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Time-Resolved FUSE Spectroscopy of the LMC WC + O Binaries Br 22 and Br 32
Using the Far Ultraviolet Spectroscopic Explorer satellite, we obtainedseries of spectra for two of the three known WC + O binaries in the LMC,Br 22, and Br 32 (HD 36521). Compared to Br 22, we detect a higher ratioof C IV to He II lines in Br 32, which could indicate a more advancedevolutionary stage for the latter. The orbit of the O star in Br 32 hasbeen determined from its P V absorption lines. We find that continuumfluxes in both systems are substantially diluted by a third-lightsource. The maximum extension of the black absorption troughs in P Cygniprofiles provides the terminal velocities of the WC winds: 3775+/-125 kms-1 for Br 22 and 4400+/-150 km s-1 for Br 32.From the phase-dependent displacements of the blue absorption edges ofprominent emission lines we estimate the half-opening angles of thewind-wind collision zones and their Coriolis deflections. In bothbinaries we fitted, via iterative procedure, the phase-dependent changesin the O VI λ1032-1037 and C III λ1175 profiles as afunction of the wind, stellar, and orbital parameters. This allowed usto isolate the excess emission produced in the wind-wind collision zoneand reproduce profile changes caused by atmospheric eclipses. A strongextra emission component is observed in Br 22 (P=14.9 days), while it isnegligible in Br 32 (P=1.9 days).Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by The JohnsHopkins University under NASA contract NAS5-32985.
| Highly ionized plasma in the Large Magellanic Cloud: evidence for outflows and a possible galactic wind
Based on an analysis of the interstellar highly ionized species CIV,SiIV, NV and OVI observed in the Far Ultraviolet Spectroscopic Explorer(FUSE) and Hubble Space Telescope/Space Telescope Imaging Spectrograph(HST/STIS) E140M spectra of four hot stars in the Large Magellanic Cloud(LMC), we find evidence for a hot LMC halo fed by energetic outflowsfrom the LMC disc and even possibly an LMC galactic wind. Signatures forsuch outflows are the intermediate- and high-velocity components(vLSR >~ 100kms-1) relative to the LMC discobserved in the high- and low-ion absorption profiles. The stellarenvironments produce strong, narrow (T <~ 2 × 104K)components of CIV and SiIV associated with the LMC disc; in particularthey are likely signatures of HII regions and expanding shells. Broadcomponents are observed in the profiles of CIV, SiIV and OVI with theirwidths implying hot, collisionally ionized gas at temperatures of a fewtimes 105K. There is a striking similarity in the OVI/CIVratios for the broad LMC and high-velocity components, suggesting muchof the material at vLSR >~ 100kms-1 isassociated with the LMC. The velocity of the high-velocity component islarge enough to escape altogether the LMC, polluting the intergalacticspace between the LMC and the Milky Way. The observed high-ion ratios ofthe broad LMC and high-velocity components are consistent with thoseproduced in conductive interfaces; such models are also favoured by theapparent kinematically coupling between the high and the weakly ionizedspecies.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE). FUSE is operated for NASA by the JohnsHopkins University under NASA contract NAS5-32985. Based on observationsmade with the NASA/ESA Hubble Space Telescope, obtained at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc. under NASA contract no.NAS5-26555.E-mail: nlehner@nd.edu
| Effects of Metallicity on the Rotational Velocities of Massive Stars
Recent theoretical predictions for low-metallicity massive stars predictthat these stars should have drastically reduced equatorial winds (massloss) while on the main sequence, and so should retain most of theirangular momentum. Observations of both the Be/(B+Be) ratio and theblue-to-red supergiant ratio appear to have a metallicity dependencethat may be caused by high rotational velocities. We have analyzed 39archival Hubble Space Telescope Imaging Spectrograph (STIS),high-resolution, ultraviolet spectra of O-type stars in the MagellanicClouds to determine their projected rotational velocities Vsini. Ourmethodology is based on a previous study of the projected rotationalvelocities of Galactic O-type stars using International UltravioletExplorer (IUE) short-wavelength prime (SWP) camera high-dispersionspectra, which resulted in a catalog of Vsini values for 177 O-typestars. Here we present complementary Vsini values for 21 LargeMagellanic Cloud and 22 Small Magellanic Cloud O-type stars based onSTIS and IUE UV spectroscopy. The distribution of Vsini values forO-type stars in the Magellanic Clouds is compared to that of GalacticO-type stars. Despite the theoretical predictions and indirectobservational evidence for high rotation, the O-type stars in theMagellanic Clouds do not appear to rotate faster than their Galacticcounterparts.
