Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
RR Lyrae Atmospherics: Wrinkles Old and New. A Preview
I report some results of an echelle spectroscopic survey of RR Lyraestars begun in 2006 that I presented in my Henry Norris Lecture of 2010January 4. Topics include (1) atmospheric velocity gradients, (2)phase-dependent envelope turbulence as it relates to Peterson'sdiscoveries of axial rotation on the horizontal branch and to Stothers'explanation of the Blazhko effect, (3) the three apparitions of hydrogenemission during a pulsation cycle, (4) the occurrence of He I lines inemission and absorption, (5) detection of He II emission and metallicline doubling in Blazhko stars, and finally (6) speculation about whathelium observations of RR Lyrae stars in omega Centauri might tell usabout the putative helium populations and the horizontal branch of thatstrange globular cluster.This paper preserves the substance and style of remarks that accompaniedthe author's PowerPoint presentation of the 2009 Henry Norris Russelllecture.
| 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.
| Spectroscopic Confirmation of the Pisces Overdensity
We present spectroscopic confirmation of the "Pisces Overdensity," alsoknown as "Structure J," a photometric overdensity of RR Lyrae starsdiscovered by the Sloan Digital Sky Survey at an estimated photometricdistance of ~85 kpc. We measure radial velocities for eight RR Lyraestars within Pisces. We find that five of the eight stars haveheliocentric radial velocities within a narrow range of -87 kms-1 < vr < -67 km s-1,suggesting that the photometric overdensity is mainly due to aphysically associated system, probably a dwarf galaxy or a disruptedgalaxy. Two of the remaining three stars differ from one another by only9 km s-1, but it would be premature to identify them asa second system.This Letter includes data gathered with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| A new method for determining physical parameters of fundamental mode RR Lyrae stars from multicolour light curves
We present a new method for determining physical parameters of RRabvariables exclusively from multicolour light curves. Our method is aninverse photometric Baade-Wesselink analysis which, using a non-linearleast-squares algorithm, searches for the effective temperature(Teff) and pulsational velocity (Vp) curves andother physical parameters that best fit the observed light curves,utilizing synthetic colours and bolometric corrections from staticatmosphere models. The Teff and Vp curves areinitially derived from empirical relations then they are varied by thefitting algorithm. The method yields the variations and the absolutevalues of the radius, the effective temperature, the visual brightnessand the luminosity of individual objects. Distance and mass are alsodetermined. The method is tested on nine RRab stars subjected toBaade-Wesselink analyses earlier by several authors. The physicalparameters derived by our method using only the light-curve data ofthese stars are well within their possible ranges defined by directBaade-Wesselink and other techniques. A new empirical relation betweenthe IC magnitude and the pulsational velocity is alsopresented, which allows to construct the Vp curve of an RRabstar purely from photometric observations to an accuracy of about 3.5kms-1.
| 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.
| Stellar evolution through the ages: period variations in galactic RRab stars as derived from the GEOS database and TAROT telescopes
Context: The theory of stellar evolution can be more closely tested ifwe have the opportunity to measure new quantities. Nowadays,observations of galactic RR Lyr stars are available on a time baselineexceeding 100 years. Therefore, we can exploit the possibility ofinvestigating period changes, continuing the pioneering work started byV. P. Tsesevich in 1969. Aims: We collected the available times ofmaximum brightness of the galactic RR Lyr stars in the GEOS RR Lyrdatabase. Moreover, we also started new observational projects,including surveys with automated telescopes, to characterise the O-Cdiagrams better. Methods: The database we built has proved to be a verypowerful tool for tracing the period variations through the ages. Weanalyzed 123 stars showing a clear O-C pattern (constant, parabolic orerratic) by means of different least-squares methods. Results: Clearevidence of period increases or decreases at constant rates has beenfound, suggesting evolutionary effects. The median values are β =+0.14 d Myr-1 for the 27 stars showing a period increase andβ = -0.20 d Myr-1 for the 21 stars showing a perioddecrease. The large number of RR Lyr stars showing a period decrease(i.e., blueward evolution) is a new and intriguing result. There is anexcess of RR Lyr stars showing large, positive β values. Moreover,the observed β values are slightly larger than those predicted bytheoretical models.Tables 3, 4, 5 and Figs. 2, 3 are only available in electronic form athttp://www.aanda.org
| 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.
