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Resim Yükleyin
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Ýlgili Makaleler
The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in theinterstellar medium. Data from FUSE were used to analyze 26 lines ofsight lying in both the galactic disk and halo, including lines toWolf-Rayet stars and through known supernova remnants. The equivalentwidths of the fluorine resonance lines at 951.871 and 954.827 Åwere measured or assigned upper limits and combined with a nitrogencurve of growth to obtain F I column densities. These column densitieswere then used to calculate fluorine depletions. Comparisons are made tothe previous study of F I by Federman and coworkers and implications forF I formation and depletion are discussed.
| The Homogeneity of Interstellar Elemental Abundances in the Galactic Disk
We present interstellar elemental abundance measurements derived fromSpace Telescope Imaging Spectrograph echelle observations of 47 sightlines extending up to 6.5 kpc through the Galactic disk. These pathsprobe a variety of interstellar environments, covering ranges of nearly4 orders of magnitude in molecular hydrogen fraction f(H2)and more than 2 in mean hydrogen sight-line density. Coupling the current data with Goddard HighResolution Spectrograph data from 17 additional sight lines and thecorresponding Far Ultraviolet Spectroscopic Explorer and Copernicusobservations of H2 absorption features, we explore magnesium,phosphorus, manganese, nickel, copper, and germanium gas-phase abundancevariations as a function of : density-dependentdepletion is noted for each element, consistent with a smooth transitionbetween two abundance plateaus identified with warm and cold neutralinterstellar medium depletion levels. The observed scatter with respectto an analytic description of these transitions implies that totalelemental abundances are homogeneous on length scales of hundreds ofparsecs, to the limits of abundance measurement uncertainty. Theprobable upper limit we determine for intrinsic variability at any is 0.04 dex, aside from an apparent 0.10 dexdeficit in copper (and oxygen) abundances within 800 pc of the Sun.Magnesium dust abundances are shown to scale with the amount of siliconin dust, and in combination with a similar relationship between iron andsilicon, these data appear to favor the young F and G star values ofSofia & Meyer as an elemental abundance standard for the Galaxy.Based on observations with the NASA/ESA.
| FUSE Determination of a Low Deuterium Abundance along an Extended Sight Line in the Galactic Disk
We present a study of the deuterium abundance along the extended sightline toward HD 90087 with the Far Ultraviolet Spectroscopic Explorer(FUSE). HD 90087 is a O9.5 III star located in the Galactic disk at adistance of ~2.7 kpc away from the Sun. Both in terms of distance andcolumn densities, HD 90087 has the longest and densest sight lineobserved in the Galactic disk for which a deuterium abundance has beenmeasured from ultraviolet absorption lines so far. Because manyinterstellar clouds are probed along this sight line, possiblevariations in the properties of individual clouds should be averagedout. This would yield a deuterium abundance that is characteristic ofthe interstellar medium on scales larger than the Local Bubble. The FUSEspectra of HD 90087 show numerous blended interstellar and stellarfeatures. We have measured interstellar column densities of neutralatoms, ions, and molecules by simultaneously fitting the interstellarabsorption lines detected in the different FUSE channels. As far aspossible, saturated lines were excluded from the fits in order tominimize possible systematic errors. IUE (International UltravioletExplorer) archival data are also used to measure neutral hydrogen. Wereport D/O=(1.7+/-0.7)×10-2 andD/H=(9.8+/-3.8)×10-6 (2 σ). Our new resultsconfirm that the gas-phase deuterium abundance in the distantinterstellar medium is significantly lower than the one measured withinthe Local Bubble. We supplement our study with a revision of the oxygenabundance toward Feige 110, a moderately distant (~200 pc) sdOB star,located ~150 pc below the Galactic plane. Excluding saturated lines fromthe fits of the FUSE spectra is critical; this led us to derive an O Icolumn density about 2 times larger than the one previously reported forFeige 110. The corresponding updated D/O ratio on this sight line isD/O=(2.6+/-1.0)×10-2 (2 σ), which is lower thanthe one measured within the Local Bubble. The data set available nowoutside the Local Bubble, which is based primarily on FUSE measurements,shows a contrast between the constancy of D/O and the variability ofD/H. As oxygen is considered to be a good proxy for hydrogen within theinterstellar medium, this discrepancy is puzzling.
