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Determination of Stellar Ellipticities in Future Microlensing Surveys
We propose a method that can determine the ellipticities of source starsof microlensing events produced by binary lenses. The method is based onthe fact that the products of the caustic-crossing timescale, Δt,and the cosine of the caustic incidence angle of the source trajectory,κ, of the individual caustic crossings are different for eventsinvolving an elliptical source, while the products are the same forevents associated with a circular source. The productΔt⊥=Δtcosκ corresponds to thecaustic-crossing timescale when the incidence angle of the sourcetrajectory is κ=0. For the unique determination of the sourceellipticity, resolutions of at least three caustic crossings arerequired. Although this requirement is difficult to achieve under thecurrent observational setup based on alert/follow-up mode, it will bepossible with the advent of future lensing experiments that will surveywide fields continuously at high cadence. For typical Galactic bulgeevents, the difference in Δt⊥ between causticcrossings is of the order of minutes, depending on the sourceorientations and ellipticities. Considering the monitoring frequency ofthe future lensing surveys of ~6 times hr-1 and the improvedphotometry, especially of the proposed space-based survey, we predictthat ellipticity determinations by the proposed method will be possiblefor a significant fraction of multiple caustic-crossing binary lensevents involving source stars having nonnegligible ellipticities.
| Surface Temperature and Synthetic Spectral Energy Distributions for Rotationally Deformed Stars
Extreme deformation of a stellar surface, such as that produced by rapidrotation, causes the surface temperature and gravity to varysignificantly with latitude. Thus, the spectral energy distribution(SED) of a nonspherical star could differ significantly from the SED ofa spherical star with the same average temperature and luminosity.Calculation of the SED of a deformed star is often approximated as acomposite of several spectra, each produced by a plane-parallel model ofgiven effective temperature and gravity. The weighting of these spectraover the stellar surface, and hence the inferred effective temperatureand luminosity, will be dependent on the inclination of the rotationaxis of the star with respect to the observer, as well as thetemperature and gravity distribution on the stellar surface. Here wecalculate the surface conditions of rapidly rotating stars with atwo-dimensional stellar structure and evolution code and compare theeffective temperature distribution to that predicted by von Zeipel'slaw. We calculate the composite spectrum for a deformed star byinterpolating within a grid of intensity spectra of plane-parallel modelatmospheres and integrating over the surface of the star. This allows usto examine the SED for effects of inclination and degree of deformationbased on the two-dimensional models. Using this method, we find that thededuced variation of effective temperature with inclination can be asmuch as 3000 K for an early B star, depending on the details of theunderlying model. As a test case for our models, we examine the rapidlyrotating star Achernar (α Eri, HD 10144). Recent interferometricobservations have determined the star to be quite oblate. Combined withthe ultraviolet SED measured by the OAO 2 satellite, we are able to makedirect comparisons with observations.
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| The polar wind of the fast rotating Be star Achernar. VINCI/VLTI interferometric observations of an elongated polar envelope
Context: .Be stars show evidence of mass loss and circumstellarenvelopes (CSE) from UV resonance lines, near-IR excesses, and thepresence of episodic hydrogen emission lines. The geometry of theseenvelopes is still uncertain, although it is often assumed that they areformed by a disk around the stellar equator and a hot polar wind.Aims: .We probe the close environment of the fast rotating Be starAchernar at angular scales of a few milliarcseconds (mas) in theinfrared, in order to constrain the geometry of a possible polar CSE. Methods: .We obtained long-baseline interferometric observations ofAchernar with the VINCI/VLTI beam combiner in the H and K bands, usingvarious telescope configurations and baseline lengths with a wideazimuthal coverage. Results: .The observed visibility measurementsalong the polar direction are significantly lower than the visibilityfunction of the photosphere of the star alone, in particular at lowspatial frequencies. This points to the presence of an asymmetricdiffuse CSE elongated along the polar direction of the star. To ourdata, we fit a simple model consisting of two components: a 2Delliptical Gaussian superimposed on a uniform ellipse representing thedistorted photosphere of the fast rotating star. Conclusions: .Weclearly detected a CSE elongated along the polar axis of the star, aswell as rotational flattening of the stellar photosphere. For theuniform-ellipse photosphere we derive a major axis of θ_eq = 2.13± 0.05 mas and a minor axis of θ_pol = 1.51 ± 0.02mas. The relative near-IR flux measured for the CSE compared to thestellar photosphere is f = 4.7 ± 0.3%. Its angular dimensions areloosely constrained by the available data at ρ_eq = 2.7 ± 1.3mas and ρ_pol = 17.6 ± 4.9 mas. This CSE could be linked tofree-free emission from the radiative pressure driven wind originatingfrom the hot polar caps of the star.
