Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
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.
| 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.
| 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.
| Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| Variable Stars in the Field of NGC 6882/6885: The Case of V381 Vulpeculae and V382 Vulpeculae
We present photometric and spectroscopic results for two reportedδ Scuti stars in the field of NGC 6882/6885. We find that V381 Vulhas a period of 0.1185 days and is a δ Scuti variable, aspreviously reported. The spectra of V382 Vul shows it to be a B3 starand therefore not a δ Scuti. All evidence points to V382 Vul beinga β Cephei star with a period of 0.1808 days. Additionally, wereport five new variables and eight suspected variable stars. Of thefive new variables, two are pulsators and three are eclipsing binarysystems. In our search for new variable stars, we use a ``robust medianstatistic'' that is proven to be better at finding low-amplitudevariables than the traditional error curve approach.
| Asteroseismology of the β Cephei star ν Eridani - IV. The 2003-2004 multisite photometric campaign and the combined 2002-2004 data
We report on the second multisite photometric campaign devoted to νEridani (ν Eri). The campaign, carried out from 2003 September 11 to2004 February 16, was very nearly a replica of the first campaign,2002-2003: the five telescopes and photometers we used were the same asthose in the first campaign, the comparison stars and observingprocedure were identical, and the numbers and time baselines of the datawere comparable.For ν Eri, analysis of the new data adds four independent frequenciesto the nine derived previously from the 2002-2003 data: three in therange 7.20-7.93 d-1 and a low one, equal to 0.614d-1. Combining the new and the old data results in twofurther independent frequencies, equal to 6.7322 and 6.2236d-1. Altogether, the oscillation spectrum is shown to consistof 12 high and two low frequencies. The latter have u amplitudes abouttwice as large as the v and y amplitudes, a signature of highradial-order g modes. Thus, we confirm the suggestion, put forward onthe basis of the data of the first campaign, that ν Eri is both aβ Cephei and a slowly pulsating B (SPB) star.Nine of the 12 high frequencies form three triplets, of which two arenew. The triplets represent rotationally split l= 1 modes, although incase of the smallest-amplitude one this may be questioned. Meanseparations and asymmetries of the triplets are derived with accuracysufficient for meaningful comparison with models.The first comparison star, μ Eri, is shown to be an SPB variable withan oscillation spectrum consisting of six frequencies, three of whichare equidistant in period. The star is also found to be an eclipsingvariable. The eclipse is a transit, probably total, the secondary isfainter than the primary by several magnitudes, and the system is widelydetached.The second comparison star, ξ Eri, is confirmed to be a δ Scutivariable. To the frequency of 10.8742 d-1 seen already in thedata of the first campaign, another, equal to 17.2524 d-1, isadded.
| 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.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| Herbig Ae/Be Stars in nearby OB Associations
We have carried out a study of the early-type stars in nearby OBassociations spanning an age range of ~3-16 Myr, with the aim ofdetermining the fraction of stars that belong to the Herbig Ae/Be class.We studied the B, A, and F stars in the nearby (<=500 pc) OBassociations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1,with membership determined from Hipparcos data. We also included in ourstudy the early-type stars in the Trumpler 37 cluster, part of the CepOB2 association. We obtained spectra for 440 Hipparcos stars in theseassociations, from which we determined accurate spectral types, visualextinctions, effective temperatures, luminosities and masses, usingHipparcos photometry. Using colors corrected for reddening, we find thatthe Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearlydifferent regions in the JHK diagram. Thus, we use the location on theJHK diagram, as well as the presence of emission lines and of strong 12μm flux relative to the visual, to identify the Herbig Ae/Be stars inthe associations. We find that the Herbig Ae/Be stars constitute a smallfraction of the early-type stellar population even in the youngerassociations. Comparing the data from associations with different agesand assuming that the near-infrared excess in the Herbig Ae/Be starsarises from optically thick dusty inner disks, we determined theevolution of the inner disk frequency with age. We find that the innerdisk frequency in the age range 3-10 Myr in intermediate-mass stars islower than that in the low-mass stars (<1 Msolar) inparticular, it is a factor of ~10 lower at ~3 Myr. This indicates thatthe timescales for disk evolution are much shorter in theintermediate-mass stars, which could be a consequence of more efficientmechanisms of inner disk dispersal (viscous evolution, dust growth, andsettling toward the midplane).
