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Fundamental parameters of Be stars located in the seismology fields of COROT
In preparation for the COROT space mission, we determined thefundamental parameters (spectral type, temperature, gravity, V sin i) ofthe Be stars observable by COROT in its seismology fields (64 Be stars).We applied a careful and detailed modeling of the stellar spectra,taking into account the veiling caused by the envelope, as well as thegravitational darkening and stellar flattening due to rapid rotation.Evolutionary tracks for fast rotators were used to derive stellar massesand ages. The derived parameters will be used to select Be stars assecondary targets (i.e. observed for 5 consecutive months) and short-runtargets of the COROT mission. Furthermore, we note that the main part ofour stellar sample falls in the second half of the main sequence lifetime, and that in most cases the luminosity class of Be stars isinaccurate in characterizing their evolutionary status.
| 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.
| Unstable quasi g-modes in rotating main-sequence stars
This paper studies the oscillatory stability of uniformly rotatingmain-sequence stars of mass 3-8 M_ȯ by solving the linearizednon-adiabatic, non-radial oscillation equations with a forcing term andsearching for resonant response to a complex forcing frequency. By usingthe traditional approximation, the solution of the forced oscillationequations becomes separable, whereby the energy equation is madeseparable by approximation. It is found that the κ-mechanism inrotating B-stars can destabilize not only gravity- or pressure modes,but also a branch of low frequency retrograde (in corotating frame)oscillations in between the retrograde g-modes and toroidal r-modes.These unstable quasi-g (or "q-modes") hardly exhibit rotationalconfinement to the equatorial regions of the star, while theoscillations are always prograde in the observer's frame, all incontrast to g-modes. The unstable q-modes occur in a few narrow periodbands (defined by their azimuthal index m) and seem to fit theoscillation spectra observed in SPB stars rather well. The unstableq-mode oscillation spectrum of a very rapidly rotating 8 M_ȯ starappears similar to that of the well-studied Be-star μ Cen. Theunstable q-modes thus seem far better for explaining the observedoscillation spectra in SPB-stars and Be-stars than do normal g-modes.
| 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.
| Rotation, pulsations and outbursts in the Be star υ Cygni (HD 202904)
υ Cyg is a Be star that shows variations at all timescales. Wemonitored its spectrum for several years from 1998 to 2004 and, inparticular, during a spectroscopic multisite campaign in 2000. In thispaper we present and analyse the data. We observed several outburstsincluding an intense one in 2000. Moreover, we found several periods ofshort-term variations, including two frequencies at 2.95 and 2.6 cd-1, which are well reproduced by models of non radialpulsations with a retrograde mode with ℓ=3 and m=3 and a zonal modewith ℓ=3 or 4 and m=0, respectively. The stellar rotation isprobably also identified at f1.5 c d-1, which iscoherent with the rotation frequency deduced from our determination ofstellar parameters. The peak-to-peak amplitude of variations also seemsto vary in time, maybe due to a beating effect between closefrequencies, but the resolution in time of our data does not allow us toseparate such close frequencies. Finally, a longer timescale variationmay be present, with a period around 11 years, which could be associatedwith a binary companion.
| Spectroscopy and Photometry of Be Stars During the Past Decade
Not Available
| 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.
| A Period and a Prediction for the Of?p Spectrum Alternator HD 191612
The observational picture of the enigmatic O-type spectrum variable HD191612 has been sharpened substantially. A symmetrical, low-amplitudelight curve with a period near 540 days has recently been reported fromHipparcos photometry. This period satisfies all of the spectroscopysince at least 1982, including extensive new observations during 2003and 2004, and it has predicted the next transition during 2004September-October. Measurements of the Hα equivalent width reveala sharp emission peak in the phase diagram, in contrast to theapparently sinusoidal light curve. The He II absorption-line strength isessentially constant, while He I varies strongly, possibly filled in byemission in the O6 state, thus producing the apparent spectral typevariations. The O8 state appears to be the ``normal'' one. Twointermediate O7 observations have been obtained, which fall at theexpected phases, but these are the only modern observations of thetransitions so far. The period is too long for rotation or pulsation;although there is no direct evidence as yet for a companion, a model inwhich tidally induced oscillations drive an enhanced wind nearperiastron of an eccentric orbit appears promising. Further observationsduring the now predictable transitions may provide a critical test.Ultraviolet and X-ray observations during both states will likely alsoprove illuminating.
