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Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| Influence of the Coriolis force on the instability of slowly pulsating B stars
This paper explores the effect of rotation on the κ-mechanisminstability of slowly pulsating B stars. A new non-adiabatic code, whichadopts the so-called traditional approximation to treat the Coriolisforce, is used to investigate the influence exerted by rotation over thestability of stellar models covering the mass range2.5Msolar<=M*<= 13.0Msolar. Theprincipal finding is that, for all modes considered apart from theprograde sectoral (PS) class, rotation shifts the κ-mechanisminstability toward higher luminosities and effective temperatures; theseshifts are accompanied by broadenings in the extent of instabilitystrips. Such behaviour is traced to the shortening of mode periods underthe action of the Coriolis force. Instability strips associated with PSmodes behave rather differently, being shifted to marginally lowerluminosities and effective temperatures under the influence of rotation.The implications of these results are discussed in the context of theobservational scarcity of pulsation in B-type stars having significantrotation; various scenarios are explored to explain the apparentdichotomy between theory and observations. Furthermore, the possiblesignificance of the findings to Be stars is briefly examined.
| 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.
| Variable Star Network: World Center for Transient Object Astronomy and Variable Stars
Variable Star Network (VSNET) is a global professional-amateur networkof researchers in variable stars and related objects, particularly intransient objects, such as cataclysmic variables, black-hole binaries,supernovae, and gamma-ray bursts. The VSNET has been playing apioneering role in establishing the field of transient object astronomy,by effectively incorporating modern advances in observational astronomyand global electronic networks, as well as collaborative progress intheoretical astronomy and astronomical computing. The VSNET is now oneof the best-featured global networks in this field of astronomy. Wereview the historical progress, design concept, associated technology,and a wealth of scientific achievements powered by VSNET.
| Properties and nature of Be stars. XXII. Long-term light and spectral variations of the new bright Be star HD 6226
Photometric and spectroscopic monitoring of the B star HD 6226 resultedin the finding that this object is a new bright Be star with a clearpositive correlation between the brightness and emission-line strength.The emission-line episodes are relatively short and seem to repeatfrequently which makes this star an ideal target for studying the causesof the Be phenomenon. The general character of the light variations, thelow v sin i = 70 km s-1 and the very pronounced lineasymmetries of the He I 6678 line, seen both outside and duringemission-line episodes, are all attributes which make HD 6226phenomenologically very similar to the well-known Be star ω CMa.Radial velocities of the deepest parts of the metallic and He I 6678absorption lines vary with a strict period of 2.61507 d over the wholetime interval covered by the observations, the velocities of the broadouter wings of the same lines varying in anti-phase and with a loweramplitude. This periodicity could not be found in the radial-velocityvariations of the sharp core of Hα. There is some indication ofvariability on a time scale of 24-29 days but our data are insufficientto prove that conclusively. A comparison of the line spectrum obtainedoutside emission episodes with synthetic spectra, standard dereddeningof U BV magnitudes and Hipparcos parallax all agree with the conclusionthat HD 6226 is a star with the following basic properties:Teff ,= 17 000 K, log g = 3.0 [cgs], mass of 5m.s and radiusof 11 Rȯ. The strong emission-line episodes may appearregularly, in a cycle of 630 days but with different durations ofindividual cycles. HD 6226 is probably one of the first B stars forwhich the Be nature was predicted on the basis of the character of itslight and colour changes.Based on spectral and photometric observations from the followingobservatories: Castanet-Tolosan, Dominion Astrophysical Observatory,Heidelberg, Hvar, Ondřejov, Skalnaté Pleso andStará Lesná and also on photoelectric photometry by AAVSOmembers.Tables 2 and 4 are available only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/669
| Classical Be Stars
Recent results for classical Be stars are reviewed and links to generalastrophysics are presented. Classical Be stars are B-type stars close tothe main sequence that exhibit line emission over the photosphericspectrum. The excess is attributed to a circumstellar gaseous componentthat is commonly accepted to be in the form of an equatorial disk. Since1988, when the last such review was published, major progress has beenmade. The geometry and kinematics of the circumstellar environment canbe best explained by a rotationally supported relatively thin disk withvery little outflow, consistent with interferometric observations. Thepresence of short-term periodic variability is restricted to the earliertype Be stars. This variation for at least some of these objects hasbeen shown to be due to nonradial pulsation. For at least one star,evidence for a magnetic field has been observed. The mechanismsresponsible for the production and dynamics of the circumstellar gas arestill not constrained. Observations of nonradial pulsation beatingphenomena connected to outbursts point toward a relevance of pulsation,but this mechanism cannot be generalized. Either the evidence that Bestars do not form a homogeneous group with respect to disk formation isgrowing or the short-term periodic variability is less important thanpreviously thought. The statistics of Be stars investigated in openclusters of the Milky Way and the Magellanic Clouds has reopened thequestion of the evolutionary status of Be stars. The central B star is afast rotator, although theoretical developments have revived thequestion of how high rotational rates are, so the commonly quoted meanvalue of about 70%-80% of the critical velocity may just be a lowerlimit. Be stars are in a unique position to make contributions toseveral important branches of stellar physics, e.g., asymmetricmass-loss processes, stellar angular momentum distribution evolution,astroseismology, and magnetic field evolution.
