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An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| The Discordance of Mass-Loss Estimates for Galactic O-Type Stars
We have determined accurate values of the product of the mass-loss rateand the ion fraction of P+4, M˙q(P+4), for asample of 40 Galactic O-type stars by fitting stellar wind profiles toobservations of the P V resonance doublet obtained with FUSE, ORFEUSBEFS, and Copernicus. When P+4 is the dominant ion in thewind [i.e., 0.5<~q(P+4)<=1], M˙q(P+4)approximates the mass-loss rate to within a factor of <~2. Theorypredicts that P+4 is the dominant ion in the winds of O7-O9.7stars, although an empirical estimator suggests that the range O4-O7 maybe more appropriate. However, we find that the mass-loss rates obtainedfrom P V wind profiles are systematically smaller than those obtainedfrom fits to Hα emission profiles or radio free-free emission bymedian factors of ~130 (if P+4 is dominant between O7 andO9.7) or ~20 (if P+4 is dominant between O4 and O7). Thesediscordant measurements can be reconciled if the winds of O stars in therelevant temperature range are strongly clumped on small spatial scales.We use a simplified two-component model to investigate the volumefilling factors of the denser regions. This clumping implies thatmass-loss rates determined from ``ρ2'' diagnostics havebeen systematically overestimated by factors of 10 or more, at least fora subset of O stars. Reductions in the mass-loss rates of this size haveimportant implications for the evolution of massive stars andquantitative estimates of the feedback that hot-star winds provide totheir interstellar environments.
| A Survey of N IV and O IV Features near 3400 Å in O2-O5 Spectra
We have conducted a survey of little-known N IV and O IV multiplets near3400 Å in an extensive sample of well-classified, very earlyO-type spectra. The initial motivation was to search for additionaluseful classification criteria for these types, but an unexpected resultis the high sensitivity of these features to evolutionary CNOprocessing. We have found a useful discriminant between O2 and latertypes in the relative strengths of the O IV multiplets, one of which issubject to selective emission in the hottest spectra; the overallstrengths of these lines also decrease between spectral types O4 and O5.More remarkable, however, are the variations in the N/O ratios amongboth individual stars and clusters. For instance, several O4 If+ spectrahave very large ratios, while main-sequence stars in the Carina Nebulagenerally have smaller values than others of the same spectral types inother regions. These effects correspond to different degrees of mixingof processed material as a function of evolutionary age and initialrotational velocities; the second effect provides significant furtherevidence that very massive stars mix while still on the main sequence.Thus, further analysis of these features will likely provide valuablediagnostics of important evolutionary parameters.
| A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.
| A Survey of Unidentified EGRET Sources at Very High Energies
The Whipple Observatory 10 m γ-ray telescope has been used tosurvey the error boxes of EGRET unidentified sources in an attempt tofind counterparts at energies of 350 GeV and above. Twenty-oneunidentified sources detected by EGRET (more than 10% of the totalnumber) have been included in this survey. In no case is a statisticallysignificant signal found in the EGRET error box, which implies that, atleast for this sample, the γ-ray spectra of these sources steepenbetween 100 MeV and 350 GeV. For each EGRET source location, we listcandidate associations and derive upper limits on the integralγ-ray flux above 350 GeV.
