational error of sigmaV >~ 0.018 mag.
| Absolute Magnitudes and Kinematic Parameters of the Subsystem of RR Lyrae Variables
The statistical parallax technique is applied to a sample of 262 RRabLyrae variables with published photoelectric photometry, metallicities,and radial velocities and with measured absolute proper motions.Hipparcos, PPM, NPM, and the Four-Million Star Catalog (Volchkov et al.1992) were used as the sources of proper motions; the proper motionsfrom the last three catalogs were reduced to the Hipparcos system. Wedetermine parameters of the velocity distribution for halo [(U_0, V_0,W_0) = (-9 +/- 12, -214 +/- 10, -16 +/- 7) km/s and (sigma_U, sigma_V,sigma_W) = (164 +/- 11, 105 +/- 7, 95 +/- 7) km/s] and thick-disk [(U_0,V_0, W_0) = (-16 +/- 8, -41 +/- 7, -18 +/- 5) km/s and (sigma_U,sigma_V, sigma_W) = (53 +/- 9, 42 +/- 8, 26 +/- 5) km/s] RR Lyrae, aswell as the intensity-averaged absolute magnitude for RR Lyrae of thesepopulations: = 0.77 +/- 0.10 and = +1.11 +/-0.28 for the halo and thick-disk objects, respectively. The metallicitydependence of the absolute magnitude of RR Lyrae is analyzed(=(0.76 +/- 0.12) + (0.26 +/- 0.26) x ([Fe/H] + 1.6) = 1.17 +0.26 x [Fe/H]). Our results are in satisfactory agreement with the_(RR)-[Fe/H] relation from Carney et al. (1992)(_(RR) = 1.01 + 0.15 x [Fe/H]) obtained by Baade-Wesselink'smethod. They provide evidence for a short distance scale: the LMCdistance modulus and the distance to the Galactic center are 18.22 +/-0.11 and 7.4 +/-±0.5 kpc, respectively. The zero point ofthe distance scale and the kinematic parameters of the RR Lyraepopulations are shown to be virtually independent of the source ofabsolute proper motions used and of whether they are reduced to theHipparcos system or not.
| 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.
| Kinematics of Metal-poor Stars in the Galaxy. II. Proper Motions for a Large Nonkinematically Selected Sample
We present a revised catalog of 2106 Galactic stars, selected withoutkinematic bias and with available radial velocities, distance estimates,and metal abundances in the range -4.0<=[Fe/H]<=0.0. This updateof the 1995 Beers & Sommer-Larsen catalog includes newly derivedhomogeneous photometric distance estimates, revised radial velocitiesfor a number of stars with recently obtained high-resolution spectra,and refined metallicities for stars originally identified in the HKobjective-prism survey (which account for nearly half of the catalog)based on a recent recalibration. A subset of 1258 stars in this cataloghave available proper motions based on measurements obtained with theHipparcos astrometry satellite or taken from the updated AstrographicCatalogue (second epoch positions from either the Hubble Space TelescopeGuide Star Catalog or the Tycho Catalogue), the Yale/San Juan SouthernProper Motion Catalog 2.0, and the Lick Northern Proper Motion Catalog.Our present catalog includes 388 RR Lyrae variables (182 of which arenewly added), 38 variables of other types, and 1680 nonvariables, withdistances in the range 0.1 to 40 kpc.
| About statistical-parallax algorithms
The results obtained by two algorithms of statistical parallaxes appliedto a large sample of RR Lyrae stars are compared. They show nosignificant difference and confirm the validity of the earlier approachbased on the principle of maximum likelihood.
| The absolute magnitudes of RR Lyraes from HIPPARCOS parallaxes and proper motions
We have used HIPPARCOS proper motions and the method of StatisticalParallax to estimate the absolute magnitude of RR Lyrae stars. Inaddition we used the HIPPARCOS parallax of RR Lyrae itself to determineit's absolute magnitude. These two results are in excellent agreementwith each other and give a zero-point for the RR Lyrae M_v,[Fe/H]relation of 0.77+/-0.15 at [Fe/H]=-1.53. This zero-point is in goodagreement with that obtained recently by several groups usingBaade-Wesselink methods which, averaged over the results from thedifferent groups, gives M_v = 0.73+/-0.14 at [Fe/H]=-1.53. Taking theHIPPARCOS based zero-point and a value of 0.18+/-0.03 for the slope ofthe M_v,[Fe/H] relation from the literature we find firstly, thedistance modulus of the LMC is 18.26+/-0.15 and secondly, the mean ageof the Globular Clusters is 17.4+/-3.0 GYrs. These values are comparedwith recent estimates based on other "standard candles" that have alsobeen calibrated with HIPPARCOS data. It is clear that, in addition toastrophysical problems, there are also problems in the application ofHIPPARCOS data that are not yet fully understood. Table 1, whichcontains the basic data for the RR Lyraes, is available only at CDS. Itmay be retrieved via anonymous FTP at cdsarc.u-strasbg.fr (130.79.128.5)or via the Web at http://cdsweb.u-strasbg.fr/Abstract.html
| Early evolution of the Galactic halo revealed from Hipparcos observations of metal-poor stars
The kinematics of 122 red giant and 124 RR Lyrae stars in the solarneighborhood are studied using accurate measurements of their propermotions obtained by the Hipparcos astrometry satellite, combined withtheir published photometric distances, metal abundances, and radialvelocities. A majority of these sample stars have metal abundances of(Fe/H) = -1 or less and thus represent the old stellar populations inthe Galaxy. The halo component, with (Fe/H) = -1.6 or less, ischaracterized by a lack of systemic rotation and a radially elongatedvelocity ellipsoid. About 16 percent of such metal-poor stars have loworbital eccentricities, and we see no evidence of a correlation between(Fe/H) and e. Based on the model for the e-distribution of orbits, weshow that this fraction of low-e stars for (Fe/H) = -1.6 or less isexplained by the halo component alone, without introducing the extradisk component claimed by recent workers. This is also supported by theabsence of a significant change in the e-distribution with height fromthe Galactic plane. In the intermediate-metallicity range, we find thatstars with disklike kinematics have only modest effects on thedistributions of rotational velocities and e for the sample at absolutevalue of z less than 1 kpc. This disk component appears to constituteonly 10 percent for (Fe/H) between -1.6 and -1 and 20 percent for (Fe/H)between -1.4 and -1.
