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Towards an understanding of the Of?p star HD191612: phase-resolved multiwavelength observations
We present the analysis of phase-resolved X-ray and optical observationsof the peculiar hot star HD191612 (Of?p). This star is known to displayline-profile variations that are recurrent with a period of 538d and itsspectrum was found to present the signature of a magnetic field. In theX-rays, it is slightly overluminous compared to the canonicalLX/LBOL) relation and appears brighter when theoptical lines are strongest. Our XMM-Newton observations further revealthat the X-ray spectrum of HD191612 exhibits rather broad lines and isdominated by a `cool' (0.2-0.6keV) thermal component, twocharacteristics at odds with the proposed magnetic rotator model. Wealso report for the first time the low-level variability of the metallic(absorption/emission) lines and HeII absorptions that appear to beassociated with radial-velocity shifts. Finally, we compare our resultswith observations of the early-type stars and discuss several possiblescenarios.Based on observations collected at the Haute-Provence Observatory(France) and with XMM-Newton, an ESA Science Mission with instrumentsand contributions directly funded by ESA Member States and the USA(NASA).E-mail: naze@astro.ulg.ac.be ‡Post-doctoral Researcher FNRS (Belgium). §Research Associate FNRS (Belgium). ¶Operated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555.
| Early-type stars observed in the ESO UVES Paranal Observatory Project - I. Interstellar NaI UV, TiII and CaII K observations*
We present an analysis of interstellar NaI (λair=3302.37 and 3302.98 Å), TiII(λair= 3383.76Å) and CaII K (λair= 3933.66 Å) absorptionfeatures for 74 sightlines towards O- and B-type stars in the Galacticdisc. The data were obtained from the Ultraviolet and Visual EchelleSpectrograph Paranal Observatory Project, at a spectral resolution of3.75 km s-1 and with mean signal-to-noise ratios per pixel of260, 300 and 430 for the NaI, TiII and CaII observations, respectively.Interstellar features were detected in all but one of the TiIIsightlines and all of the CaII sightlines. The dependence of the columndensity of these three species with distance, height relative to theGalactic plane, HI column density, reddening and depletion relative tothe solar abundance has been investigated. We also examine the accuracyof using the NaI column density as an indicator of that for HI. Ingeneral, we find similar strong correlations for both Ti and Ca, andweaker correlations for Na. Our results confirm the general belief thatTi and Ca occur in the same regions of the interstellar medium (ISM) andalso that the TiII/CaII ratio is constant over all parameters. We henceconclude that the absorption properties of Ti and Ca are essentiallyconstant under the general ISM conditions of the Galactic disc.
| The Evolution of Supernovae in Circumstellar Wind-Blown Bubbles. I. Introduction and One-Dimensional Calculations
Mass loss from massive stars (>~8 Msolar) can result inthe formation of circumstellar wind-blown cavities surrounding the star,bordered by a thin, dense, cold shell. When the star explodes as acore-collapse supernova (SN), the resulting shock wave will interactwith this modified medium around the star, rather than the interstellarmedium. In this work we first explore the nature of the circumstellarmedium around massive stars in various evolutionary stages. This isfollowed by a study of the evolution of SNe within these wind-blownbubbles. The evolution depends primarily on a single parameter Λ,the ratio of the mass of the dense shell to that of the ejectedmaterial. We investigate the evolution for different values of thisparameter. We also plot approximate X-ray surface brightness plots fromthe simulations. For very small values Λ<<1 the effect ofthe shell is negligible, as one would expect. Values of Λ<~1affect the SN evolution, but the SN ``forgets'' about the existence ofthe shell in about 10 doubling times or so. The remnant density profilechanges, and consequently the X-ray emission from the remnant will alsochange. The initial X-ray luminosity of the remnant is quite low, butinteraction of the shock wave with the dense circumstellar shell canincrease the luminosity by 2-3 orders of magnitude. As the reflectedshock begins to move inward, X-ray images will show the presence of adouble-shelled structure. Larger values result in more SN energy beingexpended to the shell. The resulting reflected shock moves quickly backto the origin, and the ejecta are thermalized rapidly. The evolution ofthe remnant is speeded up, and the entire remnant may appear bright inX-rays. If Λ>>1, then a substantial amount of energy may beexpended in the shell. In the extreme case the SN may go directly fromthe free expansion to the adiabatic stage, bypassing the Sedov stage.Our results show that in many cases the SNR spends a significant amountof time within the bubble. The low density within the bubble can delaythe onset of the Sedov stage and may end up reducing the amount of timespent in the Sedov stage. The complicated density profile within thebubble makes it difficult to infer the mass-loss properties of thepre-SN star by studying the evolution of the resulting SNR.
