Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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.
| Chemical composition of Galactic OB stars. II. The fast rotator ζ Ophiuchi
ζ Oph, HD 149757, is an O9.5Vnn star with a very high projected rotational velocity (v sin i ≥340 km s-1). It is also a classical runaway star due to itshigh proper motion. We perform a quantitative analysis of its opticalspectrum in order to measure important observables of the star such asits mass, effective temperature, luminosity and He, C, N, and Oabundances. Comparing these observed values to those predicted by therotating evolutionary models of the Geneva group we find that none ofthe two sets of models (those of 2000 and 2003) is capable ofreproducing the characteristics of the star. Nevertheless, due to itsrunaway nature, the reason for this discrepancy may be that the star isnot the result of the evolution of a single object, but the product ofthe evolution of a close binary system.
| 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}.
| Terminal Velocities of Luminous, Early-Type Stars in the Small Magellanic Cloud
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS)are used to determine terminal velocities for 11 O and B-type giants andsupergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IVresonance lines. Using archival data from observations with the GoddardHigh-Resolution Spectrograph and the International Ultraviolet Explorertelescope, terminal velocities are obtained for a further five B-typesupergiants. We discuss the metallicity dependence of stellar terminalvelocities for supergiants, finding no evidence for a significantscaling between Galactic and SMC metallicities forTeff<30,000 K, consistent with the predictions ofradiation-driven wind theory. A comparison of thev&infy;/vesc ratio between the SMC and Galacticsamples, while consistent with the above statement, emphasizes that theuncertainties in the distances to galactic OB-type stars are a seriousobstacle to a detailed comparison with theory. For the SMC sample thereis considerable scatter in v&infy;/vesc at agiven effective temperature, perhaps indicative of uncertainties instellar masses.
| 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.
| Stars with ISM Polarization Observed with HPOL. II
Polarization data are given for stars whose polarizations are mostlyinterstellar which were observed with the University of Wisconsinspectropolarimeter (HPOL) during 1995-2003. Several cases are found forwhich K in the Serkowski Law for ISM polarization is higher than allowedby published formulas.
| 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
| A study of RV in Galactic O stars from the 2MASS catalogue
We present new measurements of the interstellar reddening parameterRV=AV/E(B-V) towards 185 O stars, using J, H,Ks photometry from the 2MASS project. The results arecombined with data from the literature of 95 stars where RVhas been derived with the same technique, 22 of which in common with ourpresent sample from the 2MASS project catalogue. The averageRV from these 258 O stars is of 3.19 +/- 0.50. All objectswhose RV departs from this value by more than 2 sigma havebeen recognized. Ten objects have RV higher than this valueand two lower. It is found that anomalous RV can scarcely beassociated with anomalies in the general interstellar medium, e.g. withdifferent behaviour in different spiral arms. They are clearly linked tolocal cloud effect. In the Cygnus region RV values follow thebehaviour of the general interstellar medium, while in the Carina arm,in spite of the relatively larger distance, local cloud effects prevail.An explanation for this is suggested. The relatively few stars of oursample whose Hipparcos parallaxes are reliable, are found to havedistances systematically smaller than the distances derived by thespectroscopic parallaxes. We argue that this effect is consistent withthe recently claimed discovery of grey extinction towards OB stars.This publication makes use of data products from the Two Micron All SkySurvey (2MASS), which is a joint project of the University ofMassachusetts and the Infrared Processing and Analysis Center/CaliforniaInstitute of Technology, funded by the National Aeronautics and SpaceAdministration and the National Science Foundation.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/905
| Chemical composition of Galactic OB stars. I. CNO abundances in O9 stars
We present NLTE abundances of CNO for a sample of four O9 stars in theGalaxy, together with new determinations of their stellar parameters,T_eff, log g, epsilon (He) and microturbulence. These new analyses takeinto account the effect of line-blocking in the spectral synthesis withour classical NLTE, plane-parallel and hydrostatic model atmospheres.The sample includes three O9 He normal stars: two dwarfs, HD 214680 andHD 34078, and one supergiant, HD 209975, and one fast rotating giantwith a preliminary high He overabundance, HD 191423 with epsilon(He)=0.20. We find first that the consideration of microturbulence inthe spectral synthesis for the fast rotator leads to a considerablylower He abundance, epsilon (He)=0.12. The CNO abundances of the threeHe normal stars are in good agreement with the values in the literaturefor Galactic B dwarfs with no evidence of mixing, and show that they allhave the same chemical composition. We also discuss however the possibleCNO contamination of the supergiant HD 209975. For the fast rotator wefind that the abundances show the trend of the CNO contamination: a Noverabundance together with C and O depletion. The N/C and N/O ratios ofour stars as a function of their projected rotational velocities areconsistent with the predictions of the recent evolutionary models ofMeynet & Maeder (\cite{Mey&Mae00}). The INT is operated on theisland of La Palma by the RGO in the Spanish Obervatorio de El Roque delos Muchachos of the Instituto de Astrofísica de Canarias.
