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Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
| The chemistry of transient microstructure in the diffuse interstellar medium
Transient microstructure in the diffuse interstellar medium (ISM) hasbeen observed towards Galactic and extragalactic sources for decades,usually in lines of atoms and ions, and, more recently, in molecularlines. Evidently, there is a molecular component to the transientmicrostructure. In this paper, we explore the chemistry that may arisein such microstructure. We use a photodissociation region (PDR) code tomodel the conditions of relatively high density, low temperature, verylow visual extinction and very short elapsed time that are appropriatefor these objects. We find that there is a well-defined region ofparameter space where detectable abundances of molecular species mightbe found. The best matching models are those where the interstellarmicrostructure is young (<100 yr), small (~100 au) and dense(>104 cm-3).
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| Variable Na I Absorption toward ρ Leonis: Biased Neutral Formation in the Diffuse Interstellar Medium?
We present multiepoch KPNO coudé feed telescope observations ofinterstellar Na I and Ca II absorption toward the bright star ρ Leo.Comparisons of the Na I profiles observed over a period of 8 yr revealsignificant temporal variations in the Na I column in at least onecomponent, implying that there is ``structure'' at scales on the orderof the proper motion (~12 AU). Archival Hubble Space Telescope (HST)Goddard High Resolution Spectrograph observations of the C Ifine-structure excitation in the variable component suggest that thedensity is <~20 cm-3, significantly lower than thedensities inferred in past H I 21 cm and Na I studies. We suggest thatthe bulk of the trace neutral species are in the density peaks within aninterstellar cloud. The patchy distribution of these species naturallygives rise to the large fluctuations seen on scales of 10-1000 AU inpast temporal and binary studies. This picture predicts that the scalesover which fluctuations will be observed vary as a function of theionization rate of a species.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedfrom the data archive at the Space Telescope Science Institute. STScI isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS 5-26555.
| Detection of CaI and CH absorption at the velocity of the variable interstellar component towards κ Velorum
< I report the detection of interstellar CaI and CH at thevelocity of the previously reported variable absorption componenttowards κ Velorum (HD 81188). The very high spectral resolution(R~ 900 000) has resolved the intrinsic line profiles, yielding akinetic temperature of Tk= 190+170-150K, and a line-of-sight tubulent velocity of vt=0.26+0.08-0.13 km s-1, for the densestmaterial. The CaI/CaII ratio implies a spatial density ofnH>~ 103 cm-3, although this mayconsiderably underestimate the density in the variable region itself.The CH column density suggests a path-length of ~102 to103 au, supporting suggestions that variable interstellarcomponents such as this are formed in extended structures (sheets orfilaments) aligned with the line of sight.
| Density Variations over Subparsec Scales in Diffuse Molecular Gas
We present high-resolution observations of interstellar CN, CH,CH+, Ca I, and Ca II absorption lines toward the multiplestar systems HD 206267 and HD 217035. Substantial variations in CNabsorption are observed among three sight lines of HD 206267, which areseparated by distances of order 10,000 AU; smaller differences are seenfor CH, CH+, and Ca I. Gas densities for individual velocitycomponents are inferred from a chemical model, independent ofassumptions about cloud shape. While the component densities can differby factors of 5.0 between adjacent sight lines, the densities are alwaysless than 5000 cm-3. Calculations show that the deriveddensity contrasts are not sensitive to the temperature or reaction ratesused in the chemical model. A large difference in the CH+profiles (a factor of 2 in column density) is seen in the lower densitygas toward HD 217035.
| Rapid Temporal Variations of Interstellar Absorption Lines in the Carina Nebula
Two observations of the complex interstellar absorption-line profiles inthe UV spectrum of CPD -59°2603, obtained 22 months apart in theR~110,000 mode of the Space Telescope Imaging Spectrograph on the HubbleSpace Telescope, reveal substantial variations in the intensities ofseveral high-velocity components, including the virtual disappearance ofone of them. These are the first observations of interstellar temporalvariations in dominant ions to our knowledge. The most probableinterpretation is motion across the line of sight of discrete absorbingfeatures in the near foreground of the O-type star, within the CarinaNebula. Ground-based survey data also indicate that the extremeinterstellar profiles throughout this region originate relatively nearthe most massive stars, suggesting an origin related to their stellarwinds. The morphological similarity of these profiles to alow-ionization, narrow-line system in a quasi-stellar object (QSO)spectrum is pointed out. Time variability may provide a critical testamong different interpretations of such QSO systems. Based onobservations with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| 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
| Research Note Hipparcos photometry: The least variable stars
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/297
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| kappa Velorum: another variable interstellar sightline?.