| 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.
| Stellar and circumstellar activity of the Be star omega CMa. II. Periodic line-profile variability
The rapid line-profile variability of the early-type and pole-on Be staromega CMa between 1996 and 2002 is characterizedacross the complete optical spectrum, for quiescent phases as well asfor outbursts. Owing to different and changing line-profile variabilitypatterns, amplitudes and gamma -velocities are different from line toline and are variable on a time scale of months. A comprehensive timeseries analysis was performed on the modes of a set of selected lines(after individual seasonal normalization to avoid biases). At a highlevel of confidence, only the well-known 1.37-d period could be found inphotospheric lines not contaminated by the disk. Outside majoroutbursts, when the star is at its photometric ground state, the phasecoherence of the variability is very robust. During strong outbursts,when the star is visually bright, the period may either be very slightlydifferent or phase jumps may occur. The present observations do not havethe sampling necessary to distinguish between these possibilities.Harmanec's (\citeyear{1998A&A...334..558H}) report of continuous,cyclic period variations cannot be confirmed. Arguments are presentedthat temporary period changes may be related to interactions between thephotospheric non-radial pulsation and the disk when (during outbursts)these two domains are in contact with one another. This result does notseem to be an artifact of the also previously reported transientperiodicities near 1.49-d, which are prominent during outbursts and seemto be anchored in the exo-photosphere. However, if not properly takeninto account, they may easily lead to false conclusions about multipleor variable periods. In the Be star mu Cen, which has a similarspectral type, outbursts are triggered by the beating of two or morenon-radial pulsation modes \citep{1998hcsp.proc..343R}. Since omega CMa,too, undergoes outbursts although its photospheric variability issingle-periodic, the case of mu Cen cannot be generalized to theactivity of all early-type Be stars or to the Be phenomenon at large.Based on observations collected at the European Southern Observatory atLa Silla, Chile (ESO proposals No. 55.D-0502, 56.D-0381, 58.D-0697,64.H-0548, 62.H-0319 and 267.D-5702).
| 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.
| Stellar and circumstellar activity of the Be star omega CMa. I. Line and continuum emission in 1996-2002
Echelle spectroscopy and mostly unaided-eye photometry of the southernBe star omega CMa were obtained in the period1996-2003. The monitoring is bracketed by two brightenings by 0.4m-0.5m.The results of a literature search suggest that such phases occur aboutonce a decade and have various commonalities. Along with thesephotometric events goes enhanced line emission. This is due to anincreased total mass of the disk as well as to a change in its densityprofile. The models by Poeckert & Marlborough(\cite{1978ApJS...38..229P}, \cite{1979ApJ...233..259P}) imply that theenhanced continuum flux originates from the inner disk. Higher-orderBalmer line emission is correlated with brightness. The increase inHα is retarded by some months, possibly indicating a time delayin filling up and ionizing the outer disk. In the (U-B) vs. (B-V) colourdiagram and the D54 vs. D34 Balmer decrementdiagram the path from the ground to the bright state is distinct fromthe return path. This could result from the bulk of the disk matterbeing in the outer (inner) disk during the photometric ground (high)state, while the two transitions between the two states are both due tochanges progressing radially outward. Some mu Cen-like outbursts(Rivinius et al. \cite{1998A&A...333..125R}) seem to occur in allphases. It is conceivable that the build-up of the inner disk is causedby more frequent or more effective outbursts. During the photometricbright state various other phenomena gain in prominence and suggest thisto be a phase of increased activity. Of particular interest, butpossibly only apparently related to this phase, are absorptioncomponents at redshifts well beyond the range covered by the combinationof rotation and nonradial pulsation.Based on observations collected at the European Southern Observatory atLa Silla, Chile, ESO (proposal nos. 55.D-0502, 56.D-0381, 58.D-0697,62.H-0319, 64.H-0548).