| Iron abundances derived from RR Lyrae light curves and low-dispersion spectroscopy
With the aid of the All Sky Automated Survey (ASAS) database on theGalactic field, we compare the iron abundances of fundamental mode RRLyrae stars derived from the Fourier parameters with those obtained fromlow-dispersion spectroscopy. We show from a set of 79 stars, distinctfrom the original calibrating sample of the Fourier method and selectedwithout quality control, that almost all discrepant estimates are theresults of some defects or peculiarities either in the photometry or inthe spectroscopy. Omitting objects deviating by more than 0.4 dex, theremaining subsample of 64 stars yields Fourier abundances that fit thespectroscopic ones with σ=0.20 dex. Other, more stringentselection criteria and different Fourier decompositions lead to smallersubsamples and concomitant better agreement, down to σ=0.16 dex.Except perhaps for two variables among the 163 stars, comprised of theASAS variables and those of the original calibrating set of the Fouriermethod, all discrepant values can be accounted for by observationalnoise and insufficient data coverage. We suggest that the agreement canbe further improved when new, more accurate spectroscopic data becomeavailable for a test with the best photometric data. As a by-product ofthis analysis, we also compute revised periods and select Blazhkovariables.
| Predicted and Empirical Radii of RR Lyrae Stars
We present new theoretical period-radius-metallicity relations for RRLyrae stars. Current predictions are based on a large set of nonlinear,convective models that cover a broad range of chemical abundances andinput parameters. We also provide new and homogeneous estimates ofangular diameters for a sample of field RR Lyrae stars using a recentcalibration of the Barnes-Evans surface brightness relation. Predictedand empirical radii are, within the errors, in reasonable agreement, butin the short-period range the latter present a larger scatter. As aworking hypothesis we suggest that this discrepancy might be due to theoccurrence either of nonlinear features such as bumps or a steep risingbranch. A new distance determination for RR Lyr itself is in very goodagreement with the Hubble Space Telescope trigonometric parallax andwith the pulsation parallax.
| 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].
| The Metallicity Dependence of the Fourier Components of RR Lyrae Light Curves Is the Oosterhoff-Arp-Preston Period Ratio Effect in Disguise
The correlation of particular Fourier components of the light curves ofRR Lyrae variables with metallicity, discovered by Simon and later byKovacs and his coworkers, is shown to have the same explanation as theperiod ratios (period shifts in logP) between RRab Lyrae variables thathave the same colors, amplitudes, and light-curve shapes but differentmetallicities. A purpose of this paper is to demonstrate that the modelthat predicts the period-metallicity relations in the mediatingparameters of colors, amplitudes, and light-curve shapes also explainsthe Simon-Kovacs et al. correlation between period, φ31,and metallicity. The proof is made by demonstrating that the combinationof the first- and third-phase terms in a Fourier decomposition of RRablight curves, called φ31 by Simon & Lee, variesmonotonically across the RR Lyrae instability strip in the same way thatamplitude, color, and rise time vary with period within the strip. Thepremise of the model is that if horizontal branches at the RR Lyraestrip are stacked in luminosity according to the metallicity, then therenecessarily must be a logperiod shift between RR Lyrae stars withdifferent metallicities at the same φ31 values. However,there are exceptions to the model. The two metal-rich globular clustersNGC 6388 and NGC 6441, with anomalously long periods of their RR Lyraestars for their amplitudes, violate the period-metallicity correlationsboth in amplitudes and in φ31 values (for NGC 6441 whereφ31 data exist). The cause must be related to theanomalously bright horizontal branches in these two clusters for theirmetallicities. The effect of luminosity evolution away from the zero-agehorizontal branch, putatively causing noise in the metallicity equation,is discussed. It is clearly seen in the amplitude-period correlationsbut apparently does not exist in the φ31-periodcorrelation in the data for the globular cluster M3 analyzed by Jurcsikand coworkers and by Cacciari and Fusi Pecci, for reasons not presentlyunderstood. Clarification can be expected from study of precisionphotometric data of evolved RR Lyrae stars in globular clusters ofdifferent metallicity when their Fourier components are known.