| Structural Properties of Pulsating Star Light Curves Through Fuzzy Divisive Hierarchical Clustering
Not Available
| The Absolute Magnitude and Kinematics of RR Lyrae Stars Via Statistical Parallax
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2110L&db_key=AST
| Determination of [Fe/H] from the light curves of RR Lyrae stars.
We present an accurate and robust method for the calculation of [Fe/H]from the light curves of RRab stars. The method introduces aconsiderable improvement relative to our previously published formulae.First of all, it uses an improved and extended data base for the lightcurves and more accurate, very recent iron abundances. Secondly, the newdata base makes it possible to show that the basic relation between[Fe/H] and the Fourier parameters is linear and contains only the periodand one of the Fourier phases, most importantly φ_31_. Last but notleast, we derive interrelations among the Fourier parameters which helpus to filter out peculiar stars where more caution is needed inaccepting the calculated abundance. The applicability of the method isdemonstrated on independent samples of globular cluster stars.Peculiarities encountered in Blazhko variables and in some other casesare also discussed.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The Metallicities and Kinematics of RR Lyrae Variables.II. Galactic Structure and Formation from Local Stars
Abstract image available at:http://adsabs.harvard.edu/abs/1995AJ....110.2288L
| Kinematics of metal-poor stars in the galaxy
We discuss the kinematic properties of a sample of 1936 Galactic stars,selected without kinematic bias, and with abundances (Fe/H) is less thanor equal to -0.6. The stars selected for this study all have measuredradial velocities, and the majority have abundances determined fromspectroscopic or narrow-/intermediate-band photometric techniques. Incontrast to previous examinations of the kinematics of the metal-poorstars in the Galaxy, our sample contains large numbers of stars that arelocated at distances in excess of 1 kpc from the Galactic plane. Thus, amuch clearer picture of the nature of the metal-deficient populations inthe Galaxy can now be drawn.
| A new method for the determination of [Fe/H] in RR Lyrae stars.
The Fourier parameters of the V light curves of the field RRab stars areused to fit their [Fe/H]. The method is based on the assumption that theobserved light curves depend only on a few physical parameters,including the chemical composition. We give two formulae which estimatethe observed [Fe/H] with an accuracy of 0.23-0.18dex. Each of theseexpressions consists of a second order polynom of 2-4 Fourier parametersand the period. The method is a powerful tool in estimating themetallicity when spectroscopic data are not available.
| Pulsation and stability of RR Lyrae stars. 1: Instability strip
In order to provide a detailed analysis of RR Lyrae instability striptopology, an extensive grid of nonlinear, nonlocal, and time-dependentconvective models of RR Lyrae stars has been computed at fixed mass (M =0.65 solar mass) and chemical composition (Y = 0.299, Z = 0.001). Fourseries of envelope models at different luminosity levels (log(L/solarluminosity) = 1.81, 1.72, 1.61, 1.51) and on a large range of effectivetemperatures (5700 K less than Te less than 8000 K) have beeninvestigated. The nonlinear modal stability has been evaluated atlimiting amplitude for both the fundamental and the first overtone. Theequations governing both dynamical and convective interactions have beenintegrated in time until the initial perturbations and the nonlinearfluctuations due to superposition of higher order modes settled down.The theoretical observables obtained by the present survey (radius,luminosity, velocity and temperature amplitudes, periods) describe thepulsation characteristics of the models at full amplitude, hence theycan be properly compared with observations. A linear, nonadiabaticsurvey of the first three modes of RR Lyrae models has been alsocomputed to supply the static structure of the envelope to the nonlinearstability analysis. Several numerical simulations have been performed totest both the numerical accuracy (boundary conditions, time-step size,zoning) and the adequacy of the physical assumption (efficiency of theturbulent regime, artificial viscosity dependence, convective structureinitialization) adopted to describe the coupling between dynamical andconvective fields. The structure of the instability strip shows severalstriking features concerning the width in temperature of the regionwhere only the first overtone is unstable. Indeed, the fundamental blueedge, moving from higher to lower luminosity levels, becomes redder, incontrast to previous findings but in agreement with globular clustersobserved data. Moreover, using an improved treatment of the convectivetransport, the first-overtone red edge has been directly evaluated andhence also the width of the 'either-or' region (i.e., the region whereboth the fundamental and the first overtone are simultaneouslyunstable). It has been found that the periods of the nonlinearconvective models are systematically smaller than the correspondingperiods of both linear and nonlinear radiative models. The differencesbetween linear and nonlinear results are smaller than 2% of the period,however. This effect has been explained as a consequence of the changesinduced during the phases of maximum compression by the convectivetransport on the adiabatic exponent, and on the density inversionlocated in coincidence with the hydrogen ionization reigon.