| Confirmation of a Stellar Microjet in the Rosette H II Region (NGC 2244)
A possible microjet from a low-mass but young star, which has alreadyshed its cocoon, could be rendered observable by the Lyman photon fluxin the interior of the Rosette Nebula. Outside this environment it maynot have been observable at optical wavelengths. The kinematics of thisproposed monopolar microjet from an F8 Ve star have been investigated byspatially resolved, long-slit, spectral observations with the ManchesterEchelle Spectrometer on the San Pedro Martir telescope (Mexico). Theflow is shown to be approaching to give a radial velocity differencefrom the host nebula of -56 km s-1. An outflow velocity of,at the most, a few hundreds of km s-1 is therefore indicated.If the flow velocity is taken as 200 km s-1, which is foundin other microjets, then this jet's inclination to the sky is ~16°.The mass in the outflowing ionized gas is estimated from the surfacebrightness of the Hα emission as ~6×1027 g togive an estimated mass-loss rate of 10-8 Msolaryr-1, which, along with the detection of the outflowvelocity, confirms its microjet identification even though an uncertainfilling factor was used in these calculations. The hottest cluster star,which is also in the neighborhood of the microjet, is found alone toemit marginally sufficient Lyman photons to account for the ionizationof the jet, although direct observations of the local electron densityfrom optical line ratios are required to confirm this pointconclusively.
| O stars with weak winds: the Galactic case
We study the stellar and wind properties of a sample of Galactic Odwarfs to track the conditions under which weak winds (i.e. mass lossrates lower than 10-8 Mȯ yr-1)appear. The sample is composed of low and high luminosity dwarfsincluding Vz stars and stars known to display qualitatively weak winds.Atmosphere models including non-LTE treatment, spherical expansion andline blanketing are computed with the code CMFGEN (Hillier & Miller1998, ApJ, 496, 407). Both UV and Hα lines are used to derive windproperties while optical H and He lines give the stellar parameters. Wefind that the stars of our sample are usually 1 to 4 Myr old. Mass lossrates of all stars are found to be lower than expected from thehydrodynamical predictions of Vink et al. (2001, A&A, 369, 574). Forstars with log {L}/{Lȯ} 5.2, the reduction is byless than a factor 5 and is mainly due to the inclusion of clumping inthe models. For stars with log {L}/{Lȯ} 5.2 thereduction can be as high as a factor 100. The inclusion of X-rayemission (possibly due to magnetic mechanisms) in models with lowdensity is crucial to derive accurate mass loss rates from UV lines,while it is found to be unimportant for high density winds. The modifiedwind momentum - luminosity relation shows a significant change of slopearound this transition luminosity. Terminal velocities of low luminositystars are also found to be low. Both mass loss rates and terminalvelocities of low L stars are consistent with a reduced line forceparameter α. However, the physical reason for such a reduction isstill not clear although the finding of weak winds in Galactic starsexcludes the role of a reduced metallicity. There may be a link betweenan early evolutionary state and a weak wind, but this has to beconfirmed by further studies of Vz stars. X-rays, through the change inthe ionisation structure they imply, may be at the origin of a reductionof the radiative acceleration, leading to lower mass loss rates. Abetter understanding of the origin of X-rays is of crucial importancefor the study of the physics of weak winds.
| Quantitative H and K band spectroscopy of Galactic OB-stars at medium resolution
In this paper we have analyzed 25 Galactic O and early B-stars by meansof H and K band spectroscopy, with the primary goal to investigate towhat extent a lone near-IR spectroscopy is able to recover stellar andwind parameters derived in the optical. Most of the spectra have beentaken with subaru-ircs, at an intermediate resolution of 12 000, andwith a very high S/N, mostly on the order of 200 or better. In order tosynthesize the strategic H/He lines, we have used our recent,line-blanketed version of fastwind (Puls et al. 2005, A&A, 435,669). In total, seven lines have been investigated, where for two starswe could make additional use of the Hei2.05 singlet which has beenobserved with irtf-cshell. Apart from Brγ and Heii2.18, the otherlines are predominately formed in the stellar photosphere, and thusremain