| Infrared Observations of the Candidate LBV 1806-20 and Nearby Cluster Stars1,
We report near-infrared photometry, spectroscopy, and speckle imaging ofthe hot, luminous star we identify as candidate LBV 1806-20. We alsopresent photometry and spectroscopy of three nearby stars, which aremembers of the same star cluster containing LBV 1806-20 and SGR 1806-20.The spectroscopy and photometry show that LBV 1806-20 is similar in manyrespects to the luminous ``Pistol star,'' albeit with some importantdifferences. They also provide estimates of the effective temperatureand reddening of LBV 1806-20 and confirm distance estimates, leading toa best estimate for the luminosity of this star of greater than5×106Lsolar. The nearby cluster stars havespectral types and inferred absolute magnitudes that confirm thedistance (and thus luminosity) estimate for LBV 1806-20. If we dropkinematic measurements of the distance(15.1+1.8-1.3 kpc), we have a lower limit on thedistance of greater than 9.5 kpc and on the luminosity of greater than2×106Lsolar, based on the cluster stars. Ifwe drop both the kinematic and cluster star indicators for distance, anammonia absorption feature sets yet another lower limit to the distanceof greater than 5.7 kpc, with a corresponding luminosity estimate ofgreater than 7×105 Lsolar for the candidateLBV 1806-20. Furthermore, on the absis of very high angular resolutionspeckle images, we determine that LBV 1806-20 is not a cluster of starsbut is rather a single star or binary system. Simple arguments based onthe Eddington luminosity lead to an estimate of the total mass of LBV1806-20 (single or binary) exceeding 190Msolar. We discussthe possible uncertainties in these results and their implications forthe star formation history of this cluster.Based on data obtained at the Palomar Observatory 200 inch telescope,which is operated by the California Institute of Technology, the JetPropulsion Laboratory, and Cornell University.This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| WR 20a Is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System
We present a high-precision I-band light curve for the Wolf-Rayet binaryWR 20a, obtained as a subproject of the Optical Gravitational LensingExperiment. Rauw et al. have recently presented spectroscopy for thissystem, strongly suggesting extremely large minimum masses of 70.7+/-4.0and 68.8+/-3.8 Msolar for the component stars of the system,with the exact values depending strongly on the period of the system. Wedetect deep eclipses of about 0.4 mag in the light curve of WR 20a,confirming and refining the suspected period of P=3.686 days andderiving an inclination angle of i=74.5d+/-2.0d. Using these photometricdata and the radial velocity data of Rauw et al., we derive the massesfor the two components of WR 20a to be 83.0+/-5.0 and 82.0+/-5.0Msolar. Therefore, WR 20a is confirmed to consist of twoextremely massive stars and to be the most massive binary known with anaccurate mass determination.Based on observations obtained with the 1.3 m Warsaw telescope at LasCampanas Observatory, which is operated by the Carnegie Institute ofWashington.
| A CNO Dichotomy among O2 Giant Spectra in the Magellanic Clouds
From a survey of the 3400 Å region in the earliest O-type spectra,we have found that two of the four O2 giants observed in the LargeMagellanic Cloud have O IV lines there that are stronger than the N IVlines, while the other two have the opposite. A Small Magellanic Cloudcounterpart also has N IV stronger than O IV. Inspection of the bluespectra of these stars shows that the former pair have weaker N lines inall ionization states (III, IV, and V) present as well as lines of C IVλ4658, while the latter three have stronger N lines and greaterHe/H. Space ultraviolet observations of two of the N-strong stars show NV wind profiles substantially stronger than those of C IV, while in theN-weak stars the C IV features are equal to or stronger than the N V.The N-strong stars are now reclassified as ON2 III(f*), newly definingthat category. These characteristics strongly suggest a larger fractionof processed material in the atmospheres of the ON2 stars, which weconfirm by modeling the optical spectra. In the context of currentmodels, it is in turn implied that the ON2 stars are in a more advancedevolutionary state than the others, and/or that they had higher initialrotational velocities. The recent formulation of the effects of rotationon massive stellar evolution introduces an additional fundamentalparameter, which the CNO abundances are in principle able to constrain.We present some illustrative comparisons with current Genevaevolutionary models for rotating massive stars. It is possible thatthese very hot, nitrogen-rich objects are products of homogeneousevolution. Our results will provide motivation for further physicalmodeling of the atmospheres and evolutionary histories of the mostmassive hot stars.
| Constraints on the Ionization Balance of Hot-Star Winds from FUSE Observations of O Stars in the Large Magellanic Cloud
We present Far Ultraviolet Spectroscopic Explorer (FUSE) spectra for 25O stars in the Large Magellanic Cloud. We analyze wind profiles for theresonance lines from C III, N III, S IV, P V, S VI, and O VI in the FUSErange using a ``Sobolev with exact integration'' method. In addition,the available data from either IUE or the Hubble Space Telescope for theresonance lines of Si IV, C IV, and N V are also modeled. Becauseseveral of the FUSE wind lines are unsaturated, the analysis providesmeaningful optical depths (or equivalently, mass-loss rate timesionization fractions, Mq) as a function of normalized velocity,w=v/v&infy;. Ratios of Mq (which are independent of M)determine the behavior of the relative ionization as a function of w.The results demonstrate that, with the exception of O VI in all starsand S VI in the later stars, the ionization in the winds shifts towardlower ionization stages at higher w (contrary to the expectations of thenebular approximation). This result implies that the dominant productionmechanism for O VI and S VI in the late O stars differs from the otherions. Using the Vink et al. relationship between stellar parameters andmass-loss rate, we convert the measurements into mean ionizationfractions for each ion, 