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Modelling eclipsing binaries with pulsating components: Phase dependence of observed pulsation amplitudes
We investigate the phase dependence of the light variations due topulsations, as modulated with light variations due to the eclipses. Thestudy is based, both on a simplified model and on the rigorous lightcurve (LC) synthesis software of Wilson and Devinney. We model AB Cas asa prototype, using both of these approaches. The phase dependence of thedifferential light variations for total systems is also discussed .
| Amplitude variability or close frequencies in pulsating stars - the δ Scuti star FG Vir
The nature of the observed amplitude variability of several modes in theδ Scuti star FG Vir is examined. This is made possible by theextensive photometry obtained during 2002-04, as well as the longphotometric time-base starting in 1992. In this star, three frequenciesshow strong amplitude and phase variations. In the power spectrum, thesefrequencies also show up as frequency doublets. However, since trueamplitude variability of a single frequency can also lead to (false)frequency doublets in the power spectrum, a specific test examining indetail the observed amplitude and the phase variations of an assumedsingle frequency is applied. For the frequencies at 12.15 and 23.40cycle d-1, it is shown that the amplitude variability of asingle mode can be ruled out. In particular, an important property ofbeating between two modes is fulfilled: the amplitude and the phase varysynchronously with a phase shift close to 90°. The origin of theamplitude variability of a third mode, namely, near 19.86 cycled-1, is not clear due to the long beat period of 20+ yr, forwhich the amplitude-phase test suffers from gaps in the coverage.However, even for this frequency, the amplitude variations can beexpressed well by a mathematical two-mode model.If we examine these three close frequency pairs together with other(usually more widely separated) close frequencies in FG Vir, 18 pairs offrequencies with separations closer than 0.10 cycle d-1 havebeen detected. It is shown that the majority of the pairs occur near thetheoretically expected frequencies of radial modes. Mode identificationsare available for only a few modes: the only detected radial mode at12.15 cycle d-1 is part of a close pair.It is shown that accidental agreements between the frequencies ofexcited modes can be ruled out because of the large number of detectedclose frequency doublets.
| Acoustic oscillations in centrifugally flattened polytropic star .
We present numerical calculations of axisymmetric acoustic modes inpolytropic models of star deformed by the centrifugal force. In therange of flatness and frequencies considered, we found that, as flatnessincreases, (i) differences with perturbative methods becomes rapidlysignificant (ii) the structure of the spectrum is strongly modified(iii) the mode amplitude at the surface tends to concentrate near theequator.
| Direct imaging photometry with the MOST satellite
Canada's first space telescope, MOST (Microvariablity and Oscillationsof Stars) was successfully launched on June 30, 2003 with a primarymission to perform ultra-high-precision photometry to detect acousticoscillations in solar-like stars. MOST has the ability to observe singlefields for uninterrupted periods of up to two months and targets can beobserved either through Fabry lens imaging or Direct imaging. Thisreport reviews the Direct imaging capabilities of the MOST satellite andthe extraction of accurate stellar photometry.MOST is a Canadian Space Agency mission, operated jointly by Dynacon, Inc., and the Universities of Toronto and British Columbia, with assistance from the University of Vienna.
| Analysis of main-sequence A-type stars showing radial velocity variability.
We obtained high-resolution spectroscopic data for 33 bright (V < 8)A-type HIPPARCOS programme stars at the ``Observatoire deHaute-Provence'' (OHP) in December 2004. All our targets show someindication of radial velocity variability (Grenier et al, 1999), arelocated in or near the lower part of the Cepheid instability strip, andare poorly studied objects. In this contribution, we present the newdata of the most interesting programme stars and we derive a physicalinterpretation for their variable nature.
| Influence of overshooting and metallicity on the delta Scuti and gamma Doradus instability strips
Computations of theoretical instability strips (IS) for delta Scuti(Sct) and gamma Dor (Dor) stars are presented. The Time-DependentConvection (TDC) theory of Gabriel (1996) and Grigahcène etal. (2005) is adopted in our models. We are able to obtain the delta Sctand gamma Dor IS. We present a prospective study on the influence ofthe overshooting and metallicity on the location of these IS in the HRdiagram.