| Post-main-sequence and POST red giant branch variables with pulsation periods less than one day
Post-main-sequence (mass 1 to 3 solar masses) and post-giant branch (0.5to 1 solar mass) pulsators are discussed on the basis of four color andH beta light curves published elsewhere. The post-main-sequencevariables, called ultrashort period cepheid (USPC) (delta Sct), pulsatein the fundamental and first harmonic modes of radial pulsation and, inmany cases, in nonradial modes. The variables for which photometryallows accurate, luminosity estimates and are known to pulsatesimultaneously in the fundamental and first harmonic or in thefundamental mode alone, define a PL relation (MV = -2.80 logP - 0.60, fundamental). It is notable that the slope of this relation isin the range of slopes found for classical cepheids. Accurate Vphotometry is lacking for many of the variables known as 'anomalouscepheids', but the available data divide them into low mass,pseudocepheids (BL Her and W Vir stars) and post-main-sequence USPC(delta Sct) variables. Four USPC in NGC 5053 and six in NGC 6466, forwhich accurate photometry is available, give remarkably consistentmoduli of 16.06 +/- 0.05 and 15.98 +/- 0.08 mag, respectively, for theclusters, in which they are blue stragglers similar to SX Phe inKapteyn's star group. The assumption that the four post-giant branchvariables, called VSPC (RR Lyr), S Ari, SU Dra, and ST Leo in Kapteyn'sstar group and RR Lyr in the Groombridge 1830 group, are physicalmembers of these groups and share their V-velocities, leads to acalibration of the photometry for the derivation of reddening,luminosity, and heavy element abundance of 45 field variables. Theresulting reddenings are consistent with values obtained by othermethods and the metallicities are consistent with the most accuratelyavailable spectroscopic determinations of delta S and of Ca II K. Theluminosities of the bulk of the variables confirm Sandage's (1993)relation between MV and (Fe/H). Four or five of the fieldvariables are probably binary, including BB Vir which Kinman &Carreta (1992) have independently noted as double. The PL relation forUSPC (delta Sct) variables intersects the horizontal branch (HB) near P= 0.3d and at least two field very short period cepheid (VSPC) (RR Lyr)star, FW Lup (0.484 d) and ST Pic (0.486 d) may be first overtonepulsators of the USPC (delta Sct) variety. A dozen field VSPC (RR Lyr)stars populate a (Fe/H), MV relation with the same slope asthe other stars but displaced 0.7 mag toward higher luminosities. Theonly cluster variable found to populate this diplaced relation is No. 9in 47 Tuc, although ST Vir, which may be a member of the Arcturus group,should also be considered. The elevated luminosities are unlikely to becaused by either evolution or errors in the photometric indices. Apossible source of these apparently young VSPC(RR Lyr) variables withhalo metallicity is in second (or third) generation globular clustersformed during an episodic collapse of the galaxy that produced metalpoor stars but in a dynamical situation that hastened the disruption ofthe clusters, currently formed, before the still older globularclusters, created under conditions that have kept them in a moredisruptive free environment.
| The very short period Cepheid (RR Lyr) variables. 2: Light and color curves of variables in the solar vicinity
Four color and H beta observations for 43 very short period Cepheids(VSPC, RRLyr) variables have been obtained with the Cerro Tololo andKitt Peak reflectors. The color systems are defined in Eggen (1982).Contemporary, photo-electric V-light curves have been used to establishthe phasing and the resulting periods used to compute the phases for thepresent observations. These phases are then adjusted to fit the V-lightcurves and the resulting periods and adjusted phases are given.
| The Oosterhoff period-metallicity relation for RR Lyrae stars at the blue fundamental edge of the instability strip.
It is argued that the division of cluster variables into two nearlydiscrete period groups, which is the Oosterhoff dichotomy, is due to acombination of the continuous variation of period with metallicity andthe blueward progression of cluster horizontal branches such as M13 outof the instability strip for intermediate metallicities for (Fe/H)between about -1.7 and -1.9. The subsequent evolutionary tracks,starting beyond the blue side of the instability strip, move into thestrip above the zero-age horizontal branch, producing longer periods inthis metallicity range. The strong variation of period with metallicityis seen both in the cluster data and in the field star data.
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