〈qi〉. Because the derived ionfractions never exceed unity, we conclude that the derived values of Mare not too small. However, q(P V), which is expected to be the dominantstage of ionization in some of these winds, is never greater than 0.20.This implies that either the calculated values of M are too large, theassumed abundance of phosphorus is too large, or the winds are stronglyclumped. The implications of each possibility are discussed.Correlations between the mean ion fractions and physical parameters suchas Teff, v&infy;, and the mean wind density,〈ρ〉, are examined. Two clear relationships emerge. First, asexpected, the mean ionization fraction of the lower ions (C III, N III,Si IV, S IV) decreases with increasing Teff. Second, the meanion fraction of S VI in the latest stars and O VI in all stars increaseswith increasing v&infy;. This reaffirms the notion, firstintroduced by Cassinelli & Olson, that O VI is producednonradiatively. Finally, we discuss specific characteristics of threestars, BI 272, BI 208, and Sk -67°166. For BI 272, the ionic speciespresent in its wind suggest it is much hotter than its available(uncertain) spectral type of O7: II-III:. In the case of BI 208, ourinability to fit its observed profiles suggests that its wind is notspherically symmetric. For Sk -67°166, quantitative measurements ofits line strengths confirm the suggestion by Walborn et al. that it is anitrogen-rich O star.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| Ultraviolet Spectroscopy of the Massive LMC Multiple Systems Sk-67^0m#circ;18 (Br 5) and HD 36402 (Br 31)
Following previous IUE-based spectroscopic studies of WR+O binaries inthe Galaxy and in the SMC, we present a similar study of the twosystems, Br 5 [O3 If^*(+O) + O8-B0 I(+OB?)] and Br 31 [WC4(+O?) + O8 I:]in the Large Magellanic Cloud. We detect wind eclipse effects in theWC4+O (P = 3.033 d) pair in Br 31 similar to, but weaker than thoseobserved in the Small Magellanic Cloud system Sk 188 (WO4+O4 V). Alow-amplitude ( ~ 0 km s-1) variation in the radial velocity of UVphotospheric absorption lines and the O V 1371 emission with the 3 dayperiod is detected. The radial velocity variations of the photosphericlines may be due to the superposition of the stationary set ofabsorption lines belonging to the O8 I: star and a broader set of linesbelonging to the O-type companion in the close binary pair. The UVcontinuum energy distribution of Br 31 also supports the optical resultsthat the system contains at least 3 bright stars, one of which is a lateO-type supergiant. Contrasting with Br 31, the absence of significant SiIV 1400 Å emission in the UV spectrum of Br 5 contradicts theresults from optical spectroscopy that imply that it is triple, with thepresence of a late O-type supergiant in the system. Orbitalphase-coverage of the IUE observations does not allow the detection ofpossible atmospheric eclipse effects in Br 5, with P = 2.001 d, butradial velocity variations attributable to orbital motion of the O3 If^*star are detected.
| Far Ultraviolet Spectroscopic Explorer Atlas of OB Stars in the Magellanic Clouds
An atlas of the 900-1200 Å region in the spectra of 47 OB stars inthe Large and Small Magellanic Clouds, observed at high resolution bythe Far Ultraviolet Spectroscopic Explorer (FUSE), is presented anddiscussed. The systematic trends in the numerous stellar-wind featuresin this region, some from species (and ionizations) not represented atlonger wavelengths, are charted as a function of the optical spectraltypes. The FUSE sample is by far the most powerful to date for thatpurpose. A special effort has been made to verify the spectral types ofall stars included in the atlas, in a number of cases with new opticalobservations that are also illustrated, to avoid uncertainties from thatsource in the stellar-wind trends. A new O2 star has been found in theprocess. Most of these stars have been previously observed at longerultraviolet wavelengths by the Hubble Space Telescope and in the opticalfrom the ground with high-resolution, digital instruments; thus verycomprehensive physical modeling of these OB atmospheres and winds nowbecomes possible. This atlas will serve as a guide to the FUSEMagellanic Cloud OB database for that purpose. The Magellanic Cloudsample provides a very important complement to the FUSE database ofGalactic OB counterparts (Pellerin et al. 2002), both because the lowerextinction and interstellar H2 absorption toward the Cloudstars allow a much clearer view of the stellar spectra below 1100Å, and because of the metallicity differences among the threegalaxies. In particular, most wind features in the SMC spectra are seento be significantly weaker than those in the LMC at the same spectraltypes. 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. Also based in part onobservations collected at the European Southern Observatory in program67.D-0238, and at the Australian National University/Siding SpringObservatory.
| Far-Ultraviolet Spectroscopic Explorer Observations of Degree-Scale Variations in Galactic Halo OVI
We report Far-Ultraviolet Spectroscopic Explorer (FUSE) observations ofinterstellar O VI absorption in the halo of the Milky Way toward 12early-type stars in the Large Magellanic Cloud (LMC) and 11 in the SmallMagellanic Cloud (SMC). The mean column densities of O VI associatedwith the Galactic halo toward the LMC and SMC arelog=14.52+0.10-0.14 and14.13+0.14-0.20, respectively, where theuncertainties represent the standard deviations of the individualmeasurements about the means. Significant variations in the O VI columndensities are observed over all of the angular scales probed by ourobservations: 0.5d-5.0d toward the LMC and 0.05d-3.3d toward the SMC.The maximum factors by which the O VI varies between sight lines towardthe LMC and SMC are ~2.8 and ~4.2, respectively. Although low-,intermediate-, and high-velocity clouds are present along most of thesight lines toward the LMC, the variations in O VI column densities arefound in all of these O VI absorbing structures. The column densityvariations trace the structure of the hot ionized medium in the Galactichalo on scales of <<80-800 pc toward the LMC and <<6-400 pctoward the SMC (assuming that the absorption arises within the first 5kpc above the Galactic plane).
