תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Automatic Detection of Expanding H I Shells in the Canadian Galactic Plane Survey Data
The identification of expanding H I shells is difficult because of theirvariable morphology. In this paper we present an automatic detector forH I shells, based on the more stable dynamical characteristics ofexpanding bubbles with radii <40 pc. The detection is performed intwo stages. First, artificial neural networks are trained to recognizethe dynamical signature of an expanding bubble in the velocity spectraof 21 cm data. The second stage consists of subsequent validations basedon the potential bubble's morphology. The technique is tested on 11known bubbles, and 10 of them are successfully detected. Conducting asystematic detection on a 48deg×9deg regionin the Perseus arm, we obtain 7100 detections with spatial distributionfollowing the stellar distribution of the Galactic disk. The estimatedradius and expansion velocity distributions for objects with R<=10 pcagree with the distributions predicted by models of adiabaticallyexpanding bubble populations. The fraction of the Perseus arm volumeoccupied by the detected objects, which can be interpreted as the smallbubbles' contribution to the Galactic porosity Q, is calculated toQR<40pc=0.007+0.025-0.003. Four newbubble cases and eight serious candidates, related to known progenitors,are proposed.
| Early-type stars in the young open cluster IC 1805. II. The probably single stars HD 15570 and HD 15629, and the massive binary/triple system HD 15558
Aims.We address the issue of the multiplicity of the three brightestearly-type stars of the young open cluster IC 1805, namely HD 15570, HD15629 and HD 15558. Methods: .For the three stars, we measured theradial velocity by fitting Gaussian curves to line profiles in theoptical domain. In the case of the massive binary HD 15558, we also useda spectral disentangling method to separate the spectra of the primaryand of the secondary in order to derive the radial velocities of the twocomponents. These measurements were used to compute orbital solutionsfor HD 15558. Results: .For HD 15570 and HD 15629, the radialvelocities do not present any significant trend attributable to a binarymotion on time scales of a few days, nor from one year to the next. Inthe case of HD 15558 we obtained an improved SB1 orbital solution with aperiod of about 442 days, and we report for the first time on thedetection of the spectral signature of its secondary star. We derivespectral types O5.5III(f) and O7V for the primary and the secondary ofHD 15558. We tentatively compute a first SB2 orbital solution althoughthe radial velocities from the secondary star should be considered withcaution. The mass ratio is rather high, i.e. about 3, and leads to veryextreme minimum masses, in particular for the primary object. Minimummasses of the order of 150 ± 50 and 50 ± 15 M_ȯ arefound respectively for the primary and the secondary. Conclusions:.We propose that HD 15558 could be a triple system. This scenario couldhelp to reconcile the very large minimum mass derived for the primaryobject with its spectral type. In addition, considering new andpreviously published results, we find that the binary frequency amongO-stars in IC 1805 has a lower limit of 20%, and that previouslypublished values (80%) are probably overestimated.
