Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| 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.
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| XMM-Newton observations of the σ Ori cluster. I. The complex RGS spectrum of the hot star σ Ori AB
We present XMM-Newton observations of the young ({} 2{-}5 Myr)cluster around the hot (O9.5V) star σ Orionis AB, aimed atobtaining a high resolution RGS spectrum of the hot star as well as EPICimaging data for the whole field. We show that the RGS spectrum ofσ Ori AB may be contaminated by weaker nearby sources whichrequired the development of a suitable procedure to extract a clean RGSspectrum and to determine the thermal structure and wind properties ofthe hot star. We also report on the detection of a flare from the B2Vpstar σ Ori E and we discuss whether the flare originated from thehot star itself or rather from an unseen late-type companion. Otherresults of this observation include: the detection of 174 X-ray sourcesin the field of σ Ori of which 76 are identified as clustermembers, including very low-mass stars down to the substellar limit; thediscovery of rotational modulation in a late-type star near σ OriAB; no detectable line broadenings and shifts ( 800 kms-1) in the spectrum of σ Ori AB together with aremarkable low value of the O VII forbidden to intercombination lineratio and unusually high coronal abundances of CNO elements.
| Discovery of X-ray flaring on the magnetic Bp-star σ Ori E
We report the detection of an X-ray flare on the Bp star σ Ori Ewith the ROSAT high resolution imager (HRI). The flare is shown to havelikely occurred on the early-type star, rather than on an hypothesizedlate-type companion. We derive flare parameters such as total energyrelease, coarse estimates of size and density, and also presentarguments for a magnetic origin of the flare. We place our observationsin the context of a magnetic character of Bp-type stars and speculate ona common physical basis and connection between Bp and Be stars.
| 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.
| Statistics of the Instability Strip of β Cephei Stars
We present a study of the β Cephei instability strip based on asample of 49 stars of this type. After deriving their effectivetemperatures and luminosities from their observed (B-V), (U-B) colorsand parallaxes we find their positions in the HR diagram to be mostlyconfined to the main sequence, and their masses to lie between 7Mȯ and 30 Mȯ. Their distribution on theHR diagram matches well with our previous theoretical instability stripwhich has an upper bound in the luminosity and rather tight boundariesin the effective temperature.
| 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.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| 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.
| 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.
| Dynamical Simulations of Magnetically Channeled Line-driven Stellar Winds. I. Isothermal, Nonrotating, Radially Driven Flow
We present numerical magnetohydrodynamic (MHD) simulations of the effectof stellar dipole magnetic fields on line-driven wind outflows from hot,luminous stars. Unlike previous fixed-field analyses, the simulationshere take full account of the dynamical competition between field andflow and thus apply to a full range of magnetic field strength andwithin both closed and open magnetic topologies. A key result is thatthe overall degree to which the wind is influenced by the field dependslargely on a single, dimensionless ``wind magnetic confinementparameter'' η*(=B2eqR2*/Mv&infy;), which characterizes the ratio between magneticfield energy density and kinetic energy density of the wind. For weakconfinement, η*<=1, the field is fully opened by thewind outflow, but nonetheless, for confinements as small asη*=1/10 it can have a significant back-influence inenhancing the density and reducing the flow speed near the magneticequator. For stronger confinement, η*>1, the magneticfield remains closed over a limited range of latitude and height aboutthe equatorial surface, but eventually is opened into a nearly radialconfiguration at large radii. Within closed loops, the flow is channeledtoward loop tops into shock collisions that are strong enough to producehard X-rays, with the stagnated material then pulled by gravity backonto the star in quite complex and variable inflow patterns. Within openfield flow, the equatorial channeling leads to oblique shocks that areagain strong enough to produce X-rays and also lead to a thin, dense,slowly outflowing ``disk'' at the magnetic equator. The polar flow ischaracterized by a faster-than-radial expansion that is more gradualthan anticipated in previous one-dimensional flow tube analyses andleads to a much more modest increase in terminal speed (less than 30%),consistent with observational constraints. Overall, the results hereprovide a dynamical groundwork for interpreting many types ofobservations-e.g., UV line profile variability, redshifted absorption oremission features, enhanced density-squared emission, and X-rayemission-that might be associated with perturbation of hot-star winds bysurface magnetic fields.