| A New Spectral Classification System for the Earliest O Stars: Definition of Type O2
High-quality, blue-violet spectroscopic data are collected for 24 starsthat have been classified as type O3 and that display the hallmark N IVand N V lines. A new member of the class is presented; it is the secondknown in the Cyg OB2 association, and only the second in the northernhemisphere. New digital data are also presented for several of the otherstars. Although the data are inhomogeneous, the uniform plots bysubcategory reveal some interesting new relationships. Several issuesconcerning the classification of the hottest O-type spectra arediscussed, and new digital data are presented for the five original O3dwarfs in the Carina Nebula, in which the N IV, N V features are veryweak or absent. New spectral types O2 and O3.5 are introduced here assteps toward resolving these issues. The relationship between thederived absolute visual magnitudes and the spectroscopic luminosityclasses of the O2-O3 stars shows more scatter than at later O types, atleast partly because some overluminous dwarfs are unresolved multiplesystems, and some close binary systems of relatively low luminosity andmass emulate O3 supergiant spectra. However, it also appears that thebehavior of He II λ4686, the primary luminosity criterion atlater O types, responds to other phenomena in addition to luminosity atspectral types O2-O3. There is evidence that these spectral types maycorrespond to an immediate pre-WN phase, with a correspondingly largerange of luminosities and masses. A complete census of spectraclassified into the original O3 subcategories considered here (notincluding intermediate O3/WN types or O3 dwarfs without N IV, N Vfeatures) totals 45 stars; 34 of them belong to the Large MagellanicCloud and 20 of the latter to 30 Doradus.
| The Global Content, Distribution, and Kinematics of Interstellar O VI in the Large Magellanic Cloud
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations ofinterstellar O VI absorption toward 12 early-type stars in the LargeMagellanic Cloud (LMC). The observations have a velocity resolution of<~20 km s-1 (FWHM) and clearly show O VI 1031.926 Åabsorption at LMC velocities toward all 12 stars. From theseobservations we derive column densities of interstellar O VI in thisnearby galaxy; the observed columns are in the rangelogN(OVI)=13.9-14.6, with a mean of 14.37 and a standard deviation of+/-38% (+0.14-0.21 dex). The observations probeseveral sight lines projected onto known superbubbles in the LMC, butthese show relatively little (if any) enhancement in O VI column densitycompared to sight lines toward relatively quiescent regions of the LMC.The observed LMC O VI absorption is broad, with Gaussian dispersionsσ~30-50 km s-1. This implies temperaturesT<~(2-5)×106, indicating that much of the broadeningis nonthermal because O VI has a very low abundance at such hightemperatures. The O VI absorption is typically displaced ~-30 kms-1 from the corresponding low-ionization absorptionassociated with the bulk of the LMC gas. The general properties of theLMC O VI absorption are very similar to those of the Milky Way halo. Theaverage column density of O VI and the dispersion of the individualmeasurements about the mean are identical to those measured for the haloof the Milky Way, even though the metallicity of the LMC is a factor of~2.5 lower than the Milky Way. The velocity dispersion measured for theLMC material is also consistent with recent measurements of the Galactichalo. The striking similarities in these quantities suggest that much ofthe LMC O VI may arise in a vertically extended distribution similar tothe Galactic halo. We discuss the measurements in the context of a halocomposed of radiatively cooling hot gas and/or turbulent mixing layers.If the observed O VI absorption is tracing a radiatively coolinggalactic fountain flow, the mass flow rate from one side of the LMC diskis of the order M~1 Msolar yr-1, with a mass fluxper unit area of the disk M/Ω~2×10-2Msolar yr-1 kpc-2.
| An Atlas of Far Ultraviolet Spectroscopic Explorer Sight Lines toward the Magellanic Clouds
We present an atlas of 57 Large Magellanic Cloud (LMC) and 37 SmallMagellanic Cloud (SMC) observations obtained with the Far UltravioletSpectroscopic Explorer (FUSE) satellite. The atlas highlights 12interstellar absorption-line transitions at a resolution of ~15 kms-1. These transitions cover a broad range of temperatures,ionization states, and abundances. The species included are O VI, whichprobes hot (T~3×105 K) ionized gas; C III and Fe III,which probe warm (T~104 K) ionized gas; Si II, P II, C II, FeII, and O I, which probe warm neutral gas; and six different molecularhydrogen transitions, which trace cold (T<=500 K) gas. We includeSchmidt Hα CCD images of the region surrounding each sight lineshowing the morphology of warm ionized gas in the vicinity, along withcontinuum images near each FUSE aperture position. We present severalinitial scientific results derived from this dataset on the interstellarmedium of the Magellanic Clouds and Galactic halo. O VI absorption atMagellanic Cloud velocities appears along nearly all sight lines,regardless of optical emission-line morphology. The velocity field ofLMC disk material is probed using P II λ1152.8 absorption and isseen to be consistent with recent H I results. While the velocitystructure of the SMC is complex, two absorption features are clearlyseparated in the SMC data-a strong absorption complex between +100 and+130 km s-1, and a weaker feature near +180 kms-1. The velocity separation between these complexes varieswith position, being greater on average in the southwest portion of theSMC. A lower velocity absorption component seen the nine sight linestoward the bright H II region N66 in the SMC may be the result of anoutflow or an old SNR within this nebular complex. Absorption in Fe IIand O I at ~+60 km s-1 and ~+120 km s-1 appearalong many LMC sight lines. They are attributed, respectively, to anintermediate-velocity cloud and a high-velocity cloud in the Milky Wayhalo. Both features are dramatically stronger toward the eastern half ofthe LMC and are not correlated with each other or with LMC Hαmorphology. The lower velocity of the SMC and broader absorption linescomplicate the detection of intermediate and high velocity Galacticabsorption along SMC sight lines. This work contains data obtained forthe Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated bythe Johns Hopkins University. Financial support has been provided byNASA contract NAS 5-32985.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds
We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer(FUSE) survey of H2 along 70 sight lines to the Small andLarge Magellanic Clouds, using hot stars as background sources. FUSEspectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92%of SMC) exhibit absorption lines from the H2 Lyman and Wernerbands between 912 and 1120 Å. Our survey is sensitive toN(H2)>=1014 cm-2 the highest columndensities are logN(H2)=19.9 in the LMC and 20.6 in the SMC.We find reduced H2 abundances in the Magellanic Cloudsrelative to the Milky Way, with average molecular fractionsH2>=0.010+0.005-0.002 forthe SMC andH2>=0.012+0.006-0.003 forthe LMC, compared with H2>=0.095 for theGalactic disk over a similar range of reddening. The dominantuncertainty in this measurement results from the systematic differencesbetween 21 cm radio emission and Lyα in pencil beam sight lines asmeasures of N(H I). These results imply that the diffuse H2masses of the LMC and SMC are 8×106 and2×106 Msolar, respectively, 2% and 0.5% ofthe H I masses derived from 21 cm emission measurements. The LMC and SMCabundance patterns can be reproduced in ensembles of model clouds with areduced H2 formation rate coefficient,R~3×10-18 cm3 s-1, and incidentradiation fields ranging from 10-100 times the Galactic mean value. Wefind that these high-radiation, low formation rate models can alsoexplain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitationratios in the Clouds. We use H2 column densities in lowrotational states (J=0 and 1) to derive kinetic and/or rotationaltemperatures of diffuse interstellar gas, and we find that thedistribution of rotational temperatures is similar to Galactic gas, with=82+/-21 K for clouds withN(H2)>=1016.5 cm-2. There is only aweak correlation between detected H2 and far-infrared fluxesas determined by IRAS, perhaps as a result of differences in the surveytechniques. We find that the surface density of H2 probed byour pencil beam sight lines is far lower than that predicted from thesurface brightness of dust in IRAS maps. We discuss the implications ofthis work for theories of star formation in low-metallicityenvironments. This work is based on data obtained for the GuaranteedTime Team by the NASA-CNES-CSA FUSE mission operated by the JohnsHopkins University. Financial support to US participants has beenprovided by NASA contract NAS 5-32985.
| Multi-frequency variations of the Wolf-Rayet system HD193793 (WC7pd+O4-5) III. IUE observations
The colliding-wind binary system WR 140 (HD 193793, WC7pd+O4-5, P = 7.94yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35short-wavelength high-resolution spectra. An absorption-lineradial-velocity solution is obtained from the photospheric lines of theO component, by comparison with a single O star. The resulting orbitalparameters, e = 0.87 +/- 0.05, omega = 31degr +/- 9degr andKO star = 25 +/- 15 km s-1, confirm the largeeccentricity of the orbit, within the uncertainties of previous opticalstudies. This brings the weighted mean UV-optical eccentricity to e =0.85 +/- 0.04. Occultation of the O-star light by the WC wind and theWC+O colliding-wind region results into orbital modulation of theP-Cygni profiles of the C ii, C iv and Si iv resonance lines. Nearperiastron passage, the absorption troughs of those resonance-lineprofiles increase abruptly in strength and width, followed by a gradualdecrease. In particular, near periastron the blue black-edges of theP-Cygni absorption troughs shift to larger outflow velocities. Wediscuss that the apparently larger wind velocity and velocity dispersionobserved at periastron could be explained by four phenomena: (i)geometrical resonance-line eclipse effects being the main cause of theobserved UV spectral variability, enhanced by sightline crossing of theturbulent wind-wind collision zone; (ii) the possibility of anorbital-plane enhanced WC7 stellar wind; (iii) possible common-envelopeacceleration by the combined WC and O stellar radiation fields; and (iv)possible enhanced radiatively driven mass loss due to tidal stresses,focused along the orbiting line of centers.
| Empirical Verification of the Fe II Oscillator Strengths in the FUSE Bandpass
We report empirical determinations of atomic oscillator strengths, orf-values, for 11 ground-state transitions of Fe II in the wavelengthrange 1050<~λ<~1150 Å. We use ultravioletabsorption-line observations of interstellar material toward stars inthe Galaxy and the Magellanic Clouds taken with Copernicus, the GoddardHigh Resolution Spectrograph on board the Hubble Space Telescope, andthe Far-Ultraviolet Spectroscopic Explorer. We derive absoluteoscillator strengths by a combination of the apparent optical depth,component-fitting, and curve-of-growth fitting techniques. Our derivedoscillator strengths are generally in excellent agreement with recenttheoretical calculations by Raassen & Uylings using the orthogonaloperator technique. However, we identify three of the 11 transitionsstudied here whose f-values seem to be incompatible with thesecalculations by as much as a factor of 2. We suggest revisions to thesef-values based on our analysis. Based on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Science Institute. STScI is operated by the Associationof Universities for Research in Astronomy, Inc. under the NASA contractNAS 5-26555.