| 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.
| The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high-resolution Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) observations of O Iλ1356 and H I Lyα absorption in 36 sight lines that probe avariety of Galactic disk environments and include paths that range overnearly 4 orders of magnitude in f(H2), over 2 orders ofmagnitude in , and that extend up to 6.5 kpc inlength. Since the majority of these sight lines have also been observedby the Far Ultraviolet Spectroscopic Explorer (FUSE), we have undertakenthe study of gas-phase O/H abundance ratio homogeneity using the currentsample and previously published Goddard High Resolution Spectrograph(GHRS) results. Two distinct trends are identified in the 56 sight linesample: an apparent decrease in gas-phase oxygen abundance withincreasing mean sight-line density () and a gapbetween the mean O/H ratio for sight lines shorter and longer than about800 pc. The first effect is a smooth transition between two depletionlevels associated with large mean density intervals; it is centered near=1.5cm-3 and is similar to trendsevident in gas-phase abundances of other elements. Paths less dense thanthe central value exhibit a mean O/H ratio of log10(O/H)=-3.41+/-0.01 (or 390+/-10ppm), which is consistent with averages determined for several longlow-density paths observed by STIS (André et al. 2003) and shortlow-density paths observed by FUSE (Moos et al. 2002). Sight lines ofhigher mean density exhibit an average O/H value of log10(O/H)=-3.55+/-0.02 (284+/-12ppm). The data points for low- paths are scatteredmore widely than those for denser sight lines, because O/H ratios forsuch paths shorter than 800 pc are generally about 0.10 dex lower thanthe values for longer ones. Scenarios that would be consistent withthese results include a recent infall of metal-poor gas onto the localGalactic disk and an interstellar environment toward Orion that isconducive to reducing the apparent gas-phase oxygen abundance.Based on observations with the NASA/ESA Hubble Space Telescope (HST) andthe NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE). HSTspectra were obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985.
| 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.
| A Dozen Colliding-Wind X-Ray Binaries in the Star Cluster R136 in the 30 Doradus Region
We analyzed archival Chandra X-ray observations of the central portionof the 30 Doradus region in the Large Magellanic Cloud. The imagecontains 20 X-ray point sources with luminosities between5×1032 and 2×1035 ergs s-1(0.2-3.5 keV). A dozen sources have bright WN Wolf-Rayet or spectraltype O stars as optical counterparts. Nine of these are within ~3.4 pcof R136, the central star cluster of NGC 2070. We derive an empiricalrelation between the X-ray luminosity and the parameters for the stellarwind of the optical counterpart. The relation gives good agreement forknown colliding-wind binaries in the Milky Way Galaxy and for theidentified X-ray sources in NGC 2070. We conclude that probably allidentified X-ray sources in NGC 2070 are colliding-wind binaries andthat they are not associated with compact objects. This conclusioncontradicts earlier studies where it was argued, using ROSAT data, thattwo earlier discovered X-ray sources are accreting black hole binaries.Five of the 18 brightest stars in R136 are not visible in our X-rayobservations. These stars either are single, have low-mass companions,or have very wide orbits. The resulting binary fraction among early-typestars is then unusually high (at least 70%).