| A pulsational approach to near-infrared and visual magnitudes of RR Lyr stars
In this paper, we present an improved theoretical scenario concerningnear-infrared and visual magnitudes of RR Lyr variables, as based onup-to-date pulsating models. New relations connecting V and K absolutemagnitudes with periods, mass, luminosity and metal content arediscussed separately for fundamental and first-overtone pulsators. Wealso show that the V-K colours are predicted to supply tight constraintson the pulsator intrinsic luminosity. On this basis, we revisit the caseof the prototype variable RR Lyr, showing that the parallax inferred bythis new pulsational approach appears in close agreement with HubbleSpace Telescope absolute parallax. Moreover, available K and Vmeasurements for field and cluster RR Lyr variables with known reddeningand metal content are used to derive a relation connecting the Kabsolute magnitude to period and metallicity (MK-[Fe/H]-logP)as well as a new calibration of the MV-[Fe/H] relation. Thecomparison between theoretical prescriptions and observations suggeststhat RR Lyr stars in the field and in galactic globular clusters (GGCs)should have quite similar evolutionary histories. The comparison betweentheory and observations also discloses a general agreement that supportsthe reliability of the current pulsational scenario. On the contrary,current empirical absolute magnitudes based on the Baade-Wesselink (BW)method suggest relations with a zero-point which is fainter than ispredicted by pulsation models, together with a milder metallicitydependence. However, preliminary results based on a new calibration ofthe BW method provided by Cacciari et al. (2000) for RR Cet and SW Andappear in a much better agreement with the pulsational predictions.
| 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.
| 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.
| Systematics of RR Lyrae Statistical Parallax. III. Apparent Magnitudes and Extinctions
We sing the praises of the central limit theorem. Having previouslyremoved all other possible causes of significant systematic error in thestatistical-parallax determination of RR Lyrae absolute magnitudes, weinvestigate systematic errors from two final sources of input data:apparent magnitudes and extinctions. We find corrections due to each ofabout 0.05 mag, i.e., about half the statistical error. However, theseare of opposite sign and so approximately cancel out. Theapparent-magnitude system that we previously adopted from Layden et al.was calibrated to the photoelectric photometry of Clube & Dawe.Using Hipparcos photometry and archival modern ground-based photometry,we show that the Clube & Dawe system is about 0.06 mag too bright.Extinctions were previously based on the map of Burstein & Heiles,which was constructed from H I maps. We argue that extinctions shouldrather be estimated using the new map of Schlegel, Finkbeiner, &Davis based on COBE and IRAS measurements of dust emission. Thissubstitution increases the mean estimated extinction by about 0.05 mag,primarily because of a difference in the zero point of the two maps. Ourfinal estimate for the absolute magnitude is M_V = 0.77 +/- 0.13 at[Fe/H] = -1.60 for a pure sample of 147 halo RR Lyrae stars, or M_V =0.80 +/- 0.11 at [Fe/H] = -1.71 if we incorporate kinematic informationfrom 716 nonkinematically selected non-RR Lyrae stars from Beers &Sommer-Larsen. These are 2 and 3 sigma fainter than recentdeterminations of M_V based on main-sequence fitting of clusters usingHipparcos measurements of subdwarfs by Reid and Gratton et al. Sincestatistical parallax is being cleared of systematic errors and since theprobability of a more than 2 sigma statistical fluctuation is less than1/20, we conclude that these brighter determinations may be in error. Inthe course of these three papers, we have corrected six systematicerrors whose absolute values total 0.20 mag. Had these, contrary to theexpectation of the central limit theorem, all lined up one way, theycould have resolved the conflict in favor of the brighterdeterminations. In fact, the net change was only 0.06 mag.
| 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
| The slope of the RR Lyrae Mv-(Fe/H) relation
We review the available observational data to show that the slope of theRR Lyrae Mv-(Fe/H) relation is 0.18 +/- 0.03. The recent claim by Feastthat, because of biases, the true slope is much steeper is notjustified.
