Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Multi-periodic photospheric pulsations and connected wind structures in HD 64760
We report on the results of an extended optical spectroscopic monitoringcampaign on the early-type B supergiant HD 64760(B0.5 Ib) designed to probe the deep-seated origin of spatial windstructure in massive stars. This new study is based on high-resolutionechelle spectra obtained with the Feros instrument at ESO La Silla. 279spectra were collected over 10 nights between February 14 and 24, 2003.From the period analysis of the line-profile variability of thephotospheric lines we identify three closely spaced periods around 4.810h and a splitting of ±3%. The velocity - phase diagrams of theline-profile variations for the distinct periods reveal characteristicprograde non-radial pulsation patterns of high order corresponding topulsation modes with l and m in the range 6{-}10. A detailed modeling ofthe multi-periodic non-radial pulsations with the Bruce and Kyliepulsation-model codes (Townsend 1997b, MNRAS, 284, 839) favors eitherthree modes with l=-m and l=8,6,8 or m=-6 and l=8,6,10 with the secondcase maintaining the closely spaced periods in the co-rotating frame.The pulsation models predict photometric variations of 0.012{-}0.020 magconsistent with the non-detection of any of the spectroscopic periods byphotometry. The three pulsation modes have periods clearly shorter thanthe characteristic pulsation time scale and show small horizontalvelocity fields and hence are identified as p-modes. The beating of thethree pulsation modes leads to a retrograde beat pattern with tworegions of constructive interference diametrically opposite on thestellar surface and a beat period of 162.8 h (6.8 days). This beatpattern is directly observed in the spectroscopic time series of thephotospheric lines. The wind-sensitive lines display features ofenhanced emission, which appear to follow the maxima of the photosphericbeat pattern. The pulsation models predict for the two regionsnormalized flux amplitudes of A=+0.33,-0.28, sufficiently large to raisespiral co-rotating interaction regions (Cranmer & Owocki 1996, ApJ,462, 469). We further investigate the observed Hα wind-profilevariations with a simple rotating wind model with wind-densitymodulations to simulate the effect of possible streak lines originatingfrom the localized surface spots created by the NRP beat pattern. It isfound that such a simple scenario can explain the time scales and somebut not all characteristics of the observed Hα line-profilevariations.
| Looking for Discrete UV Absorption Features in the Early-Type Eclipsing Binaries μ1 Scorpii and AO Cassiopeiae
A search for discrete absorption components in the ultraviolet spectraof the early-type binaries μ1 Scorpii and AO Cassiopeiaehas been undertaken by analyzing material secured with the InternationalUltraviolet Explorer satellite during an exclusively assigned intervalof nearly 50 hr. While the spectra of μ1 Sco definitely donot show the presence of such lines, the spectra of AO Cas do confirmthem and permit us to draw some conclusions about where they may beformed.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability
We present the results of a long-term monitoring campaign of theHα line in a sample of bright OB supergiants (O7.5-B9) which aimsat detecting rotationally modulated changes potentially related to theexistence of large-scale wind structures. A total of 22 objects weremonitored during 36 nights spread over six months in 2001-2002.Coordinated broad-band photometric observations were also obtained forsome targets. Conspicuous evidence for variability in Hα is foundfor the stars displaying a feature contaminated by wind emission. Mostchanges take place on a daily time-scale, although hourly variations arealso occasionally detected. Convincing evidence for a cyclical patternof variability in Hα has been found in two stars: HD 14134 and HD42087. Periodic signals are also detected in other stars, butindependent confirmation is required. Rotational modulation is suggestedfrom the similarity between the observed recurrence time-scales (in therange 13-25 d) and estimated periods of stellar rotation. We callattention to the atypical case of HD 14134, which exhibits a clear12.8-d periodicity, both in the photometric and in the spectroscopicdata sets. This places this object among a handful of early-type starswhere one may observe a clear link between extended wind structures andphotospheric disturbances. Further modelling may test the hypothesisthat azimuthally-extended wind streams are responsible for the patternsof spectral variability in our target stars.
| Photospheric and stellar wind variability in ɛ Ori (B0 Ia)
We provide direct observational evidence for a link between photosphericactivity and perturbations in the dense inner-most stellar wind regionsof the B supergiant star ɛ Ori. The results, which are relevantto our understanding of the origin of wind structure, are based on amulti-spectral line analysis of optical time-series data secured in 1998using the HEROS spectrograph on the ESO Dutch 0.9-m telescope in LaSilla. A period of 1.9 days is consistently identified in Balmer,He I absorption, and weak metal lines such as Si III and C II. Theprimary characteristic is a large-amplitude swaying of the centralabsorption trough of the line, with differential velocities in linesformed at varying depths in the atmosphere. The variance resulting fromthe ``S-wave'' velocity behaviour of the lines is constrained within± the projected rotation velocity (80 km s-1) inthe weakest absorption lines, but extends blue-ward to over -200 kms-1 in Hα. A second (superimposed) 1.9 day signal ispresent at more extended blue-ward velocities (to -300 kms-1) in lines containing stronger circumstellar components.Inspection of archival optical data from 1996 provides evidence thatthis modulation signal has persisted for at least 2.5 years. Non-radialpulsational modelling is carried out in an attempt to reproduce the keyobservational characteristics of the line profile variability. Onlylimited success is obtained with prograde (m=-1) modes. The principalS-wave pattern cannot be matched by these models and remains enigmatic.Based on observations obtained as part of the MUSICOS 98 campaign fromESO La Silla, Chile.
