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Long term photometric monitoring with the Mercator telescope. Frequencies and mode identification of variable O-B stars
Aims. We selected a large sample of O-B stars that were considered as(candidate) slowly pulsating B, β Cep, and Maia stars after theanalysis of their hipparcos data. We analysed our new seven passbandgeneva data collected for these stars during the first three years ofscientific operations of the mercator telescope. We performed afrequency analysis for 28 targets with more than 50 high-qualitymeasurements to improve their variability classification. For thepulsating stars, we tried both to identify the modes and to search forrotationally split modes. Methods: . We searched for frequenciesin all the geneva passbands and colours by using two independentfrequency analysis methods and we applied a 3.6 S/N-level criterion tolocate the significant peaks in the periodograms. The modes wereidentified by applying the method of photometric amplitudes for which wecalculated a large, homogeneous grid of equilibrium models to perform apulsational stability analysis. When both the radius and the projectedrotational velocity of an object are known, we determined a lower limitfor the rotation frequency to estimate the expected frequency spacingsin rotationally split pulsation modes. Results: . We detected 61frequencies, among which 33 are new. We classified 21 objects aspulsating variables (7 new confirmed pulsating stars, including 2 hybridβ Cep/SPB stars), 6 as non-pulsating variables (binaries or spottedstars), and 1 as photometrically constant. All the Maia candidates werereclassified into other variability classes. We performed modeidentification for the pulsating variables for the first time. The mostprobable ℓ value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes,respectively, including only 4 unambiguous identifications. For 7 starswe cannot rule out that some of the observed frequencies belong to thesame rotationally split mode. For 4 targets we may begin to resolveclose frequency multiplets. Based on observations collected with the p7 photometer attached to theFlemish 1.2-m mercator telescope situated at the Roque de los Muchachosobservatory on La Palma (Spain). Section [see full textsee full text],including Figs. is only available in electronic form athttp://www.aanda.org, and Tables 2 and 3 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/243
| An asteroseismic study of the β Cephei star β Canis Majoris
Aims.We present the results of a detailed analysis of 452 ground-based,high-resolution high S/N spectroscopic measurements spread over 4.5years for β Canis Majoris with the aim of determining thepulsational characteristics of this star, and then using them to deriveseismic constraints on the stellar parameters. Methods: .Wedetermined pulsation frequencies in the Si III 4553 Å line withFourier methods. We identified the m-value of the modes by taking thephotometric identifications of the degrees ℓ into account. To thisend we used the moment method together with the amplitude and phasevariations across the line profile. The frequencies of the identifiedmodes were used for a seismic interpretation of the structure of thestar. Results: .We confirm the presence of the three pulsationfrequencies already detected in previous photometric datasets:f1 = 3.9793~c d-1 (46.057~μHz), f2 =3.9995~c d-1 (46.291~μHz), and f3 = 4.1832~cd-1 (48.417~μHz). For the two modes with the highestamplitudes, we unambiguously identify (ℓ_1,m_1) = (2,2) and(ℓ_2,m_2) = (0,0). We cannot conclude anything for the third modeidentification, except that m3 > 0. We also deduce anequatorial rotational velocity of 31 ± 5~km s-1 forthe star. We show that the mode f1 must be close to anavoided crossing. Constraints on the mass (13.5 ± 0.5~μHz),age (12.4 ± 0.7 Myr), and core overshoot (0.20 ± 0.05 H_P)of β CMa are obtained from seismic modelling using f1and f_2.