| Bright OB stars in the Galaxy. III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined Hα, IR and radio analysis
Context: .Recent results strongly challenge the canonical picture ofmassive star winds: various evidence indicates that currently acceptedmass-loss rates, {dot M}, may need to be revised downwards, by factorsextending to one magnitude or even more. This is because the mostcommonly used mass-loss diagnostics are affected by "clumping"(small-scale density inhomogeneities), influencing our interpretation ofobserved spectra and fluxes. Aims: .Such downward revisions wouldhave dramatic consequences for the evolution of, and feedback from,massive stars, and thus robust determinations of the clumping propertiesand mass-loss rates are urgently needed. We present a first attemptconcerning this objective, by means of constraining the radialstratification of the so-called clumping factor. Methods: .To thisend, we have analyzed a sample of 19 Galactic O-type supergiants/giants,by combining our own and archival data for Hα, IR, mm and radiofluxes, and using approximate methods, calibrated to more sophisticatedmodels. Clumping has been included into our analysis in the"conventional" way, by assuming the inter-clump matter to be void.Because (almost) all our diagnostics depends on the square of density,we cannot derive absolute clumping factors, but only factors normalizedto a certain minimum. Results: .This minimum was usually found tobe located in the outermost, radio-emitting region, i.e., the radiomass-loss rates are the lowest ones, compared to {dot M} derived fromHα and the IR. The radio rates agree well with those predicted bytheory, but are only upper limits, due to unknown clumping in the outerwind. Hα turned out to be a useful tool to derive the clumpingproperties inside r < 3{ldots}5 Rstar. Our most importantresult concerns a (physical) difference between denser and thinnerwinds: for denser winds, the innermost region is more strongly clumpedthan the outermost one (with a normalized clumping factor of 4.1± 1.4), whereas thinner winds have similar clumping properties inthe inner and outer regions. Conclusions: .Our findings arecompared with theoretical predictions, and the implications arediscussed in detail, by assuming different scenarios regarding the stillunknown clumping properties of the outer wind.
| 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 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.
| Bright OB stars in the Galaxy. II. Wind variability in O supergiants as traced by Hα
We investigate the line-profile variability (lpv) of Hα for alarge sample of O-type supergiants (15 objects between O4 and O9.7), inan objective, statistically rigorous manner. We employed the TemporalVariance Spectrum (TVS) analysis, developed for the case of photosphericabsorption lines and modified by us to take into account the effects ofwind emission. By means of a comparative analysis we place constraintson the properties of this variability - quantified in terms of a meanand a newly defined fractional amplitude of deviations - as a functionof stellar and wind parameters. The results of our analysis show thatall the stars in the sample show evidence of significant lpv inHα, mostly dominated by processes in the wind. The variationsoccur between zero and 0.3 v_&infy; (i.e., below 1.5 R_star ), in goodagreement with results from similar studies. A comparison between theobservations and corresponding line-profile simulations indicates thatfor stars with intermediate wind densities the properties of theHα variability can be explained by simple models consisting ofcoherent or broken shells (blobs) uniformly distributed over the windvolume, with an intrinsic scatter in the maximum density contrast ofabout a factor of two. For stars at lower and higher wind densities, onthe other hand, we found certain inconsistencies between theobservations and our predictions, most importantly concerning the meanamplitude and the symmetry properties of the TVS. This disagreementmight be explained by the presence of coherent large-scale structures,partly confined in a volume close to the star. Interpreted in terms of avariable mass-loss rate, the observed variations of Hα indicatechanges of ±4% with respect to the mean value of dot M for starswith stronger winds and of ± 16% for stars with weaker winds. Theeffect of these variations on the corresponding wind momenta is ratherinsignificant (less than 0.16 dex), increasing only the local scatterwithout affecting the Wind Momentum Luminosity Relationship.
| 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}.
| Total to Selective Extinction Ratios and Visual Extinctions from Ultraviolet Data
We present determinations of the total to selective extinction ratio R_Vand visual extinction A_V values for Milky Way stars using ultravioletcolor excesses. We extend the analysis of Gnacinski and Sikorski (1999)by using non-equal weights derived from observational errors. We presenta detailed discussion of various statistical errors. In addition, weestimate the level of systematic errors by considering differentnormalization of the extinction curve adopted by Wegner (2002). Ourcatalog of 782 R_V and A_V values and their errors is available in theelectronic form on the World Wide Web.
| 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
| 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.