| UBV photometry of Galactic foreground and LMC member stars - III. LMC member stars - a new data base
New UBV photometry for 878 luminous member stars of the Large MagellanicCloud (LMC) and 13 stars of uncertain membership is presented. The datawill be available at Centre de Données astronomiques deStrasbourg. Including former observations now UBV data are available foraltogether 2470 luminous LMC stars and 2106 foreground stars plus 65stars of uncertain membership. The observations have been used alreadyfor several investigations dealing e.g. with interstellar reddeninglines and intrinsic colours, the dust distribution and the calibrationof charge-coupled device exposures.
| Ultraviolet Spectral Classification of O and B Stars in the Large Magellanic Cloud
In this extension of our previous ultraviolet classification study ofSmall Magellanic Cloud stars, we construct an ultraviolet classificationsystem for stars of the Large Magellanic Cloud, a galaxy with ametallicity intermediate to those of the Milky Way and the SMC.Employing the same method as was used for the SMC, we compile a set ofLMC spectra drawn from the International Ultraviolet Explorer archivesand use these spectra to construct a framework of reliable UV spectralclassification criteria. We classify 143 LMC IUE spectra and identifythe LMC standard stars that define our UV classification system.Representative spectra are presented to illustrate the patterns that theUV spectral classification features exhibit. The LMC and SMC UVclassification systems are intended to be applicable to UV spectra ofstars in external galaxies for which no MK standards have been defined.Such UV spectra are expected to be collected from spaceborne instrumentssuch as the Space Telescope Imaging Spectrograph, aboard the HubbleSpace Telescope.
| Spectroscopic binaries in the Large Magellanic Cloud.
Not Available
| HSTUV measurements of wind structure and velocities in Local Group OB stars
Archival HST FOS and GHRS data sets have been used to collectultraviolet evidence for large- and small-scale stellar wind structurein extragalactic Local Group OB stars (i.e. SMC, LMC including R136,M31, M33 and NGC 6822). By comparison with previous studies of GalacticOB stars, wind activity is principally diagnosed in individualspectrograms via the presence of `narrow absorption components' andsaturated `black' absorption troughs in the resonance line doublets.Their characteristics broadly suggest that these stars share the samephysical mechanisms for perturbing the winds as those that act inGalactic stars. Both of these spectral indicators are also used toprovide reliable measures of wind terminal velocities. These velocitiesare directly compared with previously published Galactic values, withoutreliance on model profile fitting. Relative to Galactic OB stars, themost discrepant terminal velocities (and wind line profiles) result frommain-sequence early O-type stars in the SMC.
| The Pistol Star
We present new near-infrared data and analysis, which indicate that thePistol Star is one of the most luminous stars known, adding another testpoint for massive star formation and stellar evolution theories. Weestimate an extinction of A_K = 3.2 +/- 0.5 using the near-infraredcolors of the star and of surrounding stars in the young Quintupletcluster. Using our wind/atmosphere code, we find two families of modelsthat fit the spectral energy distribution and detailed line profiles.The lower luminosity models give L = 10^6.6+/-0.2 L_ȯ and T_eff =10^4.15+/-0.01 K, while the higher luminosity models give L =10^7.2+/-0.2 L_ȯ and T_eff = 10^4.33+/-0.01 K; the error inluminosity assumes an uncertainty of +/-0.5 in A_K, while the error inT_eff is constrained by detailed line modeling. The models also reveal ahelium enriched surface. As previously existing stellar evolution modelsdo not extend to such high luminosities, we employ new evolutionarytracks for very massive stars to determine the initial mass and age ofthe Pistol Star, and estimate M_initial = 200-250 M_ȯ and an age of1.7-2.1 Myr. The inferred luminosity and temperature place the star in asparsely populated zone in the H-R diagram where luminous blue variables(LBVs) are often found. This is consistent with our evolutionary models,which predict that the star is in an unstable evolutionary stage. Weinterpret the star and its surrounding nebula as an LBV that hasrecently ejected large amounts of material. Our K-band speckle-imagingdata reveal the star to be single down to a projected separation of 110AU.
| Ultraviolet Imaging Telescope Observations of the Magellanic Clouds
We present wide-field far-ultraviolet (FUV; 1300-1800 Å) images ofthe Large and Small Magellanic Clouds (LMC, SMC). These data wereobtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-1(1990 December 1-10) and Astro-2 (1995 March 2-18) missions; the imagesprovide an extensive FUV mosaic of the SMC and contain numerous regionsin the LMC, covering a wide range of stellar densities and current starformation activity. A total of 47 LMC/Lucke-Hodge and 37 SMC/Hodge OBassociations are completely or partially included in the observedfields. FUV data can identify the hottest OB stars more easily than canoptical photometry, and these stars dominate the ionizing flux, which iscorrelated to the observed Hα flux of the associated H ii regions.Of the H ii regions in the catalog of Davies, Elliott, & Meaburn(DEM), the UIT fields completely or partially include 102 DEM regions inthe LMC and 74 DEM regions in the SMC. We present a catalog of FUVmagnitudes derived from point-spread function photometry for 37,333stars in the LMC (the UIT FUV magnitudes for 11,306 stars in the SMCwere presented recently by Cornett et al.), with a completeness limit ofm_UV ~ 15 mag and a detection limit of m_UV ~ 17.5. The averageuncertainty in the photometry is ~0.1 mag. The full catalog withastrometric positions, photometry, and other information is alsoavailable from publicly accessible astronomical data archives. We dividethe catalog into field stars and stars that are in DEM regions. Weanalyze each of these two sets of stars independently, comparing thecomposite UV luminosity function of our data with UV magnitudes derivedfrom stellar evolution and atmosphere models in order to derive theunderlying stellar formation parameters. We find a most probable initialmass function (IMF) slope for the LMC field stars of Gamma = -1.80 +/-0.09. The statistical significance of this single slope for the LMCfield stars is extremely high, though we also find some evidence for afield star IMF slope of Gamma ~ -1.4, roughly equal to the Salpeterslope. However, in the case of the stars in the DEM regions (the starsin all the regions were analyzed together as a single group), we findthree IMF slopes of roughly equal likelihood: Gamma = -1.0, -1.6, and-2.0. No typical age for the field stars is found in our data for timeperiods up to a continuous star formation age of 500 Myr, which is themaximum age consistent with the completeness limit magnitude of thecatalog's luminosity function. The best age for the collection ofcluster stars was found to be t_0 = 3.4 +/- 1.9 Myr; this is consistentwith the age expected for a collection of OB stars from many differentclusters.