| An Ultrasoft Transient X-Ray Source in the Norma Region Discovered with Ginga
A new X-ray source was discovered in the Norma region with Ginga in 1988April. The source position, derived from one-dimensional scan data, andits spectral properties are presented. The X-ray emission was dominantbelow ˜ 6 keV and not found in other observations. Thus, it isthought to be an ultrasoft transient X-ray source. The X-ray spectrum,constructed by a scan fit technique, was well represented by eitherblackbody, thermal bremsstrahlung, power-law, or multicolor disk models.Although all of the spectral models were acceptable from a statisticalpoint of view, the ultrasoft nature of the spectrum and the transientbehavior suggest that this source is a candidate source of black holebinaries.
| On the Hipparcos parallaxes of O stars
We compare the absolute visual magnitude of the majority of bright Ostars in the sky as predicted from their spectral type with the absolutemagnitude calculated from their apparent magnitude and the Hipparcosparallax. We find that many stars appear to be much fainter thanexpected, up to five magnitudes. We find no evidence for a correlationbetween magnitude differences and the stellar rotational velocity assuggested for OB stars by Lamers et al. (1997, A&A, 325, L25), whosesmall sample of stars is partly included in ours. Instead, by means of asimulation we show how these differences arise naturally from the largedistances at which O stars are located, and the level of precision ofthe parallax measurements achieved by Hipparcos. Straightforwardlyderiving a distance from the Hipparcos parallax yields reliable resultsfor one or two O stars only. We discuss several types of bias reportedin the literature in connection with parallax samples (Lutz-Kelker,Malmquist) and investigate how they affect the O star sample. Inaddition, we test three absolute magnitude calibrations from theliterature (Schmidt-Kaler et al. 1982, Landolt-Börnstein; Howarth& Prinja 1989, ApJS, 69, 527; Vacca et al. 1996, ApJ, 460, 914) andfind that they are consistent with the Hipparcos measurements. AlthoughO stars conform nicely to the simulation, we notice that some B stars inthe sample of \citeauthor{La97} have a magnitude difference larger thanexpected.
| A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.
| The Remarkable Alternating Spectra of the Of?p Star HD 191612
The spectrum of HD 191612 has been found to display large, recurrentvariations between two highly reproducible, peculiar states; at leastfour transformations have occurred since 1950. In one state, thespectral type is O6-O7, with C III λ4650 emission comparable to NIII λ4640 (the definition of the Of?p category) and P Cygniprofiles at He II λ4686 and Hα. In the other state, thespectral type is O8, with the C III emission absent, very strong N IIIλ4097 absorption, broad He II λ4686 absorption with narrowcentral emission (a profile that may be unprecedented in this line amongknown O-type spectra), and a broad asymmetrical absorption at Hα.One observing sequence over several consecutive nights shows no spectralvariations, practically ruling out a short-period, interacting binary asthe origin of the phenomenon; moreover, no significant radial velocityvariations have been found. Although the sporadic observational recordprior to the discovery of the variations in early 2001 precludesdefinite conclusions, it is possible that a given state is maintainedfor a decade or longer, but one transformation occurred within 13months, and the data obtained during 2002 suggest an event with ashorter timescale.The Of?p category currently contains only five members: three in theGalaxy and two in the Small Magellanic Cloud. The other two Galacticmembers also display bizarre and unexplained phenomena; in the case ofHD 108, they are strikingly similar to those described here. Because oftheir relatively high X-ray luminosities, all three Galactic objectshave been suggested to have collapsed companions. If the spectralvariations of HD 108 and HD 191612 are due to binary interactions, theyare likely multiyear, eccentric systems like WR 140 and η Carinae.The axisymmetric shell ejections of HD 148937 could have a similarorigin. Alternatively, these stars may be rapid rotators or in anunstable evolutionary transitional stage. Further intensivespectroscopic monitoring is required to reveal their nature.