| On the Absolute Magnitudes of the O Stars
The conclusion published in 1992 by Garmany & Stencel from a studyof northern OB associations, that the absolute magnitudes of the O starsshow ``a large scatter ... intrinsic to the classification system,'' iscritically examined. It is found that the differences between theirderived absolute magnitudes of O stars and this author's 1973calibration exhibit large systematic effects in several associations,ranging from -0.74 to +1.02 mag with substantially smaller dispersions.Of course, when these results are combined, the scatter equals the fullrange of the systematic effects. To investigate the possibility ofdistance errors, the Garmany & Stencel B0-B2.5 stars in the sameassociations are subjected to the same analysis. The results for the Bstars show no significant systematic differences, eliminating errors inthe association distances derived by Garmany & Stencel from the Bstars as the source of the differences found for the O stars. It isnoteworthy that the dispersions in the absolute magnitudes of the Bstars within a given association are similar to or larger than those ofthe O stars. An examination of the distribution on the sky of the starsshows that the O and B stars in the discrepant associations aregenerally not colocated; such was already known to be the case for theimportant Perseus OB1 association. It is suggested that despite theirefforts to improve them, significant problems remain with theassociation memberships adopted by Garmany & Stencel; the relativelysmall dispersions of the O star absolute magnitudes even in thediscrepant cases indicate that they belong to different, usually moredistant associations near the lines of sight to the B associations withwhich they have been mistakenly connected. Several individual cases ofunrecognized multiple systems and classification errors are also foundin the Garmany & Stencel sample. It is concluded that the scatter inthe absolute magnitudes of the O stars is not as large as found byGarmany & Stencel, and not larger than that of the B stars.
| A Spectroscopic Reconnaissance of Ultraviolet-bright Stars
We have carried out spectroscopic observations and made preliminaryclassifications of 62 UV-bright stars identified by H. H. Lanning onplates taken by A. Sandage. The goal was to search for ``interesting''objects, such as emission-line stars, hot subdwarfs, and high-gravitystars. Our targets were grouped into two samples, a bright(mB<13) sample of 35 stars observed with the Kitt Peak 2.1m telescope and a faint (13
| Stellar populations in Seyfert 2 galaxies. I. Atlas of near-UV spectra
We have carried out a uniform spectroscopic survey of Seyfert 2 galaxiesto study the stellar populations of the host galaxies. New spectra havebeen obtained for 79 Southern galaxies classified as Seyfert 2 galaxies,7 normal galaxies, and 73 stars at a resolution of 2.2 Å over thewavelength region 3500-5300 Å. Cross-correlation between thestellar spectra is performed to group the individual observations into44 synthesis standard spectra. The standard groups include a solarabundance sequence of spectral types from O5 to M3 for dwarfs, giants,and supergiants. Metal-rich and metal-weak F-K giants and dwarfs arealso included. A comparison of the stellar data with previouslypublished spectra is performed both with the individual spectra and thestandard groups. For each galaxy, two distinct spatial regions areconsidered: the nucleus and the external bulge. Spectroscopic variationsfrom one galaxy to another and from the central to the external regionare briefly discussed. It is found that the central region of a Seyfert2 galaxy, after subtracting the bulge stellar population, always shows anear-UV spectrum similar to one of three representative categories: a)many strong emission lines and only two visible absorption lines (Ca IiK and G band) (Sey2e); b) few emission lines, many absorption lines, anda redder continuum than the previous category (Sey2a); c) an almost flatcontinuum and high-order Balmer lines seen in absorption (Sey2b). Theproportion of Seyfert 2 galaxies belonging to each class is found to be22%, 28%, and 50% respectively. We find no significative differencesbetween morphology distributions of Seyfert 2 galaxies with Balmer linesdetected in absorption and the rest of the sample. This quick lookthrough the atlas indicates that half of Seyfert 2 galaxies harbour ayoung stellar population (about or less than 100 Myr) in their centralregion, clearly unveiled by the high order Balmer series seen inabsorption. Based on observations collected at the European SouthernObservatory, Chile (ESO 65.P-0014(A)). Tables 1-3 and 8 and Fig. A.1(Appendix A) are only available in electronic form athttp://www.edpsciences.org