| Abundance analysis of prime B-type targets for asteroseismology. I. Nitrogen excess in slowly-rotating β Cephei stars
Seismic modelling of the β Cephei stars promises major advances inour understanding of the physics of early B-type stars on (or close to)the main sequence. However, a precise knowledge of their physicalparameters and metallicity is a prerequisite for correct modeidentification and inferences regarding their internal structure. Herewe present the results of a detailed NLTE abundance study of nine primetargets for theoretical modelling: γ Peg,δ Cet, ν Eri,β CMa, ξ1 CMa,V836 Cen, V2052 Oph,β Cep and DD (12) Lac (hereafter 12Lac). The following chemical elements are considered: He, C,N, O, Mg, Al, Si, S and Fe. Our curve-of-growth abundance analysis isbased on a large number of time-resolved, high-resolution opticalspectra covering in most cases the entire oscillation cycle of thestars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars,three of which have severe constraints on their equatorial rotationalvelocity, Ω R, from seismic or line-profile variation studies:β Cep (Ω R ~ 26 km s-1),V2052 Oph (Ω R ~ 56 km s-1),δ Cet (Ω R < 28 km s-1) andξ1 CMa (Ω R sin i 10 kms-1). The existence of core-processed material at the surfaceof such largely unevolved, slowly-rotating objects is not predicted bycurrent evolutionary models including rotation. We draw attention to thefact that three stars in this subsample have a detected magnetic fieldand briefly discuss recent theoretical work pointing to the occurrenceof diffusion effects in β Cephei stars possibly capable of alteringthe nitrogen surface abundance. On the other hand, the abundances of allthe other chemical elements considered are, within the errors,indistinguishable from the values found for OB dwarfs in the solarneighbourhood. Despite the mild nitrogen excess observed in someobjects, we thus find no evidence for a significantly higherphotospheric metal content in the studied β Cephei stars comparedto non-pulsating B-type stars of similar characteristics.
| High-precision elements of double-lined spectroscopic binaries from combined interferometry and spectroscopy. Application to the β Cephei star β Centauri
Aims.We present methodology to derive high-precision estimates of thefundamental parameters of double-lined spectroscopic binaries. We applythe methods to the case study of the double-lined β Cephei starβ Centauri. We also present a detailed analysis of βCentauri's line-profile variations caused by its oscillations.Methods: .High-resolution spectral time series and visual orinterferometric data with a good phase distribution along the orbitalperiod are required. We point out that a systematic error in the orbitalamplitudes, and any quantities derived from them, occurs if the radialvelocities of blended component lines are computed without spectraldisentangling. This technique is an essential ingredient in thederivation of the physical parameters if the goal is to obtain aprecision of only a few percent. We have devised iteration schemes toobtain the orbital elements for systems whose lines are blendedthroughout the orbital cycle. Results: .We derive the componentmasses and dynamical parallax of β Centauri with a precision of 6%and 4%, respectively. Modelling allowed us to refine the mass estimatesto 1% precision resulting in M_1=10.7± 0.1 M_ȯ andM_2=10.3± 0.1 M_ȯ, and to derive the age of the system asbeing (14.1± 0.6)× 106 years. We deduce twooscillation frequencies for the broad-lined primary of β Centauri:f_1=7.415 c d-1 and f_2=4.542 c d-1 or one oftheir aliases. The degrees of these oscillation modes are higher than 2for both frequencies, irrespective of the alias problem. No evidence ofoscillations in the narrow-lined secondary was found. Conclusions: .We propose that our iteration schemes be used in anyfuture derivations of the spectroscopic orbital parameters ofdouble-lined binaries with blended component lines to whichdisentangling can be successfully applied. The combination of parametersresulting from the iteration schemes with high-precision estimates ofthe orbital inclination and the angular semi-major axis frominterferometric or visual measurements allows a complete solution of thesystem.
| Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars*
We present the results of a magnetic survey of a sample of eightβCephei stars and 26 slowly pulsating B (SPBs) stars with the FOcalReducer low dispersion Spectrograph at the Very Large Telescope. A weakmean longitudinal magnetic field of the order of a few hundred Gauss isdetected in the βCephei star ξ1CMa and in 13 SPBstars. The star ξ1CMa becomes the third magnetic staramong the βCephei stars. Before our study, the star ζCas wasthe only known magnetic SPB star. All magnetic SPB stars for which wegathered several magnetic field measurements show a field that varies intime. We do not find a relation between the evolution of the magneticfield with stellar age in our small sample. Our observations imply thatβCephei and SPB stars can no longer be considered as classes ofnon-magnetic pulsators, but the effect of the fields on the oscillationproperties remains to be studied.