| An Inside-Out View of Bubbles
Fast stellar winds can sweep up ambient media and form bubbles. Theevolution of a bubble is largely controlled by the content and physicalconditions of the shocked fast wind in its interior. This hot gas wasnot clearly observed until the recent advent of Chandra and XMM-NewtonX-ray observatories. To date, diffuse X-ray emission has beenunambiguously detected from two circumstellar bubbles blown by WR stars,four planetary nebulae, and two superbubbles blown by young clusters.Model fits to the X-ray spectra show that the circumstellar bubbles aredominated by hot gas with low temperatures ( < 3×10^6 K), whilethe interstellar bubbles contain significant fractions of hotter gas (< 5×10^6 K) . In all cases, large discrepancies in the X-rayluminosity are found between observations and conventional models ofbubbles. Future theoretical models of bubbles need to re-examine thevalidity of heat conduction and take into account realistic microscopicprocesses such as mass loading from dense clumps/knots and turbulentmixing. Chandra ACIS-S observation of NGC 6888 will shed light on theseastrophysical processes.
| Ring nebulae around massive stars throughout the Hertzsprung-Russell diagram
Massive stars evolve across the H-R diagram, losing mass along the wayand forming a variety of ring nebulae. During the main sequence stage,the fast stellar wind sweeps up the ambient interstellar medium to forman interstellar bubble. After a massive star evolves into a red giantor a luminous blue variable, it loses mass copiously to form acircumstellar nebula. As it evolves further into a WR star, the fast WRwind sweeps up the previous mass loss and forms a circumstellar bubble.Observations of ring nebulae around massive stars not only arefascinating, but also are useful in providing templates to diagnose theprogenitors of supernovae from their circumstellar nebulae. In thisreview, I will summarize the characteristics of ring nebulae aroundmassive stars throughout the H-R diagram, show recent advances in X-rayobservations of bubble interiors, and compare supernovae's circumstellarnebulae with known types of ring nebulae around massive 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.
| Faint X-Ray Sources Resolved in the ASCA Galactic Plane Survey and Their Contribution to the Galactic Ridge X-Ray Emission
The X-ray emission from the central region of the Galactic plane,|l|<~45° and |b|<~0.4d, was studied in the 0.7-10 keV energyband with a spatial resolution of ~3' with the Advanced Satellite forCosmology and Astrophysics (ASCA) observatory. We developed a newanalysis method for the ASCA data to resolve discrete sources from theextended Galactic ridge X-ray emission (GRXE). We successfully resolved163 discrete sources with an X-ray flux down to 10-12.5 ergscm-2 s-1 and determined the intensity variationsof the GRXE as a function of the Galactic longitude with a spatialresolution of about 1°. The longitudinal intensity variation in theenergy band above 4 keV, for which there is little absorption in theGalactic plane, shows a large enhanced feature within |l|<~30°.This suggests a strong enhancement of X-ray emissivity of the GRXEinside the 4 kpc arm of the Galaxy. Searches for identifications of theresolved X-ray sources with cataloged X-ray sources and optical starsshow that the 66% are unidentified. Spectral analysis of each sourceshows that a large number of the unidentified sources have hard X-rayspectra obscured by the Galactic interstellar medium. We classified thesources into several groups by the flux, the hardness and the softnessof the spectra, and performed further detailed analysis for the spectrasummed within each group. Possible candidates of X-ray origins of theseunidentified sources are discussed based on the grouping spectralanalysis. Also, we derived the logN-logS relations of the resolvedsources in the energy bands below and above 2 keV separately. ThelogN-logS relation of the Galactic X-ray sources above 2 keV wasobtained for the first time with this study. It is represented by apower-law with an index of -0.79+/-0.07 after correction for thecontribution of extragalactic X-ray sources. This flat power-lawrelation suggests that the spatial distribution of the X-ray sourcesshould have an armlike structure in which the solar system is included.The integrated surface brightness of the resolved sources is about 10%of the total GRXE in both energy bands. The approximately 90% of theemission remaining is still unresolved.
