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Light curve analysis of variable stars using Fourier decomposition and principal component analysis
Context: Ongoing and future surveys of variable stars will require newtechniques to analyse their light curves as well as to tag objectsaccording to their variability class in an automated way. Aims: We show the use of principal component analysis (PCA) andFourier decomposition (FD) method as tools for variable star light curveanalysis and compare their relative performance in studying the changesin the light curve structures of pulsating Cepheids and in theclassification of variable stars. Methods: We havecalculated the Fourier parameters of 17 606 light curves of a variety ofvariables, e.g., RR Lyraes, Cepheids, Mira Variables and extrinsicvariables for our analysis. We have also performed PCA on the samedatabase of light curves. The inputs to the PCA are the 100 values ofthe magnitudes for each of these 17 606 light curves in the databaseinterpolated between phase 0 to 1. Unlike some previous studies, Fouriercoefficients are not used as input to the PCA. Results: Weshow that in general, the first few principal components (PCs) areenough to reconstruct the original light curves compared to the FDmethod where 2 to 3 times more parameters are required to satisfactorilyreconstruct the light curves. The computation of the required number ofFourier parameters on average needs 20 times more CPU time than thecomputation of the required number of PCs. Therefore, PCA does have someadvantages over the FD method in analysing the variable stars in alarger database. However, in some cases, particularly in finding theresonances in fundamental mode (FU) Cepheids, the PCA results show nodistinct advantages over the FD method. We also demonstrate that the PCAtechnique can be used to classify variables into different variabilityclasses in an automated, unsupervised way, a feature that has immensepotential for larger databases in the future.
| A Photometric Study of ZZ Microscopium
New photometric (uvby?) observations of ZZ Mic are described.Reddening values O-C values, and we confirmed a sign that the period isincreasing.
| The field high-amplitude SX Phoenicis variable BL Camelopardalis: results from a multisite photometric campaign. I. Pulsation
Context: BL Cam is an extreme metal-deficient field high-amplitude SXPhe-type variable where a very complex frequency spectrum is detected,with a number of independent nonradial modes excited, unusual among thehigh-amplitude pulsators in the Lower Classical Instability Strip. Aims: An extensive and detailed study has been carried out toinvestigate the pulsational content and properties of this object. Methods: The analysis is based on 283 h of CCD observations obtainedin the Johnson V filter, during a long multisite photometric campaigncarried out along the Northern autumn-winter of 2005-2006. Additionally,multicolour BI photometry was also collected to study the phase shiftsand amplitude ratios, between light curves obtained in differentfilters, for modal discrimination of the main excited modes. Results: The detailed frequency analysis revealed a very rich anddense pulsational content consisting of 25 significant peaks, 22 of themcorresponding to independent modes: one is the already known mainperiodicity f0 = 25.5765 cd-1 (? V = 153mmag) and the other 21 are excited modes showing very small amplitudes.Some additional periodicities are probably still remaining in theresiduals. This represents the most complex spectrum ever detected in ahigh-amplitude pulsator of this type. The majority of the secondarymodes suspected from earlier works are confirmed here and, additionally,a large number of new peaks are detected. The amplitude of the mainperiodicity f0 seems to be stable during decades, but themajority of the secondary modes show strong amplitude changes from oneepoch to another. The suspected fundamental radial nature of the mainperiodicity of BL Cam is confirmed, while the secondary peakf1 = 25.2523 cd-1 is identified as a nonradialmixed mode g4 with ? = 1. The radial double-mode nature,claimed by some authors for the main two frequencies of BL Cam, is notconfirmed. Nevertheless, the frequency f6 = 32.6464cd-1 could correspond to the first radial overtone.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| GSC 7672:2238: a binary system near the Delta Scuti star AI Vel
We report the results of a photometric and spectroscopic study of aneclipsing binary star in the field of the Delta Scuti variable AI~Vel.Time-series CCD photometry was performed allowing almost complete phasecoverage. Our period search gave an orbital period of 0.9719d. The lightcurve is typical of short-period Algol stars and suggests the presenceof phase-related increase of brightness (?=0.1-0.2) whichcan be due to mass transfer phenomena. Another possibility is thepresence of a spotted region. Spectroscopic observation shows the BalmerH? absorption line, typical of Algol systems with a transient orabsent disc.