| The Pistol Star
Results of an spectroscopic investigation of the Pistol star arepresented. The near-infrared spectra and photometry data are fit withstellar wind models to find that the star is extraordinarily luminous, L= 106.7±0.5 Lȯ, making it one of themost luminous stars known. Coupled with the relatively cool temperature,Teff = 10^{4.17_{ - 0.06}^{ + 0.19} } K, the star is clearlyin violation of the Humphreys-Davidson limit. The derived line of sightvelocity of the star assures its membership in the Quintuplet cluster.This, along with the inferred extinction, places the star at theGalactic Center.
| Kinematics of the very young nebula N59 at the edge of the supershell LMC4
The dynamics of the nebula N59 (B053540-6736), located at the boundaryof the supershell LMC4 in the Large Magellanic Cloud, is studied using ascanning Fabry-Perot interferometer. It is shown that the nebulae NGC2032 and 2035, which form the bright core of the H II region N59A(B053530-6736), belong to a single H II region which looks divided dueto the presence of a heavy dust lane. This bright core presents anexpansion motion of 24 km s(-1) . The kinetic energy involved in thismotion is of about 1.5 x 10(49) erg. This value is compatible eitherwith a supernova explosion origin or with a formation by the winds ofinterior massive stars. Since no clear traces of a SN explosion havebeen found in this nebula and since the stellar content of N59A(B053530-6736) is rich in blue stars, we conclude that these stars,mainly the very massive star HDE 269810 (R122), and probably other starshidden by the dust lane, are sufficient in providing the wind power todrive the expansion motion. The dust lane seems to be mixed in with thenebular gas and the stars, suggesting a site where star formation maystill take place. An extended shell, probably ionized by the star R122,has been detected at the same velocities as the slab, at blueshiftedvelocity, seen in the foreground absorption. The star R122 contributesto the high excitation of the faint diffuse gas, and perhaps of somefarther nebulae. To the East, the SNR 0536-676 remains as a trace of theexistence of another massive star which had already exploded. Thekinematics of N59B (B053610-6736) which contains the SNR 0536-676, isalso studied, corroborating the results of previous studies. Based onobservations collected at the European Southern Observatory
| O VI + Ly beta + C II from Starburst and Poststarburst Galaxies. I. Stellar Library and Evolutionary Synthesis Profiles
Evolutionary synthesis models of a stellar population in thefar-ultraviolet are presented. The spectra include the lines O VI lambdalambda 1032, 1038, Ly beta , and C II lambda lambda 1036, 1037. They arebased on a stellar library built with observations of O and B starscollected with Copernicus and the Hopkins Ultraviolet Telescope (HUT).This library is used as input into an evolutionary synthesis code. Theline profile of O VI + Ly beta + C II is computed for different starformation histories (instantaneous burst and continuous star formation)and different assumptions about the initial mass function (IMF). Themetallicity is near the solar value. O VI lambda lambda 1032, 1038 is avery sensitive indicator of the presence or absence of O stars. O VIdevelops a P Cygni profile when formed in stellar winds of the mostmassive stars. When these stars are absent, no O VI is formed. Incontrast, Ly beta and C II are very sensitive indicators for B stars. Ifthese stars dominate, as is the case in poststarburst galaxies, Ly betaand C II are present as strong absorption features, and they are formedin the photosphere of B stars. An equivalent width of Ly beta + C IIlarger than 1 Angstroms always indicates a population younger than 1Gyr. Because of the universal strength of O VI in O stars, O VI is not agood discriminator between instantaneous versus continuous starformation for ages in the starburst phase, but the absence of O VI andthe presence of stellar Ly beta and C II is a good indicator of a shortburst duration and for the galaxy being in a poststarburst phase.Application of this technique to starburst or poststarburst galaxieswill require careful attention to interstellar absorption.
| O-star mass-loss and wind momentum rates in the Galaxy and the Magellanic Clouds Observations and theoretical predictions.