| δ Ceti Is Not Monoperiodic: Seismic Modeling of a β Cephei Star from MOST Space-based Photometry
The β Cephei star δ Ceti was considered one of the fewmonoperiodic variables in its class. Despite (or perhaps because of) itsapparently simple oscillation spectrum, it has been challenging andcontroversial to identify this star's pulsation mode and constrain itsphysical parameters seismically. Broadband time-resolved photometry ofδ Ceti spanning 18.7 days with a duty cycle of about 65% obtainedby the Microvariability and Oscillations of Stars (MOST) satellite-thefirst scientific observations ever obtained by MOST-reveals that thestar is actually multiperiodic. Besides the well-known dominantfrequency of f1=6.205886 day-1, we have discoveredin the MOST data its first harmonic 2f1 and three otherfrequencies (f2=3.737, f3=3.673, andf4=0.318 day-1), all detected with asignal-to-noise ratio (S/N)>4. In retrospect, f2 was alsopresent in archival spectral line-profile data but at lower S/N. Wepresent seismic models whose modes match exactly the frequenciesf1 and f2. Only one model falls within the commonpart of the error boxes of the star's observed surface gravity andeffective temperature from photometry and spectroscopy. In this model,f1 is the radial (l=0) first overtone, and f2 isthe g2 (l=2, m=0) mode. This model has a mass of 10.2+/-0.2Msolar and an age of 17.9+/-0.3 Myr, making δ Ceti anevolved β Cephei star. If f2 and f3 arerotationally split components of the same g2 mode, then thestar's equatorial rotation velocity is either 27.6 km s-1 orhalf this value. Given its vsini of about 1 km s-1, thisimplies that we are seeing δ Ceti nearly pole-on.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon Inc., the University of Toronto Institutefor Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
| Effects of moderately fast shellular rotation on adiabatic oscillations
We investigate adiabatic oscillations for δ Scuti star models,taking a moderate rotation velocity (around 100 {km s-1})into account. The resulting oscillation frequencies include correctionsfor rotation up to second order in the rotation rate including those ofnear degeneracy. Effects of either a uniform rotation or a rotationprofile assuming local angular momentum conservation of the formΩ=Ω(r) on oscillation frequencies are compared. As expected,important differences (around 3 {μ Hz}) are obtained in the g andmixed-mode regions. For higher-frequency p modes, differences rangebetween 1 {μ Hz} and 3 {μ Hz}. Such differences are likely to bedetectable with future space missions such as COROT, where precisions infrequency around 0.5 {μ Hz} are expected to be reached.
| Multiperiodicity in the large-amplitude rapidly-rotating β Cephei star HD 203664
Aims.We made a seismic study of the young massive β Cephei star HD203664 with the goal of constraining its interior structure.Methods.Ourstudy is based on a time series of 328 new Geneva 7-colour photometricdata of the star spread over 496.8 days.Results.The data confirm thefrequency of the dominant mode of the star, which we refined tof_1=6.02885 c d-1. The mode has a large amplitude of 37 mmagin V and is unambiguously identified as a dipole mode (&ea;=2) from itsamplitude ratios and non-adiabatic computations. Besides f_1, wediscovered two additional new frequencies in the star with amplitudesabove 4σ: f_2=6.82902 c d-1 and f_3=4.81543 cd-1, or one of their daily aliases. The amplitudes of thesetwo modes are only between 3 and 4 mmag, which explains why they werenot detected before. Their amplitude ratios are too uncertain for modeidentification. Conclusions.We show that the observed oscillationspectrum of HD 203664 is compatible with standard stellar models butthat we have insufficient information for asteroseismic inferences.Among the large-amplitude β Cephei stars, HD 203664 stands out asthe only one rotating at a significant fraction of its critical rotationvelocity (~ 40%).
| Asteroseismology of the β Cephei star 12 (DD) Lacertae: photometric observations, pulsational frequency analysis and mode identification
We report a multisite photometric campaign for the β Cephei star 12Lacertae. 750 h of high-quality differential photoelectricStrömgren, Johnson and Geneva time-series photometry were obtainedwith nine telescopes during 190 nights. Our frequency analysis resultsin the detection of 23 sinusoidal signals in the light curves. Ten ofthose correspond to independent pulsation modes, and the remainder arecombination frequencies. We find some slow aperiodic variability such asthat seemingly present in several β Cephei stars. We perform modeidentification from our colour photometry, derive the spherical degree lfor the five strongest modes unambiguously and provide constraints on lfor the weaker modes. We find a mixture of modes of 0 <=l<= 4. Inparticular, we prove that the previously suspected rotationally splittriplet within the modes of 12 Lac consists of modes of different ltheir equal frequency splitting must thus be accidental.One of the periodic signals we detected in the light curves is argued tobe a linearly stable mode excited to visible amplitude by non-linearmode coupling via a 2:1 resonance. We also find a low-frequency signalin the light variations whose physical nature is unclear; it could be aparent or daughter mode resonantly coupled. The remaining combinationfrequencies are consistent with simple light-curve distortions.The range of excited pulsation frequencies of 12 Lac may be sufficientlylarge that it cannot be reproduced by standard models. We suspect thatthe star has a larger metal abundance in the pulsational driving zone, ahypothesis also capable of explaining the presence of β Cepheistars in the Large Magellanic Cloud.