| A comprehensive study of the SX Phoenicis star BL Camelopardalis
Context.BL Cam is an extreme field multiperiodic short-periodlow-metallicity high-amplitude SX Phe-type variable where its probablycomplex pulsational content is subject of controversies. Aims.Acomprehensive study has been carried out to investigate the nature andpulsational properties of this object. Methods.The analysis isbased on new photometric data collected during the last few years atdifferent sites, as well as on all available previously published data.Frequency analyses have been performed on a number of reliable data setsto analyse the pulsational content. In addition, the classical O-Cmethod was used to study the behaviour of the main period. Results.Our frequency analysis confirms some aspects on themultiperiodicity of BL Cam, previously found by other authors, showing anumber of secondary modes close to the main frequency f_0=25.5769cd-1. The secondary modes present eventual amplitudevariations. However, the main frequency exhibits no significant changesin its amplitude and seems to correspond to the fundamental mode ofradial pulsation. Abrupt changes sometimes observed in the light curvescould be due to spurious activity on or close to the star's photosphere.We confirm the existence of a secondary frequency at 31.6 or 32.6cd-1, with variable amplitude, and with the possibility ofboth frequencies being intrinsic to the star. More than one hundred ofnew times of light maxima have been determined in the present work.These together with those available in the literature allow us a morediscriminating analysis of the O-C diagram. This shows that the observedvariations of the main period can be described by two terms: (i) asecular increase of the main period at a rate ofdP/Pdt=117(±3)×10-9 yr-1; and (ii) aperturbation from a companion star in a rather eccentric orbit with aperiod of 10.5(±0.2) yr causing a light time semi-amplitude of148(±12) s.
| Pulsational behaviour of the SX Phe variable BL Cam.
The pulsational behaviour of the field high-amplitude SX Phe variable BLCam is investigated here on the basis of new photometric data collectedduring the last few years at different sites together with a revision ofall the available photometric data sets in the literature. Our resultsconfirm the existence of multiperiodicity in this star previously foundby other authors. In addition to the main frequency f_0=25.5769 c/d andits harmonics 2f_0 and 3f_0, with stable amplitude, a secondaryfrequency f_1 exists in the region 31-32 c/d with variable amplitude.Microvariability takes also place in this star, in the region close tof_0, with some additional secondary peaks shown as significant togetherwith some frequency combinations of these modes with f_0.Concerning f_1,it is shown that there is a possibility of the existence of twoindependent peaks (around 31.6 c/d and 32.6 c/d), nonradial and withvariable amplitudes.
| Amplitude ratios as mode characterizors in delta Scuti stars .
Seismology of delta Scuti star s holds great potential for testingtheories of stellar structure and evolution. The ratio of modeamplitudes in light and in equivalent width of spectral lines can beused for mode identification. However, the amplitude ratios (AR)predicted from theory are usually inconsistent with observations. Wehere present the first results from a campaign aimed at calibratingobservationally the absolute values of the AR.
| Cluster Ages Experiment (CASE): SX Phe stars from the globular cluster ? Centauri
We present an analysis and interpretation of oscillation spectra for all69 SX Phoenicis stars discovered in the field of the cluster. For mostof the stars we have reliable absolute magnitudes and colours. Exceptfor one or perhaps two objects, the stars are cluster members. Theirpulsational behaviour is very diverse. Multiperiodic variability with atleast part of the excited modes being non-radial is most common butthere are also many cases of high-amplitude, presumably radial mode,pulsators. In a number of such cases we have evidence for two radialmodes being excited. Parameters of radial mode pulsators are in mostcases consistent with standard evolutionary models for stars in the massrange 0.9-1.15Msolar. However, in four cases we have evidencethat the masses are significantly lower than expected. Three objectsshow frequency triplets that may be interpreted in terms of rotationalfrequency splitting of l= 1 modes. Implied equatorial velocities ofrotation are from 10 to over 100 km s-1. Nearly all measuredfrequencies fall in the ranges predicted for unstable modes. Two casesof low-frequency variability are interpreted as being caused by tidaldistortion induced by close companions.