fairly uncontaminated from more complex physical processes,particularly clumping. First we investigated the predicted behaviour ofthe strategic lines. In contradiction to what one expects from theoptical in the O-star regime, almost all photospheric H/Hei/Heii H/Kband lines become stronger if the gravity decreases. Concerning H andHeii, this finding is related to the behaviour of Stark broadening as afunction of electron density, which in the line cores is different formembers of lower (optical) and higher (IR) series. Regarding Hei, thepredicted behaviour is due to some subtle NLTE effects resulting in astronger overpopulation of the lower level when the gravity decreases.We have compared our calculations with results from the alternative NLTEmodel atmosphere code cmfgen (Hillier & Miller 1998, ApJ, 496, 407).In most cases, we found reasonable or nearly perfect agreement. Only theHei2.05 singlet for mid O-types suffers from some discrepancy, analogouswith findings for the optical Hei singlets. For most of our objects, weobtained good fits, except for the line cores of Brγ in earlyO-stars with significant mass-loss. Whereas the observations showBrγ mostly as rather symmetric emission lines, the models predicta P Cygni type profile with strong absorption. This discrepancy (whichalso appears in lines synthesized by cmfgen) might be an indirect effectof clumping. After having derived the stellar and wind parameters fromthe IR, we have compared them to results from previous optical analyses.Overall, the IR results coincide in most cases with the optical oneswithin the typical errors usually quoted for the correspondingparameters, i.e., an uncertainty in T_eff of 5%, in log g of 0.1 dex andin {dot M} of 0.2 dex, with lower errors at higher wind densities.Outliers above the 1-σ level where found in four cases withrespect to log g and in two cases for {dot M}.
| Lower mass loss rates in O-type stars: Spectral signatures of dense clumps in the wind of two Galactic O4 stars
We have analyzed the far-ultraviolet spectrum of two Galactic O4 stars,the O4If+ supergiant HD 190429A and the O4V((f)) dwarf HD 96715, usingarchival FUSE and IUE data. We have conducted a quantitative analysisusing the two NLTE model atmosphere and wind codes, tlusty and cmfgen,which incorporate a detailed treatment of NLTE metal line blanketing.From the far-UV spectrum, we have derived the stellar and windparameters and the surface composition of the two stars. The surface ofHD 190429A has a composition typical of an evolved O supergiant(nitrogen-rich, carbon and oxygen-poor), while HD 96715 exhibits surfacenitrogen enhancement similar to the enrichment found in SMC O dwarfswhich has been attributed to rotationally-induced mixing. Followingstudies of Magellanic Cloud O stars, we find that homogeneous windmodels could not match the observed profile of O vλ1371 andrequire very low phosphorus abundance to fit the Pvλλ1118-1128 resonance lines. We show, on the other hand,that we are able to match the O v and P v lines using clumped windmodels. In addition to these lines, we find that N ivλ1718 isalso sensitive to wind clumping. For both stars, we have calculatedclumped wind models that match well all these lines from differentspecies and that remain consistent with Hα data. In particular, wehave achieved an excellent match of the P v resonance doublet,indicating that our physical description of clumping is adequate. Thesefits therefore provide a coherent and thus much stronger evidence ofwind clumping in O stars than earlier claims. We show that the successof the clumped wind models in matching these lines results fromincreased recombination in the clumps, hence from a better descriptionof the wind ionization structure. We find that the wind of these twostars is highly clumped, as expressed by very small volume fillingfactors, namely f_&infy; = 0.04 for HD 190429A and f_&infy; = 0.02 forHD 96715. In agreement with our analysis of SMC stars, clumping startsdeep in the wind, just above the sonic point. The most crucialconsequence of our analysis is that the mass loss rates of O stars needto be revised downward significantly, by a factor of 3 and more. Theselower mass loss rates will affect substantially the evolution of massivestars. Accounting for wind clumping is essential when determining thewind properties of O stars. Our study therefore calls for a fundamentalrevision in our understanding of mass loss and of O-type star stellarwinds.
| 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.