| A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We report on a comprehensive search for Wolf-Rayet (W-R) stars in theSMC using interference filter imaging. Photometry of over 1.6 millionstellar images on multiple, overlapping fields covering 9.6deg2 found the previously known W-R stars at very highsignificance levels, two known Of-type stars, plus additionalcandidates, which we examined with slit spectroscopy. We discovered twonew Wolf-Rayet stars, both of type ``WN3+abs,'' bringing the totalnumber in the SMC to 11. We discuss their spectra, as well asreclassifying the previously known ones with our new data. Our surveyalso revealed four newly found Of-type stars, including one of the O5f?ptype, which is one of the earliest type stars known in the SMC. Anothernewly identified Of star is AV 398 (O8.5 If), a star often used inextinction studies under the assumption that it is of early B type. Werecover S18 (AV 154), a B[e] star whose spectrum currently lacks He IIλ4686 emission but which must have had strong emission a yearearlier; we compare this star to S Dor, suggesting that it is indeed aluminous blue variable. We also find a previously unknown symbiotic starwhose spectrum is nearly identical to the Galactic symbiotic AG Dra.More important, perhaps, than any of these discoveries is thedemonstration that there is not a significant number of W-R starswaiting to be discovered in the SMC. The number of W-R stars is a factorof 3 times lower in the SMC (per unit luminosity) than in the LMC. Thisstrongly suggests that at the low metallicity that characterizes the SMConly the most massive stars can evolve to W-R type.
| S Doradus variables in the Galaxy and the Magellanic Clouds
The goal in writing this paper is five fold: (1) to summarize thescientific achievements in the 20th century on S Dor variables (orLBVs); (2) to present an inventory of these variables in the Galaxy andthe Magellanic Clouds with a description of their physical state andinstability properties; (3) to emphasize the photometric achievements ofthe various types of instabilities. Generally this seems to be aneglected item resulting in a number of misunderstandings continuouslywandering through literature; (4) to investigate the structure of the SDor-area on the HR-diagram; (5) to estimate the total numbers of S Dorvariables in the three stellar systems. The position of the strongactive S Dor variables in minimum brightness obey the following linearrelation on the HR-diagram:log L/Lsun = 1.37 log T_eff -0.03. The relatively small dispersion of less active and supposed ex-and dormant S Dor variables with respect to this relation is twice aslarge at the blue side than at the red side. This might be caused byevolution to the WR stage and/or to high rotation. S Dor variables canbe subject to five types of instabilities: the very rare genuineeruptive episodes (the ``SD-eruptions''), two different brighteningphases caused by slow pulsations (the ``SD-phases''): one on a timescale of years, the other on a time scale of decades at a more or lessconstant luminosity and two types of microvariations: one on a timescale of weeks, the other on a time scale of about 100 d. So far, noperiodicities of light curve characteristics of any of theseinstabilities have ever been found. The durations of active andnon-active stages are estimated for about half of the sample based onscattered magnitude estimations such as from historical records, and onmodern monitoring campaigns. It would be a misunderstanding to believethat all S Dor variables should be always spectacular. It is estimatedthat most of them will not be spectacular at all for at least 70% oftheir lifetime as an S Dor variable. Tables 1 to 6 and 8 to 17 are onlyavailable in electronic form at http://www.edpsciences.org, Table 7 isonly available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/366/508. Figures 2--10,12, 14, 15, 17--19 are only available in electronic form athttp://www.edpsciences.org, see Note added in proof
| Clouds of turbulent interstellar plasma in the spiral arms of the galaxy
It is shown that the interstellar turbulent plasma in the arms of theGalaxy has two components: component BII, which is extremely nonuniformand associated with HII regions and supernova remnants, and componentBI, which is more homogeneous and corresponds to the numerousStrömgren zones formed by 07-B0 stars. The following parameters areobtained for component BI: cloud size <= 50 kpc, mean electrondensity in the clouds N_e = 0.1-1 cm^-3, characteristic thickness of thecloud layer 300-500 pc, volume density of the cloud distribution ~= 1 x10^-6 pc^-3, scale height of the cloud distribution above the Galacticplane ~= 80-250 pc. For N_e < N_e,_rit ~= 0.7 cm^-3, the turbulenceis weak; the level of turbulence grows with N_e.