| The eclipsing binary PV Cas: a case of strong differential rotation in depth
In this study, we show that only models with differential rotation, asopposed to non-rotation or solid-body rotation for the components of PVCas, are in satisfactory agreement with the observations (includingconstraints on the apsidal advance rate and the synchronous rotation ofthe components in addition to their luminosities and radii). Internalrotation profiles found for solar and metal-rich chemical compositionsare similar to each other in that only a small part of the outermostregions is rotating very slowly (veq~ 65kms -1)and the rest is rotating very rapidly. Confirmation of Ap-likevariations in the light curve of the system leads us to search for aconnection between this type (and similar types) of internal rotationand chemically peculiar stars. The temperature difference between theblue sides of the main sequence for the normal and for the magnetic Apstars may arise from such a connection, since we find a similardifference between the models with this kind of rotation and thenon-rotating models of somewhat lower mass.
| An asteroseismic study of the β Cephei star θ Ophiuchi: spectroscopic results
We present the results of a detailed analysis of 121 ground-basedhigh-resolution, high signal-to-noise ratio spectroscopic measurementsspread over 3 yr for the β Cephei star θ Ophiuchi. Wediscovered θ Oph to be a triple system. In addition to thealready known speckle B5 companion of the B2 primary, we showed thepresence of a low-mass spectroscopic companion and we derived an orbitalperiod of 56.71 d with an eccentricity of 0.1670. After removing theorbit we determined two frequencies for the primary in the residualradial velocities: f1= 7.1160 cd-1 andf2= 7.4676 cd-1. We also found the presence off3= 7.3696 cd-1 by means of a two-dimensionalfrequency search across the SiIII 4567-Åprofiles. We identifiedthe m-value of the main mode with frequency f1 by taking intoaccount the photometric identifications of the degrees l. By means ofthe moment method and the amplitude and phase variations across the lineprofile, we derived (l1, m1) = (2, -1). Thisresult allows us to fix the mode identifications of the whole quintupletfor which three components were detected in photometry. This is ofparticular use for our forthcoming seismic modelling of the primary. Wealso determined stellar parameters of the primary by non-localthermodynamic equilibrium hydrogen, helium and silicon line profilefitting and we obtained Teff= 24000 K and logg= 4.1, which isconsistent with photometrically determined values.
| An asteroseismic study of the β Cephei star θ Ophiuchi: photometric results
We have carried out a three-site photometric campaign for the βCephei star θ Oph from 2003 April to August. 245 h ofdifferential photoelectric uvy photometry were obtained during 77 clearnights. The frequency analysis of our measurements has resulted in thedetection of seven pulsation modes within a narrow frequency intervalbetween 7.116 and 7.973 c d-1. No combination or harmonicfrequencies have been found. We have performed a mode identification ofthe individual pulsations from our colour photometry that shows thepresence of one radial mode, one rotationally split l= 1 triplet andpossibly three components of a rotationally split l= 2 quintuplet. Wediscuss the implications of our findings and point out the similarity ofthe pulsation spectrum of θ Oph to that of another β Cepheistar, V836 Cen.