| Integrated optics for astronomical interferometry. VI. Coupling the light of the VLTI in K band
Aims. Our objective is to prove that integrated optics (IO) is not onlya good concept for astronomical interferometry but also a workingtechnique with high performance. Methods: . We used thecommissioning data obtained with the dedicated K-band integrated opticstwo-telescope beam combiner that now replaces the fiber coupler MONA inthe VLTI/VINCI instrument. We characterize the behavior of this IOdevice and compare its properties to other single mode beam combinerlike the previously used MONA fiber coupler. Results: . The IOcombiner provides a high optical throughput, a contrast of 89% with anight-to-night stability of a few percent. Even if a dispersive phase ispresent, we show that it does not bias the measured Fourier visibilityestimate. An upper limit of 5×10-3 for the cross-talkbetween linear polarization states has been measured. We take advantageof the intrinsic contrast stability to test a new astronomical procedurefor calibrating diameters of simple stars by simultaneously fitting theinstrumental contrast and the apparent stellar diameters. This methodreaches an accuracy with diameter errors on the order of previous onesbut without the need of an already known calibrator. Conclusions:. These results are an important step for IO, since they prove itsmaturity in an astronomical band where the technology has been speciallydeveloped for astronomical convenience. It paves the way to incomingimaging interferometer projects.
| α Eridani: rotational distortion, stellar and circumstellar activity
We explore the geometrical distortion and the stellar and circumstellaractivity of α Eri (HD 10144), the brightest Be star in the sky. Wepresent a thorough discussion of the fundamental parameters of theobject for an independent determination of its rotational distortion. Weused stellar atmosphere models and evolutionary tracks calculated forfast rotating early-type stars. If the star is a rigid rotator, itsangular velocity rate is Ω/Ωc ≃ 0.8, sothat its rotational distortion is smaller than the one inferred fromrecent interferometric measurements. We then discuss the stellar surfaceactivity using high resolution and high S/N spectroscopic observationsof He i and Mg ii lines, which concern a period of Hα lineemission decline. The variations in the He i lines are interpreted asdue to non-radial pulsations. Time series analysis of variations wasperformed with the cleanest algorithm, which enabled us to detect thefollowing frequencies: 0.49, 0.76, 1.27 and 1.72 c/d and pulsationdegrees ℓ (3{-}4) for ν = 0.76 c/d; ℓ (2{-}3) forν = 1.27 c/d and ℓ (3{-}4) for ν = 1.72 c/d. The studyof the absolute deviation of the He i λ6678 Å spectral linerevealed mass ejection between 1997 and 1998. We conclude that thelowest frequency found, ν = 0.49 c/d, is due to the circumstellarenvironment, which is present even at epochs of low emission in thewings of He i λ6678 Å and Mg ii λ4481 Å lineprofiles, as well as during nearly normal aspects of the Hα line.This suggests that there may be matter around the star affecting somespectral regions, even though the object displays a B-normal like phase.The long-term changes of the Hα line emission in α Eri arestudied. We pay much attention to the Hα line emission at theepoch of interferometric observations. The Hα line emission ismodeled and interpreted in terms of varying structures of thecircumstellar disc. We conclude that during the epoch of interferometricmeasurements there was enough circumstellar matter near the star toproduce λ 2.2 μm flux excess, which could account for theoverestimated stellar equatorial angular diameter. From the study of thelatest BrightleftharpoonsBe phase transition of α Eri we concludedthat the Hα line emission formation regions underwent changes sothat: a) the low Hα emission phases are characterized by extendedemission zones in the circumstellar disc and a steep outward matterdensity decline; b) during the strong Hα emission phases theemitting regions are less extended and have a constant densitydistribution. The long-term variations of the Hα line in αEri seem to have a 14-15 year cyclic BrightleftharpoonsBe phasetransition. The disc formation time scales, interpreted as the periodsduring which the Hα line emission increases from zero to itsmaximum, agree with the viscous decretion model. On the other hand, thetime required for the disc dissipation ranges from 6 to 12 years whichquestions the viscous disc model.