| Rotational mixing in early-type main-sequence stars
We present quantitative observational investigations into the importanceof rotationally induced mixing in late-O stars. First, we conductnon-LTE, hydrostatic, plane-parallel H/He model-atmosphere analyses ofthe optical spectra of three of the most rapidly rotating late-Onear-main-sequence stars known: HD 93521 (O9.5 V), HD 149757 (ζOphiuchi; O9.5 V), and HD 191423 (ON9 III: n), all of which haveequatorial rotation velocities of ~430kms-1 and[formmu8]ωe/ωe(crit)~=0.9. Theanalysis allows for the expected (von Zeipel) variation ofTeff and logg with latitude. These three stars are found toshare very similar characteristics, including substantially enhancedsurface-helium abundances [formmu9](y~=0.2). Secondly, we compare thedistribution of projected rotational velocities for ON andmorphologically normal dwarf O stars, and demonstrate that the ON starsare drawn from a population with more rapid rotation. The resultsprovide qualitative support for rotationally induced mixing, althoughthere remain discrepancies between atmospheric and evolutionary models(which we show employ inappropriate mass-loss rates for late-Omain-sequence stars). We show that the most rapid rotator known, HD191423, is an ON star, and note the implied disparity between O/ONmorphology and surface helium abundance; we discuss consequences for theinterpretation of spectral morphology in O-type main-sequence stars. Wedemonstrate a new, purely spectroscopic, method of distancedetermination for rapid rotators, and thereby confirm that HD 93521 liesat ~2kpc, and is not, as previously suggested, a low-mass Population IIstar. Finally, our models contradict earlier claims of stronglydifferential surface rotation, and are consistent with uniform angularvelocity at the surface.
| H I Spectra and Column Densities toward HVC and IVC Probes
We show 21 cm line profiles in the direction of stars and extragalacticobjects, lying projected on high- and intermediate-velocity clouds (HVCsand IVCs). About half of these are from new data obtained with theEffelsberg 100 m telescope, about a quarter are extracted from theLeiden-Dwingeloo Survey (LDS), and the remaining quarter were observedwith other single-dish telescopes. H I column densities were determinedfor each HVC/IVC. Paper I of this series uses these in combination withoptical and ultraviolet high-resolution measurements to deriveabundances. Here an analysis is given of the difference and ratio of N(HI) as observed with a 9' versus a 35' beam. For HVCs and IVCs the ratioN(H I-9')/N(H I-35') lies in the range 0.2-2.5. For low-velocity gasthis ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, butit appears that the correction for stray radiation is slightly off). Thesmaller range for the low-velocity gas may be caused by confusion in theline of sight, so that a low ratio in one component can be compensatedby a high ratio in another-for 11 low-velocity clouds fitted by onecomponent the distribution of ratios has a larger dispersion. Comparisonwith higher angular resolution data is possible for 16 sight lines.Eight sight lines with H I data at 1'-2' resolution show a range of0.75-1.25 for N(H I-2')/N(H I-9'), while in eight other sight lines N(HI-Lyα)/N(H I-9') ranges from 0.74 to 0.98.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| 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
| A Search for Interstellar Bubbles surrounding Massive Stars in Perseus OB1
We have examined the interstellar medium in the vicinity of massivestars belonging to the Per OB1 association based on neutral hydrogen 21cm observations obtained with the 100 m radio telescope at Effelsberg(HPBW=8.4′) and complementary data from the Leiden-Dwingeloo H ISurvey (HPBW=36'). The higher angular resolution H I observationsallowed us to discover probable wind-blown bubbles related to fourmassive stars in the association, namely, HD 14442 [O5n(f)p], HD 14947[O5If+], HD 13022 [O9.5II-III((n))], and HD 13338 [O9.5V], while thedetection of a wind-blown bubble associated with HD 16691 [O5If+] isless conclusive. A clear H I shell coincident in position with two B1IIIstars (HD 15233 and Hilt 311) was also detected. Some of these featuresalso have infrared and/or molecular counterparts. The energetics of thestructures related to each massive star is analyzed. The new H Iinterstellar bubbles appear to be similar to the ones found surroundingWolf-Rayet stars and other Of stars. The large-scale maps obtained usingthe lower angular resolution H I data show that most of the early-typestars belonging to Per OB1 are placed in a region of low H I emission.The association could have blown a H I shell of about 350×550 pcin size. This large H I shell has an infrared counterpart.