| 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.
| Non-radial pulsation, rotation and outburst in the Be star omega Orionis from the MuSiCoS 1998 campaign
omega Ori (HD 37490, HR1934) is a Be star known to have presented variations. Inorder to investigate the nature and origin of its short-term andmid-term variability, a study is performed of several spectral lines(Hα , Hdelta , ion {He}i 4471, 4713, 4921, 5876, 6678, ion {C}{ii}4267, 6578, 6583, ion {Mg}{ii} 4481, ion {Si}{iii} 4553 and ion {Si}{ii}6347), based on 249 high signal-to-noise high-resolution spectra takenwith 8 telescopes over 22 consecutive nights during the MuSiCoS (MultiSIte COntinuous Spectroscopy) campaign in November-December 1998. Thestellar parameters are revisited and the projected rotational velocity(vsin i = 179 km s-1) is redetermined using several methods.With the MuSiCoS 98 dataset, a time series analysis of line-profilevariations (LPVs) is performed using the Restricted Local Cleanest (RLC)algorithm and a least squares method. The behaviour of the velocity ofthe centroid of the lines, the equivalent widths and the apparent vsinifor several lines, as well as Violet and Red components of photosphericlines affected by emission (red ion {He}i lines, ion {Si}{ii} 6347, ion{C}{ii} 6578, 6583) are analyzed. The non-radial pulsation (NRP) modelis examined using phase diagrams and the Fourier-Doppler Imaging (FDI)method. The LPVs are consistent with a NRP mode with l = 2 or 3, |m| = 2with frequency 1.03 c d-1. It is shown that an emission lineoutburst occurred in the middle of the campaign. Two scenarios areproposed to explain the behaviour of a dense cloud, temporarily orbitingaround the star with a frequency 0.46 c d-1, in relation tothe outburst. Based on observations taken during the MuSiCoS 98 campaignat OHP (France), La Silla (ESO, Chile, ID 62.H-0270), Mount Stromlo(Australia), Xinglong Station (China), Kitt Peak (USA), MCT/LNA (Brazil)and INT (Isaac Newton Group, La Palma Island).
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| 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.
| Line Forces in Keplerian Circumstellar Disks and Precession of Nearly Circular Orbits
We examine the effects of optically thick line forces on orbitingcircumstellar disks, such as occur around Be stars. For radiallystreaming radiation (e.g., as from a point source), line forces areeffective only if there is a strong radial velocity gradient, as occurs,for example, in a line-driven stellar wind. However, we emphasize herethat, within an orbiting disk, the radial shear of the azimuthalvelocity leads to strong line-of-sight velocity gradients alongnonradial directions. As such, we show that, in the proximity of astellar surface extending over a substantial cone angle, the nonradialcomponents of stellar radiation can impart a significant line force tosuch a disk, even in the case of purely circular orbits with no radialvelocity. Given the highly supersonic nature of orbital velocityvariations, we use the Sobolev approximation for the line transfer,extending to the disk case the standard CAK formalism developed forline-driven winds. We delineate the parameter regimes for whichradiative forces might alter disk properties; but even when radiativeforces are small, we analytically quantify higher-order effects in thelinear limit, including the precession of weakly elliptical orbits. Wefind that optically thick line forces, both radial and azimuthal, canhave observable implications for the dynamics of disks around Be stars,including the generation of either prograde or retrograde precession inslightly eccentric orbits. However, our analysis here suggests a netretrograde effect, in apparent contradiction with observed long-termvariations of violet/red line profile asymmetries from Be stars, whichare generally thought to result from prograde propagation of a one-arm,disk-oscillation mode. We also conclude that radiative forces may alterthe dynamical properties at the surface of the disk where disk windsoriginate, and in the outer regions far from the star, and may even makelow-density disks vulnerable to being blown off completely.
