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The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. II.Type II Cepheids and Anomalous Cepheids in the Large Magellanic Cloud
In the second part of the OGLE-III Catalog of Variable Stars (OIII-CVS)we present 197 type II Cepheids and 83 anomalous Cepheids in the LargeMagellanic Cloud (LMC). The sample of type II Cepheids consists of 64 BLHer stars, 96 W Vir stars and 37 RV Tau stars. Anomalous Cepheids aredivided into 62 fundamental-mode and 21 first-overtone pulsators. Theseare the largest samples of such types of variable stars detectedanywhere outside the Galaxy.We present the period-luminosity and color-magnitude diagrams of starsin the sample. If the boundary period between BL Her and W Vir stars isadopted at 4 days, both groups differ significantly in (V-I) colors. Weidentify a group of 16 peculiar W Vir stars with different appearance ofthe light curves, brighter and bluer than ordinary stars of that type.Four of these peculiar W Vir stars show additional eclipsing modulationsuperimposed on the pulsation light curves. Four other stars of thattype show long-period secondary variations which may be ellipsoidalmodulations. It suggests that peculiar W Vir subgroup may be related tobinarity. In total, we identified seven type II Cepheids simultaneouslyexhibiting eclipsing variations which is a very large fraction comparedto classical Cepheids in the LMC. We discuss diagrams showing Fourierparameters of the light curve decomposition against periods. Three sharpfeatures interpreted as an effect of resonances between radial modes aredetectable in these diagrams for type II Cepheids.
| The Variable Stars of the Draco Dwarf Spheroidal Galaxy: Revisited
We present a CCD survey of variable stars in the Draco dwarf spheroidalgalaxy. This survey, which has the largest areal coverage since theoriginal variable star survey by Baade & Swope, includes photometryfor 270 RR Lyrae (RRL) stars, 9 anomalous Cepheids (ACs), 2 eclipsingbinaries, and 12 slow, irregular red variables, as well as 30 backgroundQSOs. Twenty-six probable double-mode RRL stars were identified.Observed parameters, including mean V and I magnitudes, V amplitudes,and periods, have been derived. Photometric metallicities of the ab-typeRRL stars were calculated according to the method of Jurcsik &Kovacs, yielding a mean metallicity of lang[Fe/H]rang = -2.19± 0.03. The well-known Oosterhoff intermediate nature of the RRLstars in Draco is reconfirmed, although the double-mode RRL stars, withone exception, have properties similar to those found in Oosterhoff typeII globular clusters. The period-luminosity relation of the ACs isrediscussed with the addition of the new Draco ACs.
| The anomalous Cepheid XZ Ceti
Aims. XZ Ceti is the only known anomalous Cepheid in the Galactic field.Being the nearest and brightest such variable star, a detailed study ofXZ Ceti may shed light on the behaviour of anomalous Cepheids whoserepresentatives have been mostly detected in external galaxies. Methods: CCD photometric and radial velocity observations have beenobtained. The actual period and amplitude of pulsation were determinedby Fourier analysis. The long timescale behaviour of the pulsationperiod was studied by the method of the O-C diagram using the archivalHarvard photographic plates and published photometric data. Results: XZ Ceti differs from the ordinary classical Cepheids in severalrespects. Its most peculiar feature is cycle-to-cycle variability of thelight curve. The radial velocity phase curve is not stable either. Thepulsation period is subjected to strong changes on various timescales,including a very short one. The ratio of amplitudes determined from thephotometric and radial velocity observations indicates that this Cepheidperforms an overtone pulsation, in accord with the other known anomalousCepheid in our Galaxy, BL Boo (V19 in the globular cluster NGC 5466). Conclusions: . Continued observations are necessary to study thedeviations from regularity to determine their timescale, as well as toconfirm the binarity of XZ Ceti and to study its role in the observedpeculiar behaviour.