We present ultra-high-resolution (R = 900,000) observations ofinterstellar Na i and K i absorption lines towards kappa Vel (HD 81188)which show clear evidence for temporal variation between 1994 and 2000.Specifically, the column densities of K0 and Na0in the main velocity component have increased by 40 and 16 per cent,respectively, over this period. Earlier work had suggested that thiscomponent actually consists of two unresolved sub-components; thisresult is confirmed here, and the overall line profile is found to beconsistent with only one of these sub-components having increased instrength since 1994. We argue that this variation is consistent withthe line of sight gradually probing a cold, dense interstellar filamentof the kind recently proposed by Heiles to explain other observations ofsmall-scale structure in the interstellar medium.
| The optical interstellar spectrum of Vel (HD 81188) and a measurement of interstellar cloud turbulence
We present ultra-high resolution (R~106>) observations oflines due to interstellar Na i D, Ca ii K, K i and Ti ii towards κVel (HD 81188). Five velocity components are identified in both Na i andCa ii, and two in K i; an upper limit is recorded for Ti ii. The veryhigh spectral resolution has enabled us to resolve fully the intrinsicinterstellar line profiles, and thereby obtain reliable measurements ofthe velocity dispersion parameters (b-values) for all identifiedvelocity components. By comparing the observed b-values for Na i and Ki, which differ in mass by almost a factor of 2, it has been possible toderive the kinetic temperature (Tk) and line-of-sight rmsturbulent velocity (vt) simultaneously for the two cloudsidentified in both elements. We obtain rigorous upper limits of 0.46 and0.33 km s-1 for vt in these two clouds, which aredefinitely subsonic at the measured temperatures. The three-dimensionalturbulent velocity (≡ √ 3 × vt) is alsoalmost certainly subsonic for both components, although it might justexceed the isothermal sound speed if the kinetic temperatures were closeto the lower limits permitted by the line-profile analysis.
| The Distance to the VELA Supernova Remnant
We have obtained high-resolution Ca II and Na I absorption-line spectratoward 68 OB stars in the direction of the Vela supernova remnant. Thestars lie at distances of 190-2800 pc as determined by Hipparcos andspectroscopic parallax estimations. The presence of high-velocityabsorption attributable to the remnant along some of the sight linesconstrains the remnant distance to 250+/-30 pc. This distance isconsistent with several recent investigations that suggest that thecanonical remnant distance of 500 pc is too large.
| A Second Catalog of Orbiting Astronomical Observatory 2 Filter Photometry: Ultraviolet Photometry of 614 Stars
Ultraviolet photometry from the Wisconsin Experiment Package on theOrbiting Astronomical Observatory 2 (OAO 2) is presented for 614 stars.Previously unpublished magnitudes from 12 filter bandpasses withwavelengths ranging from 1330 to 4250 Å have been placed on thewhite dwarf model atmosphere absolute flux scale. The fluxes wereconverted to magnitudes using V=0 for F(V)=3.46x10^-9 ergs cm^-2 s^-1Å^-1, or m_lambda=-2.5logF_lambda-21.15. This second catalogeffectively doubles the amount of OAO 2 photometry available in theliterature and includes many objects too bright to be observed withmodern space observatories.
| On the normal spectral energy distribution of stars: Spectral types O9-B5
The normal energy distributions for fifteen spectral subtypes from O9 toB5 for luminosity classes V, IV, and III are derived. Threephotometrically uniform catalogs served as the source of thespectrophotometric data used. Synthetic color indices for all spectraltypes are calculated using the energy distribution curves obtained.Comparison of these indices with the expected normal color indicessuggests that the energy distributions derived are reliable.
| The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Comparison of the ACRS and PPM Catalogs with the FK5 in the Southern Hemisphere
A comparison of the Astrographical Catalog of Reference Stars (ACRS) andthe Positions and Proper Motions Catalog (PPM) with the FK5 in thesouthern hemisphere, is presented. To this aim, the positions of FK5stars uniformly spread over the celestial sphere were astrographicallytaken. These positions were reduced in the ACRS and PPM systems and thencompared with those from the FK5 for the epoch of observation. The (FK5- ACRS) and (FK5 - PPM) systematic differences thus obtained, in thedeclination range from -30o down to the South Pole, for themean epoch of 1994.50, are shown.