| Application of time-dependent convection models to the photometric mode identification in delta Scuti stars
We apply the Time-Dependent Convection (TDC) treatment of Gabriel\cite{Gabriel1996} and Grigahcène \cite{Grigahcene} to the modeidentification and seismic study of delta Sct stars. We compare thenon-adiabatic phase-lags obtained with TDC and Frozen Convection (FC)treatments and show that they are very different at the red side of theinstability strip. Finally, we compare the phase differences betweenlight and velocity curves observed for the star 1 Mon with thetheoretical predictions of TDC and FC models. The much better agreementfound with the TDC models enables us to identify the modes of this starwith a higher degree of confidency.
| 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.
| Improved oscillator strengths and wavelengths in Hf II, with applications to stellar elemental abundances
Aims. We present new and improved radiative lifetimes for eight levelsin Hf I and 18 levels in Hf II, along with oscillator strengths andwavelengths for 195 transitions in Hf II. With these data we determinethe abundance of hafnium in two chemically peculiar stars: the hot-Amstar HR 3383 and the HgMn star χ Lupi, and discuss the implicationsof the new data to the hafnium abundance for the Sun and the metal-poorgalactic halo stars CS 22892-052 and CS 31082-001. Methods: . Theoscillator strengths are derived by combining radiative lifetimesmeasured with the laser induced fluorescence technique and branchingfractions determined from intensity calibrated Fourier transformspectra. The hafnium abundance in the two sharp-lined peculair stars isdetermined by comparison of spectra obtained from instruments onboardthe Hubble Space Telescope with synthetic spectra, while the abundanceof hafnium in the solar photosphere and the metal-poor halo stars isdiscussed in terms of rescaling previous investigations using the new gfvalues. Results: . The abundance enhancement of hafnium has beendetermined in HR 3383 to be +1.7 dex and that for χ Lupi A is +1.3dex. In the course of the analysis we have also determined an abundanceenhancement for molybdenum in HR 3383 to be +1.2 dex, which is similarto that known for χ Lupi A. The abundances in the metal-poor halostars CS 31082-001 and CS 22892-052 were rescaled to log ɛ(Hf) =-0.75 and -0.82 respectively, with smaller 1σ uncertainty. Thishas the effect of improving the theoretical fits of r-processnucleosynthesis to abundance data for heavy elements. The change of gfvalues also implies that the hafnium abundance in the solar photosphereshould be reduced by up to 0.2 dex, thereby inducing a discrepancy withthe meteoritic hafnium abundance.
| The chemical composition of δ Scuti*
We present chemical abundances in the photosphere of δ Scuti(δ Sct) - a prototype of the class of pulsating variables -determined from the analysis of a spectrum obtained by using the 2-mtelescope at the Peak Terskol Observatory and a high-resolutionspectrometer with R= 52000, a signal-to-noise ratio 250 and fromInternational Ultraviolet Explorer spectra. The abundance pattern ofδ Sct consists of 49 chemical elements. The abundances of 19elements have not been investigated previously. The abundances of Pr andNd obtained from the lines of the second and third spectra are equal.The abundances of heavy elements indicate the overabundances withrespect to the solar system values up to 1 dex. The abundance pattern ofδ Sct is similar to the abundance patterns of Am-Fm or δ Deltype stars.A splitting of the cores of all clean lines is observed for the spectraof δ Sct and HD 57749. This can signify evidence of non-radialpulsations in these stars.
| Search for pulsating pre-main-sequence stars in NGC6383
A search for pulsating pre-main-sequence (PMS) stars was performed inthe young open cluster NGC 6383 using CCD time-series photometry inJohnson B and V filters. With an age of only ~1.7Myr all cluster memberslater than spectral type A0 have not reached the ZAMS yet, hence beingideal candidates for investigating PMS pulsation among A- and F-typestars. In total 286 stars have been analysed using classical Fouriertechniques. From about a dozen stars within the boundaries of theclassical instability strip, two stars were found to pulsate: NGC 6383#170, with five frequencies simultaneously, and NGC 6383 #198, with asingle frequency. In addition, NGC 6383 #152 is a suspected PMS variablestar, but our data remain inconclusive. Linear, non-adiabatic modelsassuming PMS evolutionary phase and purely radial pulsation werecalculated for the two new PMS pulsators. NGC 6383 #170 appears topulsate radially in third and fifth overtones, while the other threefrequencies seem to be of non-radial nature. NGC 6383 #198 pulsatesmonoperiodically, most probably in the third radial overtone. Magnitudesand B-V colours were available in the literature for only one third ofall stars and we used them for calibrating the remaining.
| Aus den IBVS.