| 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.
| High and intermediate-resolution spectroscopy of Be stars 4481 lines
We present an atlas of Hγ , He i lambda 4471 and Mg ii lambda 4481line profiles obtained in a 10 year observation period of 116 Be stars,which enabled many of them to be observed at quite different emissionepochs. From the best fit of the observed He i lambda 4471 line profileswith non-LTE, uniform (Teff,log g) and full limb-darkenedmodel line profiles, we determined the V sin i of the program stars. Toaccount, to some degree, for the line formation peculiarities related tothe rapid rotation-induced non-uniform distributions of temperature andgravity on the stellar surface, the fit was achieved by considering(Teff,log g) as free parameters. This method produced V sin iestimations that correlate with the rotational velocities determined bySlettebak (1982) within a dispersion sigma <= 30 km s-1and without any systematic deviation. They can be considered as given inthe new Slettebak's et al. (1975) system. Only 13 program stars havediscrepant V sin i values. In some objects, this discrepancy could beattributed to binary effects. Using the newly determined V sin iparameters, we found that the ratio of true rotational velocitiesV/Vc of the program Be stars has a very low dispersion aroundthe mean value. Assuming then that all the stars are rigid rotators withthe same ratio V(/lineω)/Vc, we looked for the value of/line ω that better represents the distribution of V sini/Vc for randomly oriented rotational axes. We obtained/lineω = 0.795. This value enabled us to determine the probableinclination angle of the stellar rotation axis of the program stars. Inthe observed line profiles of Hγ , He i lambda 4471, Mg ii lambda4481 and Fe ii lambda 4351 we measured several parameters related to theabsorption and/or emission components, such as: equivalent width,residual emission and/or absorption intensity, FWHM, emission peakseparations, etc. The parameters related to the Hγ line emissionprofiles were used to investigate the structure of the nearbyenvironment of the central star. From the characteristics of thecorrelations between these quantities and the inferred inclinationangle, we concluded that in most of cases the Hγ line emissionforming regions may not be strongly flattened. Using a simplerepresentation of the radiation flux emitted by the star+envelopesystem, we derived first order estimates of physical parameterscharacterizing the Hγ line emission formation region. Thus, weobtained that the total extent of the Hγ region is Rf=~ 2.5 +/- 1.0 R* and that the density distribution in theselayers can be mimicked with a power law rho ~ R-alpha , wherealpha =2.5+2.2-0.6. The same approach enabled usto estimate the optical depth of the Hγ line emission formationregion. From its dependence with the aspect angle, we concluded thatthese regions are caracterized by a modest flattening and that the rho(equator)/rho (pole) density contrast of the circumstellar envelope nearthe star should be two orders of magnitude lower than predicted bymodels based on a priori disc-shaped circumstellar envelopes. We foundthat the separation between the emission peaks, Deltap, andthe full width at half maximum, Delta 1/2, of the Hγline emission are not only sensitive to kinematic effects, but to lineoptical depth as well. This finding agrees with previous theoreticalpredictions and confirms that Huang's (1972) relation overestimates theextent of the Hγ line emission formation region. Data obtained atCASLEO operated under agreement between the CONICET and the NationalUniversities of La Plata, Córdoba and San Juan, Argentina, at ESOLa Silla, Chile and at OHP, France.}\fnmsep\thanks{Tables 2 to 7 andFigs. 1 and 2 are only available in full in electronic form at 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/378/861}} \subtitle{Anatlas of Hγ , He {\fontsize {10pt}{12pt}\selectfont I} 4471 and Mg{\fontsize {10pt}{12pt}\selectfont II
| Statistical analysis of intrinsic polarization, IR excess and projected rotational velocity distributions of classical Be stars
