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Observational constraints on the magnetic field of RR Lyrae stars
Context: A high percentage of the astrophysically important RR Lyraestars show a periodic amplitude and/or phase modulation of theirpulsation cycles. More than a century after its discovery, this“Blazhko effect” still lacks acceptable theoreticalunderstanding. In one of the plausible models for explaining thephenomenon, the modulation is caused by the effects of a magnetic field.So far, the available observational data have not allowed us to eithersupport nor rule out the presence of a magnetic field in RR Lyrae stars.Aims: We intend to determine whether RR Lyrae stars are generallycharacterized by the presence of a magnetic field organized on a largescale. Methods: With the help of the FORS1 instrument at the ESOVLT we performed a spectropolarimetric survey of 17 relatively brightsouthern RR Lyrae stars, both Blazhko stars and non-modulated stars, anddetermined their mean longitudinal magnetic field with a typical errorbar <30 G. Results: All our measurements of the meanlongitudinal magnetic field resulted in null detections within 3?.From our data we can set an upper limit for the strength of the dipolecomponent of the magnetic fields of RR Lyrae stars to ~130 G. Because ofthe limitations intrinsic to the diagnostic technique, we cannot excludethe presence of higher order multipolar components. Conclusions:The outcome of this survey clarifies that the Blazhko modulation in thepulsation of RR Lyrae stars is not correlated with the presence of astrong, quasi-dipolar magnetic field.
| 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.
| A list of minima and maxima timings
The list contains minima of eclipsing and maxima of pulsatings stars, itcontinues the list published in OEJV 0048.
| The GEOS RR Lyr Survey
Not Available
| 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.
| Metallicity Dependence of the Blazhko Effect
The microlensing surveys, such as OGLE or MACHO, have led to thediscovery of thousands of RRLyr stars in the Galactic bulge and in theMagellanic Clouds, allowing for detailed investigation of these stars,especially the still mysterious Blazhko phenomenon. Higher incidencerate of Blazhko (BL) variables in the more metal-rich Galactic bulgethan in the LMC, suggests that occurrence of Blazhko effect correlateswith metallicity. To investigate this problem, we calibrate thephotometric method of determining the metallicity of RRab stars in theI-band and apply it to the OGLE Galactic bulge and LMC data. In bothsystems, metallicities of non Blazhko and Blazhko variables are close toeach other. The LMC Blazhko pulsators prefer slightly lowermetallicities. The different metallicities of the Galactic bulge and theLMC, cannot explain the observed incidence rates.As a by-product of our metallicity estimates, we investigate theluminosity-metallicity relation, finding a steep dependence of theluminosity on [Fe/H].
| BVI Time-Series Data of the Galactic Globular Cluster NGC 3201. I. RR Lyrae Stars
We present Johnson BV- and Kron-Cousins I-band time-series datacollected over three consecutive nights in a region of 13arcmin2 centered on the Galactic globular cluster (GGC) NGC3201. The time sampling of current CCD data allowed us to deriveaccurate light curves and, in turn, mean magnitudes and colors for asample of 53 RR Lyrae stars. To overcome the thorny problem ofdifferential reddening affecting this cluster, we derived new empiricalrelations connecting the intrinsic B-V and V-I colors of fundamental(RRab) RR Lyrae stars to the luminosity amplitude, the metallicity, andthe pulsation period. The key features of these relations are thefollowing: (1) they rely on stellar parameters, which are not affectedby reddening; (2) they supply accurate estimates of intrinsic colorsacross the fundamental instability strip and cover a wide metallicityrange; (3) they were derived by neglecting the RR Lyrae stars that areaffected by amplitude modulation. Moreover, the zero point of the E(B-V)reddening scale was empirically checked using the large sample of RRLyrae stars in M3 from Corwin & Carney, a GGC affected by avanishing reddening. According to these relations we estimatedindividual reddenings for RR Lyrae stars in our sample and the mainresults we found are the following: (1) The mean cluster reddening basedon E(B-V) color excesses is =0.30+/-0.03. This estimate isslightly higher than the mean reddening evaluations available in theliterature or based on the dust infrared map by Schlegel, Finkbeiner,& Davis, i.e., =0.26+/-0.02. Note that the angularresolution of this map is ~6', whereas for current reddening map it is~1'. (2) The mean cluster reddening based on E(V-I) color excesses is=0.36+/-0.05. This estimate is only marginally inagreement with the mean cluster reddening obtained using the reddeningmap by von Braun & Mateo and derived by adopting cluster turnoffstars, i.e., =0.25+/-0.07. On the other hand, currentintrinsic spread among individual reddenings (~0.2 mag) agrees quitewell with the estimate provided by previous authors. It is noteworthythat previous mean cluster reddenings are in very good agreement withvalues obtained using the empirical relations for intrinsic RR Lyraecolors provided by Kovacs & Walker. (3) According to currentindividual E(B-V) and E(V-I) reddenings and theoretical predictions forhorizontal-branch stars, we found that the true distance modulus forthis cluster is 13.32+/-0.06 mag. This determination is somehowsupported by the comparison between predicted and empirical pulsationamplitudes. (4) The comparison between present luminosity amplitudes andestimates available in the literature discloses that approximately 30%of fundamental RR Lyrae stars are affected by amplitude modulation (theBlazhko effect). This finding confirms empirical evidence originallybrought out by Szeidl and by Smith. Based on observations collected atthe European Southern Observatory, La Silla, Chile.