| A Hot Envelope around the Southern Coalsack: X-Ray and Far-Ultraviolet Observations
We present Far Ultraviolet Spectroscopic Explorer and ROSAT X-rayobservations toward the Southern Coalsack. An almost complete X-ray halocan be seen around the cloud in the 0.75 and 1.5 keV images, and most ofthe observed stars show O VI absorption. Both the cloud and the starshave highly accurate distance determinations, allowing us to reliablyplace the stars and the cloud relative to each other. Using thesedistance determinations, we find no O VI-bearing gas in the foregroundof the Coalsack, while for stars in the background of the cloud, O VIabsorption is the norm. The column density of O VI correlates with the0.75 and 1.5 keV intensities. These results suggest that theX-ray-emitting hot plasma is associated with the dense cloud. We proposethat the heating of the Coalsack envelope is due to the hot gas in theinterior of the Upper Cen-Lup superbubble. The Coalsack interactionregion provides a nearby example of the hot-cold gas interfaces thoughtto be responsible for the O VI absorptions seen on many sight linesthroughout the Galaxy.
| The Detection of 13CO and Other Apparent Abundance Anomalies in the Secondary Stars of Long-Period Cataclysmic Variables
We present moderate-resolution (R>1800) infrared K-band spectra of 12long-period (Porb>6 hr) cataclysmic variables (CVs). Wedetect absorption lines from the photospheres of the secondary stars inevery system, even though two of them were undergoing outbursts. We haveattempted to assign a spectral type to each of the secondary stars, andthese classifications are generally consistent with previousdeterminations. We find evidence for abundance anomalies that includeenhancements and/or deficits for all of the species commonly found inK-band spectra of G- and K-type dwarfs. There is, however, only onecommon abundance anomaly: extremely weak CO features. Only two of the 12objects appeared to have normal levels of CO absorption. We interpretthis as evidence of low carbon abundances. In addition, we detect13CO absorption in four of the 12 objects. Depleted levels of12C and enhanced levels of 13C indicate thatmaterial that has been processed in the CNO cycle is finding its wayinto the photospheres of CV secondary stars. In systems with luminousaccretion disks, we find that the spectrum of the secondary star iscontaminated by a source that flattens (reddens) the continuum. Whilefree-free or classical accretion disk spectra are flatter than theblackbody-like spectra of G and K dwarfs, removal of such contaminationfrom the K-band data results in spectra in which the absorption featuresbecome too strong to be consistent with those of G and K dwarfs.
| 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.
| Spectroscopy of Early F Stars: γ Doradus Candidates and Possible Metallic Shell Stars
We obtained high-resolution spectroscopic observations of 34 γDoradus candidates. From the red-wavelength spectra, we determinedspectral classes, radial velocities, and projected rotationalvelocities. The spectra of seven late A or early F stars show metalliclines that have composite profiles consisting of a narrow component nearthe center of a broad line, indicating that they may be shell stars orbinaries. Several stars, including HD 152896, HD 173977, HD 175337, andHD 195068/9, show large line profile asymmetries. Two stars, HD 11443(=α Trianguli) and HD 149420, are ellipsoidal variables and notγ Doradus stars. The percentage of binary systems in our samplemay be as high as 74%.
| 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).