| Outbursts in the Be star HR 2501
The Be star HR 2501 was monitored in photometry, from1978 to 1998 in the GENEVA system and from 1990 to 1992 by the HIPPARCOSsatellite, and in spectroscopy from 1998 to 2001 by using the CORALIEspectrograph. Several mostly unrelated periods or time scalescharacterize the variability of HR 2501. First, theradial velocity data reveals that this is a new lambda Eri star, with aperiod of 0.79187 d due to non-radial pulsations or clouds close to thestellar photosphere. Second, both GENEVA and HIPPARCOS photometriesexhibit a mid- to long-term variability of characteristic time ~ 500 dand peak-to-peak amplitude ~ 0.4 mag, most probably a consequence ofthe recurrent outbursts of matter from the rapidly rotating Be startowards its disk. Third, a characteristic time of ~ 300 d between theoutbursts is shown by spectroscopy by looking at the variations of theequivalent width of Hα , Hβ and HeI (5875.6 Å)emission lines (the outbursts studied in photometry and spectroscopy areunfortunately not the same due to the non-simultaneity of themonitorings). Fourth, the V/R ratio of the double peaks in Hα andHβ show a periodic-type variation during the second of thespectroscopic outbursts, with periods of respectively 16.7 and 15.1 d,in agreement with the prediction of the dynamical evolution of a blob ofmaterial ejected from the equator of the star into the Keplerian disk.Fifth, a short-term photometric period of 0.46 d is detected during anepoch of intensive monitoring; however, it must be noted that if thisvariability is confirmed, the value of this short-term period is unsure.The other main results are: i) During the 27 months of the spectroscopicsurvey, in addition to the mentioned outbursts, the emission in Hαand Hβ lines decreased progressively until it almost completelyvanishes at the end. ii) The estimated radius of the circumstellar diskvaries and reaches 5.5, 4 and 2 stellar radii at maximum for theHα , Hβ and HeI emission regions respectively; iii) Thespectroscopic monitoring was particularly successful, and the series ofobservation of the line profiles, with the variation of the emissionfrom day to day, is quite exceptional in the studies of Be stars.Based on observations collected at the Swiss 40 cm, 70 cm and 120 cmtelescopes at the European Southern Observatory (La Silla, Chile) and ondata from the ESA HIPPARCOS satellite.The photometric and radial velocity data 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/401/271
| 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.
| To Be or not to Be and a 50-cm post-mortem eulogy
Not Available
| Short-period line profile and light variations in the Be star λ Eridani
We present three seasons of photometric observations and one season ofintensive high-dispersion spectroscopic observations of the Be starλ Eridani. We show that only one period, P =0.70173d, is presentin the photometry, although there are large light amplitude variationsfrom season to season. We confirm a suspicion that light outburstsrepeat at intervals of about 475d. A total of 348 echelle spectra of thestar were obtained over a 2-week observing run. We show that theperiodic variations are present in the emission wings of the heliumlines, in the emission wings of the Hα line and in the absorptioncores of Hβ and Hγ . Together with the fact that the periodicvariations appear outside the projected rotational velocity limit, thisindicates that they are associated with circumstellar materialimmediately above the photosphere and supports the idea of corotatinggas clouds. We present evidence in support of a true rotational periodof 2P =1.40346d and suggest that the mass loss in Be stars is caused bycentrifugal magnetic acceleration.
| Koordinierte Zusammenarbeit zwischen den VdS-FG 'BAV' und 'Spektroskopie' ?
