Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
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.
| 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.
| First far-UV observations of KQ Puppis with FUSE
We report the first far-ultraviolet spectrum of the emission linespectroscopic binary KQ Pup (M2Iab+B0Ve), obtained with FUSE shortlyafter conjunction at orbital phase Phi =0.13. The spectrum presents asharp flux cutoff at 1040 Å; longwards, it is dominated by a largeamount of resonant and low excitation transitions of neutral and singlyionized species, probably mostly of circumsystem origin, and by theLyman absorption bands of H2, whose strength corresponds toan interstellar H2 column density of ~ 2x 1020cm-2, indicating a fraction of molecular hydrogen2N(H2)/[N(HI)+2N(H2)] of ~ 0.4. AnN(HI)/EB-V ratio of 4.3x 1021 is derived from theIUE spectra. The long-term UV monitoring of KQ Pup with IUE, HST andFUSE reveals a large decrease of the far-UV flux since orbital phase0.82 and occupying more than 0.3 of the orbital period. This ``shell''episode is attributed to line absorption and to Lyalpha Rayleighscattering due to an extended dense cool envelope in the line of sightof the B star.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985. The HST spectrum used in thispaper was obtained from the Multimission Archive at the Space TelescopeScience Institute (MAST).
| Fuse: Distance Determination and ISM Towards V767 CEN
Not Available
| To Be or not to Be and a 50-cm post-mortem eulogy
Not Available
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| FUSE observations towards the pole-on Be star HR 5223
New spectra have been obtained for the pole-on Be star HR 5223 (HD120991) using the Far Ultraviolet Satellite Explorer (FUSE). We give acomplete description of the far-UV spectral range (920 to 1180 Å).The spectra are affected by strong blends with interstellar lines andmolecular bands that also significantly lower the energy distribution ofthe star. We produce a synthetic spectrum of the interstellar medium(ISM) to determine the column densities of several elements(H2, H I, N I, O I ...) seen towards HR 5223 and todisentangle the components due to the ISM, the photosphere and/or to thecircumstellar envelope. The line identification list is available at theCDS. Using the obtained column densities, we determine the reddening ofthe star due to the ISM only and locate the star relative to the nearbyIS clouds. The fit of the dereddened UV flux distribution with modelsthat account for the gravitational darkening due to the stellar fastrotation allowed us to estimate the stellar fundamental parameters(Teff} = 22 000 K; log g = 3.7) and its distance (d = 834+/-20 pc). The distance obtained, which has to be considered as the mostaccurate available at the moment, is in agreement with thecharacteristics of the ISM matter distribution that affects the observedspectrum of the star and with the detecting limits of the HIPPARCOSsatellite. Table \ref{tab:lilvcen} is only available in electronic format http://www.edpsciences.org
| 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).
| 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.
| 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
| On the Variability of O4-B5 Luminosity Class III-V Stars
We investigate the Hipparcos Satellite photometry of O4-B5 luminosityclass III-V stars. Some for which further study is desirable areidentified. These stars in general are more variable than cooler stars
| Long-term visual spectrophotometric behaviour of Be stars. II. Correlations with fundamental stellar parameters and interpretation
The long-term visual spectrophotometric (SPh) behaviour of Be stars as afunction of fundamental stellar parameters is studied. Some previous SPhresults obtained by other authors are confirmed. Moreover, a tendencyfor temperature and aspect angle dependency of SPh variations is found.From the characteristics of visual SPh behaviour in Be stars we deriveconstraints for models of regions in circumstellar envelopes where thevisual continuum spectrum is formed: (i) The SPh emission and absorptionphases should not imply preferential aspect angles, as they can bothappear whatever the stellar inclination. This phenomenon cannot bealways accounted for by strongly flattened circumstellar envelopes; (ii)Radii of the visible continuum forming regions cannot be larger than afew R_*; (iii) Electron densities of these regions should not exceed N_e~ 10(13) cm(-3) ; (iv) Electron temperature of circumstellar layersproducing the SPh emission phases compare with the stellar Balmercontinuum radiation temperature and it is much lower in those producingthe SPh absorption phases. Three scenarios were studied to produce theobserved characteristics of emissions in the V magnitude and in thesecond component of Balmer discontinuity (Delta D) during the SPhemission phases: (a) expansion of a massive circumstellar shell thatpreserves circumstellar envelope flattening; (b) expansion of acircumstellar shell which increases the global flattening, so that adisc-like structure is formed; (c) continuous mass ejection thatincreases the storage of mass in a constant volume with a givenflattening. Mechanisms (a) and (b) produce a double valued (DeltaV,Delta D) SPh relation, while (c) produces a single valued relation.Only mechanisms (a) and (c) can easily produce the observed amounts ofemission Delta V and Delta D without violating the modeling constraintsfrom (i) to (iv) imposed by observations. The model SPh slopes of (DeltaV,Delta D) show the global sin i and T_eff observed dependencies. Thescenarios used to describe the double valued (Delta V,Delta D) suggestanother possible way how to build up circumstellar envelopes around Bestars.