| A radio and mid-infrared survey of northern bright-rimmed clouds
We have carried out an archival radio, optical and infrared wavelengthimaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA SkySurvey (NVSS) archive, images from the Digitised Sky Survey (DSS) andthe Midcourse Space eXperiment (MSX). The data characterise the physicalproperties of the Ionised Boundary Layer (IBL) of the BRCs. We haveclassified the radio detections as: that associated with the ionisedcloud rims; that associated with possible embedded Young Stellar Objects(YSOs); and that unlikely to be associated with the clouds at all. Thestars responsible for ionising each cloud are identified and acomparison of the expected ionising flux to that measured at the cloudrims is presented. A total of 25 clouds display 20 cm radio continuumemission that is associated with their bright optical rims. The ionisingphoton flux illuminating these clouds, the ionised gas pressure and theelectron density of the IBL are determined. We derive internal molecularpressures for 9 clouds using molecular line data from the literature andcompare these pressures to the IBL pressures to determine the pressurebalance of the clouds. We find three clouds in which the pressureexerted by their IBLs is much greater than that measured in the internalmolecular material. A comparison of external pressures around theremaining clouds to a global mean internal pressure shows that themajority of clouds can be expected to be in pressure equilibrium withtheir IBLs and hence are likely to be currently shocked byphotoionisation shocks. We identify one source which shows 20 cmemission consistent with that of an embedded high-mass YSO and confirmits association with a known infrared stellar cluster. This embeddedcluster is shown to contain early-type B stars, implying that at leastsome BRCs are intimately involved in intermediate to high mass starformation.Figure \ref{fig:images} and Table \ref{tbl:istars1} are only availablein electronic form at http://www.edpsciences.org
| Discovery of a huge magnetic field in the very young star NGC 2244-334 in the Rosette Nebula cluster
During a survey of field strengths in upper main sequence stars in openclusters, we observed the star NGC 2244-334 in the Rosette Nebulacluster and discovered an extraordinarily large mean longitudinal fieldof about -9 kG, the second largest longitudinal field known in anon-degenerate star. This star appears to be a typical Ap He-wk (Si)star of about 4 Mȯ. Spectrum synthesis using a linesynthesis code incorporating the effects of the strong magnetic fieldindicates that He is underabundant by about 1.5 dex, and C, O and Mg byabout 0.1-0.4 dex, while Si, Mn and Fe are overabundant by about 1 dex,and Cr and Ti are nearly 2 dex overabundant.Cluster membership for this star is secure, so its age is about 2×106 yr, which is less than 3% of its main sequence lifetime.This star is one of the very youngest magnetic upper main sequence starswith a well-determined age, and confirms that both magnetic fields andstrong chemical peculiarity can appear in stars which are both extremelyyoung and very close to the ZAMS.This paper is based on data obtained at the European SouthernObservatory VLT during observing runs 70.D-0352 and 270.D-5032.
| 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.
| 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.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available at the CDS, via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/433
| A review of the properties of the scattered starlight which contaminates the spectrum of reddened stars
When a star is observed behind an interstellar cloud of sufficientcolumn density, we do not observe the direct light from the star, whichis totally extinguished. Rather, we see only starlight scattered atsmall angles from the star. I use several papers published in NewAstronomy to recount the different steps which permit understanding ofhow, and under which conditions, scattered starlight can be moreimportant than direct starlight in the spectrum of a reddened star.Associated problems-the fit of the extinction curve, the nature of thescatterers (small grains, or, atoms or molecules from the gas), the2200Å bump-are also, briefly discussed.