| Kinematic model inversions of hot star recurrent DAC data - tests against dynamical CIR models
The Discrete Absorption Components (DACs) commonly observed in theultraviolet lines of hot stars have previously been modelled bydynamical simulations of Corotating Interaction Regions (CIRs) in theirline-driven stellar winds. Here we apply the kinematic DAC inversionmethod of Brown et al. to the hydrodynamical CIR models and test thereliability of the results obtained. We conclude that the inversionmethod is able to recover valuable information on the velocity structureof the mean wind and to trace movement of velocity plateaux in thehydrodynamical data, though the recovered density profile of the streamis correct only very near to the stellar surface.
| Inference of hot star density stream properties from data on rotationally recurrent DACs
The information content of data on rotationally periodic recurrentdiscrete absorption components (DACs) in hot star wind emission lines isdiscussed. The data comprise optical depths τ(w,φ) as afunction of dimensionless Doppler velocityw=(Δλ/λ0)(c/v&infy) and oftime expressed in terms of stellar rotation angle φ. This is usedto study the spatial distributions of density, radial and rotationalvelocities, and ionisation structures of the corotating wind streams towhich recurrent DACs are conventionally attributed.The simplifying assumptions made to reduce the degrees of freedom insuch structure distribution functions to match those in the DAC data arediscussed and the problem then posed in terms of a bivariaterelationship between τ(w,φ) and the radial velocityvr(r), transverse rotation rate Ω(r) and densityρ(r,φ) structures of the streams. The discussion applies tocases where: the streams are equatorial; the system is seen edge on; theionisation structure is approximated as uniform; the radial andtransverse velocities are taken to be functions only of radial distancebut the stream density is allowed to vary with azimuth. The lastkinematic assumption essentially ignores the dynamical feedback ofdensity on velocity and the relationship of this to fully dynamicalmodels is discussed. The case of narrow streams is first considered,noting the result of Hamann et al. (\cite{Ham01}) that the apparentacceleration of a narrow stream DAC is higher than the acceleration ofthe matter itself, so that the apparent slow acceleration of DACs cannotbe attributed to the slowness of stellar rotation. Thus DACs eitherinvolve matter which accelerates slower than the general wind flow, orthey are formed by structures which are not advected with the matterflow but propagate upstream (such as Abbott waves). It is then shownhow, in the kinematic model approximation, the radial speed of theabsorbing matter can be found by inversion of the apparent accelerationof the narrow DAC, for a given rotation law.The case of broad streams is more complex but also more informative. Theobserved τ(w,φ) is governed not only by vr(r) andΩ(r) of the absorbing stream matter but also by the densityprofile across the stream, determined by the azimuthal(φ0) distribution functionF0(φ0) of mass loss rate around the stellarequator. When F0(φ0) is fairly wide inφ0, the acceleration of the DAC peak τ(w,φ) inw is generally slow compared with that of a narrow stream DAC and theinformation on vr(r), Ω(r) andF0(φ0) is convoluted in the dataτ(w,φ).We show that it is possible, in this kinematic model, to recover byinversion, complete information on all three distribution functionsvr(r), Ω(r) and F0(φ0) fromdata on τ(w,φ) of sufficiently high precision and resolutionsince vr(r) and Ω(r) occur in combination rather thanindependently in the equations. This is demonstrated for simulated data,including noise effects, and is discussed in relation to real data andto fully hydrodynamic models.Figures \ref{fig:results1}, \ref{fig:results2}, \ref{fig:results7},\ref{fig:results8}, \ref{fig:results13}, \ref{fig:results14},\ref{fig:rotation1} and \ref{fig:rotation2} are only available inelectronic form at http://www.edpsciences.org
| Observational studies of wind and photospheric variability in three early-type stars
Not Available
| Observed instabilities in OB and Wolf-Rayet stars
A wealth of recent observations confirms that the fast, dense winds ofOB and Wolf-Rayet (WR) stars are highly structured, with the structuresresulting from the inherently unstable nature of a radiatively-drivenwind, as well as being triggered from the underlying photosphere. Thisreview mainly addresses variability patterns observed in the winds andphotospheric regions of presumably single stars. Schematically wedivide the detectable structures into two broad categories: small-scaleand large-scale inhomogeneities, with the former mainly of a stochasticbehavior/origin, and the latter frequently demonstrating a recurrent,even periodic, behavior. We then discuss the nature and nurture ofinstabilities, highlighting phenomenological similarities anddifferences in the variability of OB and WR stars.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Nearby Young Single Black Holes
We consider nearby young black holes formed after supernova explosionsin close binaries whose secondary components are currently observed asthe so-called runaway stars. Using data on runaway stars and makingreasonable assumptions about the mechanisms of supernova explosions andbinary breakup, we estimate the present position of nearby young blackholes. For two objects, we obtained relatively small error regions($\sim 50$-100 deg$^2$). The possibility of detecting these nearby youngblack holes is discussed.