Not Available
| 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.
| Blue Variable Stars from the MACHO Database. I. Photometry and Spectroscopy of the Large Magellanic Cloud Sample
We present the photometric properties of 1279 blue variable stars withinthe Large Megallanic Cloud. Photometry is derived from the MACHOdatabase. The light curves of the sample exhibit a variety ofquasi-periodic and aperiodic outburst behavior. A characteristic featureof the photometric variation is that the objects are reddest when atmaximum outburst. A subset of 102 objects were examinedspectroscopically. Within this subset, 91% exhibited Balmer emission inat least one epoch, in some cases with spectacular spectral variability.The variability observed in the sample is consistent with theestablishment and maintenance of the Be phenomenon.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is 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?/A+A/390/1033
| Hydrogen infrared recombination lines as a diagnostic tool for the geometry of the circumstellar material of hot stars
We have analysed the infrared hydrogen recombination lines of a sampleof well studied hot massive stars observed with the Infrared SpaceObservatory. Our sample contains stars from several classes of objects,whose circumstellar environment is believed to be dominated by anionized stellar wind (the Luminous Blue Variables) or by a densedisk-like geometry (Be stars and B[e] stars). We show that hydrogeninfrared recombination lines can be used as a diagnostic tool toconstrain the geometry of the ionized circumstellar material. The linestrengths are sensitive to the density of the emitting gas. Highdensities result in optically thick lines for which line strengths areonly dependent on the emitting surface. Low density gas producesoptically thin lines which may be characterized by Menzel case Brecombination. The ISO observations show that stellar winds aredominated by optically thin H I recombination lines, while disks aredominated by optically thick lines. Disks and winds are well separatedin a diagnostic diagram using the Hu(14-6)/Bralpha and theHu(14-6)/Pfgamma line flux ratios. This diagnostic tool is useful toconstrain the nature of hot star environments in case they are highlyobscured, for instance while they are still embedded in their natalmolecular cloud. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA.
| Search for duplicity in periodic variable Be stars
Four Be stars, HR 1960, HR 2968,HR 3237 and HR 3642, selectedaccording to their periodic variations in HIPPARCOS and GENEVAphotometries, were monitored from 1998 until 2001 with the CORALIEspectrograph. Among these stars, two are new spectroscopic binaries andone is a new lambda Eri short period variable. HR 1960 is a lowamplitude (K = 3.4 km s-1) SB1 with a period of 395.48 d inagreement with the photometric prediction. HR 3237 is a short period SB1(P = 5.1526 d). HR 3642 presents some interesting variations inphotometry and spectroscopy: indeed, a mid- and a short-term variationis present with periods of 137.99 d (Hp magnitude) and 1.13028 d (radialvelocity) respectively. The short-term variation, characteristic of thelambda Eri stars, probably implies non-radial pulsations orinhomogeneities in the corotating disc. The last star, HR 2968, is anexcellent photometric binary candidate, but no spectroscopic obviousnessof a companion has been found. Based on observations collected at theSwiss 40 cm, 70 cm and 120 cm telescopes at the European SouthernObservatory (La Silla, Chile) and on data from the ESA HIPPARCOSsatellite. The photometric and radial velocity data are only availablein electronic 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/385/488 Table 3 is onlyavailable in electronic form at http://www.edpsciences.org
| An atlas of 2.4 to 4.1 mu m ISO/SWS spectra of early-type stars
We present an atlas of spectra of O- and B-type stars, obtained with theShort Wavelength Spectrometer (SWS) during the Post-Helium program ofthe Infrared Space Observatory (ISO). This program is aimed at extendingthe Morgan & Keenan classification scheme into the near-infrared.Later type stars will be discussed in a separate publication. Theobservations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 μm, supplemented with 10 spectra acquired during the nominal mission witha similar observational setting. For B-type stars, this sample providesample spectral coverage in terms of subtype and luminosity class. ForO-type stars, the ISO sample is coarse and therefore is complementedwith 8 UKIRT