| Les etoiles, deformees par leur rotation.
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| Recent astrophysical results from the VLTI.
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| First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator α Leonis (Regulus)
We report on K-band interferometric observations of the bright, rapidlyrotating star Regulus (type B7 V) made with the CHARA Array on MountWilson, California. Through a combination of interferometric andspectroscopic measurements, we have determined for Regulus theequatorial and polar diameters and temperatures, the rotational velocityand period, the inclination and position angle of the spin axis, and thegravity darkening coefficient. These first results from the CHARA Arrayprovide the first interferometric measurement of gravity darkening in arapidly rotating star and represent the first detection of gravitydarkening in a star that is not a member of an eclipsing binary system.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| On the evolutionary status of Be stars. I. Field Be stars near the Sun
A sample of 97 galactic field Be stars were studied by taking intoaccount the effects induced by the fast rotation on their fundamentalparameters. All program stars were observed in the BCDspectrophotometric system in order to minimize the perturbationsproduced by the circumstellar environment on the spectral photosphericsignatures. This is one of the first attempts at determining stellarmasses and ages by simultaneously using model atmospheres andevolutionary tracks, both calculated for rotating objects. The stellarages (τ) normalized to the respective inferred time that eachrotating star can spend in the main sequence phase (τ_MS) reveal amass-dependent trend. This trend shows that: a) there are Be starsspread over the whole interval 0 τ/τ_MS 1 of themain sequence evolutionary phase; b) the distribution of points in the(τ/τMS,M/Mȯ) diagram indicates thatin massive stars (M 12~Mȯ) the Be phenomenon ispresent at smaller τ/τ_MS age ratios than for less massive stars(M 12~Mȯ). This distribution can be due to: i)higher mass-loss rates in massive objets, which can act to reduce thesurface fast rotation; ii) circulation time scales to transport angularmomentum from the core to the surface, which are longer the lower thestellar mass.
| Radiative transfer in moving media. II. Solution of the radiative transfer equation in axial symmetry
A new method for the formal solution of the 2D radiative transferequation in axial symmetry in the presence of arbitrary velocity fieldsis presented. The combination of long and short characteristics methodsis used to solve the radiative transfer equation. We include thevelocity field in detail using the Local Lorentz Transformation. Thisallows us to obtain a significantly better description of thephotospheric region, where the gradient of the global velocity is toosmall for the Sobolev approximation to be valid. Sample testcalculations for the case of a stellar wind and a rotating atmosphereare presented.
| Determination of stellar shape in microlensing event MOA 2002-BLG-33
We report a measurement of the shape of the source star in microlensingevent MOA 2002-BLG-33. The lens for this event was a close binary whosecentre-of-mass passed almost directly in front of the source star. Atthis time, the source star was closely bounded on all sides by a causticof the lens. This allowed the oblateness of the source star to beconstrained. We found that a/b = 1.02+0.04-0.02where a and b are its semi-major and semi-minor axes respectively. Theangular resolution of this measurement is approximately 0.04μ{arcsec}. We also report HST images of the event that confirm aprevious identification of the source star as an F8-G2 turn-offmain-sequence star.