| 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.
| Bias Properties of Extragalactic Distance Indicators. XI. Methods to Correct for Observational Selection Bias for RR Lyrae Absolute Magnitudes from Trigonometric Parallaxes Expected from the Full-Sky Astrometric Mapping Explorer Satellite
A short history is given of the development of the correction forobservation selection bias inherent in the calibration of absolutemagnitudes using trigonometric parallaxes. The developments have beendue to Eddington, Jeffreys, Trumpler & Weaver, Wallerstein,Ljunggren & Oja, West, Lutz & Kelker, after whom the bias isnamed, Turon Lacarrieu & Crézé, Hanson, Smith, andmany others. As a tutorial to gain an intuitive understanding of severalcomplicated trigonometric bias problems, we study a toy bias model of aparallax catalog that incorporates assumed parallax measuring errors ofvarious severities. The two effects of bias errors on the derivedabsolute magnitudes are (1) the Lutz-Kelker correction itself, whichdepends on the relative parallax error δπ/π and the spatialdistribution, and (2) a Malmquist-like ``incompleteness'' correction ofopposite sign due to various apparent magnitude cutoffs as they areprogressively imposed on the catalog. We calculate the bias propertiesusing simulations involving 3×106 stars of fixedabsolute magnitude using Mv=+0.6 to imitate RR Lyraevariables in the mean. These stars are spread over a spherical volumebounded by a radius 50,000 pc with different spatial densitydistributions. The bias is demonstrated by first using a fixed rmsparallax uncertainty per star of 50 μas and then using a variable rmsaccuracy that ranges from 50 μas at apparent magnitude V=9 to 500μas at V=15 according to the specifications for the Full-SkyAstrometric Mapping Explorer (FAME) satellite to be launched in 2004.The effects of imposing magnitude limits and limits on the``observer's'' error, δπ/π, are displayed. We contrast themethod of calculating mean absolute magnitude directly from theparallaxes where bias corrections are mandatory, with an inverse methodusing maximum likelihood that is free of the Lutz-Kelker bias, althougha Malmquist bias is present. Simulations show the power of the inversemethod. Nevertheless, we recommend reduction of the data using bothmethods. Each must give the same answer if each is freed from systematicerror. Although the maximum likelihood method will, in theory, eliminatemany of the bias problems of the direct method, nevertheless the biascorrections required by the direct method can be determined empiricallyvia Spaenhauer diagrams immediately from the data, as discussed in theearlier papers of this series. Any correlation of the absolute(trigonometric) magnitudes with the (trigonometric) distances is thebias. We discuss the level of accuracy that can be expected in acalibration of RR Lyrae absolute magnitudes from the FAME data over themetallicity range of [Fe/H] from 0 to -2, given the known frequency ofthe local RR Lyrae stars closer than 1.5 kpc. Of course, use will alsobe made of the entire FAME database for the RR Lyrae stars over thecomplete range of distances that can be used to empirically determinethe random and systematic errors from the FAME parallax catalog, usingcorrelations of derived absolute magnitude with distance and position inthe sky. These bias corrections are expected to be much more complicatedthan only a function of apparent magnitude because of variousrestrictions due to orbital constraints on the spacecraft.
| 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.
| 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.
| 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.