Not Available
| Multimode Resonant Coupling in Pulsating Stars
We consider evolution of an unstable acoustic mode interacting with anensemble of stable g-modes. We show that the static multimode solutiondoes not exist. We then find the condition for the stability of thestatistical equilibrium.Performing numerical integration of amplitude equations for a simplifiedsystem we find that the acoustic mode amplitude exhibits a largeirregular variability on the timescale given by the inverse of thegrowth rate.The g-mode pairs are excited in significantly wider range of detuningparameters than it is implied by the parametric instability criterionapplied to the average amplitude. However, the number of interactingg-mode pairs is reduced because the pairs differing in the detuningparameter by less than their damping rates are synchronized andeffectively act as a single pair.We apply the multimode resonant coupling theory to a realistic stellarmodel. We choose a seismic model of a delta Sct star XX Pyx. Althoughfor some l=2 modes we find amplitudes of the order of a fewmillimagnitudes, the typical amplitudes of low-degree modes are muchhigher. Taking into account the rotational splitting results in decreaseof amplitudes by a factor of few which is not enough to obtainconsistency with observations. We conclude that in this star and likelyin all evolved δ Sct stars, the resonant mode coupling cannot bethe dominant amplitude limiting effect. The nonresonant saturation ofthe driving effect must play the role.
| Modelling of the fast rotating δ Scuti star Altair
We present an asteroseismic study of the fast rotating star HD187642 (Altair), recently discovered to bea δ Scuti pulsator. We have computed models taking into accountrotation for increasing rotational velocities. We investigate therelation between the fundamental radial mode and the first overtone inthe framework of Petersen diagrams. The effects of rotation on suchdiagrams, which become important at rotational velocities above 150 {km}{s}-1, as well as the domain of validity of our seismic toolsare discussed. We also investigate the radial and non-radial modes inorder to constrain models fitting the five most dominant observedoscillation modes.
| A photometric study of the light variations of the triple system DG Leo
Multi-site and multi-year differential photometry of the triple star DGLeo reveals a complex frequency spectrum that can be modelled as thecombination of at least three δ Scuti type frequencies in therange 11.5-13 c/d (with semi-amplitudes of 2-7 mmag) and a superimposedslow variability of larger amplitude. The period of the slow variationfits very well with half the orbital period of the inner spectroscopicbinary indicating the presence of ellipsoidal variations caused by thetidally deformed components in a close configuration. These findings,together with the results of a recent spectroscopic analysis (showingthat the system consists of a pair of mild Am stars and one A-typecomponent of normal solar composition), infer that DG Leo is anextremely interesting asteroseismic target. Identification of whichcomponent(s) of this multiple system is (or are) pulsating anddetermination of the excited pulsation modes will both contribute to amuch better understanding of the non-trivial link between multiplicity,chemical composition, rotation, and pulsation in the lower part of theclassical Cepheid instability strip.
| Convection-pulsation coupling. II. Excitation and stabilization mechanisms in δ Sct and γ Dor stars
We apply here the Time Dependent Convection (TDC) treatment presented inour earlier paper in this series to the study of δ Sct and γDor pulsating stars. Stabilization of the δ Sct p-modes at the rededge of the Instability Strip (IS) and the driving of the γ Dorg-modes are explained by our models. Theoretical IS obtained withdifferent values of the Mixing Length (ML) parameter α arecompared to observations and a good agreement is obtained for αbetween 1.8 and 2. The influence of each term of our TDC treatment(perturbation of convective flux, turbulent pressure, and turbulentkinetic energy dissipation) on the eigenfrequencies and on the drivingand damping mechanisms is investigated. Finally, we show that our TDCmodels predict the likely existence of hybrid stars with both δSct p-modes and γ Dor g-modes oscillations.
