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Stellar Rotation: A Clue to the Origin of High-Mass Stars?
We present the results of a study aimed at assessing whether low- andhigh-mass stars form similarly. Our approach is (1) to examine theobserved projected rotational velocities among a large sample of newlyformed stars spanning a range in mass between 0.2 and 50Msolar and (2) to search for evidence of a discontinuity inrotational properties that might indicate a difference in the stellarformation process at some characteristic mass. Our database includesrecently published values of vsini for young intermediate- and low-massstars in Orion, as well as new observations of O stars located in youngclusters and OB associations. We find that the median of the quantityvobs/vc (observed rotational speed divided byequatorial breakup velocity) is typically about 0.15 and shows noevidence of a discontinuity over the full range of stellar masses, whilethe quantity Jsini/M (derived angular momentum per unit mass) exhibits aslow, monotonic rise (J/M~M0.3) with increasing mass with noevidence of a discontinuity. We suggest that these observations are mostsimply interpreted as indicative of a single stellar formation andangular momentum regulation mechanism, one that results in rotationrates well below breakup and angular momenta per unit mass that differsystematically by no more than a factor of 3-4 over a mass rangespanning a factor of 250.
| On the diffuse bands related to the C2 interstellar molecule
The recently published idea that intensities of some weak diffuseinterstellar bands (DIBs) are related to the C2 molecule column densityhave been examined. We use a set of high quality echelle spectra ofheavily reddened stars, acquired at the Bohyunsan Optical AstronomicalObservatory (South Korea), with a resolution R=30 000. The high quality(high S/N ratio) of our spectra is proved by the fact that despite usingthe most widely used Phillips (2, 0) band of the C2 molecule (near 8760Å), we can trace the (3, 0) Phillips band (near 7725 Å) aswell. Equivalent widths of four (5176, 5542, 5546 and 5769 Å) outof 16 examined DIBs demonstrate relatively good correlation with C2column density. However, a majority of the studied DIBs, alreadyreported as "C2" ones, most likely are not related to this simplestcarbon molecule. A removal of peculiar objects like HD 34078 from theanalyzed sample does not substantially change the level of correlations.
| Interstellar Ca II Line Intensities and the Distances of the OB stars
We show that the equivalent widths of the well-known interstellar Ca IIH and K lines can be used to determine the distances to OB stars in ourGalaxy. The equivalent widths, measured in the spectra of 147 early-typestars, are strongly related to the Hipparcos parallaxes of thoseobjects. The lines fitted to the parallax-equivalent width data aregiven by the formulae π=1/[2.78EW(K)+95] and π=1/[4.58EW(H)+102],where π is in arcseconds and EW is in milliangstroms. The form of theformulae, yielding a finite parallax even for zero absorption, showsthat space within ~100 pc of the Sun contains very little Ca II, whichis in agreement with the known dimensions of the Local Bubble. Using CaII lines for distance determination does not require the knowledge ofthe absolute magnitude of the object; it is thus well suited for targetsfor which the absolute calibration is either not precise (OBsupergiants) or not available at all (peculiar objects). We alsodemonstrate that neither the reddening E(B-V) nor the equivalent widthsof interstellar K I and CH lines are suitable candidates for distanceestimation, their relation with parallaxes being far less tight than forCa II.
| 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.
| 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.