| The double-mode nature of the HADS star GSC 00144-03031 and the Petersen diagram of the class
The double-mode pulsation of GSC 00144-03031 has been detected whensearching for COROT targets. A very large dataset composed of 4722photometric measurements was collected at six observatories in Europeand America. There is no hint of the excitation of additional modes(down to 0.6 mmag) and therefore GSC 00144-03031 seems to be a puredouble-mode pulsator, with a very short fundamental radial mode (P = 84min). From uvbyβ photometry and evolutionary tracks it appears tobe a Pop. I star with M = 1.75 Mȯ, located in the middleof the instability strip, close to the Zero-Age Main Sequence. We alsodiscovered other new double-mode pulsators in the databases oflarge-scale projects: OGLE BW2_V142, OGLE BW1_V207, ASAS3 094303-1707.3,ASAS3 000116-6037.0, NSVS 3234596 and NSVS 3324715. An observationalPetersen diagram is presented and explained by means of new models. Acommon sequence connecting Pop. I stars from the shortest to the longestperiods is proposed and the spreads in the period ratios are ascribed todifferent metallicities (at the shortest periods) and to differentmasses (at the longest ones).
| Asteroseismology of HADS stars: V974 Oph, a radial pulsator flavoured by nonradial components
The analysis of a dense time-series on V974 Oph disclosed the richpulsational content (at least five independent terms) of thishigh-amplitude (0.60 mag in B-light) delta Scts star. A mode with afrequency very close to the main one (probably the fundamental radialmode) has been detected: such a doublet is not a common feature in starsof the same class. Also another term can be considered a radial one, butthe high ratio (0.786) raises some problems that can be solved only byadmitting very low metallicity. It is quite evident that someundetectable terms are again hidden in the noise, as the least-squaresfit leaves a rms residual much higher than the observational noise. Allthat considered, nonradial modes seem to play a key rôle in thelight variability of V974 Oph. Revealing an unsuspected asteroseismicinterest, V974 Oph provides a link between low- and high-amplitude deltaScts stars.Based on observations collected at Europan Southern Observatory, LaSilla, Chile
| Long term behaviour of the double-mode high-amplitude delta Scuti star RV Arietis
Stability of both the fundamental and first overtone oscillations of RVAri was investigated by different methods which led to concordantresults. The fundamental period of RV Ari has been decreasing veryslowly with a rate of (1/P0)(dP0/dt)=-0.6x10-8 y-1 while the first overtone period has beenincreasing a little faster with a rate of(1/P1)(dP1/dt) =+0.9x 10-8y-1, if we assume linear period changes over the past 65years. Although the very slow change in the periods is consistent withthe evolutionary theories, the fact that the signs of the period changesfor the two modes are opposite indicates that the dominating part of theperiod variation may be the result of non-evolutionary effects. Theamplitudes of the fundamental and overtone oscillations have undergonesmall changes (several mmag) on a time scale of years, and a slightincrease in the amplitude of the first overtone pulsation is suspected.An additional frequency at f2=13.6116 cd-1 isfound. Table 2 is only available in electronic form at\http://www.edpsciences.org
| δ Scuti stars and their related objects
δ Scuti stars are a group of stars located on or a little abovethe main sequence of H-R diagram with spectral type from A3 to F5. Theyare low amplitude single or multi period pulsators with period shorterthan 0.3 d. Within the same area there are several groups of variablesor special stars correlated with them, e.g., Dwarf Cepheids, γ Dorvariables, Blue Stragglers, Am stars, Ap stars, ROAp variables, λBoo variables and δ Del variables. In this paper a general reviewin this field, including the number of new variables discovered after1995, is presented. The most reliable period variation rates for all thehigh amplitude variables and several low amplitude variables are listed.Statistic shows the higher the rotation rate v sin i is, the lower thelight variation amplitude is. Thus within young open clusters highamplitude variables cannot be found. The amplitudes-periods distributionhave 3 peaks with the highest of 1.0 mag in V at 0.17 d in period. Forδ Scuti variables in stellar systems the shorter the averageperiod is, the lower the metallicity and the older the age of thestellar system are.