| Detection of a classical δ Scuti star in the new eclipsing binary system HIP 7666
HIP 7666 is a variable star newly discovered during the Hipparcosmission and classified as of unknown type (ESA 1997, The Hipparcos andTycho Catalogues, ESA SP-1200). During 23 nights between July 2000 andNovember 2000, over 2300 CCD observations in the V band were obtained.These data show that the new variable is a detached eclipsing binarysystem with an orbital period of 2.37229 days. In addition, one of thecomponents undergoes very short-period oscillations with a mainpulsation frequency of 24.46 or 25.47 c/d. HIP 7666 is therefore a newmember of the rare group of detached eclipsing binary systems with aδ Scuti type component.
| Convection-pulsation coupling. I. A mixing-length perturbative theory
We present in details a time-dependent convection treatment in the frameof the Mixing-Length Theory (MLT). Following the original ideas by Unno(1967, PASJ, 19, 140), this theory has been developed by Gabriel et al.(1974, Bull. Ac. Roy. Belgique, Classe des Sciences, 60, 866) andGabriel (1996, Bull. Astron. Soc. India, 24, 233). In this paper, wepresent it in a united form, we detail the basic derivations andapproximations and give final improvements. A new perturbation of theenergy closure equation is proposed for the first time, making itpossible to avoid the occurrence of short wavelength spatialoscillations of the thermal eigenfunctions. This theory accounts for theperturbation of the convective flux, the turbulent Reynolds stress andthe turbulent kinetic energy dissipation. It has been numericallyimplemented in a non-radial non-adiabatic pulsation code and the firstresults published in a Letter by Dupret et al. (2004a, A&A, 414,L17) indicate that the theory predicts the observed red border of thelower end of the instability strip and the driving mechanism of therecently discovered γ Dor stars.
| PSST: The Planet Search Survey Telescope
The Planet Search Survey Telescope is an automated small-aperture CCDimaging photometer designed to search for transits by extrasolar planetsacross the disks of their parent stars. It simultaneously observesthousands of stars with apparent R magnitudes between 10 and 13 in afield approximately 6deg×6deg. Stars in thisbrightness range are well within the capability of the high-precisionradial velocity systems that have successfully detected over 100extrasolar planets to date. The combination of the photometric transitdepth and radial velocity amplitude can provide both the radius of theplanet and a good estimate of its mass, since the orbit is nearlyedge-on. As a result, estimates of the planet's density and otherparameters can be obtained.
| Asteroseismology with robotic telescopes
Asteroseismology explores the interior of pulsating stars by analysingtheir normal mode spectrum. The detection of a sufficient number ofpulsation modes for seismic modelling of main sequence variablesrequires large quantities of high-precision time resolved photometry.Robotic telescopes have become an asset for asteroseismology because oftheir stable instrumentation, cost- and time-efficient operation and thepotentially large amounts of observing time available. We illustratethese points by presenting selected results on several types ofpulsating variables, such as δ Scuti stars (main sequence andpre-main sequence), γ Doradus stars, rapidly oscillating Ap starsand β Cephei stars, thereby briefly reviewing recent successstories of asteroseismic studies of main sequence stars.
| The science case of the PEPSI high-resolution echelle spectrograph and polarimeter for the LBT
We lay out the scientific rationale for and present the instrumentalrequirements of a high-resolution adaptive-optics Echelle spectrographwith two full-Stokes polarimeters for the Large Binocular Telescope(LBT) in Arizona. Magnetic processes just like those seen on the Sun andin the space environment of the Earth are now well recognized in manyastrophysical areas. The application to other stars opened up a newfield of research that became widely known as the solar-stellarconnection. Late-type stars with convective envelopes are all affectedby magnetic processes which give rise to a rich variety of phenomena ontheir surface and are largely responsible for the heating of their outeratmospheres. Magnetic fields are likely to play a crucial role in theaccretion process of T-Tauri stars as well as in the acceleration andcollimation of jet-like flows in young stellar objects (YSOs). Anotherarea is the physics of active galactic nucleii (AGNs) , where themagnetic activity of the accreting black hole is now believed to beresponsible for most of the behavior of these objects, including theirX-ray spectrum, their notoriously dramatic variability, and the powerfulrelativistic jets they produce. Another is the physics of the centralengines of cosmic gamma-ray bursts, the most powerful explosions in theuniverse, for which the extreme apparent energy release are explainedthrough the collimation of the released energy by magnetic fields.Virtually all the physics of magnetic fields exploited in astrophysicsis somehow linked to our understanding of the Sun's and the star'smagnetic fields.