| Core Overshoot: An Improved Treatment and Constraints from Seismic Data
We present a comprehensive set of stellar evolution models for Procyon Ain an effort to guide future measurements of both traditional stellarparameters and seismic frequencies toward constraining the amount ofcore overshoot in Procyon A and possibly other stars. Currentobservational measurements of Procyon A when combined with traditionalstellar modeling only place a large upper limit on overshoot ofαOV<1.1. By carrying out a detailed pulsationanalysis, we further demonstrate how p- and g-mode averaged spacings canbe used to gain better estimates of the core size. For both p- andg-modes, the frequency spacings for models without overshoot are clearlyseparated from the models with overshoot. In addition, measurements ofthe l=0 averaged small p-mode spacings could be used to establishProcyon A's evolutionary stage. For a fixed implementation of overshootand under favorable circumstances, the g-mode spacings can be used todetermine the overshoot extent to an accuracy of +/-0.05HP.However, we stress that considerable confusion is added due to theunknown treatment of the overshoot region. This ambiguity might beremoved by analyzing many different stars. A simple nonlocal convectiontheory developed by Kuhfuss is implemented in our stellar evolution codeand contrasted with the traditional approaches. We show that this theorysupports a moderate increase of the amount of convective overshoot withstellar mass of ΔαOV~=+0.10 between 1.5 and 15Msolar. This theory places an upper limit on Procyon A's coreovershoot extent of ~0.4HP, which matches the limit imposedby Roxburgh's integral criterion.
| Asteroseismology: Past, Present and Future
Asteroseismology studies stars with a wide variety of interior andsurface conditions. For two decades asteroseismic techniques have beenapplied to many pulsating stars across the HR diagram. Asteroseismologyis now a booming field of research with stunning new discoveries; Ihighlight a personal selection of these in this review, many of whichare discussed in more detail elsewhere in these proceedings. For manyyears the Nainital-Cape Survey for northern roAp stars has been runningat ARIES, so I emphasise new spectroscopic results for roAp stars andpoint out the outstanding prospects for the planned ARIES 3-m telescopeat Devastal. High precision spectroscopy has revolutionised theasteroseismic study of some types of stars - particularly solar-likeoscillators and roAp stars - while photometry is still the best way tostudy the frequency spectra that are the basic data of asteroseismology.New telescopes, new photo-meters and space missions are revolutionisingasteroseismic photometry. In addition to the ground-based potential ofasteroseismic spectroscopy, India has the knowledge and capability forspace-based asteroseismic photometry. The future for asteroseismology isbright indeed, especially for Indian astronomers.
| Catalog of Galactic β Cephei Stars
We present an extensive and up-to-date catalog of Galactic β Cepheistars. This catalog is intended to give a comprehensive overview ofobservational characteristics of all known β Cephei stars, coveringinformation until 2004 June. Ninety-three stars could be confirmed to beβ Cephei stars. We use data from more than 250 papers publishedover the last nearly 100 years, and we provide over 45 notes onindividual stars. For some stars we reanalyzed published data orconducted our own analyses. Sixty-one stars were rejected from the finalβ Cephei list, and 77 stars are suspected to be β Cepheistars. A list of critically selected pulsation frequencies for confirmedβ Cephei stars is also presented.We analyze the β Cephei stars as a group, such as the distributionsof their spectral types, projected rotational velocities, radialvelocities, pulsation periods, and Galactic coordinates. We confirm thatthe majority of the β Cephei stars are multiperiodic pulsators. Weshow that, besides two exceptions, the β Cephei stars with highpulsation amplitudes are slow rotators. Those higher amplitude starshave angular rotational velocities in the same range as thehigh-amplitude δ Scuti stars (Prot>~3 days).We construct a theoretical HR diagram that suggests that almost all 93β Cephei stars are main-sequence objects. We discuss theobservational boundaries of β Cephei pulsation and the physicalparameters of the stars. We corroborate that the excited pulsation modesare near to the radial fundamental mode in frequency and we show thatthe mass distribution of the stars peaks at 12 Msolar. Wepoint out that the theoretical instability strip of the β Cepheistars is filled neither at the cool nor at the hot end and attempt toexplain this observation.
| Metallicity of mono- and multiperiodic β Cephei stars
Analyzing IUE ultraviolet spectra of β Cep pulsating stars wenoticed that multiperiodic variables have a larger mean metal abundancein the photosphere, [ m/H] , than monoperiodic ones. We applystatistical tests to verify this dichotomy. We obtain that, with a largeprobability, the multiperiodic β Cep stars have greater values of [m/H] . This result is consistent with the linear non-adiabatic theory ofpulsation of early B-type stars.