| Long-term visual spectrophotometric behaviour of Be stars
The long-term spectrophotometric variations of 49 Be stars are studiedusing the U and V magnitudes of the UBV system, the total Balmerdiscontinuity D and the visible gradient Phi _rb. BCD spectrophotometricand photometric data in five different photometric systems, obtained inmost cases since 1950 and reduced to the BCD system, were used. The(U,D), (V,D), (Phi _rb,D) and (Phi _rb,V) correlations obtained differfrom star to star and they can be single or double-valued. They differclearly for Be phases or Be-shell phases. Be stars with small Vsin ishowing the ``spectrophotometric shell behaviour'': D > D_*, werefound. This finding implies either that strongly flattened models ofcircumstellar envelopes are in doubt for these stars, or that not all Bestars are rapid rotators. Comparison of observed variations with thosepredicted for model Be stars with spherical circumstellar envelopes ofvariable densities and dimensions implies that spectrophotometricpatterns of Be phases are due to circumstellar envelopes in low opacityregimes, while those of spectrophotometric shell phases are due tocircumstellar envelopes in high opacity regimes. In a given star, theenvelope regions responsible for the observed variations of D and Phi_rbin spectrophotometric shell phases seem to be smaller and denser thanthose producing the observed variations of these parameters inspectrophotometric Be phases. The high positive RV found in strong shellphases might favor the formation of compact circumstellar layers nearthe star. Figure 6 is only available in electronic form at CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Investigation of the variability of bright Be stars using HIPPARCOS photometry
The high accuracy and the homogeneity of Hipparcos data for bright starshave allowed us to quantify the degree of variability of Be stars. Thisdegree has been found to be highly dependent on the temperature of thestar. Rapid variability is the main feature of the 86% of early Be andless than 20% of late Be stars taking into account the limit ofdetection considered. In addition to Be stars reported in the Hipparcoscatalogue (ESA 1997) as short-period variables, we have been able toenlarge the number of detections as well as to confirm periodspreviously determined. Be stars that show larger amplitude rapidvariations are proposed as candidates for a search of multiperiodicityi.e. as non-radial pulsators. We have also searched for the presence ofoutbursts and fading events in the Hipparcos data. Outbursts have beenfrequently and preferentially detected in early Be stars with rather lowto moderate v sini while fading events seem to be more conspicuous instars with higher v sini. Mid-term and long-term variations have alsobeen investigated. Several stars have shown some evidence of temporaryquasi-periodic oscillations ranging between 10 and 200 days. Finallyinformation concerning long-term variations is reported. Cycles shorterthan or equal to the Hipparcos mission have mainly been detected instars earlier than B6. Long-term time scales of late Be stars areconfirmed to be longer by far. Tables 1 and 2 are only available inelectronic form at CDS via ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| The spur-like radio feature in Centaurus
We present a study of the radio emission of the Centaurus region atgalactic latitudes 2(deg) <= b <= 18(deg) , using pre-existing408MHz continuum data and results of new 1.42GHz continuum and neutralhydrogen line observations. It is shown that the extensive spur-likeradio feature visible in this region has a non-thermal nature and isactually formed by several previously unidentified individual sources.Five new supernova remnant (SNR) candidates have been detected atrelatively high latitudes. Three of them, as well as two formercandidates reported by Duncan et al. (1995), were confirmed through thecomputation of their spectral indices. One of the new SNRs, G315.4+12.9,might be associated with the open cluster NGC 5460.
| A spectral analysis of HDE 269445 from optical and infrared observations.