| The All Sky Automated Survey. Catalog of Variable Stars. III. 12h-18h Quarter of the Southern Hemisphere
This paper describes the third part of the photometric data from the 9arcdeg x 9arcdeg ASAS camera monitoring the whole southern hemisphere inV-band. Preliminary list of variable stars based on observationsobtained since January 2001 is presented. Over 3200000 stars brighterthan V=15 mag on 18000 frames were analyzed and 10453 were found to bevariable (1718 eclipsing, 731 regularly pulsating, 849 Mira and 7155other stars). Light curves have been classified using the improvedautomated algorithm, which now takes into account 2MASS colors and IRASinfrared fluxes. Basic photometric properties are presented in thetables and some examples of thumbnail light curves are printed forreference. All photometric data are available over the INTERNET athttp://www.astrouw.edu.pl/\gp/asas/asas.html orhttp://archive.princeton.edu/\asas.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Synergies in Variability Studies of Stars, Supernovae, and Active Galactic Nuclei
We introduce some basic concepts concerning multiband investigations ofvariable sources such as stars, supernovae, and active galactic nuclei.These topics have been widely discussed during the internationalworkshop Variability with Wide Field Imagers. We summarize the mainconclusions of this Workshop and briefly outline possible avenues forfuture broad collaborations.
| Preparing the COROT space mission: Incidence and characterisation of pulsation in the lower instability strip
By pursuing the goal to find new variables in the COROT field-of-view wecharacterised a sample of stars located in the lower part of theinstability strip. Our sample is composed of stars belonging to the diskpopulation in the solar neighbourhood. We found that 23% of the starsdisplay multiperiodic light variability up to a few mmag in amplitude,i.e., easily detectable on a single night of photometry. uvbybetaphotometry fixed most of the variables in the middle of the instabilitystrip and high-resolution spectroscopy established that they have vsin i>100 km s-1. An analysis of the Rodríguez &Breger (\cite{Rodr01}) sample (delta Sct stars in the whole Galaxy)shows slightly different features, i.e., most delta Sct stars have a0.05-mag redder (b-y)0 index and lower vsin i values.Additional investigation in the open cluster NGC 6633 confirms the sameincidence of variability, i.e., around 20%. The wide variety ofpulsational behaviours of delta Sct stars (including unusual objectssuch as a variable beyond the blue edge or a rapidly rotatinghigh-amplitude pulsator) makes them very powerful asteroseismic tools tobe used by COROT. Being quite common among bright stars, delta Sctstars are suitable targets for optical observations from space.Based on observations collected at the S. Pedro Martír, SierraNevada, La Silla, Haute-Provence, South African and Roque de LosMuchachos observatories.
| Automated spectroscopic abundances of A and F-type stars using echelle spectrographs. II. Abundances of 140 A-F stars from ELODIE
Using the method presented in Erspamer & North (\cite{erspamer},hereafter Paper I), detailed abundances of 140 stars are presented. Theuncertainties characteristic of this method are presented and discussed.In particular, we show that for a S/N ratio higher than 200, the methodis applicable to stars with a rotational velocity as high as 200 kms-1. There is no correlation between abundances and Vsin i,except a spurious one for Sr, Sc and Na which we explain by the smallnumber of lines of these elements combined with a locally biasedcontinuum. Metallic giants (Hauck \cite{hauck}) show larger abundancesthan normal giants for at least 8 elements: Al, Ca, Ti, Cr, Mn, Fe, Niand Ba. The anticorrelation for Na, Mg, Si, Ca, Fe and Ni with Vsin isuggested by Varenne & Monier (\cite{varenne99}) is not confirmed.The predictions of the Montréal models (e.g. Richard et al.\cite{richard01}) are not fulfilled in general. However, a correlationbetween left [(Fe)/(H)right ] and log g is found for stars of 1.8 to 2.0M_sun. Various possible causes are discussed, but the physical realityof this correlation seems inescapable.Based on observations collected at the 1.93 m telescope at theObservatoire de Haute-Provence (St-Michel l'Observatoire, France) andCORALIE.Based on observations collected at the Swiss 1.2 m Leonard Eulertelescopes at the European Southern Observatory (La Silla, Chile).Tables 5 and 6 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/398/1121