| Long-term spectroscopic and photoelectric variability of the peculiar runaway O7I fp star HD 108
The results of fifteen-year-long observations of optical spectra andthree-color BVR photoelectric photometry of the runaway star HD 108 arepresented. We detected periodic variations in the radial velocities ofabsorption and emission lines and optical variability on a long timescale (several years). Our search for periodicities in theradial-velocity variations of all available spectroscopic observationsrevealed the period P = 1627.6 days, which is most likely a result ofthe binary nature of the star. Long-term optical light variations areaccompanied by color variations in the star. The motion of the star incolor-magnitude diagrams may be due mainly to a change of the Paschencontinuum in the stellar envelope. The enhanced absorption in thecontinuum and the reddening of the star in 1996-1997 are assumed to beproduced by the commenced envelope ejection. The detected long-termphotometric variability may be a direct result of the influence of themassive unseen secondary component on the primary optical component.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. II. Photometry
Abundant {HIPPARCOS photometry over 3 years of 141 O and Wolf-Rayetstars, including 8 massive X-ray binaries, provides a magnificentvariety of light curves at the sigma ~ 1-5% level. Among the mostinteresting results, we mention: optical outbursts in HD 102567 (MXRB),coinciding with periastron passages; drastic changes in the light curveshape of HD 153919 (MXRB); previously unknown long-term variability ofHD 39680 (O6V:[n]pe var) and WR 46 (WN3p); unusual flaring of HDE 308399(O9V); ellipsoidal variations of HD 64315, HD 115071 and HD 160641;rotationally modulated variations in HD 66811=zeta Pup (O4Inf) and HD210839=lambda Cep (O6I(n)fp); dust formation episode in WR 121 (WC9). Ina statistical sense, the incidence of variability is slightly higheramong the WR stars, which might be explained by the higher percentage ofknown binary systems. Among the presumably single WR stars, thecandidate runaways appear to be more variable then the rest. Based ondata from the ESA Hipparcos astrometry satellite
| Wolf-Rayet stars and O-star runaways with HIPPARCOS. I. Kinematics
Reliable systemic radial velocities are almost impossible to secure forWolf-Rayet stars, difficult for O stars. Therefore, to study the motions- both systematic in the Galaxy and peculiar - of these two relatedtypes of hot, luminous star, we have examined the Hipparcos propermotions of some 70 stars of each type. We find that (a) both groupsfollow Galactic rotation in the same way, (b) both have a similarfraction of ``runaways'', (c) mean kinetic ages based on displacementand motion away from the Galactic plane tend to slightly favour thecluster ejection over the the binary supernova hypothesis for theirformation, and (d) those with significant peculiar supersonic motionrelative to the ambient ISM, tend to form bow shocks in the direction ofthe motion. Based on data from the ESA Hipparcos astrometry satellite.Table~1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| Coupled mass and angular momentum loss of massive main sequence stars
We investigate the interaction of mass loss and rotation during corehydrogen burning in massive stars. We compute their main sequenceevolution assuming rigid rotation, and carry angular momentum as apassive quantity in the stellar interior but incorporate its effect onthe stellar mass loss rate. We consider the example of a 60M_sun starassuming various initial rotation rates. We show that rotation maysubstantially enhance the total main sequence mass loss of massivestars. Furthermore, we argue that the surface layers of rotating massivemain sequence stars may reach the limit of hydrostatic stability(``Omega -limit'') by achieving a considerable fraction of theirEddington luminosity. We show that this process is not catastrophic forthe star, but rather that the coupling of mass and angular momentum losslimits the mass loss rate dot M_Ω of main sequence stars at theOmega -limit. dot M_Ω is determined through the angular momentumloss imposed by the Omega -limit rather than by atomic physics. For our60M_sun sequences, it is dot M_Ω =~ 10(-5) M_sun yr(-1) . We finda convergence of the rotational velocities of main sequence stars of agiven initial mass at the Omega -limit, but a strong dependance of theirmass at core hydrogen exhaustion from the initial rotation rate. Sincethen also the post-main sequence evolution depends on the initial amountof angular momentum, we argue that this is a third independent initialparameter for the evolution of massive stars, as important as initialmass and metallicity. We briefly discuss observable consequences of thecoupling of mass and angular momentum loss, e.g. a significant declineof the projected rotational velocity v sin i towards the cool end of themain sequence, a period of strongly enhanced and aspherical mass loss,disks or rings in the equatorial plane of the star reminiscent ofB[e]-stars, and highly bipolar circumstellar structures.