| A New Spectral Classification System for the Earliest O Stars: Definition of Type O2
High-quality, blue-violet spectroscopic data are collected for 24 starsthat have been classified as type O3 and that display the hallmark N IVand N V lines. A new member of the class is presented; it is the secondknown in the Cyg OB2 association, and only the second in the northernhemisphere. New digital data are also presented for several of the otherstars. Although the data are inhomogeneous, the uniform plots bysubcategory reveal some interesting new relationships. Several issuesconcerning the classification of the hottest O-type spectra arediscussed, and new digital data are presented for the five original O3dwarfs in the Carina Nebula, in which the N IV, N V features are veryweak or absent. New spectral types O2 and O3.5 are introduced here assteps toward resolving these issues. The relationship between thederived absolute visual magnitudes and the spectroscopic luminosityclasses of the O2-O3 stars shows more scatter than at later O types, atleast partly because some overluminous dwarfs are unresolved multiplesystems, and some close binary systems of relatively low luminosity andmass emulate O3 supergiant spectra. However, it also appears that thebehavior of He II λ4686, the primary luminosity criterion atlater O types, responds to other phenomena in addition to luminosity atspectral types O2-O3. There is evidence that these spectral types maycorrespond to an immediate pre-WN phase, with a correspondingly largerange of luminosities and masses. A complete census of spectraclassified into the original O3 subcategories considered here (notincluding intermediate O3/WN types or O3 dwarfs without N IV, N Vfeatures) totals 45 stars; 34 of them belong to the Large MagellanicCloud and 20 of the latter to 30 Doradus.
| 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
| An atlas of 2.4 to 4.1 mu m ISO/SWS spectra of early-type stars
We present an atlas of spectra of O- and B-type stars, obtained with theShort Wavelength Spectrometer (SWS) during the Post-Helium program ofthe Infrared Space Observatory (ISO). This program is aimed at extendingthe Morgan & Keenan classification scheme into the near-infrared.Later type stars will be discussed in a separate publication. Theobservations consist of 57 SWS Post-Helium spectra from 2.4 to 4.1 μm, supplemented with 10 spectra acquired during the nominal mission witha similar observational setting. For B-type stars, this sample providesample spectral coverage in terms of subtype and luminosity class. ForO-type stars, the ISO sample is coarse and therefore is complementedwith 8 UKIRT Larcmin -band observations. In terms of the presence ofdiagnostic lines, the Larcmin -band is likely the most promising of thenear-infrared atmospheric windows for the study of the physicalproperties of B stars. Specifically, this wavelength interval containsthe Bralpha , Pfgamma , and other Pfund lines which are probes ofspectral type, luminosity class and mass loss. Here, we present simpleempirical methods based on the lines present in the 2.4 to 4.1 mu minterval that allow the determination of i) the spectral type of Bdwarfs and giants to within two subtypes; ii) the luminosity class of Bstars to within two classes; iii) the mass-loss rate of O stars and Bsupergiants to within 0.25 dex. Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA. The appendix 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/384/473
| The standard theory of extinction and the spectrum of stars with very little reddening
This paper examines the relationship between spectra of stars of samespectral type with extremely low reddenings. According to the standardtheory, the relationship between the spectrum of stars with samespectral type and small, but different reddenings should be different inthe optical and in the UV. This difference is not observed: the ratio ofthe spectra of two stars in directions where the reddening is largeenough to be detected and low enough not to give a noticeable2200Å bump is an exponential of /1/λ from the near-infraredto the far-UV. This result is in conformity with the ideas introduced inpreceding papers: the exponential optical extinction extends to the UV,and the spectrum of stars with enough reddening is contaminated by lightscattered at close angular distance from the stars. An application willbe the determination of the spectrum of a non-reddened star from thespectrum of a star of same spectral type with little reddening.
| The spectrum of HD46223
The spectrum of HD46223 was established from the optical to the far UVand normalized by the spectrum of a non-reddened star of same spectraltype. The resulting spectrum is separated into two components. One isthe direct starlight. The second is an additional component of lightscattered at small angles. In the optical the spectrum is dominated bydirect starlight which decreases exponentially due to the linearextinction ~e-2E(B-V)/λ. Scattered light begins to benoticeable in the near-UV. The near-UV rise of the scattered light isinterrupted in the 2200Å bump region. The wavelength dependence ofthe scattered light is established in the far-UV, where scattered lightdominates the extinction curve. A 1/λ4 dependence isfound, proving the presence of grains small compared to UV wavelengths.The mathematic expressions of the different components mentioned abovegive a good fit to the extinction curve in the direction of the star. Ona mathematic standpoint the fit can be completed by a Lorentzien for the2200Å bump region. The physical interpretation of the bump may bemore difficult to achieve since the paper shows the possibility thatonly scattered light is extinguished in the bump region. Consequencesfor the grain size distribution which is necessary to explain thedifferent aspects of scattering in interstellar clouds, for the value ofRV, and problems raised by this interpretation of thespectrum of HD46223, are considered at the end of this paper.