We present the results of statistical analyses of a sample of 627 Bestars. The parameters of intrinsic polarization (p*),projected rotational velocity (v sin i), and near IR excesses have beeninvestigated. The values of p* have been estimated for a muchlarger and more representative sample of Be stars (~490 objects) thanpreviously. We have confirmed that most Be stars of early spectral typehave statistically larger values of polarization and IR excesses incomparison with the late spectral type stars. It is found that thedistributions of p* diverge considerably for the differentspectral subgroups. In contrast to late spectral types (B5-B9.5), thedistribution of p* for B0-B2 stars does not peak at the valuep*=0%. Statistically significant differences in the meanprojected rotational velocities (/line{vsin i}) are found for differentspectral subgroups of Be stars in the sense that late spectral typestars (V luminosity class) generally rotate faster than early types, inagreement with previously published results. This behaviour is, however,not obvious for the III-IV luminosity class stars. Nevertheless, thecalculated values of the ratio vt/vc of the truerotational velocity, vt, to the critical velocity forbreak-up, vc, is larger for late spectral type stars of allluminosity classes. Thus, late spectral type stars appear to rotatecloser to their break-up rotational velocity. The distribution of nearIR excesses for early spectral subgroups is bi-modal, the position ofthe second peak displaying a maximum value E(V-L)~ 1 . m 3for O-B1.5 stars, decreasing to E(V-L)~0. m8 for intermediatespectral types (B3-B5). It is shown that bi-modality disappears for latespectral types (B6-B9.5). No correlations were found betweenp* and near IR excesses and between E(V-L) and vsin i for thedifferent subgroups of Be stars. In contrast to near IR excesses, arelation between p* and far IR excesses at 12 mu m is clearlyseen. A clear relation between p* and vsin i (as well asbetween p* and /line{vsin i}/vc) is found by thefact that plots of these parameters are bounded by a ``triangular"distribution of p*: vsin i, with a decrease of p*towards very small and very large vsin i (and /line{vsini}/vc) values. The latter behaviour can be understood in thecontext of a larger oblateness of circumstellar disks for the stars witha rapid rotation. From the analysis of correlations between differentobservational parameters we conclude that circumstellar envelopes forthe majority of Be stars are optically thin disks with the range of thehalf-opening angle of 10degr
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Surface trapping and leakage of low-frequency g modes in rotating early-type stars - II. Global analysis
A global analysis of the surface trapping of low-frequency non-radial gmodes in rotating early-type stars is undertaken within the Cowling,adiabatic and traditional approximations. The dimensionless pulsationequations governing these modes are reviewed, and the boundaryconditions necessary for solution of the equations are considered; inparticular, an outer mechanical boundary condition, which does notenforce complete wave trapping at the stellar surface, is derived anddiscussed in detail. The pulsation equations are solved for a7-Msolar model star over a range of rotation rates, using anumerical approach. The results of the calculations confirm the findingsof the preceding paper in the series: modes with eigenfrequencies belowa cut-off cannot be fully trapped within the star, and exhibit leakagein the form of outwardly propagating waves at the surface. The dampingrates resulting from leakage are calculated for such `virtual' modes,and found to be appreciably larger than typical growth rates associatedwith opacity-driven pulsation. Furthermore, it is demonstrated that thesurface perturbations generated by virtual modes are significantlychanged from those caused by fully trapped modes; the latter resultsuggests differences in the line-profile variations exhibited by thesetwo types of mode. The findings are discussed in the context of the 53Per, SPB and pulsating Be classes of variable star. Whilst wave leakagewill probably not occur for overstable g modes in the 53 Per and slowlyrotating SPB stars, the adoption of the new outer mechanical boundarycondition may still affect the pulsational stability of these systems.Wave leakage for overstable modes remains a possibility in Be stars andthe more rapidly rotating SPB stars.