| First VLTI/MIDI observations of a Be star: Alpha Arae
We present the first VLTI/MIDI observations of the Be star alpha Ara (HD158 427), showing a nearly unresolved circumstellar disk in the N band.The interferometric measurements made use of the UT1 and UT3 telescopes.The projected baselines were 102 and 74 meters with position angles of 7° and 55°, respectively. These measurements put an upper limiton the envelope size in the N band under the uniform disk approximationof φmax= 4±1.5 mas, corresponding to 14R*, assuming R*=4.8 Rȯ and theHipparcos distance of 74 pc. On the other hand the disk density must belarge enough to produce the observed strong Balmer line emission. Inorder to estimate the possible circumstellar and stellar parameters wehave used the SIMECA code developed by Stee et al. (1995, A&A, 300,219) and Stee & Bittar (2001, A&A, 367, 532). Optical spectrataken with the échelle instrument Heros and the ESO-50 cmtelescope, as well as infrared ones from the 1.6m Brazilian telescopewere used together with the MIDI spectra and visibilities. Theseobservations place complementary constraints on the density and geometryof the alpha Ara circumstellar disk. We discuss the potential truncationof the disk by a companion and we present spectroscopic indications of aperiodic perturbation of some Balmer lines.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Models for the Rapidly Rotating Be Star Achernar
We present models for the Be star Achernar (α Eri), which recentinterferometric observations have shown has a photospheric shape that issignificantly distorted by the effects of rotation. The models aretwo-dimensional, axisymmetric configurations, constructed using a newversion of the self-consistent field (SCF) method for computing thestructure of a rapidly, differentially rotating star. Our revised SCFtechnique does not suffer from the computational difficulties thataffected previous implementations of the method, yielding convergedstellar models regardless of mass. For models with masses like those ofmain sequence stars of mid- to early-B spectral type, it is possible toreproduce Achernar's inferred equatorial and polar dimensions through acombination of rotational flattening/distension and suitable inclinationof the rotation axis. However, while matching Achernar's apparent shape,these models are discrepant in other respects, being (on average) coolerand more rapidly rotating than observations indicate.
| Physical Parameters of Southern B- and Be-Type Stars
In this paper we present new results on stellar fundamental parametersfor early B and Be field stars observed in the southern hemisphere:effective temperature, superficial gravity, and projected stellarrotation velocity. The estimation of their projected rotation velocitiesis made by two successive methods. We first obtain an initial valuebased on Fourier transforms of the He I λ4471 line for 34 B andBe field stars with magnitudes in the range0.5<=mv<=10, followed by a more accurate fittingprocedure of observed lines with non-LTE model line profiles. Thisprocedure yields stellar rotation velocity estimates that are inagreement with those of the literature. We derive also Teffand logg values by fitting equivalent widths and profiles of NLTE modelspectra to the observed ones. Finally, we give estimates of stellarages, masses, and bolometric luminosities derived from interpolations inthe evolutionary tracks calculated by Schaller.
| First observations with an H-band integrated optics beam combiner at the VLTI
This paper reports results obtained with the Very Large TelescopeInterferometer Commissioning Instrument (VLTI-VINCI) where the fiberinterferometric coupler MONA was temporarily replaced by an integratedoptics beam combiner (IONIC). The two-beam combiner operating in the Hatmospheric band was tested at Paranal during two weeks in July 2002.Fringes were recorded on several stars with the siderostats and the UnitTelescopes (UT). We demonstrate that integrated optics allows beamcombination with a high interferometric efficiency, stability andspatial filtering. The instrumental visibility of VLTI+IONIC is betterthan 85%, which is just 5% under the visibility of the IONIC devicealone. During this run, the flux injection in the IONIC combiner was notoptimized since we used the K-band MONA optics. However, we obtainfringes with the 35 cm siderostats on a star of magnitude mH=2.47(κ2 Scl). It allows us to extrapolate a magnitudearound mH6 with 8 m telescopes without adaptive optics and in thesame poor injection conditions.Based on observations collected at the European Southern Observatory,Paranal, Chile (public commissioning data).