| Absolute Magnitude Calibrations of Population I and II Cepheids and Other Pulsating Variables in the Instability Strip of the Hertzsprung-Russell Diagram
The status of the absolute magnitude calibrations is reviewed for thelong period Cepheids of population I and II, RR Lyrae stars, evolved"above horizontal branch" (AHB1) variables (periods 0.8 to 3 days),dwarf Cepheids of both populations (the Delta Scuti and SX Phoenicusvariables), and the anomalous Cepheids (AC). Evidence shows that theperiod-color and period-luminosity (P-L) relations for population ICepheids in the Galaxy and in the Large and Small Magellanic Clouds havedifferent slopes and zero points. This greatly complicates use ofCepheids for the extragalactic distance scale. Strategies are discussedto patch the problem. A consensus exists for the long distance scale forRR Lyrae stars whose calibrations favor MV(RR) = 0.52 at[Fe/H] = -1.5. Exceptions exist for "second parameter" clusters wherethe variation of the morphology of the horizontal branch withmetallicity is anomalous, the most blatant being NGC 6388 and NGC 6441.The status and calibrations of ABH1 and AC show that differentevolutionary paths and masses explain the difference P-L relations forthem. AC appear predominantly in the dwarf spheroidal galaxies, but arealmost absent in Galactic globular clusters. AHB1 stars are absent indwarf spheroidals but are present in globular clusters. The differencemay be used to study the formation of the remote Galactic halo if it ispartially made by tidal disruption of companion dwarf spheroidals.
| Observational study of the anomalous Cepheid XZ Ceti
Photometric and radial velocity observations show that the brightestknown anomalous Cepheid, XZ Ceti pulsates in the first overtone. ThisCepheid, peculiar in the Galactic field shows strong period changesappearing on a short time scale.
| 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.
| 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.
| The Intermediate-Period Cepheid Strip Stars
To explore the population of variable stars that fall between RR Lyraestars and classical Cepheids, we have obtained extensive photometricobservations for 56 stars with periods between 0.6 and 1.0 days. Forthese stars and an additional 10, we have examined the form andstability of the light curves to explore the variety of types of starsin this period regime. We conclude that the majority of the sample areBailey type ab RR Lyrae stars. However, a surprising number thatpossibly exhibit the Blazhko effect are found. There are a half-dozenother stars that do not appear to be in either of these two groups. Oneis the well-known multimode pulsator AC And. We suggest that two of theothers may be anomalous Cepheids and two may be type II Cepheids.
| Fourier decomposition and frequency analysis of the pulsating stars with P<1 d in the OGLE database. I. Monoperiodic Delta Scuti, RRc and RRab variables. Separation criteria and particularities
The OGLE database is revisited to investigate in more detail theproperties of the Fourier parameters. Methodological improvements led usto identify a clear separation among High-Amplitude delta Scuti (HADS),RRc and RRab stars. The bimodal distribution of the R21parameter in HADS stars is explained as a contamination effect from RRcstars: there is evidence that all stars with 0.200.55 d; evolutionary phases or the combination ofdifferent physical conditions (not only metallicity) are suggested toexplain this separation, observed also in the cluster RRab stars. Thestellar parameters of RRc stars in a given globular cluster showdifferent tendencies than those of RRc stars from different clusters.Tables~3, 4 and 5 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)} or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/371/986 or via e-mail fromthe author.
| 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.
| 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 impact of HIPPARCOS on the RR Lyrae Distance Scale
Not Available
| Radial velocities and iron abundances of field RR Lyraes. I.