| Line formation in Be star circumstellar disks Shear broadening, shell absorption, stellar obscuration and rotational parameter
We improve the theory of Horne & Marsh on shear broadening inaccretion disks of CVs and adapt it to Be star circumstellar disks.Stellar obscuration and shell absorption are taken into account indetail. It is shown that shell absorption is already present in thoseemission lines where the central depression does not drop below thestellar continuum. The model profiles are fitted to observed symmetricHα net emission lines with low equivalent width. The derived diskradii range from Rd = 5.3 R_* to Rd = 18 R_* andthe surface emissivity varies as ~ R-m with 1.6 < m <3.5. The comparison between model profiles of rotational parameterj>(1)/(2) with the optically thick Hα profile of HR 5440 rulesout the range of j>(1)/(2). This can be understood by the lack ofvelocity shear in the outer disk regions. We conclude that Keplerianrotation (j=(1)/(2)) is a valid approximation. Based on observationscollected at the German-Spanish Astronomical Center (DSAZ), Calar Alto,operated by the Max-Plank-Institut für Astronomie Heidelbergjointly with the Spanish National Commission for Astronomy. Based onobservations collected at the Observatoire de Haute-Provence (OHP),CNRS, France.
| The nature of RX J0052.1-7319
The nature of the X-ray source RX J0052.1-7319 is discussed fromobservational data obtained from ROSAT observations performed in 1995and 1996. An accurate position is derived from ROSAT HRI observations ofthe source performed in 1995. The 6'' error circle containstwo OGLE microlensing optical variables of which one has previously beenidentified with a 14.5 mag Be-type star in the Small Magellanic Cloud.During the October 1996 observation RX J0052.1-7319 was found to beextremely bright (with a count rate of ~ 1.1+/-0.1\ s-1) and15.3+/-0.1 second X-ray pulsations have been discovered during thisobservation. This would indicate for a high-mass X-ray binary nature ofthe source. During the 1995 observation the X-ray source detected at theposition of RX J0052.1-7319 was a factor ~ 200 fainter. Thecorresponding luminosity has changed from ~ 5.2*E37\ erg\s-1 to ~ 2.6*E35\ erg\ s-1 assuming SMCmembership of the source. It is unclear whether the so-far unidentifiedsecond optical variable contributes to the X-ray flux of the source.
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| Simultaneous photometry and spectroscopy of the Be star 28 (omega) CMa - II. Line profile modelling
We analyse a series of high-resolution line profiles of He I 6678 in theperiodic Be star 28 CMa. A simple zeroth-order approximation to theeigenfunctions is used to model the line profile variations in terms ofnon-radial pulsation. In addition, we use the best available theory forthe eigenfunction in a rotating star. In all cases the calculated fitsto the observed profiles are poor. We conclude that non-radial pulsationis almost certainly not responsible for the line profile variations inthis star. In an attempt to understand the very large line profilechanges, but negligible light variation, we consider a `patch' model.This consists of a circular area on the photosphere having the sametemperature, but a different intrinsic line width. A simple model ofthis kind produces a good fit to the line profile variations and to thephotometry. We conclude that modulation of the profile by a patch on, orclose to, the photosphere may offer a plausible starting point forunderstanding the periodic variations in some Be stars.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Time-resolved spectroscopyHiifill of the peculiar Hα variable Be star HD 76534
We present time-resolved spectroscopy of the Be star HD 76534, which wasobserved to have an Hα outburst in 1995, when the line went fromphotospheric absorption to emission at a level of more than two timesthe continuum within 2.5 hours. To investigate the short-term behaviourof the spectrum of HD 76534 we have obtained 30 spectra within two hoursreal-time and searched for variations in the spectrum. Within the levelsof statistical significance, no variability was found. Rather thanperiodic on short time scales, the Hα behaviour seems to becommonly episodic on longer (> 1 year) time scales, as an assessmentof the existing data on the Hα line and the Hipparcos photometrysuggests. HD 76534 underwent only 1 photometric outburst in the 3 yearspan that the star was monitored by the Hipparcos satellite.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Eridano |
| Ascensione retta: | 05h09m08.80s |
| Declinazione: | -08°45'15.0" |
| Magnitudine apparente: | 4.27 |
| Distanza: | 537.634 parsec |
| Moto proprio RA: | 0.4 |
| Moto proprio Dec: | -2.6 |
| B-T magnitude: | 4.002 |
| V-T magnitude: | 4.211 |
Cataloghi e designazioni:
|
INHABITED SKY