| The origin of massive O-type field stars. I. A search for clusters
We present a study aimed at clarifying the birthplace for 43 massiveO-type field stars. In this first paper we present the observationalpart: a search for stellar clusters near the target stars. We derivestellar density maps at two different resolving scales, viz. 0.25pc and 1.0 pc from NTT and TNG imaging and the 2MASS catalogue.These scales are typical for cluster sizes. The main result is that thelarge majority of the O-type field population are isolated stars: only12% (5 out of 43) of the O-type field stars is found to harbour asmall-scale stellar cluster. We review the literature and aim atcharacterizing the stellar field of each O-type field star with theemphasis on star formation and the presence of known young stellarclusters. An analysis of the result of this paper and a discussion ofthe O-type field population as products of a dynamical ejection event ispresented in an accompanying paper.Based on observations collected at the European Southern Observatory,Chile, and at the Italian Telescopio Nazionale Galileo (TNG) operated onthe island of La Palma by the Centro Galileo Galilei of the CNAA(Consorzio Nazionale per l'Astronomia e l'Astrofisica) at the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.Table 2 and Figs. 4 to 17 are available in electronic form athttp://www.edpsciences.org
| 10 MK Gas in M17 and the Rosette Nebula: X-Ray Flows in Galactic H II Regions
We present the first high spatial resolution X-ray images of twohigh-mass star forming regions, the Omega Nebula (M17) and the RosetteNebula (NGC 2237-2246), obtained with the Chandra X-Ray ObservatoryAdvanced CCD Imaging Spectrometer instrument. The massive clusterspowering these H II regions are resolved at the arcsecond level intomore than 900 (M17) and 300 (Rosette) stellar sources similar to thoseseen in closer young stellar clusters. However, we also detect softdiffuse X-ray emission on parsec scales that is spatially and spectrallydistinct from the point-source population. The diffuse emission hasluminosity LX~=3.4×1033 ergs s-1in M17 with plasma energy components at kT~=0.13 and ~=0.6 keV (1.5 and7 MK), while in Rosette it has LX~=6×1032ergs s-1 with plasma energy components at kT~=0.06 and ~=0.8keV (0.7 and 9 MK). This extended emission most likely arises from thefast O star winds thermalized either by wind-wind collisions or by atermination shock against the surrounding media. We establish that onlya small portion of the wind energy and mass appears in the observeddiffuse X-ray plasma; in these blister H II regions, we suspect thatmost of it flows without cooling into the low-density interstellarmedium. These data provide compelling observational evidence that strongwind shocks are present in H II regions.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| 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.
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| Tomographic Separation of Composite Spectra. X. The Massive Close Binary HD 101131
We present the first orbital elements for the massive close binary HD101131, one of the brightest objects in the young open cluster IC 2944.This system is a double-lined spectroscopic binary in an ellipticalorbit with a period of 9.64659+/-0.00012 days. It is a young system ofunevolved stars (approximately 2 million yr old) that are well withintheir critical Roche surfaces. We use a Doppler tomography algorithm toreconstruct the individual component optical spectra, and we applywell-known criteria to arrive at classifications of O6.5 V((f)) and O8.5V for the primary and secondary, respectively. We compare thereconstructed spectra of the components to single-star spectrumstandards to determine a flux ratio off2/f1=0.55+/-0.08 in the V band. Both componentsare rotating faster than synchronously. We estimate the temperatures andluminosities of the components from the observed spectralclassifications, composite V magnitude, and cluster distance modulus.The lower limits on the masses derived from the orbital elements and thelack of eclipses are 25 and 14Msolar for the primary andsecondary, respectively. These limits are consistent with the somewhatlarger masses estimated from the positions of the stars in theHertzsprung-Russell diagram and evolutionary tracks for single stars.
| UBVI and Hα Photometry of the Young Open Cluster NGC 2244
New UBVI and Hα photometry has been performed for the young opencluster NGC 2244. We classified 30 OB stars as being members of thecluster using proper-motion data and spectral types from previousinvestigators, along with photometric diagrams obtained in this study.We measured Hα emission strength of the stars by Hαphotometry and set up a selection criterion to select pre-main-sequence(PMS) stars with Hα emission. Fourteen PMS stars and seven PMScandidates were found using the criterion. In addition, six stars foundnear the positions of ROSAT HRI X-ray sources were assumed to be PMSstars and the optical counterparts of these X-ray sources. We determineda reddening of =0.47+/-0.04 for the cluster and atotal-to-selective extinction ratio of RV=3.1+/-0.2. Thedistance modulus derived was V0-MV=11.1. Bycomparing our photometric results with theoretical evolution models, wederived a main-sequence turnoff age of 1.9 Myr and a PMS age spread ofabout 6 Myr. The slope of the initial mass function, Γ, calculatedin the mass range 0.5<=logm<=2.0 could be flat(Γ=-0.7+/-0.1).