We present systemic velocities and iron abundances for 56 RR Lyraes, themajority of which have been observed by the HIPPARCOS satellite.Comparison between our systemic velocities and previous valuesidentifies several binary candidates only one of which, TU UMa, waspreviously suspected of being a binary. However, spectra of the unusualRR Lyrae BB Vir show no evidence of line doubling and hence do notsupport the recent claims that this star may have a Blue HorizontalBranch companion. Comparison between our abundances and previousdeterminations shows reasonable agreement except with the recent work of\cite[Layden (1994)]{Lay94} where we find systematic differences.Several of the stars included on the HIPPARCOS observing list as RRLyraes are shown to be mis-classified. Of particular interest are thestars V363 Cas and AT And which, by analogy with XZ Cet, may beanomalous Cepheids. The Appendix is available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Structural Properties of Pulsating Star Light Curves Through Fuzzy Divisive Hierarchical Clustering
Not Available
| The Fundamental Parameters of Field Horizontal-Branch Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2291G&db_key=AST
| Spectral Characteristics of Field Horizontal-Branch Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112.2286C&db_key=AST
| 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
| 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.
| 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
| The long-period field RR Lyrae stars
Fourier decompositions have been performed on several of the VR lightcurves obtained for 16 faint variable stars with less-than-1-dayperiods, where the trends of slower rises to maxima and flatter topsthan those characteristic of Bailey-type ab RR Lyrae stars are evident.While the Fourier coefficients in such cases are essentially constantfor periods shorter than 3/4-day for the Bayley-type ab RR Lyrae stars,the amplitude ratios R(12), R(13), and R(14) decrease, and the phi(21)and phi(23) phase differences increase at longer period. When theFourier parameters for known anomalous Cepheids are compared with thoseof RR Lyrae stars, the scatter is found to preclude an unambiguoussegregation of the anomalous Cepheids from the RR Lyrae stars.
| The Longest Period RR Lyrae Stars
Not Available
| The unusual pulsating variable XZ Ceti
The peculiar pulsating variable XZ Ceti has a period of 0.8231 day,suggesting that it is a Bailey type ab RR Lyrae star, but it has a lightcurve shape and amplitude which is like that of the Bailey type cvariables. Fourier decomposition analysis verifies that the parametersof its light curve match those of the type c variables, except for itsunacceptably long period. A new photometric light curve which verifiesthis long period has beem obtained along with energy distributions fromspectrum scans, which are used to derive a temperature and approximatesurface gravity of this star. These results are used to constructpulsation models so as to attempt to identify the nature of XZ Ceti. Themodels suggest the possibility that XZ Ceti is an anomalous Cepheid or,perhaps, an overtone BL Her star.
| Finding charts for field horizontal-branch A-type stars.
Not Available
| Scanner observations of field horizontal-branch A stars
Measurements have been conducted regarding the energy distributions of16 field horizontal-branch A stars, plus nine others of miscellaneoustypes, at 20 wavelengths between 3450 and 6970 A using the HCO scannerson 0.9 m telescopes at Kitt Peak and Cerro Tololo Observatories. Theobservations are reported here as absolute energy distributions on thesystem of Hayes and Latham. The (internal) measurement error (std. dev.)is 0.025 mag per observation, averaged over all wavelengths.
| The Anomalous Pulsator XZ Ceti
Not Available
| The light curves of RR Lyrae field stars
Fourier decompositions have been made of the light curves of a largesample of RR Lyrae field stars. The coefficients have been tabulated.Following the scheme of an earlier investigation of classical Cepheids,certain combinations of the low-order coefficients - phi21, R21, andphi31 - are plotted against period. The Bailey-type c pulsators standout from the type ab stars, particularly on the R21 plot which is foundto be a more sensitive discriminator of Bailey type than is thetraditionally employed amplitude-period diagram. The RR Lyrae plots ofphi21, R21, and phi31 are compared with those previously obtained forclassical Cepheids. It is noted that, while the Cepheid plots display atightly defined progression with period, reflecting the influence of amodal resonance, in the RR Lyrae case there is much more scatter.However, some evidence is shown to exist for a Cepheid-like progressionappearing among the longer period RR Lyrae pulsators and culminating inthe unique small-amplitude variable XZ Ceti.
| Fourier decomposition of RR Lyrae pulsations.
Not Available
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