| Stellar and wind parameters of Galactic O-stars. The influence of line-blocking/blanketing
We have re-analyzed the Galactic O-star sample from \citet{puls96} bymeans of line-blanketed NLTE model atmospheres in order to investigatethe influence of line-blocking/blanketing on the derived parameters. Theanalysis has been carried out by fitting the photospheric and wind linesfrom H and He. In most cases we obtained a good fit, but we have alsofound certain inconsistencies which are probably related to a stillinadequate treatment of the wind structure. These inconsistenciescomprise the line cores of Hγ and Hβ insupergiants (the synthetic profiles are too weak when the mass-loss rateis determined by matching Hα) and the ``generalizeddilution effect'' (cf. \citealt{vo89}) which is still present in He I4471 of cooler supergiants and giants.Compared to pure H/He plane-parallel models we found a decrease ineffective temperatures which is largest at earliest spectral types andfor supergiants (with a maximum shift of roughly 8000 K). This findingis explained by the fact that line-blanketed models of hot stars havephotospheric He ionization fractions similar to those from unblanketedmodels at higher Teff and higher log g. Consequently, anyline-blanketed analysis based on the He ionization equilibrium resultsin lower Teff-values along with a reduction of either log gor helium abundance (if the reduction of log g is prohibited by theBalmer line wings). Stellar radii and mass-loss rates, on the otherhand, remain more or less unaffected by line-blanketing.We have calculated ``new'' spectroscopic masses and compared them withprevious results. Although the former mass discrepancy \citep{h92}becomes significantly reduced, a systematic trend for masses below 50Msun seems to remain: The spectroscopically derived valuesare smaller than the ``evolutionary masses'' by roughly 10Msun. Additionally, a significant fraction of our samplestars stays over-abundant in He, although the actual values were foundto be lower than previously determined.Also the wind-momentum luminosity relation (WLR) changes because oflower luminosities and almost unmodified wind-momentum rates. Comparedto previous results, the separation of the WLR as a function ofluminosity class is still present but now the WLR for giants/dwarfs isconsistent with theoretical predictions.We argue that the derived mass-loss rates of stars withHα in emission are affected by clumping in the lowerwind region. If the predictions from different and independenttheoretical simulations (\citealt {Vink00, Paul03, puls03a}) that theWLR should be independent of luminosity class were correct, a typicalclumping factor <ρ2>/<ρ>2 ≈5 should be derived by ``unifying'' the different WLRs.Based upon observations obtained at the INT and the European SouthernObservatory, La Silla, Chile. The INT is operated on the island of LaPalma by the ING in the Spanish Observatorio de El Roque de losMuchachos of the Instituto de Astrofísica de Canarias.Appendix A in only available in electronic form athttp://www.edpsciences.org
| Bright OB stars in the Galaxy. I. Mass-loss and wind-momentum rates of O-type stars: A pure H\alpha analysis accounting for line-blanketing
We study mass-loss and wind momentum rates of 29 Galactic O-type starswith luminosity classes I, III and V by means of a pure H\alpha profileanalysis and investigate to what extent the results compare to thoseoriginating from a state-of-the-art, complete spectral analysis. Ourinvestigation relies on the approximate method developed by\citet{Puls96} which we have modified to account for the effects ofline-blanketing. Effective temperatures and gravities needed to obtainquantitative results from such a simplified approach have been derivedby means of calibrations based on most recent spectroscopic NLTEanalyses and models of Galactic stars by \citet{Repo03} and\citet{Martins02}. Comparing (i) the derived wind-densities to thosedetermined by \citet{Repo03} for eleven stars in common and (ii) theWind-momentum Luminosity Relationship (WLR) for our sample stars tothose derived by other investigations, we conclude that our approximateapproach is actually able to provide consistent results. Additionally,we studied the consequences of ``fine tuning'' some of the direct andindirect parameters entering the WLR, especially by accounting fordifferent possible values of stellar reddening and distances. Combiningour data set with the corresponding data provided by \citet{Herrero02}and \citet{Repo03} we finally study the WLR for the largest sample ofGalactic O-type stars gathered so far, including an elaborate errortreatment. The established disagreement between the theoreticalpredictions and the ``observed'' WLRs being a function of luminosityclass is suggested to be a result of wind clumping. Different strategiesto check this hypothesis are discussed, particularly by comparing theH\alpha mass-loss rates with the ones derived from radio observations.