| Performance of the Extreme Ultraviolet Explorer Imaging Telescopes
The Extreme Ultraviolet Explorer (EUVE) observatory, successfullylaunched on 1992 June 7, provided astronomers with the first observatorydedicated to observation in the extreme ultraviolet. The scientificobjectives of EUVE include an all-sky survey in four spectral bands, adeep survey in two spectral bands, and a spectroscopic survey carriedout through a NASA guest observer program. We describe here the hardwarecomponents of the four telescopes used to carry out both the all-skysurvey and the deep survey, and we present the imaging and photometricperformance characteristics of the instruments as determined from groundcalibration and in-orbit data.
| ROSAT PSPC Observations of 27 Near--Main-Sequence B Stars
In this paper, we report on ROSAT Position Sensitive ProportionalCounter (PSPC) observations of 27 near--main-sequence B stars made withunprecedented sensitivity. Contrary to the results of previous surveys,it is found that 75% of the sample stars are X-ray sources, albeit mostat modest levels. The X-ray luminosities of the program stars range from5.6 x 1027 up to 2.2 x 1032 ergs s-1. We find that LX/LBol decreasesabruptly beyond about B0 and stabilizes at LX/LBol ~ 10-8.5 by about B2,with seven nondetections at B2 and later. For the B0 and B1 stars, ourmodeling suggests that wind attenuation of the X-ray photons issignificant, so that the emitted X-ray luminosity, corrected for thisattenuation, actually exceeds 10-7LBol in some cases. Presumably, thissituation is even more severe for O stars; thus, the well-known LX/LBol~ 10-7 law simply may be an artifact of the neglect of wind attenuation.The ROSAT PSPC observations of most of the B stars are very soft, withthe notable exception of tau Sco (B0 V). The wind emission measurefilling factors that we find for the very early B stars are rather large(roughly 0.1--1). This could be brought into line with theoreticalcalculations of the line-force instability, wind-shock mechanism if themass-loss rates of these stars are a few times higher than theorycurrently predicts. However, the X-rays from stars later than B2 requirefilling factors greater than unity and thus cannot be produced by anyradiation-driven wind-shock mechanism because there is simply not enoughwind material to produce the observed X-rays. It is possible that mid-to late-B stars represent some kind of transition to, or hybrid of, windand coronal X-ray mechanisms.
| Kinematical Structure of the Local Interstellar Medium: The Galactic Center Hemisphere
A survey of interstellar Na I D1 and D2 absorption features found in thespectra of 31 early-type stars in the first and fourth Galacticquadrants reveals a new picture of the large-scale kinematics ofinterstellar gas in the vicinity of the Sun. While absorption featuresfound in the spectra of stars in the fourth Galactic quadrant show thegas expanding from stars in the Scorpio-Centaurus OB association, starsin the first Galactic quadrant, in the range of distances covered bythis survey (74--312 pc from the Sun), show interstellar gas fallingtoward the Galactic plane with a bulk motion whose component parallel tothat plane is directed toward the Scorpio-Centaurus association. Thispicture differs completely from generally accepted models in which thenearby interstellar gas flows away from the Scorpio-Centaurusassociation toward the Galactic anticenter. Evidence for the presence ofan expanding shell-like feature that is reaching the solar system ispresented.
| Diffuse interstellar bands near 9600Å: not due to C60^+^ yet.
High spectral resolution studies of the two Diffuse Interstellar Bands(DIB) previously assigned to C60^+^ (λ9577 and λ9632)confirm that these DIBs have the same full-width-at-half-maximum andbehave similarly in dense cloud environments where the DIBs are weakenedrelative to the diffuse interstellar medium. Their relative strength,however, is less than previously reported and differs from the relativeintensities measured from matrix isolation spectroscopy. A simultaneoussearch for the two laboratory derived vibronic transitions of C60^+^ atabout 9366 and 9419 Å has not lead to the detection of the twoweak counterparts stronger than 15% of λ9577. We conclude thatfor the diffuse band spectrum to be consistent with C60^+^, not only themeasured band positions have to be slightly different in the gas-phase,but also the relative intensity of the two main lines has to differsignificantly from that of the known laboratory data.
| Behaviour of the HeI 587.6, 667.8, 706.5 and 728.1nm lines in B-type stars. On the helium stratification in the atmosphere of magnetic helium peculiar stars.