| Wind variability of B supergiants. IV. A survey of IUE time-series data of 11 B0 to B3 stars
We present the most suitable data sets available in the InternationalUltraviolet Explorer (IUE) archive for the study of time-dependentstellar winds in early B supergiants. The UV line profile variability in11 B0 to B3 stars is analysed, compared and discussed, based on 16separate data sets comprising over 600 homogeneously reducedhigh-resolution spectrograms. The targets include ``normal'' stars withmoderate rotation rates and examples of rapid rotators. A gallery ofgrey-scale images (dynamic spectra) is presented, which demonstrates therichness and range of wind variability and highlights differentstructures in the winds of these stars. This work emphasises thesuitability of B supergiants for wind studies, under-pinned by the factthat they exhibit unsaturated wind lines for a wide range of ionization.The wind activity of B supergiants is substantial and has highly variedcharacteristics. The variability evident in individual stars isclassified and described in terms of discrete absorption components,spontaneous absorption, bowed structures, recurrence, and ionizationvariability and stratification. Similar structures can occur in stars ofdifferent fundamental parameters, but also different structures mayoccur in the same star at a given epoch. We discuss the physicalphenomena that may be associated with the spectral signatures. Thediversity of wind patterns evident likely reflects the role of stellarrotation and viewing angle in determining the observationalcharacteristics of azimuthally extended structure rooted at the stellarsurface. In addition, SEI line-synthesis modelling of the UV wind linesis used to provide further information about the state of the winds inour program stars. Typically the range, implied by the line profilevariability, in the product of mass-loss rate and ion fraction (mdotq_i) is a factor of ~ 1.5, when integrated between 0.2 and 0.9 v_infty ;it can however be several times larger over localised velocity regions.At a given effective temperature the mean relative ion ratios can differby a factor of 5. The general excess in predicted (forward-scattered)emission in the low velocity regime is discussed in terms of structuredoutflows. Mean ion fractions are estimated over the B0 to B1 spectralclasses, and trends in the ionic ratios as a function of wind velocityare described. The low values obtained for the ion fractions of UVresonance lines may reflect the role of clumping in the wind.
| Evidence for a connection between photospheric and wind structure in HD 64760
We report on the results of an extended optical spectroscopic monitoringcampaign on the early-type B supergiant HD 64760(B0.5 Ib). The study is based on high-resolution echelle spectraobtained with the Landessternwarte Heidelberg's HEROS instrument at ESOLa Silla. Ninety-nine spectra were collected over 103 nights betweenJanuary 19 and May 1, 1996. The Hα line shows a characteristicprofile with a central photospheric absorption superimposed bysymmetrically blue- and red-shifted wind-emission humps. Thetime-averaged line profile is well described by a differentiallyrotating and expanding radiation-driven wind: the redistribution of thewind emission flux into a double peak profile is interpreted in terms ofthe resonance zone effect in rotating winds as first described byPetrenz & Puls (\cite{Pet96}). Detailed time-series analyses of theline profile variations across the Hα profile reveal for the firsttime in an optical data set of HD 64760 a periodic2.4-day modulation of the inner and outer flanks of the Hαemission humps. The stronger modulations of the inner flanks of theemission humps at photospheric velocities are due to complex widthvariations of the underlying photospheric Hα profile. The weakervariations of the outer flanks are in phase and reflect variations atthe base of the stellar wind. The detected 2.4-day modulation periodtogether with a second period of 1.2 days (in the red emission humponly) is in excellent agreement with the outer-wind modulation periodsas reported by Fullerton et al. (\cite{Ful97}) from intensive IUE UVtime-series observations in 1993 and 1995. The 2.4-day period is furtherdetected in the photospheric He I lambda4026 line as prograde traveling(pseudo-)absorption and emission features. The observed variabilitypattern is indicative for low-order non-radial pulsations in thephotosphere of HD 64760. The non-radial pulsationsare identified as the source of persistent, regularly spaced stellarsurface structure which is maintained throughout the photosphere - windtransition zone (this work) out into the UV regime of the terminalvelocity outflow. Based on observations collected at the EuropeanSouthern Observatory at La Silla, Chile (Proposal ID 56.D-0235).