| Harvesting Scientific Results with the VLTI
The ESO Very Large Telescope Interferometer (VLTI) has been included forthe first time in the official call for proposals requesting ESOtelescopes for the period starting in April 2004. This marks theofficial start of public interferometric observations open to thecommunity. It is the start of a new approach to interferometry as astandard astronomical technique, and a point of pride and satisfactionfor all the people who have been working with this challenging goal formany years. But it should not be forgotten that the VLTI has alreadylogged over two years of intensive commissioning, as well as someinitial science demonstration runs. Over 16,000 observations of hundredsof objects have been collected and are available publicly over the ESOarchive on the WEB. In 2003, the first scientific results of thisremarkable effort have appeared. Already more than a dozen papers basedon VLTI data have been submitted or accepted by refereed journals, witha similar volume of contributions to workshops and conferences of ascientific nature. We provide here an overview of this early scientificproduction of the VLTI, ranging from the determination of fundamentalparameters of many classes of stars to the first interferometricmeasurement of the inner regions of the nucleus of the Seyfert galaxyNGC 1068.
| Non-radially pulsating Be stars
Based on more than 3000 high-resolution echelle spectra of 27 early-typeBe stars, taken over six years, it is shown that the short-term periodicline profile variability of these objects is due to non-radialpulsation. The appearance of the line profile variability depends mostlyon the projected rotational velocity v sin i and thus, since all Bestars rotate rapidly, on the inclination i. The observed variability ofthe investigated stars is described, and for some of them line profilevariability periods are given for the first time. For two of theinvestigated stars the line profile variability was successfully modeledas non-radial pulsation with l=m=+2 already in previous works. Since Bestars with similarly low v sin i share the same variability properties,these are in general explainable under the same model assumptions. Theline profile variability of stars with higher v sin i is different fromthe one observed in low v sin i stars, but can be reproduced by thesame model, if only the model inclination is modified to more equatorialvalues. Only for a few stars with periodic line profile variability thel=m=2 non-radial pulsation mode is not able to provide a satisfyingexplanation. These objects might pulsate in different modes (e.g.tesseral ones, l != |m|). Almost all stars in the sample show traces ofoutburst-like variability, pointing to an ephemeral nature of themass-loss phenomenon responsible for the formation of the circumstellardisk of early-type Be stars, rather than a steady star-to-disk masstransfer. In addition to the variability due to non-radial pulsationpresent in most stars, several objects were found to show other periodsresiding in the immediate circumstellar environment. The presence ofthese secondary periods is enhanced in the outburst phases. Short-livedaperiodic phenomena were clearly seen in two stars. But, given theunfavourable sampling of our database to follow rapid variability oftransient nature, they might be more common. Only in two out of 27 starsshort-term spectroscopic variability was not detected at all.Based on observations collected at the European Southern Observatory atLa Silla, Chile, 55.D-0502, 56.D-0381, 58.D-0697, 62.H-0319, 63.H-0080,64.H-0548, and 267.D-5702, the German-Spanish Astronomical Centre, CalarAlto, operated by the Max-Planck-Institute for Astronomy, Heidelberg,jointly with the Spanish National Commission for Astronomy, and onobservations with the Wendelstein 80-cm and the Ondřejov 2-mtelescopes, both equipped with the HEROS spectrograph provided by theLandessternwarte Heidelberg.