| Rotation and magnetic field in the Be star omega Orionis
omega Ori is a B2IIIe star for which rotationalmodulation and non-radial pulsations (NRP) have been recentlyinvestigated from two independent observational campaigns in 1998 and1999. Putting the data of these 2 campaigns together, and adding dataobtained in 2001, we search for multiperiodicity in the line profilevariations and evidence for outbursts. From new spectropolarimetric dataobtained at the Télescope Bernard Lyot (TBL, Pic du Midi, France)in 2001 we also measure the Stokes V parameter in the polarised light.We find evidence for the presence of a weak magnetic field inomega Ori sinusoidally varying with a period of 1.29d. The equivalent widths (EW) of the wind sensitive UV resonance linesalso show a variation with the same period, which we identify as therotational period of the star. We propose an oblique rotator model andderive Bpol =530 +/- 230 G to explain the observations.Moreover, we carry out an abundance analysis and find the star to beN-enriched, a property which is shared with other magnetic stars. Wepropose omega Ori as the first known classical Bestar hosting a magnetic field.Based on observations obtained using the Musicos spectropolarimeter atthe Observatoire du Pic du Midi (France), during the MuSiCoS 98 campaign\citep{neiner}, and by \cite{balona}. Based on INES data from theInternational Ultraviolet Explorer (IUE) satellite.Table 7 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/275
| Spectroscopy of the growing circumstellar disk in the delta Scorpii Be binary
We present the results of a spectroscopic monitoring program of thebinary system delta Scorpii, whose primary became a Be star after thelast periastron encounter in the Summer of 2000. The observations covera period of 2 years (March 2001-June 2003) and are a continuation of ourprevious campaign reported in Miroshnichenko et al. (\cite{metal}). Wefound that the emission-line spectrum was gradually strengthening overthe whole monitoring period. Several short-term increases of the lineemission, possibly due to outbursts of matter ejected from the stellarsurface, were detected. The outbursts seem to be responsible for thebrightness fadings, seen afterwards. We attempt to explain the overallobserved line and continuum behaviour in terms of a circumstellar disk,growing in density and size. The disk is most likely Keplerian with anouter radius of ~10 Rstar and a mean expansion speed of ~0.4 km s-1.Tables 1-2 are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/408/305}
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Long term disc variability in the Be star o Andromedae
We present 18 years of high resolution and S/N Hα spectroscopy ofthe Be shell star o And, obtained between 1985-2002.Spectra taken during late 1985 show a pure photospheric profile, withdisc re-formation commencing in 1986; a process that is found to occurover long timescales ( ~ 103 days). Analysis of the evolutionof the properties of the Hα shell profile suggest that the disckinematics are dominated by rotational motion. It has been shown thatdisc loss in o And occurs ``inside out''; we findthat the disc also appears to be rebuilt in a similar manner, with discmaterial gradually diffusing to larger radii. The long timescale forchanges in the bulk properties of the disc, domination of rotationalover radial velocities and manner of disc loss and formation are allconsistent with the predictions of the viscous decretion disc model forBe star discs.
| 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.
| 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.
| On the role of duplicity in the Be phenomenon. I. General considerations and the first attempt at a 3-D gas-dynamical modelling of gas outflow from hot and rapidly rotating OB stars in binaries
This paper begins a new series of studies devoted to a criticalre-examination of the role of duplicity for the Be phenomenon and forthe variability patterns observed for many Be stars. Based on bothdynamical and energy considerations and a numerical gas-dynamicalmodelling, a new hypothesis of the formation of Be envelopes inbinaries, via an outflow from a rapidly rotating B star in a detachedbinary, is outlined. It is shown that such an outflow is facilitated bythe presence of a companion to the B star and leads to the formation ofan envelope but not to any significant mass exchange between the binarycomponents.
| Variability and pulsations in the Be star 66 Ophiuchi
66 Oph is a Be star seen under a moderate inclination angle that showsstrong variability from UV to IR wavelengths. A concise review oflong-term variability history is given. High resolution, high S/Nspectroscopic observations obtained in 1997, 1998 and 2001 andspectropolarimetric observations obtained in 2000 are presented. Theseobservations occurred during a long-term decrease of Hα intensity.Fundamental parameters of the star have been revisited fromBarbier-Chalonge-Divan (BCD) calibrations. New V sin i values areobtained using Fourier transforms applied to observed helium lines and arotational frequency f_rot = 1.29 c d-1 is determined. Timeseries analysis and Fourier Doppler Imaging (FDI) of He I lines (4713,4921, 5876 and 6678 Å) lead for the first time to the detection ofmulti-periodicity in 66 Oph. The two main frequencies found are f = 2.22c d-1 and f = 4.05 c d-1 . They are attributed tonon-radial pulsations and can be associated with mode degree l = 2 and l= 3, respectively. Inspection of Stokes V profiles suggests the presenceof a weak Zeeman signature but further observations are needed toconfirm the detection of a magnetic field in 66 Oph. Based onobservations taken at OHP and Pic du Midi Observatory (France), atMBT/LNA (Brazil) and on Brazilian observing time at La Silla (ESO,Chile).
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Besorolás csoportokba:
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Pozíciós és asztrometriai adatok
| Csillagkép: | Kentaur |
| Rektaszcenzió: | 13h49m37.00s |
| Deklináció: | -42°28'26.0" |
| Vizuális fényesség: | 3.04 |
| Távolság: | 161.551 parszek |
| RA sajátmozgás: | -23.9 |
| Dec sajátmozgás: | -18.9 |
| B-T magnitude: | 3.214 |
| V-T magnitude: | 3.415 |
Katalógusok és elnevezések:
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