| The Effects of Rotation and Stellar Magnetic Field in the Nebular Shapes: LBV Nebulae and Pne
Not Available
| On the early evolution of aspherical Wolf-Rayet bubbles
We present several models of the evolution of young Wolf-Rayet (WR) ringnebulae. We focus on the transient but non-negligible phase during whichthe stellar wind is accelerated, from a velocity of about 10 km s^-1 inthe red supergiant (RSG) stage to velocities of the order of 2000 kms^-1, typical of WR winds. The WR wind interacts with the free RSG windand sweeps up an accelerating shell. This shell experiences a strongRayleigh-Taylor instability and fragments, producing dense knots andfilaments. Note that this happens before it interacts with the shellswept up by the RSG wind. We also assume an anisotropic RSG wind.Initially the outer shock of the nebula is isothermal, given its lowvelocity and the high density of the RSG wind close to the star. Lateron the shock becomes adiabatic and the transition to the fast adiabaticregime occurs earlier along the polar direction, where the density islower. In this phase the post-shock gas of the adiabatic shock is hotenough to emit soft X-rays. We compare our results with the well studiedWR ring nebula NGC 6888. It shows an ellipsoidal, filamentary shell atoptical wavelengths, while in the X-ray two lobes are present inopposite zones along the major axis. We find that our simple model mayqualitatively explain these features. Our hypotheses are not specificfor NGC 6888 and we expect that the `two-lobe' X-ray morphology is acommon attribute of young ring nebulae.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
| Supernovae and Stellar Wind in the Interstellar Medium
Not Available
| A Spectral Atlas of Hot, Luminous Stars at 2 Microns
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..107..281H&db_key=AST
| Deep Hα survey of the Milky Way. III. The l=338deg area.
The Galactic plane has been observed between l=337deg and l=342deg inthe frame of an Hα Survey of the Southern Milky Way. This area isknown to be rich in radio sources but poor in Hα emission. Theanalysis of high resolution profiles of the Hα emission observedin this direction nevertheless enabled to distinguish 6 differentvelocity components: 2 faint layers of diffuse ionized hydrogen at 0 and-12km/s (V_LSR_), 2 brighter layers at -28 and -39km/s includingindividual HII regions, a faint patch at -50km/s and two isolated brightHII regions at -61km/s. Combining these Hα observations withstellar and radio data we conclude about the most probable distances forthe different components.
| Projected Rotational Velocities of O-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...463..737P&db_key=AST
| Far-Ultraviolet Stellar Photometry: Fields Centered on rho Ophiuchi and the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104..101S&db_key=AST
| Two Circumstellar Bubbles around Blue Supergiants in the LMC
During its evolution, a massive star loses mass via stellar winds. Afast stellar wind may sweep up the ambient medium into a shell,appearing as a ``ring nebula" around the central star. While ringnebulae around Wolf-Rayet stars have received considerable attention inrecent years, ring nebulae around O and B stars are far lesswell-explored. This is because very few well-defined rings around OBstars are known; in our Galaxy only two cases are known, the BubbleNebula and NGC6164-5. Last year we discovered two ring nebulae aroundblue supergiants, Sk-69 279 (O9f; V=12.8 mag) and Sk-69 271 (B2; V=12.0mag), in the Large Magellanic Cloud (Weis et al. 1995, RevMexAASC 3,237). Both nebulae have diameter ~ 19'', corresponding to ~ 5 pc. Toinvestigate the origin of these nebulae, we obtained long-slit Hα+[N II] echelle observations with the 4m telescope at the Cerro TololoInter-American Observatory. The nebula around Sk-69 279 shows line-splitindicating an expansion velocity of 27km s(-1) , while the nebula aroundSk-69 271 shows no line-split, suggesting an expansion velocity <15kms(-1) . Assuming that t = 0.5 r/v, the dynamic age of these nebulae are1x10(5) yr and >2x10(5) yr, respectively. The most tale-tellinginformation comes from the [N II]/Hα ratio. Both nebulae show [NII]/Hα ratios significantly higher than those of the background H IIemission. This behavior is typical for ring nebulae around WR stars orluminous blue variables that contain stellar nucleosynthesis processedmaterial. Therefore, we conclude that the ring nebulae around Sk-69 279and Sk-69 271 must be ``circumstellar bubbles" containing processedstellar material. These two blue supergiants must have evolved past thered supergiant phase. The chemical composition of these two ring nebulaecould place constraints on models of stellar evolution.
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