| Behaviour of the periods in double mode RR Lyrae, V26 in M15
The changes in the periods of the fundamental and first overtonepulsation of the variable V26 in the globular cluster M15 are studied.The analysis was based on published observations. The method ofinvestigation was outlined in our previous paper (Paparó et al.\cite{pap98}). The Fourier phase diagrams for both frequencies have beenconstructed. General guidelines concerning the cycle counting problemsare discussed. In both periods a sudden increase took place around JD2431500 (~1945) and the changes were the same in sign but slightlydifferent in magnitude. The period change behaviour of the fundamentalmode was noticed for the first time. Before the abrupt change thefundamental and first overtone periods were P0 = 0fd538898and P1 = 0fd402256 days and after it P0 =0fd538979 and P1 = 0fd402273 days. Although the fundamentalcomponent is very weak, there is a remarkable concordance between ourperiod after the abrupt change and the period derived by Clement &Walker (\cite{cle91}). The disagreement between the observed periodchanges and the expectation of stellar evolutionary theory stronglysuggest that period changes are dominated by short term changes governedby the physics of the star. Fast changes in oscillation frequency suchas in V26/M15 cannot take place without sudden changes in the stellarstructure. The possible physical processes are the mixing events in asmall part of the semiconvective zone (Sweigart & Renzini\cite{sr79}) or the dredge up of a small amount of helium by occasionalconvection overshooting (Cox \cite{cox98}).
| Period and amplitude variations in the high-amplitude delta Scuti star AE Ursae Majoris
We present a comprehensive investigation of the variations of period andamplitude in the high-amplitude delta Scuti star AE UMa based on our newJohnson V time-series measurements and the existing data. No additionalfrequencies were detected even though all the available data sets from1974 to 2001 were analysed. The light variations of AE UMa can bewell-reproduced with the fundamental and first-overtone radial modes andtheir coupled terms. New observations and analyses support the mostrecent results of Pócs & Szeidl (\cite{pocs}). Thefundamental period was essentially constant over the past 27 years withits standard value of 0fd 086017066 (f_0= 11.625600 cd-1),while the first overtone period decreased at a rate of(1)/(P_1)(dP_1)/(dt) = -4.3x 10-8 yr-1. Theamplitude variations in the two modes of AE UMa are detected at themilli-magnitude level on a time-scale of years. It seems that theamplitudes vary in opposite phases, implying an energy conservation orsome kind of intrinsic variability cause. We deny theover-interpretation of the period change given by Hintz et al.(\cite{hintz97}) and explore its reason. Figure~\ref{Fig:lightcurve} isonly available in electronic form at http://www.edpsciences.org
| Stability of pulsation of the double-mode high-amplitude delta Scuti star AE Ursae Majoris
Stability of both the fundamental and first overtone oscillations of AEUMa was investigated by different methods which led to concordantresults. The fundamental period of AE UMa has been essentially constantin the past 60 years consistent with the theoretical expectation (Breger& Pamyatnykh \cite{BP98}). The reported fast period decrease (Hintzet al. \cite{HHJ97}) is shown to be incorrect. The constancy of thefundamental period suggests that the star is in the post-main sequenceevolutionary state as suggested by the evolutionary theories. The firstovertone period is decreasing with a rate of (1/P_1) (dP_1/dt)=-7.310-8 y-1. The fact that the rate of period changefor two modes is quite different indicates that non-evolutionary effectsmay also generate period changes. The changes in amplitude of thefundamental and first overtone were examined by comparing theleast-squares amplitude solution for different segments of observations.Small long-term variations in the amplitudes have occurred in the past25 years.