| Optical properties of X-ray selected stars in NGC 2244 in the Rosette Nebula
We present high spatial resolution optical photometry of NGC 2244 in theRosette Nebula using the NOAO MOSAIC Imager at Kitt Peak. We obtainedBVIR and Hα images of the central cluster and surrounding nebulae,and present results for 138 sources selected from X-ray observationswith the ROSAT PSPC and HRI. Color-magnitude diagrams of the clustershow a large number of the stars are still undergoing contraction ontothe main sequence. The faintest X-ray selected cluster members have thehighest X-ray-to-optical luminosities (Lx sog7*E30 erg s-1; Lx/Lbol ~10-2 to 10-3) and indicate they are a populationof young active late-type stars. Hα emission from the X-rayemitters is also remarkable. We have extended the detections of PMSstars in NGC 2244 well into the range of K spectral types. While most ofthe cluster stars are located in the color-magnitude diagrams in betweenthe ZAMS and the 3 Myrs isochrone, significantly younger low mass starsexist and confirm earlier reports that star formation is still going onin the Rosette Nebula/NGC 2244 region. Table 3 is only available inelectronic form 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/384/890
| On the Correlation between CO Absorption and Far-Ultraviolet Nonlinear Extinction toward Galactic OB Stars
A sample of 59 sight lines to reddened Galactic OB stars was examinedfor correlations of the strength of the CO Fourth Positive(A1Π-X1Σ+) absorption bandsystem with the ultraviolet interstellar extinction curve parameters. Weused archival high-dispersion NEWSIPS IUE spectra to measure the COabsorption for comparison with parametric fits of the extinction curvesfrom the literature. A strong correlation with the nonlinear far-UVcurvature term was found with greater absorption, normalized to E(B-V),being associated with more curvature. A weaker trend with the linearextinction term was also found. Mechanisms for enhancing CO in dustenvironments exhibiting high nonlinear curvature are discussed.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The eclipsing binary V578 Mon in the Rosette nebula: age and distance to NGC 2244 using Fourier disentangled component spectra
V578 Mon is a member of the young stellar cluster NGC 2244 which isembedded in the Rosette nebula. It is a double-lined binary, consistingof two early-B type components which eclipse each other partially duringtheir 2.40848-day orbit. A novel technique of Fourier disentangling isapplied to disentangle the spectrum of the binary into the spectra ofits components, allowing a direct spectroscopic temperaturedetermination for each component. Together with an analysis of the lightcurve, the spectral disentangling process allows us to derive theorbital and fundamental stellar parameters. The hotter star (T_eff = 30000 K) has a mass of 14.5 Msun and a radius of 5.2 Rsun. The cooler one (T_eff = 26 400 K) has a mass of 10.3 Msun and a radius of 4.3 Rsun . Both stars are well insidetheir critical Roche lobe and move in an eccentric orbit (e=0.087) whichhas a semi-major axis of 22 Rsun and is seen at aninclination of 72.6degr . The rotation of both stars is synchronizedwith the orbital motion. The systemic velocity of 34.9 km s-1,confirms that the binary belongs to NGC 2244. The age of the binary,and hence of NGC 2244, is (2.3±0.2) 106 years. Fromthe fundamental stellar parameters and the interstellar absorption, thedistance to the cluster is derived to be 1.39 ±0.1 kpc, i.e.slightly lower than that found from photometric studies. Its age andspace velocity indicate that NGC 2244 is formed in the galactic plane.The high accuracy obtained (e.g. probable errors of 0.5% for the masses,1m -1.5% for the radii, despite the lack of total eclipses,and 1.5% for the temperatures) is a direct consequence of the spectraldisentangling technique, which as a by-product delivers very accurateradial velocities for all orbital phases. Based on observations obtainedat the European Southern Observatory (ESO), La Silla, Chile Table 5 isonly available in electronic form at the CDS via anonymous ftp to130.79.128.5 or at http://cdsweb.u-strasbg.fr
| Classification of O Stars in the Yellow-Green: The Exciting Star VES 735
Acquiring data for spectral classification of heavily reddened starsusing traditional criteria in the blue-violet region of the spectrum canbe prohibitively time consuming using small to medium sized telescopes.One such star is the Vatican Observatory emission-line star VES 735,which we have found excites the H II region KR 140. In order to classifyVES 735, we have constructed an atlas of stellar spectra of O stars inthe yellow-green (4800-5420 Å). We calibrate spectral type versusthe line ratio He I lambda4922:He II lambda5411, showing that this ratioshould be useful for the classification of heavily reddened O starsassociated with H II regions. Application to VES 735 shows that thespectral type is O8.5. The absolute magnitude suggests luminosity classV. Comparison of the rate of emission of ionizing photons and thebolometric luminosity of VES 735, inferred from radio and infraredmeasurements of the KR 140 region, to recent stellar models givesconsistent evidence for a main-sequence star of mass 25 M_solar and ageless than a few million years with a covering factor 0.4-0.5 by thenebular material. Spectra taken in the red (6500-6700 Å) show thatthe stellar Hα emission is double-peaked about the systemicvelocity and slightly variable. Hβ is in absorption, so that theemission-line classification is ``(e)''. However, unlike the case of themore well-known O(e) star zeta Oph, the emission from VES 735 appears tobe long-lived rather than episodic.
| CCD spectra of MK standards and a preliminary extension of the MK classification to the yellow-red region.