| 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.
| A Lower Limit to the Distance of High-Velocity Cloud Complex H
We derive a lower limit for the distance of the high-velocity cloud(HVC) complex H, which is a structure covering 480 square degrees on thesky and is centered on l = 131 deg, b = 1 deg. Considering theuncertainties in the derivation of stellar distances, we find that thedistance to the HVC is certainly larger than 3.4 kpc, probably largerthan 5 kpc, and possibly larger than 6.5 kpc. This distance limit isbased on the result that we do not find absorption associated with theHVC in IUE spectra of 17 OB stars. The three most distant of these starswere observed by us; we used the IUE archives to analyze the spectra ofthe other 14 stars. We do not have conclusive evidence that heavyelements are present in this HVC. This would require a detection ofabsorption in the spectrum of an extragalactic background source.However, the nondetections can still be considered secure, as the columndensity detection limits for the Mg II lambda lambda 2796, 2802, C IIlambda 1334, and O I lambda 1302 lines are a factor of 30-4100 below thecolumn density values expected for normal interstellar medium gas phaseabundances. Our lower limit to the distance is used to discuss possibleorigins of HVC complex H. It seems unlikely that it is associated with asuperbubble at large Galactocentric radii, an infalling dwarf galaxy, orthe outer arm. It might be an unusual Galactic fountain cloud or anintergalactic cloud.
| Dust shells around certain early-type stars with emission lines.
Not Available
| Massive stars
We describe the present state of massive star research seen from theviewpoint of stellar evolution, with special emphasis on close binaries.Statistics of massive close binaries are reasonably complete for theSolar neighbourhood. We defend the thesis that within our knowledge,many scientific results where the effects of binaries are not included,have an academic value, but may be far from reality. In chapter I, wesummarize general observations of massive stars where we focus on the HRdiagram, stellar wind mass loss rates, the stellar surface chemistry,rotation, circumstellar environments, supernovae. Close binaries can notbe studied separately from single stars and vice versa. First, theevolution of single stars is discussed (chapter I). We refer to newcalculations with updated stellar wind mass loss rate formalisms andconclusions are proposed resulting from a comparison with representativeobservations. Massive binaries are considered in chapter II. Basicprocesses are briefly described, i.e. the Roche lobe overflow and masstransfer, the common envelope process, the spiral-in process in binarieswith extreme mass ratio, the effects of mass accretion and the mergingprocess, the implications of the (asymmetric) supernova explosion of oneof the components on the orbital parameters of the binary. Evolutionarycomputations of interacting close binaries are discussed and generalconclusions are drawn. The enormous amount of observational data ofmassive binaries is summarized. We separately consider the non-evolvedand evolved systems. The latter class includes the semi-detached andcontact binaries, the WR binaries, the X-ray binaries, the runaways, thesingle and binary pulsars. A general comparison between theoreticalevolution and observations is combined with a discussion of speciallyinteresting binaries: the evolved binaries HD 163181, HD 12323, HD14633, HD 193516, HD 25638, HD 209481, Φ Per and silon Sgr; theWR+OB binary V444 Cyg; the high mass X-ray binaries Vela X-1, Wray 977,Cyg X-1; the low mass X-ray binaries Her X-1 and those with a black holecandidate; the runaway ζ Pup, the WR+compact companion candidatesCyg X-3, HD 50896 and HD 197406. We finally propose an overallevolutionary model of massive close binaries as a function of primarymass, mass ratio and orbital period. Chapter III deals with massive starpopulation synthesis with a realistic population of binaries. We discussthe massive close binary frequency, mass ratio and period distribution,the observations that allow to constrain possible asymmetries during thesupernova explosion of a massive star. We focuss on the comparisonbetween observed star numbers (as a function of metallicity) andtheoretically predicted numbers of stellar populations in regions ofcontinuous star formation and in starburst regions. Special attention isgiven to the O-type star/WR star/red supergiant star population, thepulsar and binary pulsar population, the supernova rates.