| On the Origin of the High-Ionization Intermediate-Velocity Gas toward HD 14434
We present Far-Ultraviolet Spectroscopic Explorer and Space TelescopeImaging Spectrograph observations of high-ionization interstellarabsorption toward HD 14434 (l=135.1d, b=-3.8d d~2.3 kpc), an O5.5 V starin the Perseus OB1 association. Intermediate-velocity interstellar Si IVand C IV absorption is present at VLSR=-67 km s-1,while low-ionization gas associated with the Perseus arm is detected atabout -50 km s-1. Neither N V nor O VI is detected atVLSR=-67 km s-1, although Al III and Fe III,tracers of warm ionized gas, are seen. The high-ion column densities inthe -67 km s-1 component are log[N(CIV)]=13.92+/-0.02cm-2, log[N(SiIV)]=13.34+/-0.02 cm-2,log[N(NV)]<=12.65 cm-2, and log[N(OVI)]<=13.73cm-2 (3 σ limits). The observed C IV/Si IV ratio of3.8+/-0.3 in this intermediate-velocity cloud (IVC) is similar to theGalactic average (4.3+/-1.9). Our analysis of the Si IV and C IV linewidths yields a temperature of T~10,450+/-3400 K for this component. Atthis low temperature, neither Si IV nor C IV can be produced viacollisions. We investigate several photoionization models to explain theintermediate-velocity Si IV and C IV absorption toward HD 14434.Photoionization models employing cooling of a hot (T~106 K)diffuse plasma as the source of ionizing radiation reproduce theobserved properties of the IVC toward HD 14434 quite well. The hotplasma responsible for the ionizing radiation in these models may beattributed to hot gas contained in a supershell in or near the Perseusarm or from a more generally distributed hot ionized medium.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the data archive at the Space Telescope Science Institute.STScI is operated by the Association of Universities for Research inAstronomy, Inc., under the NASA contract NAS 5-26555.
| Radio observations of interstellar bubbles surrounding massive stars}
We show radio continuum observations of the WR ring nebulae around WR101 and WR 113 obtained using the VLA and HI 21 cm line data of theinterstellar bubble around the O type stars BD +24 deg 3866 and BD+25deg 3952 obtained with the DRAO Synthesis Telescope. We review previousradio continuum and HI line results toward WR and O-type stars.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Spectropolarimetry of O supergiants
We present medium-resolution spectropolarimetry at high signal-to-noiseratio of the Hα emission line of 20 O-type supergiants. Five stars(25 per cent) of the sample show a statistically significant change inpolarization through the line. We combine our Hα data with newK-band spectropolarimetry and archival low-resolution opticalspectropolarimetry to determine the polarigenic mechanism in the starsthat show a line effect. We show that the line polarization change inthe binary systems is caused by the classical `dilution' mechanism, inwhich the Hα emission is essentially unpolarized and the continuumpolarization is caused by intrabinary scattering. We find that the lineeffect in HD 108 is also well modelled by pure dilution, but suggestthat the continuum polarization is the result of stochastic windclumping. A similar description applies to the continuum polarization ofHD 188001, although the line effect cannot be reproduced by puredilution. We use low-resolution spectropolarimetry to determine theinterstellar polarization vector to λ Cephei, and confirm thatthe intrinsic polarization of the object is very low (<0.1 per cent,corresponding to an equator:pole density ratio of <1.25). The linepolarization of this star is modelled using the TORUS three-dimensionalradiative-transfer code. We show that the line effect is a consequenceof symmetry breaking caused by the rapid rotation of the system (>200km s-1), and that the system is similar polarimetrically tothe O4 supergiant ζ Puppis. Finally, we note that the precision ofcurrent photo- and spectro-polarimetric observations is insufficient totest structured wind models, which predict a continuum polarization of~0.1 per cent.