| High-Amplitude δ Scuti and SX Phoenicis Stars: The Effects of Chemical Composition on Pulsations and the Period-Luminosity Relation
We present a theoretical study of the radially pulsating δ Scutiand SX Phoenicis variables, concentrating on the blue straggler SXPhoenicis variables found in globular clusters. We have evolved a gridof stellar models with the metal abundance of the globular cluster M55,including models with alpha-enhanced metal abundances, and tested thesemodels for radial pulsations observed in the high-amplitude δScuti and SX Phoenicis stars. Our grid includes models with globallyenriched helium content to simulate the effects of stellar collisionsand global mixing possible in blue stragglers. We find that globalenrichment of helium strongly affects the temperature and luminosity ofa given star, but the location of the instability strip blue edge andthe slope of the period-luminosity (PL) relation are unchanged. Thissuggests that the PL relation is not affected by blue stragglerformation if blue stragglers are fully mixed stellar mergers. Our blueedges and PL relations are in agreement with other theoreticaldeterminations and also with the observational PL relation of M55, butthey are not in agreement with the PL relation previously derived forhigh-amplitude δ Scuti stars in the field. Analysis of thedouble-mode variable, V41, suggests either that the star may not bepulsating in the first and second overtones as claimed or that normalstellar models may not be accurate models of blue straggler stars.
| Catching the sounds of stars. Asteroseismology, the right tool to understand stellar interiors
Contents: 1. Why measure stellar acoustic oscillations? 2. Acousticspectrum of solar-like stars. 3. Listen to solar-like stars by preciseDoppler measurements. 4. Recent progress in solar-like seismology. 5.Acoustic waves in solar-twin α Cen A. 6. Future prospects with ESOinstruments.
| Low-frequency variability and binarity of the δ Scuti star XX Pyx
We analyse 147h of single-site CCD time series photometry of themultiperiodic low-amplitude δ Scuti star XX Pyx with the aim ofinvestigating variability at low frequencies. Part of the data wereobtained in the context of the 1998 multisite campaign on XX Pyx, theresults of which were described by Handler et al. We find that periodiclow-frequency variations are present in the XX Pyx light curves, and wedetect two frequencies at fA=0.8695cycled-1 andfB=1.7352cycled-1, respectively, with amplitudesof 4.5 and 5.4mmag. The low-frequency variability is intrinsic to XXPyx, and cannot be attributed to instrumental or atmospheric effects.The near 2:1 ratio of the frequencies leads us to suggest that XX Pyx isa δ Scuti star in a binary system, with a possible binary periodof 27.6h. This is strongly supported by the detection of radial velocityvariations from the re-analysis of echelle spectra obtained by Handleret al. However, in the absence of a spectroscopic period, alternativeexplanations of the photometric variability involving pulsation are alsopossible; the variations occur close to possible combination frequenciesof the short-period (δ Scuti) variations, but high Q values of0.57 and 0.28d suggest that the variations are not a result of normal(p-)modes. They could possibly be due to g-modes excited to observableamplitudes as a result of resonance effects. Surface features (spots)are unlikely to be the cause of the variations. We searched forcombination frequencies (fi-fj), f being thenormal δ Scuti frequencies detected by Handler et al., but failedto detect any.
| Pulsation in the pre-main-sequence Herbig Ae star HD 142666
Asteroseismology of pre-main-sequence δ Scuti stars has thepotential not only to provide unprecedented constraints on models ofthese stars, but also to allow for the possibility of detectingevolutionary period changes, thus providing a direct measure of thepre-main-sequence evolutionary time-scale. In the last two years, thepublished number of such stars known has doubled from four to eight.Searches are now being conducted amongst the Herbig Ae stars, which areconsidered to be excellent candidates. We announce the discovery ofδ Scuti pulsation in one Herbig Ae star, HD 142666, which lieswithin Marconi & Palla's theoretically predicted instability stripfor pre-main-sequence stars, making this the ninth knownpre-main-sequence δ Scuti star. We also demonstrate a lack ofδ Scuti pulsation in another such star, HD 142527.
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| Sað Açýklýk: | 18h42m16.40s |
| Yükselim: | -09°03'09.0" |
| Görünürdeki Parlaklýk: | 4.72 |
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