| A Survey for H alpha Emission in Massive Binaries: The Search for Colliding Wind Candidates
I report the results of the first all-sky survey of H alpha emission inthe spectra of O-type binaries. The survey includes 26 systems, of which10 have emission that extends clearly above the continuum. This is thefirst report of emission for four of these. An additional three systemsshow small distortions in the H alpha profile that may result from weakemission. I compare the distribution of emission systems in H-R diagramsfor both binary and single stars, using a survey of single O-type starsdone by Conti (1974). Emission in main-sequence systems is extremelyrare and is completely absent in my sample of binary stars. Among binarystars, 78% of the systems containing giants show some emission, while nosingle giants in Conti's sample do. In the case of supergiants, 78% ofsingle stars show emission, while all supergiant binaries show strongemission. H alpha emission may come from a variety sources, but the factthat binaries have a higher incidence and strength of emission inpost--main-sequence stages may indicate that wind interactions are acommon source of emission in massive binaries. To ascertain whether ornot colliding winds have been observed, it will be necessary to studythe H alpha line profile throughout several orbits of each candidatecolliding wind system and look for recurring orbital-phase--relatedvariations. Such a study is underway.
| 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.
| High-Velocity Clouds
High-velocity clouds (HVCs) consist of neutral hydrogen (\HI) atvelocities incompatible with a simple model of differential galacticrotation; in practice one uses {the absolue value of} VLSR {greater orequal to} 90 km/s to define HVCs. This review describes the mainfeatures of the sky and velocity distributions, as well as the availableinformation on cloud properties, small-scale structure, velocitystructure, and observations other than in 21-cm emission. We show thatHVCs contain heavy elements and that the more prominent ones are morethan 2 kpc from the Galactic plane. We evaluate the hypotheses proposedfor their origin and reject those that account for only one or a fewHVCs. At least three different hypotheses are needed: one for theMagellanic Stream and possibly related clouds, one for the Outer ArmExtension, and one (or more) for the other HVCs. We discuss the evidencefor the accretion and the fountain model but cannot rule out either one.
| 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
| Metal line strengths of blue stragglers towards the young galactic association Perseus OB1.
We present equivalent width measurements of lines of HeI, CIII, NIII,OII, MgII and SiIV from high resolution optical spectra of eight bluestragglers towards the young galactic association Perseus OB1, togetherwith similar data from ultraviolet spectra. We have carried outintercomparisons of the observed line strengths between target stars ofsimilar effective temperatures, and comparisons between the target starsand data for other O-type stars, in order to search for possibleabundance anomalies in our target stars. We find firm evidence that twostars, HD 12323 and HD 13268, exhibit CNO bicycle processed material attheir surfaces, and we outline evolutionary scenarios to explain thisphenomenon. One further star, HD 12993, also shows marginal evidence forprocessed material at its surface. There is no evidence for abundanceanomalies in the five remaining stars, and we consider that they may notbe association members. Hence their identification as blue stragglers isin doubt.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Evidence for Mass Outflow from the Nucleus of M101: Knots, Rings, and a Geyser
Hα on-line and off-line CCD images of M101 obtained with theCanada-France-Hawaii 3.6 m telescope show the presence of two Hαbright, filled knots paired linearly across the nucleus in a north-southorientation. The knots are centered 2.4" (85 pc, assuming a distance toM101 of 7.4 Mpc) from the nucleus and lie roughly perpendicular to aneast-west molecular bar. Each knot in turn is connected to an elongated,photoionized ring lying parallel along the bar. The eastern ring,connected to the southern knot, reaches outward in a well-defined 500 x200 pc oval. The 700 x 300 pc western ring, connected to the northernknot, is more broken and dissipated in the middle. An arc containingblue stars and/or significant Hα absorption lies along thesouthern side of a dust lane extending from the nucleus westward alongthe bar. Hubble Space Telescope Planetary Camera image data show pocketsof star-forming regions to the east and to the south of the nucleuswhich are associated with the knot and ring in that half. The imagingdata, together with velocity data obtained with the Coude' feedspectrometer at Kitt Peak National Observatory show that the knots andrings are likely a bipolar outflow originating from a velocity <100km s^-1^ "geyser" which has a period of approximately 22 million yearsand is located in the nucleus. The geyser may be caused by a mass <10^6^ M_sun_ black hole orbiting within the nucleus, sweeping materialfrom the molecular bar.
| Blue stragglers in the young galactic association Perseus OB1.