| Disentangling component spectra of κ Scorpii, a spectroscopic binary with a pulsating primary. II. Interpretation of the line-profile variability
We analyse the complex short-term SiIII line-profile variability of the spectroscopic binary β Cep star κ Scorpii after orbit subtraction, before and after spectral disentangling. We refine the known oscillation frequency of the star: f1=4.99922 c d-1 and detect2f1. Variability is also found at frequencies nearf2≃ 4.85 c d-1 and f3≃ 5.69c d-1 or their aliases. These frequencies are not significantif we consider the spectra alone, but they survive our selection afterthe consideration that they were derived previously from independentground-based and space photometry by different teams. Moreover, we finddominant variability in the equivalent width with a frequency in theinterval [0.22,0.30] c d-1 which we interpret as therotational frequency frot of the star. The complex windowfunction does not allow us to determine definite values forf2, f3, frot. The variability withf1 is interpreted as a prograde non-radial oscillation modewith spherical wavenumbers (ℓ,m)=(2,-1) or (1,-1). The additionalfrequencies are explained in terms of rotational modulation superposedto the main oscillation. We also point out that we cannot disprove thevariability in κ Scorpii to originate from co-rotating structures. KOREL disentangling preserves the large-amplitude line-profile variability but its performance for complex low-amplitude variability remains to bestudied in detail.Based on observations obtained with the Coudé ÉchelleSpectrograph on the ESO CAT telescope and with the CORALIEéchelle spectrograph on the 1.2-m Euler Swiss telescope, bothsituated at La Silla, Chile.
| Orbital parameters, masses and distance to β Centauri determined with the Sydney University Stellar Interferometer and high-resolution spectroscopy
The bright southern binary star β Centauri (HR5267) has beenobserved with the Sydney University Stellar Interferometer (SUSI) andspectroscopically with the European Southern Observatory Coude AuxiliaryTelescope and Swiss Euler telescope at La Silla. The interferometricobservations have confirmed the binary nature of the primary componentand have enabled the determination of the orbital parameters of thesystem. At the observing wavelength of 442nm the two components of theprimary system have a magnitude difference of 0.15 +/- 0.02. Thecombination of interferometric and spectroscopic data gives thefollowing results: orbital period 357.00 +/- 0.07d, semimajor axis 25.30+/- 0.19mas, inclination 67.4 +/-0.3, eccentricity0.821 +/- 0.003, distance 102.3 +/- 1.7pc, primary and secondary massesM1=M2= 9.1 +/- 0.3 Msolar and absolutevisual magnitudes of the primary and secondary M1V=-3.85 +/-0.05 and M2V=-3.70 +/- 0.05, respectively. The high degree ofaccuracy of the results offers a fruitful starting point for futureasteroseismic modelling of the pulsating binary components.
| Asteroseismology of the β Cephei star ν Eridani: massive exploration of standard and non-standard stellar models to fit the oscillation data
We present the results of a detailed seismic modelling of the βCephei star ν Eridani with the Liège evolution and pulsationcodes. We selected four clearly detected, well-identified andindependent pulsation modes from the frequency spectrum obtained from arecent five-month multisite, multitechnique campaign, while previousmodelling work only took into account three frequencies. We show bymeans of a massive exploration of the parameter space that no standardstellar model both matches and excites these four observed modes, incontrast to the conclusion reached when considering only threefrequencies. Therefore, we have considered stellar models with differentmetal mixtures and different initial hydrogen abundance values. We showthat an increase in the relative number fraction of iron throughout thewhole star or a large decrease in the initial hydrogen abundance makethe stellar models matching the four selected modes satisfy allobservational constraints and we provide the general properties of thebest such physical models.