High resolution spectra of the HeI587.6, 667.8 706.5 and 728.1nm lineshave been obtained to test the prediction (notably by Vauclair et al.1991) that helium abundance should decrease with depth in helium richstars and increase in helium weak stars. A sample of B-type mainsequence stars, with expected solar abundances and non stratifiedatmospheres, have also been observed in order to compare the behaviourof the selected lines with the chemically peculiar case and with theory.We found significant discrepancies with theory for the lines HeI 706.5.and 728.1nm, and, in order to outline differences between 'normal' and'peculiar' stars, we have adopted an empirical correction to the Lorentzbroadening parameter in the Voigt profile, under the assumption of LTE.This parameter is derived from the imposition of a satisfactory fit withobservations for the relation equivalent width vs. effective temperaturefor normal B stars. For helium rich stars we confirm Vauclair et al.(1991) predictions that helium abundance decreases with depth. However,we found that helium abundance decreases with depth in helium weak starstoo, which contradicts Vauclair et al. (1991) predictions. For somepeculiar stars, the inferred helium abundance is in disagreement withthe peculiarity class reported in the General Catalogue of Ap and Amstars (Renson et al. 1991).
| The Pulkovo Spectrophotometric Catalog of Bright Stars in the Range from 320 TO 1080 NM
A spectrophotometric catalog is presented, combining results of numerousobservations made by Pulkovo astronomers at different observing sites.The catalog consists of three parts: the first contains the data for 602stars in the spectral range of 320--735 nm with a resolution of 5 nm,the second one contains 285 stars in the spectral range of 500--1080 nmwith a resolution of 10 nm and the third one contains 278 stars combinedfrom the preceding catalogs in the spectral range of 320--1080 nm with aresolution of 10 nm. The data are presented in absolute energy unitsW/m(2) m, with a step of 2.5 nm and with an accuracy not lower than1.5--2.0%.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| 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.
| The determination of T_eff_ of B, A and F main sequence stars from the continuum between 3200 A and 3600 A.
A method of determination of the effective temperature of B, A and Fmain sequence stars is proposed, using the slope of the continuumbetween 3200A and 3600A. The effective temperature calibration is basedon a sample of stars with energy distributions known from UV to the red.We have determined the Balmer jump and the effective temperatures for235 main sequence stars. The temperatures found have been compared withthose derived by Underhill et al. (1979), Kontizas & Theodossiou(1980), Theodossiou (1985), Morossi & Malagnini (1985). Thecomparison showed good agreement for most of the stars in common. On theother hand, the temperatures derived from the reddening-free colourfactor QUV, from the colour index (m1965-V) and from (B-V), given inGulati et al. (1989), are systematically lower than our temperatures,however the differences are within one-sigma error.
| The position corrections of 1400 stars observed with PA II in San Juan.
Not Available
| The local distribution of NA I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures.
| ROSAT and EUVE observations of B stars
Recent observations of the X-ray and EUV emission of non-supergiant Bstars are summarized. As compared with O stars, the X-rays of most ofthe near-main-sequence B stars are soft, and the stars show a departurefrom the Lx = 1007Lbol relation. Usingline driven wind models to provide an estimate of the densitydistribution, it is concluded that a major fraction of the wind emissionmeasure is hot, whereas in shocked wind theory less than 10 percent ofthe wind emission measure should be hot. The X-ray observations suggestthat all of the B stars are X-ray emitters with a basal X-ray luminosityof about 10-8.5Lbol. For the Be stars, the X-rayemission is that which is expected from a normal B-star wind coming fromthe poles, as in the Wind Compressed Disk (WCD) model of Be-stars. Noneof the stars, including the beta Cep stars, show noticeable variabilityin their X-rays. Extreme Ultraviolet Explorer (EUVE) observations ofepsilon CMa B2 II, find it to be the brightest object in the EUV sky at500 to 700 A. It shows a Lyman continuum flux that is a factor of 30higher than line blanketed model atmospheres. The EUVE spectra showemission lines both from high stages of ionization ( Fe IX to Fe XVI)and from low stages ( HeII and O III). The He II Lyman alpha resultsfrom recombination follwing X-ray photoionization in the wind, and the OIII resonance line is found to be present because of the Bowenfluorescence mechanism. Thus, there is and intersting coupling betweenthe wind production by the EUV photospheric emission, the production ofX-ray and line EUV emission by winds, and the production of fluorescenceby recombination in the wind; all of these processes are now observablein B stars.
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