We present new, near-IR spectroscopic observations of HDE 269445, whichwe combine with published HST and IUE ultraviolet data and optical highresolution spectra. We discuss the spectral morphology of the star fromUV to near-IR wavelengths, concentrating on profile variations in the UVand optical H and He lines. From a spectroscopic analysis with non-LTEmodel atmospheres, we derive for HDE 269445: T_*_=34000K,R_*_=43Rsun_, log˙(M)(Msun_/yr)=-4.5 andHe/H=0.4/0.6 by number. These parameters are in good agreement withthose previously derived by Pasquali et al. (1997ApJ...478..340P) whoused only ultraviolet and optical lines. Therefore, our analysisconfirms that a combination of optical and infrared lines can befruitfully used to determine stellar and wind properties whenultraviolet data are not available. Our model calculations only fit thebroad component underneath a strong core emission. We interpret thediscrepancy between the observed and the model line profiles as theeffect of a non-spherical wind which has a hot, fast polar component anda cold, slow equatorial component. The time-variability detected in someH and He lines indicates that the wind geometry may be variable and thestar has undergone changes in T_eff_ and ˙(M). These properties aresimilar to those observed for the galactic LBV AG Carinae by Leithereret al. (1994ApJ...428..292L) and we may suspect that HDE 269445 is alsoa Luminous Blue Variable. This could explain why HDE 269445 has defiedeasy spectral classification, and, although originally classified as aOfpe/WN9, it has always represented a peculiarity for this spectraltype.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| Atlas of high-resolution emission and shell lines in Be stars. Line profiles and short-term variability.
We present an atlas of high-S/N, high-resolution ({DELTA}v=6km/s) dataof Be star emission and shell profiles. We have collected profiles ofHα and of Fe II, mostly of the λ5317 transition. Theselines have been selected to provide measures for the overall emissionstrength and for the velocity field in these disks. We have collecteddata for 77 southern and equatorial programme stars, covering the period1982-1993. This is the most comprehensive overview of profile shapes inBe disks. We propose a three-dimensional scheme in which most observedprofiles can be classified. The parameters are i) inclination, ii)optical depth, and iii) the pattern of the velocity field. A search forshort-term variability (timescales between five days and a few minutes)in six stars ended with negative result. Shortest observed timescale forvariability is a few days for well-developed disks in binary systems (HR1910, HR 2142).
| On the structure of Be star disks.
We investigate the geometrical structure of the emitting part ofcircumstellar envelopes around Be stars from an empirical point of view.We use new high-resolution, high-S/N spectroscopic data of the FeIIλ5317 and some other faint FeII emission lines in 27 Be starsshowing symmetrical emission lines (class 1). We find a clearcorrelation between its total width (measuring the maximum velocities ofcircumstellar matter) and the stellar rotational velocity. Thiscorrelation means that a typical Be envelope (or, more precisely, thatpart of it which is visible in optical emission lines) is anaxisymmetric, rotationally supported disk. For empirical investigationof the vertical structure, we use the occurrence of shell lines. Wedefine, as shell criterion based on FeII lines, a Be shell star as onewith FeII central intensity F_cd_/F_*_(FeII)<1. Using this forcalibrating an appropriate parameter for the much more frequentlyobserved Hα line, we find that shell stars are those withF_p_/F_cd_(Hα)>=1.5 where F_p_ is the mean peak intensity atHα. In a sample of 114 programme stars, we find a shell starfraction of 22.8%. This number is readily transformed into a halfopening angle of Be star disks, φ=13deg. We furthermore show thatBe disks must be thin at the inner edge, and may become fairly thick atthe outer rim. This, together with the small value of φ, isevidence for a conical or concave shape, the latter typical of ahydrostatically balanced disk. Finally we provide evidence that thefamous "shell-Be" phase transitions can naturally occur in such disks asa geometrical effect if they are seen under inclination i=~70deg and iftheir outer radius is variable with time.
| Kuhle Gasringe um heisse veranderliche Sterne. Teil 2 : Die Variationen der Be-Sterne und die Erklarung der Ringbildung.
Not Available
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| V/R variability and global oscillations in Be star disks.
We present high-resolution Hα and Fe II spectroscopic data on theemission line behaviour of five selected Be stars (HR 3237, δ Cen,48 Lib, α Ara, 66 Oph), four of them showing cyclic V/R lineprofile variability in the period 1982-1993. With the exception of theshell star 48 Lib, these stars temporarily also show a characteristicasymmetric (steeple-type) profile shape in their Fe II λ5317emission line (which we have chosen as characteristical optically thinemission line). These profiles are very similar in all three stars. Incontrast, the star α Ara has been selected because it exhibitssymmetric double-peak profiles for the whole investigated period.Steeple-type profiles are cyclically variable in the sense that theyshow asymmetric line profiles for almost half a cycle, then asymmetryinversion in the second half-cycle, and re-appear with the same shapeafter a full cycle which lasts about 8-10 years. In one star, 66 Oph, wereport the detection of V/R variability onset in the course of 1988,i.e. a sudden transition from a non-varying, symmetric to a variable,asymmetric line profile shape. We demonstrate that this characteristicsteeple-type profile shape and the long-term V/R variability patternknown since long are two manifestations of the same physical phenomenon.We provide arguments in favour of the global disk oscillation scenarioas causing a large-scale perturbation of the quasi-Kepleriancircumstellar disk which precesses under the influence of thenon-spherical gravitational potential of the central star. We finallyspeculate that the onset of such distortion, as observed in 66 Oph,might be related to some viscous instability in the circumstellar disk.