| Revised Stellar Temperatures for Magellanic Cloud O Supergiants from Far Ultraviolet Spectroscopic Explorer and Very Large Telescope UV-Visual Echelle Spectrograph Spectroscopy
We have undertaken quantitative analysis of four LMC and SMC O4-9.7extreme supergiants using far-ultraviolet FUSE, ultraviolet IUE/HubbleSpace Telescope, and optical Very Large Telescope UV-Visual EchelleSpectrograph spectroscopy. Extended, non-LTE model atmospheres thatallow for the consistent treatment of line blanketing, developed byHillier & Miller, are used to analyze wind and photosphericsspectral features simultaneously. Using Hα to constrain mass-lossrates, He I-He II photospheric lines reveal stellar temperatures thatare systematically (5-7.5 kK) and substantially (15%-20%) lower thanpreviously derived from unblanketed, plane-parallel, non-LTEphotospheric studies. We have confidence in these revisions sincederived temperatures generally yield consistent fits across the entire912-7000 Å observed spectral range. In particular, we are able toresolve the UV-optical temperature discrepancy identified for AzV 232(O7 Iaf+) in the SMC by Fullerton and coworkers. Thetemperature and abundance sensitivity of far-ultraviolet, UV, andoptical lines are discussed. ``Of'' classification criteria are directlylinked to (strong) nitrogen enrichment (via N III λ4097) and(weak) carbon depletion (via C III λλ4647-4651), providingevidence for mixing of unprocessed and CNO-processed material at theirstellar surfaces. Oxygen abundances are more difficult to constrain,except via O II lines in the O9.7 supergiant, for which it is also foundto be somewhat depleted. Unfortunately, He/H is very difficult todetermine in individual O supergiants because of uncertainties inmicroturbulence and the atmospheric scale height. The effect of windclumping is also investigated, for which P V λλ1118-1128potentially provides a useful diagnostic in O star winds, unlessphosphorus can be independently demonstrated to be underabundantrelative to other heavy elements. Revised stellar properties affectexisting calibrations of (1) Lyman continuum photons-a factor of 2 lowerfor the O4 supergiant-and (2) kinetic energy released into the ISM by Osupergiants. Our results also have importance for the calibration of thewind momentum-luminosity relationship for OB stars, particularly sincethe stars studied here are among the visually brightest OB stars inexternal galaxies. Based on observations made with the NASA-CNES-CSA FarUltraviolet Spectroscopic Explorer. FUSE is operated for NASA by JohnsHopkins University under NASA contract NAS 5-32985. Also based in parton observations collected at the European Southern Observatory VeryLarge Telescopes in programs 65.H-0705 and 67.D-0238, plus archival dataobtained with the NASA-ESA Hubble Space Telescope and NASA-ESA-PPARCInternational Ultraviolet Explorer.
| On the Absolute Magnitudes of the O Stars
The conclusion published in 1992 by Garmany & Stencel from a studyof northern OB associations, that the absolute magnitudes of the O starsshow ``a large scatter ... intrinsic to the classification system,'' iscritically examined. It is found that the differences between theirderived absolute magnitudes of O stars and this author's 1973calibration exhibit large systematic effects in several associations,ranging from -0.74 to +1.02 mag with substantially smaller dispersions.Of course, when these results are combined, the scatter equals the fullrange of the systematic effects. To investigate the possibility ofdistance errors, the Garmany & Stencel B0-B2.5 stars in the sameassociations are subjected to the same analysis. The results for the Bstars show no significant systematic differences, eliminating errors inthe association distances derived by Garmany & Stencel from the Bstars as the source of the differences found for the O stars. It isnoteworthy that the dispersions in the absolute magnitudes of the Bstars within a given association are similar to or larger than those ofthe O stars. An examination of the distribution on the sky of the starsshows that the O and B stars in the discrepant associations aregenerally not colocated; such was already known to be the case for theimportant Perseus OB1 association. It is suggested that despite theirefforts to improve them, significant problems remain with theassociation memberships adopted by Garmany & Stencel; the relativelysmall dispersions of the O star absolute magnitudes even in thediscrepant cases indicate that they belong to different, usually moredistant associations near the lines of sight to the B associations withwhich they have been mistakenly connected. Several individual cases ofunrecognized multiple systems and classification errors are also foundin the Garmany & Stencel sample. It is concluded that the scatter inthe absolute magnitudes of the O stars is not as large as found byGarmany & Stencel, and not larger than that of the B stars.