| 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 Magnitudes of the RR Lyrae Stars
We present a new calibration of the relationship between the absolutemagnitudes and metal abundances, [Fe/H], of RR Lyrae stars. Temperaturesinferred from optical and near infrared color indices and the new Kuruczmodels are utilized in conjunction with Baade-Wesselink solutions toderive absolute magnitudes. Temperature inferred from the optical andnear infrared photometry are $\sim$ 200K - 300K higher for the variablesthan those given from ($V-K$) color indices and the older Kurucz models.We find $M_v$ = 0.287 [Fe/H] + 0.964. This equation gives higherluminosities ($\sim$ 0.2 mag) for the metal-poor variables than previousB-W calibrations. The RR Lyrae stars in the Large Magellanic Cloud inconjunction with our calibration yield a distance modulus of 18.53.Thus, the Cepheids and RR Lyrae stars give identical distance moduli at$\leq$ 0.1 mag level. The repercussions of the higher temperatures andhigher luminosities of RR Lyrae stars on the distances and ages ofGlobular clusters, distance to the galactic center, the Hubble constantand age of the universe, and the masses of RR Lyrae stars are discussed.(SECTION: Stars)
| Structural Properties of Pulsating Star Light Curves Through Fuzzy Divisive Hierarchical Clustering
Not Available
| Template K Light Curves for RR Lyrae Stars
We discuss a technique to compute using templates developed by fitting aFourier series to existing high-quality K light curves of field RRLyraes. We find that a series of order 2 is sufficient to model thelight curves of first-overtone RRc variables, but 4 different 6th-ordertemplates are needed for the fundamental RRab stars due to changes inthe light curves that appear to correlate with the B amplitude. Applyingthe appropriate template to single-phase observations yield estimatedvalues whose deviation from the true is randomly distributed over phase,and is of the same order of magnitude as the observational uncertainty,as long as the ephemeris phase is accurate. The addition of a secondpoint, separated by at least 0.2 in phase from the first, allows the useof template-shifting to remove deviations that may arise fromuncertainties in the ephemeris phase, and template and scaling factorselection, with final systematic errors reduced to less than 0.03 mag.We find that the use of templates yield superior results to thosederived using other techniques, which can produce values that showsystematic deviations over phase. (SECTION: Stars)
| 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 Composition of HB Stars: RR Lyrae Variables
We used moderately high-resolution, high S/N spectra to study thechemical composition of 10 field ab-type RR Lyrae stars. A newtemperature scale was determined from literature Infrared Flux Methodmeasures of subdwarfs and the Kurucz (1992) model atmospheres, and usedto calibrate colors for both dwarfs and RR Lyraes. The applicability ofKurucz (1992) model atmospheres in the analysis of RR Lyraes at minimumlight was analyzed: we found that they are able to reproduce colors,excitation and ionization equilibria as well as the wings of Halpha. Wederived abundances for 21 species. The metal abundances of the programstars span the range -2.50<[Fe/H]<+0.17$. Lines of most elementsare found to form in LTE conditions. Fe lines satisfy very well theexcitation and ionization equilibria. RR Lyraes share the typicalabundance pattern of other stars of similar [Fe/H]: alpha-elements areoverabundant by about 0.4dex and Mn is underabundant by about 0.6dex instars with [Fe/H]<-1. Significant departures from LTE are found onlyfor a few species. We used our new [Fe/H] abundances, as well as valuesfrom Butler and coworkers (corrected to our system), and from highresolution spectroscopy of globular clusters giants, to obtain a newcalibration of the DeltaS index: [Fe/H]= -0.194(\pm 0.011)DeltaS-0.08(\pm 0.18) and to update the metallicity calibration of the Ca II Kline index: [Fe/H]= 0.65(\pm 0.17)W'(K) -3.49(\pm 0.39). Finally, ournew metallicity scale was used to revise the [Fe/H] dependence of theabsolute magnitude of RR Lyrae stars, Mv: Mv = 0.20(\pm 0.03)[Fe/H] +1.06(\pm 0.04).
| 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.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Octans |
| Ascensió Recta: | 13h46m31.80s |
| Declinació: | -84°24'06.4" |
| Magnitud Aparent: | 10.949 |
| Moviment propi RA: | -31.3 |
| Moviment propi Dec: | 17.6 |
| B-T magnitude: | 11.473 |
| V-T magnitude: | 10.993 |
Catàlegs i designacions:
|
INHABITED SKY