| Near Infrared Variability of Be-Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.269.1123D&db_key=AST
| Ultraviolet spectral classification and stellar winds in a sample of Be and standard stars
Equivalent widths of 16 lines of C I, C II, C III, C IV, Si II, Si III,Si IV, Al II, Al III, Fe II, and Fe III, plus centriod and edgevelocities of the Si IV and C IV lines, were measured in InternationalUltraviolet Explorer spectra of 39 Ble-B8e and 18 B1-B8 standardnon-emission-line stars. These suggest the following: (1) Certain lineratios of Si II/III, C II/III, Al II/III, and Fe II/III are verysensitive to spectral type and represent excellent UV criteria forspectral classification. (2) UV line strengths and line ratios show thatthere are no significant differences between the photospheric linespectra of Be and normal, non-emission-line stars of corresponding type.(3) The Si IV and C IV wind lines in the Be stars are correlated withboth spectral type and luminosity class in the sense that the hotteststars have the strongest lines with the largest centroid and edgevelocities, and the giants and subgiants have stronger lines than themain-sequence stars. (4) The Si IV wind lines persist to spectral typeB8 in both the Be stars and the standard stars but are stronger in theBe stars than in the standards for the earlier types. (5) The C IV windlines persist to spectral type B8 in the Be stars, but only to B3 in thestandard stars, and are stronger in the Be stars than in the standardsat all spectral types. (6) The equivalent widths of the Si IV and C IVwind lines are only very weakly correlated with v sin i, if at all, buta threshold in v sin i near 150 kn/sec exists, below which no largeequivalent widths of Si IV or C IV may be seen. Assuming that the Bestars are all rapid rotators, such a correlation is essentially acorrelation with i and suggests that the winds from Be stars arisepreferentially from the equatorial regions. (7) Shell stars have weakerC IV absorption and smaller centroid and edge velocities than other Bestars, suggesting that they have weaker winds. Since there isconsiderable evidence that these are stars with cool, low-velocity diskswhich are being viewed edge-on or nearly edge-on, the winds may beinhibited and modified by the denser material in the equatorial regions.(8) Mg II emission is detected in about half of the program Be starswith long-wavelength IUE spectra, and seems not to be correlated withspectral type, v sin i, or strength of the Si IV wind lines. Since theMg II emission presumably originates in the cool, low-velocity envelope,and since Mg II emission also correlates with hydrogen Balmer emissionin the Be stars, this sugests that there is no strong physicalrelationship between the stellar winds and the cool disk. (9) The Beshell stars have stronger resonance lines of Si II, C I, C II, Al II, FeII, presumably formed in the cool shells, than the other Be stars andthe normal, non-emission-line stars of the same spectral types,consistent with the strong lines arising from metastable levels in theoptical spectra of these stars.
| On the rotation properties of Be stars and their envelopes.
We present the results of low resolution spectroscopy (4A) obtained atCTIO for a sample of 42 Be, 4 B and 3 O stars covering theλ3700-7000A spectral range. Equivalent widths of the Balmer lineswere measured up to H_10_. We also give the V/R state of the Hαemission line. In addition, the peak separation and the width at thebase of the Hβ and Hγ emission lines are tabulated for atotal of 243 medium resolution (1A) spectra of 51 southern Be starstaken at the Manuel Foster Observatory, Chile. We analyzed our Hαequivalent widths (Wα) together with published data for a total of122 Be and 2 Oe stars. An upper limit of Wα increasing with theprojected rotational velocity vsini is shown by the data. The Wαvalues depend strongly on spectral type, being lower for later-type Bestars. This could indicate that the maximum strength of emission dependsmainly on the spectral type and rotation velocity. The relation betweenWα(max) and vsini, together with the observed excess in number oflow vsini Be stars and the reported anti-correlation between photometricperiod and vsini give evidence for a considerable range of the truerotation velocities of Be stars: definitely there are intrinsically slowrotators among them. However, our results could also be interpreted interms of anisotropic Hα emission coming from the disk. Theanalysis of the net equivalent widths and peak separations of Hαdouble emission line profiles strongly favor a disk type emittingenvelope with a r^-j^ rotation law with j=1.4+/-0.2 which do not dependon the spectral subtype. This result is interpreted as evidence ofradial motions in the envelopes of Be stars. The mean radial extensionsof the regions which emit Balmer lines were determined to range between30% (H_10_) and 60% (Hβ) of the Hα emitting envelope in thecase j=1. The mean electron density within the envelope has been foundto vary only by a factor of 4. We compare our results with earlierinvestigations.
| Near-IR excess of Be stars.