Larcmin -band observations. In terms of the presence ofdiagnostic lines, the Larcmin -band is likely the most promising of thenear-infrared atmospheric windows for the study of the physicalproperties of B stars. Specifically, this wavelength interval containsthe Bralpha , Pfgamma , and other Pfund lines which are probes ofspectral type, luminosity class and mass loss. Here, we present simpleempirical methods based on the lines present in the 2.4 to 4.1 mu minterval that allow the determination of i) the spectral type of Bdwarfs and giants to within two subtypes; ii) the luminosity class of Bstars to within two classes; iii) the mass-loss rate of O stars and Bsupergiants to within 0.25 dex. Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA. The appendix is 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/384/473
| A spectroscopic search for variability of Be stars in the SMC
Two low-v sin i early-type Be stars in the area of the young cluster NGC330 in the Small Magellanic Cloud (SMC) were monitored with the VLT andthe UVES echelle spectrograph during 8 consecutive nights. Exposuretimes between 45 and 60 min yielded a signal-to-noise ratio of about 80.The temporal variance in the line profiles does not significantly differfrom the one of the continuum in both sets of about two dozen spectra.Four more stars were observed only once or twice; their line profiles,too, were not asymmetric. This is in marked contrast to the Galaxy,where short-term line profile variability would normally have beendetected with high probability in at least one out of any two randomlyselected Be stars of early spectral sub-class and low v sin i. Somepossible repercussions on the understanding of the Be phenomenon arebriefly discussed. Based on observations collected at the EuropeanSouthern Observatory, Paranal, Chile (ESO Progr. No. 267.D-5703).
| Short-period line profile variations in the Be star μ Centauri
We present new intensive photometric observations of the Be star μCen for several seasons which support a period close to 1d. We alsopresent high-resolution spectroscopic data consisting of 302 spectra in1999 and 864 spectra in 2000, all obtained within a two-week observingrun in each season. We use stacked grey-scale plots of spectra, fromwhich the mean line profile has been removed, to examine the profilevariations. We find that most nights show one residual absorptionfeature, moving from blue to red, visible in all helium and metal linesand also clearly visible in Hα and other lines formed in thecircumstellar environment. We therefore conclude that this feature is ofcircumstellar origin. In addition, a residual absorption feature movingfrom red to blue is sometimes seen at irregular intervals. We find thatthe residual absorption feature frequently strays outside the projectedrotational velocity limit and that on occasions it remains well withinthis limit. The radial velocity data reproduce only two of the sixfrequencies previously found in the star. We point out that this by nomeans implies that the star is a multiperiodic, non-radial pulsator.Photometric data obtained over several seasons indicate a period veryclose to 1d and not the 0.5-d period found from the radial velocities.We describe an outburst which occurred during the run and which resultedin increased Hα emission two nights later. It is clear thatoutbursts in Be stars are localized events and that the gas released byoutbursts is probably responsible for localized increased absorption,resulting in periodic light and line profile variations.
| Short-period line profile variations in the Be star ɛ Cap
We present new high-dispersion spectroscopic data for the Be starɛ Cap. The purpose of these data is to study the short-periodline profile variations. By using a two-dimensional period-findingtechnique, we confirm that the photometric period of 0.99d is present inthe helium line profiles. We show that the variations are not easilyexplained by non-radial pulsation and suggest that corotatingcircumstellar material is responsible.
| Emission activity of the Be star 28 CMa: entering a new cycle?
A new major emission phase of the southern Be star 28 CMa is reported.It manifests itself as a brightening by more than 0.4m in the V band anda drop of the Balmer line emission. The spectroscopic behaviour fits theoutburst scheme derived for mu Cen, but takes place on a much longertime scale, thereby making 28 CMa an ideal target for detailed studiesof Be star outbursts. On this basis, some spectroscopic phenomenaexpected to appear within a few months are mentioned.