Not Available
| 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.
| 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.
| A survey of vacuum-ultraviolet extinction curves based on International Ultraviolet Explorer spectra
This paper presents the interstellar extinction curves derived from theIUE spectra with the aid of the already published 'artificialstandards'. The variety of possible shapes of the curves, demonstratedearlier on spectra from the TD/1 satellite, is fully confirmed.
| 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.
| 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.
| New Perspectives on AX Monocerotis
AX Moncerotis is a 232d, noneclipsing, interacting binary star thatconsists of a K giant, a Be-like giant, and large amounts ofcircumstellar material. The K star is almost certainly a synchronousrotator and is probably in contact with its critical lobe. The Be starwas believed to be a rapid rotator based on extremely wide absorptionlines, but new spectra show that these lines arise from thecircumstellar environment. Hydrogen emission, also circumstellar, ismany times stronger than the continuum. Near-ultraviolet light curvesexhibit a 0.5 mag dip near phase 0.75, but there is no such variabilityat longer wavelengths. Gas flow trajectories from the cusp of the K startoward the Be star provide a simple explanation for the photometric andspectroscopic behavior. We may have found a decreasing orbital period,but more data are necessary to confirm this result. We present severalmodels for AX Mon based on (1) new and archival visible photometry, (2)archival ultraviolet spectroscopy, (3) new and archival visiblespectroscopy, (4) new visible polarimetry, and (5) new radio photometry.Future observations, including optical interferometry, are proposed.
| Equivalent width of NA I and K I lines and reddening.
The profile, radial velocity and equivalent width of the interstellarlines of Na I (5890.0, 5895.9Å) and K I (7699.0Å) have beenobtained from Echelle+CCD observations at resolving powerλ/{DELTA}λ~16,500 for 32 O and early B stars sufferingfrom a reddening between E_B-V_=0.06 and 1.57. The data have been usedto search for and calibrate a relation between equivalent width andreddening. When the interstellar lines show a single and sharpcomponent, useful relations to estimate reddening from equivalent widthshave been derived. The relation for Na I is most sensitive in the range0.0<=E_B-V_<=0.4, and the one for K I takes over at higherreddening. Good quality equivalent width measurements allow E_B-V_ to beestimated with an accuracy of about 0.05 mag. For multi-componentprofiles of Na I and K I lines the estimate of reddening is moreambiguous with a general scatter of 0.15mag. Close blends of multiplecomponents allow only an estimate of an upper limit to E_B-V_.
| 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
| Two-dimensional Ultraviolet Spectral Typing of O-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...460..906P&db_key=AST
| Derivation of the Galactic rotation curve using space velocities
We present rotation curves of the Galaxy based on the space-velocitiesof 197 OB stars and 144 classical cepheids, respectively, which rangeover a galactocentric distance interval of about 6 to 12kpc. Nosignificant differences between these rotation curves and rotationcurves based solely on radial velocities assuming circular rotation arefound. We derive an angular velocity of the LSR of{OMEGA}_0_=5.5+/-0.4mas/a (OB stars) and {OMEGA}_0_=5.4+/-0.5mas/a(cepheids), which is in agreement with the IAU 1985 value of{OMEGA}_0_=5.5mas/a. If we correct for probable rotations of the FK5system, the corresponding angular velocities are {OMEGA}_0_=6.0mas/a (OBstars) and {OMEGA}_0_=6.2mas/a (cepheids). These values agree betterwith the value of {OMEGA}_0_=6.4mas/a derived from the VLA measurementof the proper motion of SgrA^*^.
| Terminal Velocities and the Bistability of Stellar Winds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...455..269L&db_key=AST
| The Initial Mass Function and Massive Star Evolution in the OB Associations of the Northern Milky Way
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...454..151M&db_key=AST
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