The near-IR excess emission of 144 Be stars is derived from visual andnear-IR observations. The quasi-simultaneous nature of the observationsprovide colour excesses that are independent of temporal variations.Colour-colour diagrams are used to identify stars with excess coloursmarkedly different from the bulk of the sample stars. The near-IRemission of four stars that have markedly different colours isattributed to the presence of a binary companion or thermal dustemission. The percentage of stars with a significant excess increaseswith wavelength. The excess emission increases with wavelength and thelargest excesses occur in stars of earlier spectral type. The near-IRexcess colours are examined and compared to theoretical excess colourscalculated from a simple bremsstrahlung emitting disc model with aradial density distribution of the form ρ{prop.to}r^-β^. Theeffect of model parameters on the excess colours is discussed. Theobserved excesses for the bulk of the stars are well fit bycircumstellar discs with radii greater than ~10R_*_ and with a densityindex β, in the range 2.0-5.0. This is very similar to the range ofvalues previously determined by Waters et al. from IRAS far-IRobservations. A small number of stars cannot be reconciled with discswith a constant density index out to 10R_*_. It is argued that thecircumstellar plasma around these stars has a change in structure at~2-10R_*_. The possibilities of disc truncation or a change in thedensity index as the cause of the structure change are discussed.
| Line formation in Be star envelopes. 1: Inhomogeneous density distributions
Based on 3D radiative line transfer calculations for inhomogeneous Bestar circumstellar envelopes, theoretical H alpha emission line profilesare presented. The results show that the so-called winebottle-typeemission line profiles and shell profiles can be generated by the sameoptically thick Keplerian disk viewed at different inclination angles i.The H alpha variability of HR 2825 in the eighties can be explained byline splitting over-crossing due to non-coherent scattering broadeningcaused b a density variation in the envelope. Huang's law isre-investigated; its restricted application for winebottle-type andshell line profiles is discussed. Finally the calculated emission lineprofiles are compared with observed H alpha line profiles.
| ROSAT X-ray survey of an area 10 degrees square around the active radio galaxy Centaurus A
Diffuse X-ray material extends in an irregular, broad filament for morethan 5 degrees north east of the giant radio elliptical, NGC 5128 (CenA). In the opposite direction the X-ray extension is clearly traced forabout 2 degrees, making an overall line of X-rays of greater than 7degon the sky originating from Cen A. The direction of this line coincidesalmost exactly with the direction of the X-ray jet, and its associatedradio jet, in the interior of Cen A. At a slightly different angle,within the accuracy of measurement along the X-ray counter jet, isencountered the radio and X-ray galaxy NGC 5090. Its presence in theX-ray ejection from Cen A supports the precedent from other associationsthat NGC 5090 is physically associated with Cen A. In the hardest X-raysthere is some indication that the radio lobes from Cen A are edgebrightened. Positions and known identifications of sources in the fieldare given. Some preliminary estimates of the properties of thisextensive X-ray material which has apparently been ejected from thisactive galaxy are given.
| A catalogue of radii of Be star line emitting regions
A bibliographic catalog of the radii of the line-emitting regions aroundBe stars is presented. The table also provides the separation of theemission peaks, the wavelength of the line used, observing date, and theV sin i value given by the author.
| A homogeneous catalog of new UBV and H-beta photometry of B- and A-type stars in and around the Scorpius-Centaurus OB association
B- and A-type stars in and near the Sco-Cen OB association areinvestigated with UBV and H-beta photometry to acquire data relevant tothe luminosity function of Sco-Cen. The measurements generally consistof two 10-s integrations of U, B, V, (W, N) filters, and theobservations are corrected iteratively for atmospheric extinction andinstrumental response. The data presented give the mean V magnitude,mean B-V, mean U-B, and the estimated uncertainties for these values.The catalog provides a homogeneous catalog of data for a large fieldwith stellar objects delineating membership to the association Sco-Cenand that affect the luminosity function of the aggregate.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Kentaur |
| Rektaszcenzió: | 13h53m56.90s |
| Deklináció: | -47°07'41.0" |
| Vizuális fényesség: | 6.1 |
| Távolság: | 10000000 parszek |
| RA sajátmozgás: | -7.2 |
| Dec sajátmozgás: | -3.3 |
| B-T magnitude: | 5.802 |
| V-T magnitude: | 5.868 |
Katalógusok és elnevezések:
|
INHABITED SKY