| On the wavelength drift of spectral features from structured hot star winds
Spectral lines formed in stellar winds from OB stars are observed toexhibit profile variations. Discrete Absorption Components (DACs) show aremarkably slow wavelength drift with time. In a straightforwardinterpretation, this is in sharp contradiction to the steep velocity lawpredicted by the radiation-driven wind theory, and by semi-empiricalprofile fitting. In the present paper we re-discuss the interpretationof the drift rate. We show that the Co-rotating Interaction Region (CIR)model for the formation of DACs does not explain their slow drift rateas a consequence of rotation. On the contrary, the apparent accelerationof a spectral CIR feature is even higher than for the correspondingkinematical model without rotation. However, the observations can beunderstood by distinguishing between the velocity field of the matterflow, and the velocity law for the motion of the patterns in which theDAC features are formed. If the latter propagate upstream against thematter flow, the resulting wavelength drift mimics a much sloweracceleration although the matter is moving fast. Additional to the DACs,a second type of recurrent structures is present in observed OB starspectra, the so-called modulations. In contrast to the DACs, thesestructures show a steep acceleration compatible with the theoreticallypredicted velocity law. We see only two possible consistent scenarios.Either, the wind is accelerated fast, and the modulations are formed inadvected structures, while the DACs come from structures which arepropagating upstream. Or, alternatively, steep and shallow velocity lawsmay co-exist at the same time in different spatial regions or directionsof the wind.
| A search for the cause of cyclical wind variability in O stars. Simultaneous UV and optical observations including magnetic field measurements of the O7.5III star xi Persei
We present the results of an extensive observing campaign on the O7.5III star xi Persei. The UV observations were obtainedwith the International Ultraviolet Explorer. xi Perwas monitored continuously in October 1994 during 10 days at ultravioletand visual wavelengths. The ground-based optical observations includemagnetic field measurements, Hα and He I lambda 6678 spectra, andwere partially covered by photometry and polarimetry. We describe amethod to automatically remove the variable contamination of telluriclines in the groundbased spectra. The aim of this campaign was to searchfor the origin of the cyclical wind variability in this star. Wedetermined a very accurate period of 2.086(2) d in the resonance linesof Si Iv and in the subordinate N Iv and Hα line profiles. Theepochs of maximum absorption in the UV resonance lines due to discreteabsorption components (DACs) coincide in phase with the maxima inblue-shifted Hα absorption. This implies that the periodicvariability originates close to the stellar surface. The phase-velocityrelation shows a maximum at -1400 km s-1. The general trendof these observations can be well explained by the corotatinginteraction region (CIR) model. In this model the wind is perturbed byone or more fixed patches on the stellar surface, which are mostprobably due to small magnetic field structures. Our magnetic fieldmeasurements gave, however, only a null-detection with a 1sigma errorbarof 70 G in the longitudinal component. Some observations are moredifficult to fit into this picture. The 2-day period is not detected inthe photospheric/transition region line He I lambda 6678. The dynamicspectrum of this line shows a pattern indicating the presence ofnon-radial pulsation, consistent with the previously reported period of3.5 h. The edge variability around -2300 km s-1 in thesaturated wind lines of C Iv and N V is nearly identical to the edgevariability in the unsaturated Si Iv line, supporting the view that thistype of variability is also due to the moving DACs. A detailed analysisusing Fourier reconstructions reveals that each DAC actually consists of2 different components: a ``fast'' and a ``slow'' one which merge athigher velocities. Based on observations obtained using theInternational Ultraviolet Explorer, collected at NASA Goddard SpaceFlight Center and Villafranca Satellite Tracking Station of the EuropeanSpace Agency.