| Identification of Variable Stars in Indus
Not Available
| A Test for Large-Scale Systematic Errors in Maps of Galactic Reddening
Accurate maps of Galactic reddening are important for a number ofapplications, such as mapping the peculiar velocity field in the nearbyuniverse. Of particular concern are systematic errors which vary slowlyas a function of position on the sky, as these would induce spuriousbulk flow. We have compared the reddenings of Burstein & Heiles (BH)and those of Schlegel, Finkbeiner, & Davis (SFD) to independentestimates of the reddening, for Galactic latitudes |b|>10^deg. Ourprimary source of Galactic reddening estimates comes from comparing thedifference between the observed B-V colors of early-type galaxies, andthe predicted B-V color determined from the B-V-Mg_2 relation. We havefitted a dipole to the residuals in order to look for large-scalesystematic deviations. There is marginal evidence for a dipolar residualin the comparison between the SFD maps and the observed early-typegalaxy reddenings. If this is due to an error in the SFD maps, then itcan be corrected with a small (13%) multiplicative dipole term. Weargue, however, that this difference is more likely to be due to a small(0.01 mag) systematic error in the measured B-V colors of the early-typegalaxies. This interpretation is supported by a smaller, independentdata set (globular cluster and RR Lyrae stars), which yields a resultinconsistent with the early-type galaxy residual dipole. BH reddeningsare found to have no significant systematic residuals, apart from theknown problem in the region 230^deg
| 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.
| 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.
| 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.
| The metallicities and kinematics of RR Lyrae variables, 1: New observations of local stars
In order to study the structure and formation history of the galaxy, wehave obtained low-to-moderate dispersion spectra of 302 nearby RR Lyraevariables of Bailey type 'ab'. We derived abundances, typically accurateto 0.15-0.20 dex and calibrated to the Zinn & West (1984) globularcluster metallicity scale, from the pseudoequivalent widths of the Ca IIK, H delta, H gamma, and H beta lines. Radial velocities accurate tobetween 2 and 30 km/s were obtained from the spectra and from theliterature. Distances accurate to between 5% and 20% were derived frompublished apparent magnitudes and Burstein & Heiles (1982)reddenings. The metallicity distribution of the RR Lyrae stars peaks at(Fe/H)K approximately equals -1.5, and is narrower than thatof the Ryan & Norris (1991) subdwarfs, as expected since the mostmetal-rich and metal-poor progenitors preferentially appear as stablered and blue horizontal branch stars, rather than as RR Lyrae. Themetal-rich tail of the RR Lyrae distribution extends to(Fe/H)K approximately equals 0, and a qualitative analysis ofthe distribution of distances from the galactic plane shows that thestars in this tail (i.e., (Fe/H)K greater than -1.0) are moreconcentrated to the plane than the more metal-poor stars. The abundancedistribution of the local RR Lyrae stars is in excellent agreement withthe changing abundance distributions of distant RR Lyrae stars as afunction of galactocentric distance, as derived by Suntzeff et al.(1991), who ascribed this change to systematic variation in horizontalbranch morphology (probably age variations) with galactocentricdistance. The abundance distribution of the local RR Lyrae stars alsoagrees well with those of the distant RR Lyrae stars as a function ofdistance from the galactic plane. There is no evidence for an abundancegradient in this direction, suggesting that gaseous dissipation did notplay a major role in the formation of the outer halo.
| 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.
| The reddening of type AB RR Lyrae stars
The paper modifies Sturch's (1966) method for deriving, from observednear-minimum light colors, EB-V values for RR Lyrae starswith Bailey's types for a or b light curves in order to avoid the use ofreddening and metallicity-sensitive U-B colors. The Delta(S), (Fe/H)calibration is examined, and blanketing corrections are derived fromsynthetic colors of models by Kurucz (1975, 1979). Sturch's finding thatthe intrinsic blanketing-corrected near-minimum light B-V color of avariable can be estimated from its period is verified. The inherentaccuracy of the method is discussed and found to depend appreciably oncycle-to-cycle color variations. Comparisons are made with otherreddening determinations. The blanketing corrections determined bySturch from delta(U-B) values are found to make his EB-Vvalues less than those found in the present study, while his estimatedintrinsic colors make his EB-V values greater than thosefound in this study.
| Detection of a galactic color gradient for blue horizontal-branch stars of the halo field and implications for the halo age and density distributions
Abstract image available at:http://adsabs.harvard.edu/abs/1991ApJ...375..121P
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| Costellazione: | Indiano |
| Ascensione retta: | 21h11m29.90s |
| Declinazione: | -45°04'28.4" |
| Magnitudine apparente: | 9.994 |
| Moto proprio RA: | -71.4 |
| Moto proprio Dec: | -84.7 |
| B-T magnitude: | 10.329 |
| V-T magnitude: | 10.022 |
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