Moderate spectral resolution, very high signal-to-noise optical spectrahave been obtained for eight blue stragglers towards the youngassociation Per OB1. These have been used to estimate projectedrotational and radial velocities. The line profiles of hydrogen andneutral and ionized helium have been analysed using non-LTE modelatmosphere calculations to deduce effective temperatures and logarithmicsurface gravities. Helium abundances have also been derived, althoughfor some targets this quantity was not well constrained. In addition,stellar radii, luminosities, and masses have been estimated both byspectroscopic methods and by comparison with evolutionary tracks. Thespectroscopic estimates were based on absolute magnitudes deduced fromeither assuming a unique distance modulus, or from a spectraltype-absolute magnitude calibration; the latter leads to a range ofdistance moduli of 1.4 magnitudes. Both sets of spectroscopic masseswere systematically smaller than the evolutionary masses, butluminosities deduced from spectral types gave a smaller and lessscattered set of mass discrepancies. None of these stars is definitelygenerically related to Per OB1 and they may therefore represent a moreheterogeneous group than has previously been thought, containingpossibly one helium and nitrogen enriched spectroscopic binary, twostars belonging to other associations/clusters, two runaway stars(nitrogen enhanced and possibly helium rich), and a group of O-typestars beyond Per OB1 containing two very fast rotators which may behelium enriched.
| The threshold for stellar winds in hot main-sequence stars
The profiles of ultraviolet resonance lines of C IV were surveyed in asample of 29 cluster and association members in the spectral type rangeO9-B2 III-V, together with a few field stars of interest. Thetemperatures and gravities of the stars were taken from the modelatmosphere analysis by Grigsby, Morrison, & Anderson (1992), and theluminosities were estimated on the basis of cluster and associationdistances from the recent literature. A parameter Pw wasdefined in order to describe the degree and assymetry of the C IVprofile. This parameter, together with total C IV equivalent width, wasfound to be well correlated with stellar luminosity and temperature. Afew anomalous stars were noted: tau Sco, HD 66665, HD 13621, and the ONstars HD12323 and HD 201345. The results suggest a sudden onset ofobservable mass loss at T(effective) = 27,500 +/- 500 K, log (L/solarluminosity) = 4.4 +/- 0.12, in agreement with the previous study byPrinja (1989). At T(effective) = 28,000 K and log g = 4, our non-LTEmodel atmospheres show an enhancement in the ground-state population ofC(+3) in their topmost layer, which could be responsible for initiationof the winds via radiation pressure on the C(+3) ions, or for the onsetof visibility of C(+3) ions in the wind because of an increase in theoptical depth in the C IV lines in the outermost layers.
| A study of some stars with circumstellar dust envelopes. I
We present the results of a study of circumstellar dust envelopes of 36stars of early(O-B-A) types in the directions of the associations CasOB1, Cas OB2, Per OB1, and Ori OB1. We determine the absorption at 1640Å, the linear radius of the dust envelopes, the mean value of thecoefficient k, and the masses of the envelopes. They differsignificantly from one another.
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| Takýmyýldýz: | Kahraman |
| Sað Açýklýk: | 02h02m30.13s |
| Yükselim: | +55°37'26.4" |
| Görünürdeki Parlaklýk: | 8.914 |
| Uzaklýk: | 10000000 parsek |
| özdevim Sað Açýklýk: | -2.7 |
| özdevim Yükselim: | -5.8 |
| B-T magnitude: | 8.863 |
| V-T magnitude: | 8.91 |
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