| Extended optical spectroscopic monitoring of wind structure in HD 152408
New perspectives are provided on significant spatial structure andtemporal variability in the near-star wind regions (i.e. < 3 R_star )of the massive luminous star HD 152408 (classified as O8:Iafpe orWN9ha). This study is primarily based on the analysis of high-qualityéchelle spectra secured during 21 nights between 1999 July toAugust, using the Landessternwarte-developed (fibre-fed) FEROSinstrument on the ESO 1.52-m telescope. These extended time-series data,with a total simultaneous wavelength coverage of lambda lambda3600 -9200Å, were exploited to monitor absorption and emission fluctuations(of ~ 5-10% of the line flux) in several He i and Balmer lines, togetherwith more deep-seated (near-photosphere) disturbances in weaker metallicemission and absorption lines. Organised large-scale wind structure inHD 152408 is principally betrayed by sequential episodes of discreteabsorption and emission features, which migrate from near zero velocityto almost the wind terminal velocity. This evolution is extremely slow,however, typically spanning ~ 4 days for an individual episode. Wedemonstrate that the blue-shifted sorption episodes in He i are veryclosely mirrored (in velocity and time) by absorption features (i.e.reduced not enhanced flux) in the blue wings of the mainly recombinationformed broad Hα emission line. The implication is that there isdetailed balancing between ground state photoionization andrecombination in the substantially optically thick Balmer lines.Surprisingly, the velocity behaviour of the red-ward and blue-wardmigrating features is highly asymmetric, such that the mean accelerationof the former is less than 50% of the latter. Fourier analysis reveals amodulation time-scale for the wind activity of ~ 7.7 days, plus itsharmonic at 3.9 days. The longer period is ~ 28 times greater than thecharacteristic radial wind flow time of HD 152408. We also detect a ~1.5 day periodic variation in the radial velocity of the weak C ivlambda lambda 5801, 5812 absorption lines, which are the closestapproximation to ``pure'' photospheric lines in the optical spectrum ofHD 152408. The wind-formed optical lines of HD 152408 are also affectedby fluctuations in the central peak emission, particularly evident inHα where the equivalent width may vary by up to 20%. Data securedbetween 1995 and 1999 reveal, however, that the epoch-to-epoch meanprofiles are remarkably similar. Non-LTE steady-state stellar atmospheremodels are used to synthesis profiles to match representative Hαand He i lambda 5876 line profiles. Only a slow (tailored) velocity law(compared to beta =1) provides a good match to the Hα emissionpeak and wings, but the models predict excess He i absorption. Theobserved extreme Hα emission variations can be reproduced by thesynthetic profiles with an implied +/-10% variation in mass-loss rate.The results on optical line profile variability in HD 152408 arediscussed in the context of models for co-rotating interaction regions(CIRs) in the wind. Several constraints are provided that argue againstsimple velocity fields in such streams, including (i) the slowacceleration of features to high velocities, within ~ 3 R_star , (ii)the strong asymmetry in projected acceleration of the approaching andreceding stream material, (iii) Balmer line absorption effects in theapproaching material, (iv) the rise of localised features from very lowvelocities, and (v) the stability of the large-scale CIRs againstturbulent small-scale wind structure. We suggest that it may be worthexploring hydrodynamic simulations of CIRs that incorporate differentvelocity fields on the leading (faster accelerating; blue-wardabsorption) and trailing (slower accelerating; red-ward emission) edgesof the spiral structures. Based on observations collected at theEuropean Southern Observatory, Chile (ESO 63.H--0080(A)).
| 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 spectral variability of HD 192639 and its implications for the star's wind structure
We report the analysis of an extensive set of spectroscopic data of theO(f) supergiant HD 192639. A Fourier analysis of our time-series revealsa recurrent variability with a ``period'' of roughly 4.8 days which ismost prominent in the absorption components of the He {ii}\ lambda 4686and Hα P-Cygni profiles. The same periodicity is also detected inthe blue wing of several absorption lines (e.g. Hβ ). Thevariations of the absorption components correspond most probably to acyclical modulation of the amount of stellar wind material along theline of sight towards the star. The 4.8-day period affects also themorphology of the double-peaked He {ii}\ lambda 4686 and Hαemission components, although these emission components display alsovariations on other (mainly longer) time scales. The most likelyexplanation for the 4.8-day modulation is that this cycle reflects thestellar rotational period (or half this period). We find that the mostimportant observational properties can be explained - at leastqualitatively - by a corotating interaction region or a tilted confinedcorotating wind. Based on observations collected at the Observatoire deHaute Provence, France and the Ritter Observatory, Toledo, USA.
| On the origin of the O and B-type stars with high velocities. II. Runaway stars and pulsars ejected from the nearby young stellar groups
We use milli-arcsecond accuracy astrometry (proper motions andparallaxes) from Hipparcos and from radio observations to retrace theorbits of 56 runaway stars and nine compact objects with distances lessthan 700 pc, to identify the parent stellar group. It is possible todeduce the specific formation scenario with near certainty for twocases. (i) We find that the runaway star zeta Ophiuchi and the pulsarPSR J1932+1059 originated about 1 Myr ago in a supernova explosion in abinary in the Upper Scorpius subgroup of the Sco OB2 association. Thepulsar received a kick velocity of ~ 350 km s-1 in thisevent, which dissociated the binary, and gave zeta Oph its large spacevelocity. (ii) Blaauw & Morgan and Gies & Bolton alreadypostulated a common origin for the runaway-pair AE Aur and mu Col,possibly involving the massive highly-eccentric binary iota Ori, basedon their equal and opposite velocities. We demonstrate that these threeobjects indeed occupied a very small volume ~ 2.5 Myr ago, and show thatthey were ejected from the nascent Trapezium cluster. We identify theparent group for two more pulsars: both likely originate in the ~ 50 Myrold association Per OB3, which contains the open cluster alpha Persei.At least 21 of the 56 runaway stars in our sample can be linked to thenearby associations and young open clusters. These include the classicalrunaways 53 Arietis (Ori OB1), xi Persei (Per OB2), and lambda Cephei(Cep OB3), and fifteen new identifications, amongst which a pair ofstars running away in opposite directions from the region containing thelambda Ori cluster. Other currently nearby runaways and pulsarsoriginated beyond 700 pc, where our knowledge of the parent groups isvery incomplete.