| Multiperiodicities from the Hipparcos epoch photometry and possible pulsation in early A-type stars
A selection criterion based on the relative strength of the largestpeaks in the amplitude spectra, and an information criterion are used incombination to search for multiperiodicities in Hipparcos epochphotometry. The method is applied to all stars which have beenclassified as variable in the Hipparcos catalogue: periodic, unsolvedand microvariables. Results are assessed critically: although there aremany problems arising from aliasing, there are also a number ofinteresting frequency combinations which deserve further investigation.One such result is the possible occurrence of multiple periods of theorder of a day in a few early A-type stars. The Hipparcos catalogue alsocontains a number of these stars with single periodicities: such starswith no obvious variability classifications are listed, and informationabout their properties (e.g., radial velocity variations) discussed.These stars may constitute a new class of pulsators.
| 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.
| On the Variability of F1-F9 Luminosity Class III-V Stars
Hipparcos Satellite photometry of F1-F9 luminosity class III-V starsindicates that most are not particularly variable. A few stars for whichfurther study is desirable are identified.
| A revised catalogue of delta Sct stars
An extensive and up-dated list of delta Sct stars is presented here.More than 500 papers, published during the last few years, have beenrevised and 341 new variables have been added to our last list, sixyears ago. This catalogue is intended to be a comprehensive review onthe observational characteristics of all the delta Sct stars known untilnow, including stars contained in earlier catalogues together with othernew discovered variables, covering information published until January2000. In summary, 636 variables, 1149 references and 182 individualnotes are presented in this new list. Tables 1 and 2 will be accessibleonly in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Stellar Structure and Evolution: Deductions from Hipparcos
During the last decade, the understanding of fine features of thestructure and evolution of stars has become possible as a result ofenormous progress made in the acquisition of high-quality observationaland experimental data, and of new developments and refinements in thetheoretical description of stellar plasmas. The confrontation ofhigh-quality observations with sophisticated stellar models has allowedmany aspects of the theory to be validated, and several characteristicsof stars relevant to Galactic evolution and cosmology to be inferred.This paper is a review of the results of recent studies undertaken inthe context of the Hipparcos mission, taking benefit of the high-qualityastrometric data it has provided. Successes are discussed, as well asthe problems that have arisen and suggestions proposed to solve them.Future observational and theoretical developments expected and requiredin the field are also presented.
| Pulsation models of delta Scuti variables. II. delta Scuti stars as precise distance indicators
The Hipparcos period-luminosity relation for high-amplitude delta Scutistars is discussed as an average period-luminosity-colour-metallicityrelation comparing the Hipparcos sample of calibrating stars with a morerepresentative sample taken from McNamara (1997). The Hipparcos relationincludes systematic effects of both colour and metallicity and agreeswith other period-luminosity relations with an accuracy of about +/-0.10 mag. Direct stellar-evolution and pulsation modelling is used tostudy effects on the average period-luminosity relation from thelocation of the high-amplitude variables within the instability stripgiven as a range in effective temperature (colour term) and themetallicity given as [Fe/H]. Using accurate data for the instabilitystrip given by McNamara precise agreement with the Hipparcos relation isfound. By fitting the bolometric magnitude as a function of pulsationperiod, effective temperature and metallicity an accuracy of 0.02-0.03mag is obtained both within the narrow high-amplitude strip andincluding a much broader strip covering most of the full {delta Sct}instability strip. Problems in practical application of this relationare briefly considered. Using uvbybeta narrow-band photometry it seemspossible to obtain this accuracy in determinations of distances to e.g.the Galactic Bulge and the Magellanic Clouds.