A new, very fast approximate method is presented to determine mass-lossrates of O-stars from Halpha_ line profiles. The method usesH and HeII departure coefficients from unified model atmospheresparametrized in a simple way as function of wind velocity together withphotospheric NLTE line profiles as the inner boundary condition for anumerically exact radiative transfer solution to derive a windcontaminated Halpha_-profile. The method is also applied toHgamma_ to determine stellar gravities corrected for windemission. A detailed analytical discussion of Halpha_ lineformation in O-star winds is given and it is demonstrated that formervery simple approaches considering only optically thin wind emissionlead to significant systematic errors. Scaling relations and generalizedcurves of growth are presented that connect mass-loss rate, terminalvelocity, stellar parameters and Halpha_ equivalent width.The method is applied to samples of O-stars in the Galaxy, LMC and SMCand mass-loss rates are derived from Halpha_ in combinationwith terminal velocities measured from IUE and HST spectra. The resultsreveal that a tight empirical relation exists between the radiusmodified stellar wind momentum rate˙(M)vinfinity_R_*_^0.5^ and the stellar luminosity. Thevariations of this relationship between the Galaxy, LMC and SMC areexplained in terms of different abundances. Furthermore, for almost allobjects with dense winds (mostly supergiants), the commonly usedvelocity field exponent β could be derived, indicating a typicalvalue of β=~1. A comparison with the improved theory of radiationdriven winds (as presented recently by Pauldrach et al. 1994) shows thatthe observed wind momentum-luminosity relationship can be understoodqualitatively in terms of the theory. However, there exist significantsystematic discrepancies as a function of effective temperature,luminosity class and wind performance numberη=˙(M)vinfinity_c/L. We stress that thesediscrepancies would not have been detected with previous simplifiedHalpha_ -approaches. The deficiencies of the theory arediscussed and suggestions for future improvements are made.
| Gas motions in and origin of the supergiant shell LMC4.
IUE high-dispersion spectra of stars in the supergiant shell LMC4havebeen used to derive velocities and column densities of absorbing gasclouds. The HI 21-cm profiles for the LMC4region from the LMC data ofRohlfs et al. (1984) have been analysed and interpreted in cloudcomponents. Combining the absorption and emission line data, a detailedpicture emerges of the location along the line of sight of the variousgas components and of the stars. LMC4 has a systemic velocity of285km/s. A pocket of gas in the center of LMC4is found to be receding at~325km/s while over the entire area we find gas near 260km/s. These twocomponents of gas differ in various ways. We argue that the gas at325km/s most likely belongs to a shell breakup. The gas at 260km/sprobably does not belong to the LMC4but lies in front of the supershell.There is no evidence for a systematic radial expansion of the gas fromthe geometric center of LMC4. Also, there is gas near 220km/s found inseveral of the IUE spectra. Given that the velocity differs by 65km/sfrom that of the main body of LMC4and the large spatial extent wepropose this gas cloud to be in the Milky Way halo. Our results arecompared with several theoretical scenarios for superbubble structures.We conclude that the origin of LMC4 is best explained by a process ofself-propagating star formation. Our data are in agreement withexpansion velocities around 10km/s.
| The physics of massive OB stars in different parent galaxies. 1: Ultraviolet and optical spectral morphology in the Magellanic Clouds
Hubble Space Telescope/Faint Object Spectrograph (HST/FOS) and EuropeanSpace Observatory (ESO) 3.6-m/CASPEC observations have been made of 18stars ranging in spectral type from O3 through B0.5 Ia, half of them ineach of the Large and Small Magellanic Clouds, in order to investigatemassive stellar winds and evolution as a function of metallicity. Thespectroscopic data are initially presented and described here in anatlas format. The relative weakness of the stellar-wind features in theSMC early O V spectra, due to their metal deficiency, is remarkable.Because of their unsaturated profiles, discrete absorption componentscan be detected in many of them, which is generally not possible in LMCand Galactic counterparts at such early types, or even in SMC giants andsupergiants. On the other hand, an O3 III spectrum in the SMC has a weakC IV but strong N V wind profile, possibly indicating the presence ofprocessed material. Wind terminal velocities are also given andintercompared between similar spectral types in the two galaxies. Ingeneral, the terminal velocities of the SMC stars are smaller, inqualitative agreement with the predictions of radiation-driven windtheory. Further analyses in progress will provide atmospheric and windparameters for these stars, which will be relevant to evolutionarymodels and the interpretation of composite starburst spectra.
| Mission Astro 2. Un observatoire a bord d'Endeavour.
Not Available
| Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs
Strong x-ray emission detected in Large Magellanic Cloud (LMC)superbubbles has been explained as the result of interior supernovaremnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot befound using conventional criteria. We thus investigate the possibilityof using the interstellar absorption properties in the ultraviolet (UV)as a diagnostic of hidden SNR shocks. The International UltravioletExplorer (IUE) archives provide the database for this pilot study. Theycontain high-dispersion spectra of several stars in x-ray brightsuperbubbles. To distinguish the effects of SNR shocks from those oflocal stellar winds and a global hot halo around the LMC, we includedcontrol objects in different environments. We find that almost allinterstellar absorption properties can be explained by the interstellarenvironment associated with the objects. Summarizing the two mostimportant results of this study: (1) a large velocity shift between thehigh-ionization species (C IV and Si IV) and the low-ionization species(S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however,the absence of a velocity shift does not preclude the existence of SNRshocks; (2) there is no evidence that the LMC is uniformly surrounded byhot gas; hot gas is preferentially found associated with largeinterstellar structures like superbubbles and supergiant shells, whichmay extend to large distances from the plane.
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Observation and Astrometry data
| Constellation: | Dorade |
| Right ascension: | 05h35m13.90s |
| Declination: | -67°33'27.5" |
| Apparent magnitude: | 12.126 |
| Proper motion RA: | 0.9 |
| Proper motion Dec: | -0.9 |
| B-T magnitude: | 11.768 |
| V-T magnitude: | 12.097 |
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