| Stellar and circumstellar activity of the Be star omega CMa. III. Multiline non-radial pulsation modeling
The line profile variability of omega CMa is modeled for variousphotospheric absorption lines of different ions as non-radial pulsation.The retrograde pulsation suggested by \citet{1982A&A...105...65B}could be confirmed. Due to rapid rotation, the line profile variabilityappears prograde, however. The line profiles could be reproduced ingreat detail, including prominent structures like ``spikes'' and``ramps''. These features arise naturally from the pole-on orientationof the star together with high-amplitude pulsation in g-modes, i.e. withhorizontal motions being dominant. The change of the line profilevariability during outbursts (understood as the beginning of phases ofhigh brightness) reported in \citetalias{paper2} of this series can alsobe understood within the framework of non-radial pulsation if veilingeffects of the circumstellar disk are taken into account. It isconcluded that the coherent periodic line profile variability of theabsorption lines of omega CMa can be explained by non-radial pulsationin detail.Based on observations collected at the European Southern Observatory atLa Silla, Chile, 55.D-0502, 56.D-0381, 58.D-0697, 62.H-0319, 64.H-0548,and 267.D-5702.
| The spinning-top Be star Achernar from VLTI-VINCI
We report here the first observations of a rapidly rotating Be star,alpha Eridani, using Earth-rotation synthesis on the Very LargeTelescope (VLT) Interferometer. Our measures correspond to a 2a/2b =1.56+/-0.05 apparent oblate star, 2a and 2b being the equivalent uniformdisc angular diameters in the equatorial and polar direction.Considering the presence of a circumstellar envelope (CSE) we argue thatour measurement corresponds to a truly distorted star since alphaEridani exhibited negligible Hα emission during theinterferometric observations. In this framework we conclude that thecommonly adopted Roche approximation (uniform rotation and centrallycondensed mass) should not apply to alpha Eridani. This result opensnew perspectives to basic astrophysical problems, such as rotationallyenhanced mass loss and internal angular momentum distribution. Inaddition to its intimate relation with magnetism and pulsation, rapidrotation thus provides a key to the Be phenomenon: one of theoutstanding non-resolved problems in stellar physics.
| Estimation of the mass loss, opening angle and mass of Be circumstellar disks from Brmathsf γ continuum emission and interferometric measurements
Using the SIMECA code developed by Stee & Araùjo(\cite{stee1}); Stee et al. (\cite{stee2}) for Be stars we obtain acorrelation between the mass loss rates {dot M} and the Brgammacontinuum luminosity as a function of the opening angle of the disk. Weshow that this correlation is similar to those obtained by Scuderi etal. (\cite{scuderi}) for O-B supergiants. We found that the wind densityat the base of the photosphere, from a sample of 8 Be stars, liesbetween 10-13 and 10-12 g cm-3. We alsopresent a relationship between the mass of the circumstellar disk andthe 2.16 mu m flux. Finally we emphasize how interferometricmeasurements can help to estimate the wind density and we present asample of 16 Be stars with predicted visibilities that can be observedwith the VLTI.
| Multiperiodicity and NRP in STARS Stars
Not Available
| Autocorrelation Analysis of Hipparcos Photometry of Short-Period Be Stars
We have used Hipparcos epoch photometry and a form of autocorrelationanalysis to investigate the amplitude and timescale of the short-periodvariability of 82 Be stars, including 46 Be stars that were analyzed byHubert & Floquet using Fourier and CLEAN analysis and 36 other Bestars that were suspected of short-period variability. Our method hasgiven useful information for about 84% of these stars; for the rest, thetime distribution of the Hipparcos epoch photometry limits thecapability of our technique.
| The ups and downs of a stellar surface: nonradial pulsation modelling of rapid rotators
The main result of the modelling project is the confirmation thatearly-type Be stars pulsate nonradially in g-modes. Based on thedetailed modelling of ω CMa, it could be shown that this star isproto-typical, and that the periodic lpv of early type Be stars ingeneral is due to nonradial pulsation.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
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