| 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.
| Surface trapping and leakage of low-frequency g modes in rotating early-type stars - I. Qualitative analysis
A qualitative study of the surface trapping of low-frequency non-radialg modes in rotating early-type stars is undertaken within the Cowling,adiabatic and traditional approximations. A dispersion relationdescribing the local character of waves in a rotating star is derived;this dispersion relation is then used to construct propagation diagramsfor a 7-Msolar stellar model, which show the location andextent of wave trapping zones inside the star. It is demonstrated that,at frequencies below a cut-off, waves cannot be fully trapped within thestar, and will leak through the surface. Expressions for the cut-offfrequency are derived in both the non-rotating and rotating cases; it isfound from these expressions that the cut-off frequency increases withthe rotation rate for all but prograde sectoral modes. While waves belowthe cut-off cannot be reflected at the stellar surface, the presence ofa sub-surface convective region in the stellar model, owing to Heiiionization, means that they can become partially trapped within thestar. The energy leakage associated with such waves, which are assignedthe moniker virtual modes owing to their discrete eigenfrequencies,means that stability analyses which disregard their existence (byassuming perfect reflection at the stellar surface) may be in error. Theresults are of possible relevance to the 53 Per and SPB classes ofvariable star, which exhibit pulsation frequencies of the same order ofmagnitude as the cut-off frequencies found for the stellar model. It issuggested that observations either of an upper limit on variabilityperiods (corresponding to the cut-off), or of line-profile variationsowing to virtual modes, may permit asteroseismological studies of theouter layers of these systems.
| Synthetic line profiles of rotationally distorted hot-star winds
A new Monte Carlo stellar wind radiative-transfer code is presented. Thecode employs a three-dimensional opacity grid, and fully treatspolarization and multiple scattering. Either Mie or Rayleigh scatteringphase matrices may be used, and the line-transfer is treated by means ofthe Solobolev approximation. Variance reduction techniques are employedto increase computational efficiency. The results of several tests ofthe code are reported. It is confirmed that no continuum polarization isproduced in the spherically symmetric wind case, and that the lineprofiles computed match those computed using establishedradiative-transfer codes. The continuum polarization produced by alatitudinally structured low-density wind is found to be in goodagreement with that predicted by the single-scattering analyticaltreatment of Fox, while in the higher density regime the polarizationsare consistent with the multiple-scattering code given by Hillier. Twoillustrative applications of the code are described, using the windparameters of ζ Puppis [O4I(n)f] as the base model. In the firstthe effect on the line profile of a corotating spiral densityenhancement is examined. It is found that the spiral gives line profilevariations on the order of 5 per cent, and that it produces anS-wave-like pattern as a function of rotational phase. It is noted thatthe accelerations described by the spiral wave may mimic those producedby tangentially accelerating wind clumps. The variable polarizationproduced by the spiral is found to have an amplitude of 0.1 per cent,with two maxima per rotational period in phase with the line emissionmodulation. The second application investigates the profiles andpolarization produced in a clumped wind. Although the parameters of thediscrete wind clumps are necessarily arbitrary, it is found that aclumped-wind model reproduces the level of spectroscopic variabilityfound by Eversberg et al. It is shown that the wind emission `bumps'produced in the synthetic spectra often arise from the superposition invelocity space of flux from several spatially discrete wind blobs.Although the two example models may, in combination, reproduce theobserved spectroscopic variability of OB supergiants, it appears thatthe predicted polarimetric variability of these models is too weak toexplain the polarimetric observations of OB stars by Hayes and Lupie& Nordsieck. Finally, a new line polarization effect is described,resulting from line absorption of continuum photons in a rotating wind.The effect has a striking resemblance to the observations of ζPuppis presented by Harries & Howarth (1996), and it is possiblethat the Hα polarization structure observed arises from thiseffect, rather than by line dilution of a continuum polarization.
| The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way
The picture of the young stellar groups in the Canis Major-Puppis-Vela(215 deg
| Winds from Hot Stars
This review deals with the winds from "normal" hot stars such asO-stars, B- and A-supergiants, and Central Stars of Planetary Nebulaewith O-type spectra. The advanced diagnostic methods of stellar winds,including an assessment of the accuracy of the determinations of globalstellar wind parameters (terminal velocities, mass-loss rates, windmomenta, and energies), are introduced and scaling relations as afunction of stellar parameters are provided. Observational results areinterpreted in the framework of the stationary, one-dimensional (1-D)theory of line-driven winds. Systematic effects caused by nonhomogeneousstructures, time dependence, and deviations from spherical symmetry arediscussed. The review finishes with a brief description of the role ofstellar winds as extragalactic distance indicators and as tracers of thechemical composition of galaxies at high redshift.