| Asteroseismology of the β Cephei star ν Eridani: interpretation and applications of the oscillation spectrum
The oscillation spectrum of ν Eri is the richest known for anyvariable of the β Cephei type. We interpret the spectrum in termsof normal mode excitation and construct seismic models of the star. Thefrequency data combined with data on mean colours set the upper limit onthe extent of overshooting from the convective core. We use data onrotational splitting of two dipole (l= 1) modes (g1 andp1) to infer properties of the internal rotation rate.Adopting a plausible hypothesis of nearly uniform rotation in theenvelope and increasing rotation rate in the μ-gradient zone, we findthat the mean rotation rate in this zone is about three times fasterthan in the envelope. In our standard model only the modes in the middlepart of the oscillation spectrum are unstable. To account for excitationof a possible high-order g mode at ν= 0.43 cd-1 as well asp modes at ν > 6 cd-1 we have to invoke anoverabundance of Fe in the driving zone.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| Asteroseismology of the β Cep star HD 129929. I. Observations, oscillation frequencies and stellar parameters
We have gathered and analysed a timeseries of 1493 high-qualitymulticolour Geneva photometric data of the B3V β Cep star HD129929. The dataset has a time base of 21.2 years. The occurrence of abeating phenomenon is evident from the data. We find evidence for thepresence of at least six frequencies, among which we see components oftwo frequency multiplets with an average spacing of ˜0.0121 cd-1 which points towards very slow rotation. This result isin agreement with new spectroscopic data of the star and also withpreviously taken UV spectra. We provide the amplitudes of the sixfrequencies in all seven photometric filters. The metal content of thestar is Z=0.018± 0.004. All these observational results will beused to perform detailed seismic modelling of this massive star in asubsequent paper.Based on data gathered with the Swiss 0.7m telescope equipped with thephotometer P7 of the Geneva Observatory and with the FEROS spectrographattached to the ESO 2.2 m telescope, both situated at La Silla in Chile;the reduced photometric multicolour data are provided in Table 1, whichis only available 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/qcat?J/A+A/415/241
| Asteroseismology of the β Cephei star ν Eridani - II. Spectroscopic observations and pulsational frequency analysis
We undertook a multisite spectroscopic campaign for the β Cepheistar ν Eridani. A total of 2294 high-resolution spectra were obtainedfrom telescopes at 11 different observatories around the world. The timebase of dedicated multisite observations is 88 d. To this data set wehave added 148 older, previously unpublished spectra, such that theoverall time-span of the 2442 spectra is 430 d. The analysis of theradial velocity variations derived from the SiIII triplet centred on4560Å leads to 19 significant frequencies, of which sevencorrespond to independent pulsation frequencies. Five of these aremembers of multiplets with an average spacing of 0.018 +/- 0.002cd-1. Our spectroscopic results agree well with those derivedfrom a simultaneous multisite photometric campaign of the star, albeitthat we do not recover their low frequency at 0.43218 cd-1.We find three different candidate frequencies below 1 cd-1instead. We also find that the radial velocity amplitude of the mainmode has increased by some 30 per cent over the last 15 years, which isconsistent with the photometry data. We derive a relative equivalentwidth variation of 6.5 per cent, which is completely dominated by themain radial mode. The phase difference between the radial velocity andlight variations for the main frequency is , which is clearly deviantfrom the adiabatic value and confirms the radial nature of the dominantmode. The spectral line broadening leads to an upper limit of 20 kms-1 for vsini, which is consistent with the long rotationperiod derived from the frequency splittings.
| Asteroseismology of the β Cephei star ν Eridani - I. Photometric observations and pulsational frequency analysis
We undertook a multisite photometric campaign for the β Cephei starν Eridani. More than 600 h of differential photoelectric uvyVphotometry were obtained with 11 telescopes during 148 clear nights.The frequency analysis of our measurements shows that the variability ofν Eri can be decomposed into 23 sinusoidal components, eight of whichcorrespond to independent pulsation frequencies between 5 and 8cd-1. Some of these are arranged in multiplets, whichsuggests rotational m-mode splitting of non-radial pulsation modes asthe cause. If so, the rotation period of the star must be between 30 and60 d.One of the signals in the light curves of ν Eri has a very lowfrequency of 0.432 cd-1. It can be a high-order combinationfrequency or, more likely, an independent pulsation mode. In the lattercase, ν Eri would be both a β Cephei star and a slowly pulsatingB (SPB) star.The photometric amplitudes of the individual pulsation modes of ν Eriappear to have increased by about 20 per cent over the last 40 years. Sohave the amplitudes of the dominant combination frequencies of the star.Among the latter, we could only identify sum frequencies with certainty,not difference frequencies, which suggests that neither light-curvedistortion in its simplest form nor resonant mode coupling is theirsingle cause.One of our comparison stars, μ Eridani, turned out to be variablewith a dominant time-scale of 1.62 d. We believe either that it is anSPB star just leaving its instability strip or that its variations areof rotational origin.