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| Pulsating Stars in the ASAS-3 Database. I. beta Cephei Stars
We present results of an analysis of the ASAS-3 data for short-periodvariables from the recently published catalog of over 38000 stars. Usingthe data available in the literature we verify the results of theautomatic classification related to \beta Cep pulsators. In particular,we find that 14 stars in the catalog can be classified unambiguously asnew beta Cep stars. By means of periodogram analysis we derive thefrequencies and amplitudes of the excited modes. The main modes in thenew beta Cep stars have large semi-amplitudes, between 35 and 80 mmag.Up to four modes were found in some stars. Two (maybe three) new betaCep stars are members of southern young open clusters: ASAS164409-4719.1 belongs to NGC 6200, ASAS 164630-4701.2 is a member ofHogg 22, and ASAS 164939-4431.7 could be a member of NGC 6216.We also analyze the photometry of four known beta Cep stars in theASAS-3 catalog, namely IL Vel, NSV 24078, V1449 Aql and SY Equ. Finally,we discuss the distribution of beta Cep stars in the Galaxy.
| 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.
| Metallicities of the β Cephei stars from low-resolution ultraviolet spectra
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
| 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.
| 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 - 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.
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| 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.
| Asteroseismology of the β Cep star HD 129929. II. Seismic constraints on core overshooting, internal rotation and stellar parameters
We perform a detailed seismic study of the β Cep star HD 129929.Our analysis is based on the recent derivation of six pulsationfrequencies. These frequencies are unambiguously identified from theseismic modelling and the photometric amplitudes to be the radialfundamental, the ℓ=1, p1 triplet, two consecutivecomponents of the ℓ=2, g1 quintuplet. A non-adiabaticanalysis allows us to constrain the metallicity of the star to Z \in[0.016, 0.022]. In addition, the fitting of three independentfrequencies, two of which correspond to axisymmetric (m=0) modes, allowsus to constrain the core overshooting parameter toαov=0.10 ± 0.05, as well as the other globalparameters of the star. Finally, from the observation of the ℓ=1triplet and part of the ℓ=2 quintuplet, we derive constraints on theinternal rotation of this star.
| β Cep stars from a spectroscopic point of view
In this review we present the current status of line-profile-variationstudies of β Cep stars. Such studies have been performed for 26bright members of this class of pulsating stars in the past 25 years. Wedescribe all these currently available data and summarize theinterpretations based on them in terms of the excited pulsation modes.We emphasize that line-profile variations offer a much more detailedpicture of the pulsational behaviour of pulsating stars compared toground-based photometric data. The latter, however, remain necessary tounravel the often complex frequency pattern and to achieve unambiguousmode identification for multiperiodic β Cep stars and also toderive the pulsational properties of the faint members of the class. Wehighlight the statistical properties of the sample of 26 stars for whichaccurate spectroscopic studies are available and point out some futureprospects.
| 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.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe 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/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Seismic modelling of the beta Cep star EN (16) Lacertae
We perform seismic modelling of the massive beta Cep star EN Lacertae.The starting point of our analysis is the spectroscopic modeidentification recently performed. To this, we add a new updatedphotometric mode identification based upon a non-adiabatic descriptionof the eigenfunctions in the outer atmosphere. Both mode identificationsagree and this allows us to fine-tune the stellar parameters of ENLacertae with unprecedented precision. This is done by producing a hugeamount of stellar models with different parameters and selecting thosethat fulfill the frequency values and the mode identification. Our studyis the first one of its kind in which a reconcilation between observedpulsational characteristics and theoretical models can be achieved at alevel that allows accurate determination of the basic stellar parametersof a massive oscillator. We derive a mass of M=9.62+/- 0.11Msun and an age of 15.7 million years if we assume thatconvective overshooting does not occur.
| Spectroscopic mode identification for the beta Cephei star EN (16) Lacertae
We perform for the first time spectroscopic mode identification in theeclipsing binary beta Cephei star EN (16) Lac. This mode identificationis based upon a time series of 942 line profiles of the He I lambdalambda 6678 Åline in its spectrum. All three known frequenciesf1, f,SUB>2,/SUB>, f3 of the star arepresent in the line-profile variations, but we failed to find additionalmodes. Using different identification methods we find conclusiveevidence for the radial nature of the main mode and for the l=2, m=0identification of the mode with frequency f2. A uniqueidentification of the third mode is not possible from the spectra, butwe do derive that l3<3. Fits to the amplitude and phasevariability of the modes imply a rotation frequency between 0.1 and 0.4c d-1. The star's rotation axis is not aligned with theorbital axis.Based on observations gathered with the coudé spectrographattached to the 2.0 m reflector telescope at Tautenburg Observatory.