| New aspects of line-profile variability in P Cygni's optical spectrum
High-resolution photographic spectra as well as high S/N ratio CCDspectral observations of P Cygni are analyzed in terms of line-profilevariability (lpv). Four different kinds of lpv are established:systematic variability in the absorption troughs of low and intermediateexcitation lines due to propagating Discrete Absorption Components(DACs); ``swaying'' variability consisting of continuous modulations invelocity and intensity of the absorption cores and emission peaks oflines of intermediate and high excitation; red-emission-wing variabilitydue to travelling ``bumps'', and long-term (LT) variability in HI andHeI lines of relatively large optical depth. DAC propagation is a slowvariation of P Cygni's stellar wind. The components probably originatefrom large-scale, high-density (low-excitation) perturbation(s) whichdevelop in a relatively outer part of the wind (V >=0.41Vinf) but appear to be maintained, in some indirect way,by photospheric processes. The geometry of the structures is not yetclear but they could be either spherically symmetric or curved, likekinks. The ``swaying'' variability manifests itself by modulations inposition and intensity of the absorption cores and emission peaks ofalmost all lines in the optical. Simultaneous variations in emission andabsorption line-strength were also observed. The modulations are atleast partially due to variations in the number density which affect alllayers of the supersonic wind starting at its base up to layers wherethe Hff line forms (0.18 <= V <= 0.95Vinf). Thephenomenon appears to be stable over many years, though on a variabletime-scale. Suggestive evidence for a close relationship between themodulations and changes in the stellar brightness and temperature wasfound, indicating that the ``swaying'' variability is more likelycoupled to processes in the photosphere. Non-radial pulsations (NRPs)either of g-mode or of s-mode oscillations are a possible cause for thisvariability. The LT variability makes up a very slow pattern ofvariation in both the velocity of the absorption cores and the intensityof the emission peaks of the stronger HI and HeI lines. This variabilityis found only in the outer part of the wind (V <=0.82Vinf). The nature of the LT variation is not known atpresent. The red-emission-wing variability is localized in thehigh-velocity part of the emission lobes of P Cygni-type profiles, +90<= V <= +230/250 km s-1. This variability is presumablycaused by outward propagating ``bumps'', but its exact nature is stillunknown. No indication for any clear relation between different kinds oflpv was found. Even when the variations operate in one and the sameregion of the wind (in velocity space) it is not obvious whether and howthey interact. Stellar rotation does not seem to play a fundamental rolein setting the time-scale of either the DAC-induced variability or theLT variability. The relationship between the ``swaying'' variability andthe rotation is still not clear but it is possible that this variabilityis rotationally modulated. The wind variability of P Cygni appears to bequalitatively similar to the wind variability of the early B-typesupergiant HD64760 but different from the variability of O-type starwinds.
| Spectroscopic binary orbits from photoelectric radial velocities. Paper 148: HR 7955
Not Available
| Ultraviolet and Optical Line Profile Variations in the Spectrum of ɛ Persei
The rapid variable star, ɛ Per (B0.5 IV-III), displays thelargest amplitude profile fluctuations known among the growing number ofmassive, spectrum-variable stars. Here we present an analysis of acontinuous 5 day run of IUE UV spectroscopy, and we show for the firsttime that the systematic, blue-to-red moving patterns observed inhigh-quality optical spectra are also present in the UV photosphericlines. We present cross-correlation functions of the individual spectrawith that of a narrow-lined standard that produce a high signal-to-noiseratio representation of the blue-to-red moving bump patterns found inindividual lines. We then use time series analysis methods to determinethe periodic components of the profile variations (after reregisteringthe spectra to correct for binary motion). There are at least sixperiods present (ranging from 8.46 to 2.27 hr), and most of thesesignals are also found in optical line variations observed in 1986(although the relative amplitudes have changed significantly).Furthermore, analysis of a shorter time series of IUE spectra from 1984shows that similar periods were present then. We also present Hαand He I λ6678 profiles obtained with the Georgia StateUniversity Multi-Telescope Telescope, which were made simultaneouslywith IUE, and we show that the profile variations are essentiallyidentical in the UV and optical ranges. We rule out rotationalmodulation and circumstellar gas obscuration as possible causes, and wesuggest instead that the variations are the result of photosphericnonradial pulsations of relatively low degree (l=3-5). There weresignificant changes (~10%) in the equivalent widths of the UV stellarwind lines during the IUE run, and we suggest that wind strengtheningevents are related to episodes of large-amplitude, constructiveinterference between the NRP modes. Thus, intermode beating may play animportant role in promoting wind loss from massive stars.