| Asteroseismology of HD 129929: Core Overshooting and Nonrigid Rotation
We have gathered and analyzed 1493 high-quality multicolor Genevaphotometric data taken over 21 years of the B3V star HD 129929. Wedetect six frequencies, among which appear the effects of rotationalsplitting with a spacing of ~0.0121 cycles per day, which implies thatthe star rotates very slowly. A nonadiabatic analysis of theoscillations allows us to constrain the metallicity of the star to Z∈ [0.017,0.022], which agrees with a similar range derived fromspectroscopic data. We provide evidence for the occurrence of coreconvective overshooting in the star, with αov = 0.10+/- 0.05, and we rule out rigid rotation.
| Statistics of the Instability Strip of β Cephei Stars
We present a study of the β Cephei instability strip based on asample of 49 stars of this type. After deriving their effectivetemperatures and luminosities from their observed (B-V), (U-B) colorsand parallaxes we find their positions in the HR diagram to be mostlyconfined to the main sequence, and their masses to lie between 7Mȯ and 30 Mȯ. Their distribution on theHR diagram matches well with our previous theoretical instability stripwhich has an upper bound in the luminosity and rather tight boundariesin the effective temperature.
| Fast-rotating nearby solar-type stars, Li abundances and X-ray luminosities. I. Spectral classification, v sin i, Li abundances and X-ray luminosities
We present the results of high-resolution spectroscopic andhigh-precision photometric observations on a sample of 129 late-F andG-type nearby stars selected on the basis of their large rotationalvelocity. Using also data from the Hipparcos satellite, CORAVEL and fromthe ROSAT satellite database, we infer spectral types, compute radialvelocities, v sin i, Li abundances and X-ray luminosities andinvestigate the single or binary nature of the sample stars. Such acareful analysis of our sample shows a large fraction of binaries ( =~62%) and of young single disk stars. In particular, at least 9 stars canbe considered bona-fide PMS or ZAMS objects, and 30 stars are identifiedas SBs for the first time. Information on the presence of Ca II Kemission and on optical variability is given for some of the stars ofthe sample. Based on data collected at the European SouthernObservatory, La Silla, Chile. Tables 1, 3, 4 and 5 and the complete dataset are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/491
| An Extension of the Case-Hamburg OB Star Surveys
We have extended the Case-Hamburg OB star surveys to b = +- 30 degreesfor l = +- 60 degres using the Curtis Schmidt telescope and 4 degreeobjective prism at the Cerro Tololo Inter-American Observatory. Acatalog of 234 OB stars and other objects with peculiar spectra ispresented along with finding charts for those objects too faint to beincluded on the BD or CD charts. (SECTION: Stars)
| Observational status and excitation mechanisms of beta-Cephei variables.
Not Available
| Nonadiabatic observables in beta Cephei models
Using results of linear nonadiabatic calculations for oscillations ofbeta Cephei star models, we calculate amplitudes and phases for light,color and radial velocity for the unstable modes of low harmonicdegrees, l. The nonadiabatic observables are the amplitude ratios andthe phase differences for various oscillating parameters. We constructtheoretical diagrams involving these observables as well as pulsationperiods and compare them with the stellar data. Balona & Stobie(1979) showed that the diagrams based on two-color photometric data maybe used to determine the l-value of an observed mode. Our use of resultsof nonadiabatic calculations improves their method. We show, inparticular, that the diagrams employing the satellite ultravioletmeasurements are the best for discrimination between the l = 0, 1 and 2cases. The clearest separation of the domains for the three l-valuesoccurs in the diagram making use of both photometric and the radialvelocity data. The observational points fall into three theoretricaldomains and an assignment of the l-value is unambiguous. A comparison ofthe theory with the observations is also made using the Period versusAmplitude ratios diagrams in various photometric systems. The agreementis very satisfactory and, in most cases, the l-value can be determined.The nonadiabtic observables are useful not only to determine l but alsothe radial order of the observed modes as well as for constrain meanstellar parameters. As an example we consider the case of delta Ceti-asingle mode beta Cephei star. Our results point to the significantmulticolor photometric and spectroscopic data for asteroseismology.
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Osservazione e dati astrometrici
| Costellazione: | Centauro |
| Ascensione retta: | 14h46m25.76s |
| Declinazione: | -37°13'20.1" |
| Magnitudine apparente: | 8.078 |
| Distanza: | 675.676 parsec |
| Moto proprio RA: | 0.4 |
| Moto proprio Dec: | -1.2 |
| B-T magnitude: | 7.857 |
| V-T magnitude: | 8.06 |
Cataloghi e designazioni:
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