| Non-LTE abundances of magnesium, aluminum and sulfur in OB stars near the solar circle
Non-LTE abundances of magnesium, aluminum and sulfur are derived for asample of 23 low-v sin i stars belonging to six northern OBassociations of the Galactic disk within 1 kpc of the Sun. Theabundances are obtained from the fitting of synthetic line profiles tohigh resolution spectra. A comparison of our results with HII regionabundances indicates good agreement for sulfur while the cepheidabundances are higher. The derived abundances of Mg show good overlapwith the cepheid results. The aluminum abundances for OB stars aresignificantly below the cepheid values. But, the OB star results show adependence with effective temperature and need further investigation.The high Al abundances in the cepheids could be the result of mixing. Adiscussion of the oxygen abundance in objects near the solar circlesuggests that the current mean galactic oxygen abundance in this regionis 8.6-8.7 and in agreement with the recently revised oxygen abundancein the solar photosphere. Meaningful comparisons of the absolute S, Aland Mg abundances in OB stars with the Sun must await a reinvestigationof these elements with 3D hydrodynamical model atmospheres for the Sun.No abundance gradients are found within the limited range ingalactocentric distances in the present study. Such variations would beexpected only if there were large metallicity gradients in the disk.
| A new version of the moment method, optimized for mode identification in multiperiodic stars
We present a numerical version of the moment method for theidentification of non-radial pulsation modes. The new version requiresless computation time than the previous one, allowing users to considerall the information contained in the first three moments of amultiperiodic star and to identify multiple modes simultaneously. This,together with the use of a new discriminant that considers the momentscalculated at each time of observation, increases considerably thefeasibility and the accuracy of the mode identification formultiperiodic stars. Moreover, the technique is extended to rotatingpulsating stars. We apply the new version to three B stars showingmultiperiodicity.Computer code for mode identification available upon request.Tables 1 to 8 are only available in electronic form athttp://www.edpsciences.org
| A photometric mode identification method, including an improved non-adiabatic treatment of the atmosphere
We present an improved version of the method of photometric modeidentification of Heynderickx et al. (\cite{hey}). Our new version isbased on the inclusion of precise non-adiabatic eigenfunctionsdetermined in the outer stellar atmosphere according to the formalismrecently proposed by Dupret et al. (\cite{dup}). Our improvedphotometric mode identification technique is therefore no longerdependent on ad hoc parameters for the non-adiabatic effects. Itcontains the complete physical conditions of the outer atmosphere of thestar, provided that rotation does not play a key role. We apply ourmethod to the two slowly pulsating B stars HD 74560 and HD 138764 and tothe beta Cephei star EN (16) Lac. Besides identifying the degree l ofthe pulsating stars, our method is also a tool for improving theknowledge of stellar interiors and atmospheres, by imposing constraintson parameters such as the metallicity and the mixing-length parameteralpha (a procedure we label non-adiabatic asteroseismology).The non-adiabatic eigenfunctions needed for the mode identification areavailable upon request from the authors.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Boron Abundances in B-Type Stars: A Test of Rotational Depletion during Main-Sequence Evolution
Boron abundances have been derived for seven main-sequence B-type starsfrom Hubble Space Telescope STIS spectra around the B III λ2066line. In two stars, boron appears to be undepleted with respect to thepresumed initial abundance. In one star, boron is detectable but isclearly depleted. In the other four stars, boron is undetectable,implying depletions of 1-2 dex. Three of these four stars are nitrogenenriched, but the fourth shows no enrichment of nitrogen. Onlyrotationally induced mixing predicts that boron depletions areunaccompanied by nitrogen enrichments. The inferred rate of borondepletion from our observations is in good agreement with thesepredictions. Other boron-depleted nitrogen-normal stars are identifiedfrom the literature. In addition, several boron-depleted nitrogen-richstars are identified, and while all fall on the boron-nitrogen trendpredicted by rotationally induced mixing, a majority have nitrogenenrichments that are not uniquely explained by rotation. The spectrahave also been used to determine iron group (Cr, Mn, Fe, and Ni)abundances. The seven B-type stars have near-solar iron groupabundances, as expected for young stars in the solar neighborhood. Wehave also analyzed the halo B-type star PG 0832+676. We find[Fe/H]=-0.88+/-0.10, and the absence of the B III line gives the upperlimit [B/H]<-2.5. These and other published abundances are used toinfer the star's evolutionary status as a post-asymptotic giant branchstar. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal GO 07400.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Gyík |
| Rektaszcenzió: | 22h56m23.60s |
| Deklináció: | +41°36'14.0" |
| Vizuális fényesség: | 5.59 |
| Távolság: | 369.004 parszek |
| RA sajátmozgás: | -1.7 |
| Dec sajátmozgás: | -4.3 |
| B-T magnitude: | 5.398 |
| V-T magnitude: | 5.563 |
Katalógusok és elnevezések:
|