| Non-radial pulsations in the O stars XI Persei and lambda Cephei
A new time-series analysis of profile changes in the photospheric He Ilambda 4713 spectral line from data taken during 5 days in 1989 at theCalar Alto and Kitt Peak observatories has provided evidence for thepresence of a non-radial prograde p-mode in the O 7.5 giant xiPer (l = 3, P = 3.5) and probably two such modes in the O6supergiant lambda Cep (l = 3, P = 12.3 h and l = 5, P= 6.6 h). The corotating pulsation periods are in both cases muchshorter than the estimated stellar rotation period. Modeling theobserved amplitude of the line-profile changes (assuming |m|=l) yields avelocity amplitude of approximately 5 km s(-1) of the pulsation in xiPer and 6 km s(-1) in lambda Cep. Any such a pulsation by itself isunlikely to be the cause of the well-known cyclical wind variability inthese stars because the pulsation period is too short, but thecumulative action of multiple modes could cause such an effect. Weakmagnetic fields anchored at the surface remain the strongest candidatefor the origin of wind variability.
| Long- and short-term variability in O-star winds. II. Quantitative analysis of DAC behaviour
A quantitative analysis of time series of ultraviolet spectra from asample of 10 bright O-type stars (cf. Kaper et al. \cite{KH96}, Paper I)is presented. Migrating discrete absorption components (DACs),responsible for the observed variability in the UV resonance doublets,are modeled. To isolate the DACs from the underlying P Cygni lines, amethod is developed to construct a template (``least-absorption'')spectrum for each star. The central velocity, central optical depth,width, and column density of each pair of DACs is measured and studiedas a function of time. It turns out that the column density of a DACfirst increases and subsequently decreases with time when the componentis approaching its asymptotic velocity. Sometimes a DAC vanishes beforethis velocity is reached. In some cases the asymptotic DAC velocitysystematically differs from event to event. In order to determine thecharacteristic timescale(s) of DAC variability, Fourier (CLEAN) analyseshave been performed on the time series. The recurrence timescale of DACsis derived for most targets, and consistent results are obtained fordifferent spectral lines. The DAC recurrence timescale is interpreted asan integer fraction of the stellar rotation period. In some datasets thevariability in the blue edge of the P Cygni lines exhibits a longerperiod than the DAC variability. This might be related to the systematicdifference in asymptotic velocity of successive DACs. The phaseinformation provided by the Fourier analysis confirms the expectedchange in phase with increasing velocity. This supports theinterpretation that the DACs are responsible for the detectedperiodicity. The phase diagram for the O giant xi Per shows clearevidence for so-called ``phase bowing'', which is an observationalindication for the presence of curved wind structures like corotatinginteraction regions in the stellar wind. An important difference withthe results obtained for the B supergiant HD 64760 (Fullerton et al.\cite{FM97}) is that in this O star the phase bowing can be associatedwith the DACs. No other O stars in our sample convincingly show phasebowing, but this could be simply due to the absence of periodic signaland hence coherent phase behaviour at low wind velocities. Based onobservations by the International Ultraviolet Explorer, collected atNASA Goddard Space Flight Center and Villafranca Satellite TrackingStation of the European Space Agency.
| Repetitive structure in the stellar wind of HD 93843: a normal O-type star
We present the results from a 28-day IUE time-series campaign monitoringthe stellar wind of the O5-type giant HD 93843. The principal aim was tostudy variability in the wind of a star with a normal projected rotationvelocity. Systematic changes are identified, amidst continuousline-profile variability, in the absorption troughs of the Siiv and Nvresonance lines. The patterns observed have characteristic time-scalesof several days and are mimicked by fluctuations (of several 100 kms^-1) in the blue wings of the saturated Civ P Cygni profile. Fourieranalysis provides support for the repeatability of wind structures in HD93843 on a 7.1-d `period'. Power at this frequency is evident only atintermediate and high velocities (i.e., above ~0.3 of the terminalvelocity). The long modulation time-scale suggests that changes in thestar itself probably provide the physical source for triggering theonset of wind structure. Unfortunately the rotational, photometric,pulsational and magnetic properties of HD 93843 are too poorlyconstrained or known to permit a more detailed interpretation of the7.1-d wind modulation in terms of potential inhomogeneities at thestellar surface. Nevertheless, our study demonstrates that the incidenceof cyclic, possibly regular, stellar-wind variability is not restrictedto rapid rotators. Comparisons with other OB stars which have exhibitedrepetitive wind changes on `periods' of several days suggest that thetime-dependent UV properties of HD 93843 are more akin to those of theO4-type supergiant zeta Puppis.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Pupa |
| Sað Açýklýk: | 07h53m18.20s |
| Yükselim: | -48°06'11.0" |
| Görünürdeki Parlaklýk: | 4.24 |
| Uzaklýk: | 595.238 parsek |
| özdevim Sað Açýklýk: | -5.7 |
| özdevim Yükselim: | 2.5 |
| B-T magnitude: | 4.036 |
| V-T magnitude: | 4.203 |
Kataloglar ve belirtme:
|