| The behavior of the NIV 5203.2 Å line in stars with Wolf-Rayet and early O-type spectra
Not Available
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| 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
| An analysis of HST UV spectra of Cyg OB2 stars
As a first step of a vigorous program to investigate the Wind Momentum -Luminosity Relationship (WLR) of Galactic O-stars by analyzing starsbelonging to the same cluster we present UV HST observations of sixsupergiants and one giant in the Cyg OB2 association.Terminal and turbulent wind velocities, velocity laws and metal ioncolumn densities are derived and mean ionization fractions areestimated. Turbulent velocities are mostly in the range 10-14% ofv_infty . The terminal velocities agree well with the average v_inftyvs. spectral class relationship compiled by Kudritzki & Puls(\cite{kp00}). We compare the observed v_infty vs. escape velocity(depending on the diagnostics of the stellar mass) correlation with thepredictions of the radiatively driven wind theory and find betteragreement with the spectroscopic masses rather than with theevolutionary ones. The beta velocity field exponents are in the range0.7-0.8, without any trend towards larger values. We show that for asingle luminosity class there is a tight relationship between T_eff and (the mean density at the point in the stellar wind, wherehalf the wind terminal velocity is reached). In consequence, theionization fractions show the same trend with both, T_eff and : we find that N V increases with T_eff, Si Iv decreases and C Ivdoes not clearly correlate. As a byproduct, we also derive interestellarH I column densities towards Cyg OB2, which turn outto be quite large. For one object (Cyg OB2#2) we findinconsistencies making the association membership questionable. Partlybased on INES data from the IUE satellite.
| A Search for Interstellar Bubbles surrounding Massive Stars in Perseus OB1
We have examined the interstellar medium in the vicinity of massivestars belonging to the Per OB1 association based on neutral hydrogen 21cm observations obtained with the 100 m radio telescope at Effelsberg(HPBW=8.4′) and complementary data from the Leiden-Dwingeloo H ISurvey (HPBW=36'). The higher angular resolution H I observationsallowed us to discover probable wind-blown bubbles related to fourmassive stars in the association, namely, HD 14442 [O5n(f)p], HD 14947[O5If+], HD 13022 [O9.5II-III((n))], and HD 13338 [O9.5V], while thedetection of a wind-blown bubble associated with HD 16691 [O5If+] isless conclusive. A clear H I shell coincident in position with two B1IIIstars (HD 15233 and Hilt 311) was also detected. Some of these featuresalso have infrared and/or molecular counterparts. The energetics of thestructures related to each massive star is analyzed. The new H Iinterstellar bubbles appear to be similar to the ones found surroundingWolf-Rayet stars and other Of stars. The large-scale maps obtained usingthe lower angular resolution H I data show that most of the early-typestars belonging to Per OB1 are placed in a region of low H I emission.The association could have blown a H I shell of about 350×550 pcin size. This large H I shell has an infrared counterpart.
| The massive Wolf-Rayet binary LSS 1964 (= WR 29)
We present a radial velocity study of LSS 1964 (= WR 29) based ondigital spectral images obtained with the 2.15-m telescope at CASLEO,San Juan, Argentina, between 1997 and 2000. We find this star to be adouble-lined WN + O binary with a period of 3.16415 days. The WNcomponent appears to be more massive than the O type component. Nivabsorption at ~ lambda 5203 Å is found to belong to the WN7 star.