| Ultraviolet Spectrophotometry of Variable Early-Type Be and B Stars Derived from High-Resolution IUE Data
High-dispersion IUE data encode significant information about aggregateline absorptions that cannot be conveniently extracted from individualstellar spectra. Here we apply a new technique in which fluxes from eachechelle order of a short-wavelength IUE spectrum are binned together toconstruct low-resolution spectra of a rapidly varying B or Be star. Thedivision of binned spectra obtained during a ``bright-star'' phase byspectra from a ``faint-star'' phase leads to a ratioed spectrum thatcontains information about the mechanism responsible for a star'svariability. The most likely candidate mechanisms are either theperiodic or episodic occultations of the star by ejected matter or achange in photospheric structure, e.g., from pulsation. We model thevariations caused by these mechanism by means of model atmosphere andabsorbing-slab codes. Line absorptions strength changes are rathersensitive to physical conditions in circumstellar shells and ``clouds''at temperatures of 8000-13,000 K, which is the regime expected forcircumstellar structures of early B stars. To demonstrate proofs of thisconcept, we construct spectral ratios for circumstellar structuresassociated with flux variability in various Be stars: (1) Vela X-1 has abow-shock wind trailing its neutron star companion; at successive phasesand hence in different sectors, the wind exhibits spectrophotometricsignatures of a 13,000 or 26,000 K medium; (2) 88 Her undergoes episodic``outbursts'' during which its UV flux fades, followed a year later by adimming at visible wavelengths as well; the ratioed spectrum indicatesthe ``phase lag'' is a result of a nearly gray opacity that dominatesall wavelengths as the shell expands from the star and cools, permittingthe absorptions in the visible to ``catch up'' to those in the UV; and(3) ζ Tau and 60 Cyg exhibit periodic spectrum and flux changes,which match model absorptions for occulting clouds but are actually mosteasily seen from selective variations of various resonance lines. Inaddition, ratioed UV spectra of radial and large-amplitude nonradialpulsating stars show unique spectrophotometric signatures, which can besimulated with model atmospheres. An analysis of ratioed spectraobtained for a representative sample of 18 classical Be stars known tohave rapid periodic flux variations indicates that 13 of them haveratioed spectra that are relatively featureless or have signatures ofpulsation. Ratioed spectra of three others in the sample exhibitsignatures that are consistent with the presence of corotating clouds.
| A Search for High-Velocity Be Stars
We present an analysis of the kinematics of Be stars based uponHipparcos proper motions and published radial velocities. We findapproximately 23 of the 344 stars in our sample have peculiar spacemotions greater than 40 km s-1 and up to 102 kms-1. We argue that these high-velocity stars are the resultof either a supernova that disrupted a binary or ejection by closeencounters of binaries in young clusters. Be stars spun up by binarymass transfer will appear as high-velocity objects if there wassignificant mass loss during the supernova explosion of the initiallymore massive star, but the generally moderate peculiar velocities of BeX-ray binaries indicate that the progenitors lose most of their massprior to the supernova (in accordance with model predictions). Binaryformation models for Be stars predict that most systems bypass thesupernova stage (and do not receive runaway velocities) to createultimately Be+white dwarf binaries. The fraction of Be stars spun up bybinary mass transfer remains unknown, since the post-mass transfercompanions are difficult to detect.
| Evolution in circumstellar envelopes of Be stars: From disks to rings?
New series of echelle spectra were obtained to study the medium- andlong-term evolution of the disks of several Be stars. Subtle variationsin the wings of optically thin and thick emission lines suggest that theconventional, static picture of the disk being in quasi-contact with thecentral star is justified primarily (or perhaps only) after an outburstevent. Some weeks to months later, a low-density region seems to developabove the star and slowly grows outwards. A subsequent outburst maylater replenish this cavity. In fact, in two stars this more ring-likestructure is apparently at times detached far enough from the star toallow for the formation of a secondary inner disk from the ejecta of alater outburst. This behaviour is not necessarily representative of Bestars in general because in the later spectral sub-types, discrete massloss events have not so far been observed to play a major role. In thelight of the apparent life cycle of such disks, a brief discussion isgiven of the differences in strength and variability between the windsof Be and normal B stars. It seems possible to attribute thesedifferences to matter that was initially in the disk and thereforelargely shielded from the stellar radiation, but that during the courseof the inner excavation (or even complete distruction) of the diskbecomes exposed. Based on observations collected at the EuropeanSouthern Observatory at La Silla, Chile, ESO proposal No. 64.H-0548 andon observations with the Wendelstein 80-cm and the Ondřejov 2-mtelescopes, both equipped with the HEROS spectrograph provided by theLandessternwarte Heidelberg.
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