| Stability in the Light Curves of High-Amplitude delta Scuti Stars: Selected Monoperiodic Stars
We have analyzed all the reliable photometric data sets available in thebibliography for a selected sample of monoperiodic high-amplitude deltaScuti stars in order to study the stability of their light curves. Atotal of 169 data sets and more than 22,000 points have been consideredfor seven stars: ZZ Mic, EH Lib, BE Lyn, YZ Boo, SZ Lyn, AD CMi, and DYHer. The results do not reveal significant long-term changes ofamplitude of the light curves for any of these stars.
| Mode and period changes in pulsating stars near the main sequence : delta Scuti stars.
Not Available
| Le satellite Hipparcos. Une moisson de resultats astrophysiques.
Not Available
| The Impact of High Accuracy Astrometry on Asteroseismology
Accurate stellar distances -- as can be provided only by spaceastrometry -- are necessary for the determination of accurate stellarfundamental parameters (in particular luminosity and mass) of single andnon-eclipsing binary stars. These same parameters are needed, togetherwith the oscillation frequencies determined by asteroseismologicobservations, to provide stringent tests of stellar structure andevolution models. The impact on stellar structure studies of theupcoming generation of space astrometric and asteroseismologicalmissions (in particular Gaia, currently under study at the EuropeanSpace Agency) is discussed.
| Photometric Abundance Calibration of delta Scuti Stars Using HK Photometry
The hk index has been used as a metallicity indicator for RR Lyraevariable stars. It is now being applied to the shorter period deltaScuti variables. Employing spectroscopic abundances of stars withpublished hk values and photometric indices calculated from stellaratmosphere models, a three-dimensional interpolation is used todetermine [Fe/H] from intrinsic b-y, c_1, and hk values. The resulting[Fe/H], log g, and T_eff values for 10 delta Scuti stars are presented.
| The delta Scuti Star GSC 2985-01044
GSC 2985-01044 is a delta Scuti star with a period of 0.0933584 days anda V-magnitude range of 11.85-12.05 its light curve is slightly variable.The location, space motion, and other properties of this star indicatethat it is a higher amplitude delta Scuti star (or ``dwarf Cepheid'')that is a member of the old disk population. The problem of determiningthe local space densities of the various populations of the higheramplitude delta Scuti stars is discussed.
| Oscillating Blue Stragglers in the Core of 47 Tucanae
Six SX Phe variables detected in a 39 hr time series of Hubble SpaceTelescope Planetary Camera exposures in the F336W (U) filter arediscussed. Two of these stars show both the fundamental andfirst-overtone modes and are among the first double-mode SX Phe starsreported in globular clusters. One of the double-mode stars showsevidence for a third mode near the expected period for the secondovertone. Two of the other SX Phe variables in 47 Tuc are shown tooscillate simultaneously in the fourth and fifth radial overtones. Theremaining two variables both have multiple oscillations excited whichinclude nonradial modes. To support interpretation of these variables weprovide evolutionary computations using current opacities and physicsallowing comparison of the blue straggler stars (BSSs) and severalcluster-magnitude diagram features for 47 Tuc. Linear nonadiabaticoscillation analysis is performed for a significant range of envelopeand full evolutionary models in order to (1) establish the theoreticalcharacteristics of SX Phe pulsations (e.g., location of primary drivingand adiabatic nature), (2) establish the theoretical red and blue edgesof the instability strip, and (3) derive the theoretical relationsnecessary for obtaining evolutionary and pulsational masses. Massesinferred from fundamental period-period ratio relations for thedouble-mode oscillators are well above the turnoff mass in 47 Tucanaeand are consistent with expected masses for these blue stragglers basedon position in the color-magnitude diagram (CMD) and comparison withtheoretical evolutionary tracks. Combining the evolution and pulsationconstraints results in mass estimates for the four double-mode BSSs in47 Tuc of M = 1.35 +/- 0.1 to 1.6 +/- 0.2 M_?.
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