| Fundamental parameters of Galactic luminous OB stars. V. The effect of microturbulence
We study the effect of microturbulence in the line formationcalculations of H and He lines, in the parameter range typical for O andearly B stars. We are specially interested in its effect on thedetermination of stellar parameters: Teff, log g andspecially on the He abundance. We first analyze the behaviour of H andHe model lines between 4 000 and 5 000 Äwith microturbulence andfind that for O stars only He I lines and He Ii lambda 4686 are sensiblyaffected by microturbulence, and that models with lower gravities, theones suitable for supergiants, are more sensitive to it. Using a testprocedure we show that the expected changes in Teff, log gand Helium abundance due to the inclusion of microturbulence in theanalysis, are small. We analyze five stars (two late, one intermediateand two early O stars) using microturbulence velocities of 0 and 15kms-1 and confirm the result of the previous test. Theparameters obtained for 15 kms-1 differ from the ones at 0kms-1 within the limits of the standard error box of ouranalysis. Only later types reduce their He abundance, by 0.02 in epsilon. Comparing with values in the literature we find that the range of ourchanges agree with previous results. In some cases other effects can addto microturbulence, and further reduce the He abundance up to 0.04. Thequality of the line fits only improves for He I lambda 4471, but not tothe extent of completely solving the so-called dilution effect.Therefore our conclusion is that microturbulence is affecting thederivation of stellar parameters, but its effect is comparable to theadopted uncertainties. Thus it can reduce moderate He overabundances andsolve line fit quality differences, but it cannot explain by itselflarge He overabundances in O stars. The INT is operated on the island ofLa Palma by the RGO in the Spanish Obervatorio de El Roque de losMuchachos of the Instituto de Astrof\'\i sica de Canarias.
| Fundamental parameters of Galactic luminous OB stars. IV. The upper HR diagram
We present observations and analyses of seven Galactic O stars of typeO6 and earlier. The analyses are carried out using NLTE plane-parallel,hydrostatic models as well as NLTE spherical models with mass-loss. Withdetailed calculations for the former and simulations for the latter, itis shown that the flux blocking due to UV metal lines is important forthese objects, in agreement with previous studies, and the way themechanism operates is explained. We find that the plane-parallel,hydrostatic unblanketed model atmospheres have increasing difficultiesin fitting the early-type spectra of massive stars, and for 50 000 K andabove a fit seems to be impossible. The gravities derived are relativelylow even for the luminosity class V stars. These objects also show themass discrepancy found in earlier studies, indicating that sphericityand mass-loss are important, even at their higher gravities. We thenperform an analysis using spherical models with mass-loss. It is foundthat gravities should be increased by 0.1-0.25 dex, reducing, but notsolving, the mass discrepancy. We show that spectroscopic masses are inbetter agreement with the theory of radiatively driven winds thanevolutionary masses are. A helium abundance larger than solar is alsoobtained for most objects. Some additional effects (partly related topresent approximations) that have an influence in our analyses arestudied. It is found that He iI lambda 4200 is less sensitive to detailsof the model calculations than He iI lambda 4541 and thus it ispreferred for temperature determinations, with the consequence of lowereffective temperatures. It is shown that the fits to He iI lambda 4686are improved when the upward rates of the He iI resonance lines arereduced (with respect to the conventional treatment adequate for linesformed in expanding atmospheres), either by setting them in detailedbalance or by artificially adding extra opacity sources that simulateline blocking. The He iI blend with H_alpha ,is also affected. Somestars of our sample have such high mass-loss rates that the derivationof gravities from the wings of Balmer lines, in particular H_{\loggamma}, becomes doubtful. For the most extreme objects, the mass-lossrates needed to fit H_alpha are different from those needed to fitH_{\log gamma}, by a maximum factor of two. From the point of view ofindividual stars, we have analysed some of the most massive and luminousstars in the Milky Way. According to our analysis, three of them (CygOB2 #7, HD 15 570 and HD 15 558) have particularly large initial masses,close to or in excess of 100 M/Msunsun. Finally, the leastluminous object in our sample, HD 5 689, could have been erroneouslyassigned to Cas OB7 and might be a runaway star. The INT is operated onthe island of La Palma by the RGO in the Spanish Obervatorio de El Roquede los Muchachos of the Instituto de Astrof\'\i sica de Canarias.