| Optical spectroscopy of X-Mega targets - II. The massive double-lined O-type binary HD 93205
A new high-quality set of orbital parameters for the O-typespectroscopic binary HD 93205 has been obtained combining échelleand coudé CCD observations. The radial velocity orbits derivedfrom the Heii λ4686Å (primary component) and Heiλ4471Å (secondary component) absorption lines yieldsemi-amplitudes of 133+/-2 and 314+/-2kms-1 for each binarycomponent, resulting in minimum masses of 31 and 13Msolar(q=0.42). We also confirm for the binary components the spectralclassification of O3V+O8V previously assigned. Assuming for the O8Vcomponent a `normal' mass of 22-25Msolar we would derive forthe primary O3V a mass of `only' 52-60Msolar and aninclination of about 55° for the orbital plane. We have alsodetermined for the first time a period of apsidal motion for thissystem, namely 185+/-16yr using all available radial velocity data setsof HD 93205 (from 1975 to 1999). Phase-locked variations of the X-rayemission of HD 93205 consisting of a rise of the observed X-ray fluxnear periastron passage are also discussed.
| Multi-frequency variations of the Wolf-Rayet system HD193793 (WC7pd+O4-5) III. IUE observations
The colliding-wind binary system WR 140 (HD 193793, WC7pd+O4-5, P = 7.94yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35short-wavelength high-resolution spectra. An absorption-lineradial-velocity solution is obtained from the photospheric lines of theO component, by comparison with a single O star. The resulting orbitalparameters, e = 0.87 +/- 0.05, omega = 31degr +/- 9degr andKO star = 25 +/- 15 km s-1, confirm the largeeccentricity of the orbit, within the uncertainties of previous opticalstudies. This brings the weighted mean UV-optical eccentricity to e =0.85 +/- 0.04. Occultation of the O-star light by the WC wind and theWC+O colliding-wind region results into orbital modulation of theP-Cygni profiles of the C ii, C iv and Si iv resonance lines. Nearperiastron passage, the absorption troughs of those resonance-lineprofiles increase abruptly in strength and width, followed by a gradualdecrease. In particular, near periastron the blue black-edges of theP-Cygni absorption troughs shift to larger outflow velocities. Wediscuss that the apparently larger wind velocity and velocity dispersionobserved at periastron could be explained by four phenomena: (i)geometrical resonance-line eclipse effects being the main cause of theobserved UV spectral variability, enhanced by sightline crossing of theturbulent wind-wind collision zone; (ii) the possibility of anorbital-plane enhanced WC7 stellar wind; (iii) possible common-envelopeacceleration by the combined WC and O stellar radiation fields; and (iv)possible enhanced radiatively driven mass loss due to tidal stresses,focused along the orbiting line of centers.
| 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
| Modified Strömgren Sphere
A Strömgren sphere is the theoretical result of an ionizing sourcesuch as a hot star placed in a uniform medium such as neutral atomichydrogen. The steady state solution which balances ionizations andrecombinations results in a sphere of ionized hydrogen centered on thestar. We have extended the model to allow for an evacuated, sphericalcavity either centered on the star or with the star displaced withrespect to the evacuated cavity. The resulting brightness distributionsof a few representative geometries are illustrated and compared toactual H II regions of the Large Magellanic Cloud and the Milky Way. Theextra degrees of freedom provided by our model allow it to approximatethe actual H II regions better than a classic Strömgren sphere.Possible mechanisms to create the evacuated cavity are winds orsupernovae.
| 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
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