| The Ionization in the Winds of O Stars and the Determination of Mass-Loss Rates from Ultraviolet Lines
Empirical ionization fractions of C IV, N V, Si IV, and empiricalionization plus excitation fractions of C III* and N iv^* in the windsof 34 O stars and one B star have been derived. We combine the mass-lossrates derived from radio measurements and Hα with the line fittingof ultraviolet resonance lines and subordinate lines using the Sobolevplus exact integration (SEI) method. The dependence of the empiricalionization fractions, q, on the stellar effective temperature and on themean wind density is discussed. This sets constraints for the models ofionization in the winds of hot stars. The ionization and excitationfractions can be expressed in terms of an empirical radiationtemperature. This radiation temperature scales with T_eff, and we deriveempirical relations for T_rad as a function of T_eff. The radiationtemperatures are on the order of 0.5-0.9 T_eff, with significantdifferences between the ions. The derived relations between theionization fractions and the stellar parameters have an uncertainty of0.2 dex forSi IV, N V, and C iii^*, and about 0.26 dex for N iv^*. For CIV, we can only derive an expression for the mean ionization fraction inthe wind if the mass-loss rate is small, M<10^-6 M_solar yr^-1,because the C IV lines are usually saturated for higher mass-loss rates.The resulting expressions for T_rad can be used to derive the mass-lossrates from studies of ultraviolet P Cygni profiles in the range ofstellar parameters studied here: 30,000 K<~T_eff<~50,500 K,5.2<~logL/L_*<~6.4, and -7.5 M_solar<~logM<~-4.6 M_solaryr^-1. An accuracy of about a factor of 2 or better can be reached,depending on the lines that are used and the accuracy of the line fitsand the stellar parameters. The Si IV lines give the most reliablemass-loss rates, because the abundance is about the same for all Ostars, the lines saturate only for high mass-loss rates, the doubletlines only partly overlap, and the mass-loss rate is proportional to thesquare root of the column density. The radiation temperature of N Vshows a surprisingly strict relation with T_eff, with a scatter of onlyDeltaT_rad/T_eff=0.01. The mass-loss rate cannot be derived from the N Vlines, because the column density of the N V ions in the wind isindependent of M. A consistency check and a test of the method for thestars HD 14749 and HD 190429 show that the mass-loss rate derived fromthe UV lines with the ionization fractions of this paper agree very wellwith the mass-loss rate derived from new radio flux measurements.
| 1-m spectroscopy of normal OB stars
We have obtained spectra of 70 normal OB stars in the near-IR I(1-μm) band. The strongest features are those due to lines of thehydrogen Paschen series and neutral and ionized helium, which are, forthe most part, in absorption. The information content in this spectralrange is sufficient for only a rough classification of hot stars into`early O', `late O' and `B' types. Curiously, the leading He i tripletline, He i λ1.0830 μm, is usually not detectable, although ina few stars it is in emission; its behaviour generally correlates withthe leading helium singlet line, He i λ 2.058 μ m. These twofeatures appear to be present in emission only in stars with extremes ofmass loss or wind extension.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | פרסאוס |
| התרוממות ימנית: | 02h26m46.99s |
| סירוב: | +58°52'33.1" |
| גודל גלוי: | 8.048 |
| מרחק: | 2941.176 פארסק |
| תנועה נכונה: | 2.2 |
| תנועה נכונה: | -1.8 |
| B-T magnitude: | 8.483 |
| V-T magnitude: | 8.084 |
קטלוגים וכינוים:
|