| Rotational mixing in early-type main-sequence stars
We present quantitative observational investigations into the importanceof rotationally induced mixing in late-O stars. First, we conductnon-LTE, hydrostatic, plane-parallel H/He model-atmosphere analyses ofthe optical spectra of three of the most rapidly rotating late-Onear-main-sequence stars known: HD 93521 (O9.5 V), HD 149757 (ζOphiuchi; O9.5 V), and HD 191423 (ON9 III: n), all of which haveequatorial rotation velocities of ~430kms-1 and[formmu8]ωe/ωe(crit)~=0.9. Theanalysis allows for the expected (von Zeipel) variation ofTeff and logg with latitude. These three stars are found toshare very similar characteristics, including substantially enhancedsurface-helium abundances [formmu9](y~=0.2). Secondly, we compare thedistribution of projected rotational velocities for ON andmorphologically normal dwarf O stars, and demonstrate that the ON starsare drawn from a population with more rapid rotation. The resultsprovide qualitative support for rotationally induced mixing, althoughthere remain discrepancies between atmospheric and evolutionary models(which we show employ inappropriate mass-loss rates for late-Omain-sequence stars). We show that the most rapid rotator known, HD191423, is an ON star, and note the implied disparity between O/ONmorphology and surface helium abundance; we discuss consequences for theinterpretation of spectral morphology in O-type main-sequence stars. Wedemonstrate a new, purely spectroscopic, method of distancedetermination for rapid rotators, and thereby confirm that HD 93521 liesat ~2kpc, and is not, as previously suggested, a low-mass Population IIstar. Finally, our models contradict earlier claims of stronglydifferential surface rotation, and are consistent with uniform angularvelocity at the surface.
| 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
| 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.
| A spectropolarimetric survey of northern hemisphere Wolf-Rayet stars
We present a homogeneous, high signal-to-noise spectropolarimetricsurvey of 16 northern hemisphere Wolf-Rayet (WR) stars. A reduction inpolarization at emission-line wavelengths - the `line effect' - isidentified in four stars: WRs 134, 137, 139, and 141. The magnitude ofthe effect in WR 139 (V444 Cyg) is variable, while WR 136, previouslyreported to show the line effect, does not show it in our data. Assumingthe line effect generally to arise from axisymmetric distortions ofstellar winds, we show that a model in which all WRs have the sameintrinsic (equator-on) polarization, with the observed variations solelya result of inclination effects, is inconsistent with the observations.A model in which the intrinsic polarizations are uniformly distributedis more plausible, but best-fitting results are obtained if thedistribution of polarizations is biased towards small values, with only~20 per cent of stars having intrinsic polarizations greater than ~0.3per cent. Radiative transfer calculations indicate that the observedcontinuum polarizations can be matched by models with equator:poledensity ratios of 2-3. The model spectra have electron-scattering wingsthat are significantly stronger than observed (in both intensity andpolarized flux), confirming that the winds of stars showing intrinsicpolarization must be clumped on small scales as well as being distortedon large scales. We combine the results of our survey with observationsfrom the literature to give a sample of 29 stars which have bothaccurate spectropolarimetric observations and physical parametersderived from standard-model analyses. We find that the line-effect starsare clustered at high M, L in the luminosity-mass-loss rate plane(although they are unexceptional in the terminal velocity-subtype andthe surface-mass-flux-temperature planes). The mass-loss rates derivedfrom radio-continuum observations for these stars are in good accordwith the results of optical emission-line analyses, suggesting that (i)the wind structure of line-effect stars has a density contrast which iseffectively constant with radius, and (ii) the high M values may beartefacts of large-scale wind structure. Assuming that observedspectroscopic and photometric variability of the line-effect stars isrelated to the WR rotation period, we compute equatorial rotationvelocities. These velocities correspond to ~10 per cent of the corebreakup rates, and may be large enough to produce significantwind-compression effects according to the models of Ignace, Cassinelli& Bjorkman.
| 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.
| On the wind momentum problem of O-type stars in the galaxy
We have examined the wind momentum problem of O-type stars in theGalaxy. It is shown that the discrepancy between theoretical andempirical mass loss rates and terminal velocities can be reversed byusing recently updated values of force multiplier parameters. With thesenew values, the momentum problem found by former investigators isreversed so that there now appears to be more than enough radiationforce in order to accelerate the stellar winds of a sample of GalacticO-type stars.
| 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.
| 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
| The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...460..914V&db_key=AST
| O-star mass-loss and wind momentum rates in the Galaxy and the Magellanic Clouds Observations and theoretical predictions.
A new, very fast approximate method is presented to determine mass-lossrates of O-stars from Halpha_ line profiles. The method usesH and HeII departure coefficients from unified model atmospheresparametrized in a simple way as function of wind velocity together withphotospheric NLTE line profiles as the inner boundary condition for anumerically exact radiative transfer solution to derive a windcontaminated Halpha_-profile. The method is also applied toHgamma_ to determine stellar gravities corrected for windemission. A detailed analytical discussion of Halpha_ lineformation in O-star winds is given and it is demonstrated that formervery simple approaches considering only optically thin wind emissionlead to significant systematic errors. Scaling relations and generalizedcurves of growth are presented that connect mass-loss rate, terminalvelocity, stellar parameters and Halpha_ equivalent width.The method is applied to samples of O-stars in the Galaxy, LMC and SMCand mass-loss rates are derived from Halpha_ in combinationwith terminal velocities measured from IUE and HST spectra. The resultsreveal that a tight empirical relation exists between the radiusmodified stellar wind momentum rate˙(M)vinfinity_R_*_^0.5^ and the stellar luminosity. Thevariations of this relationship between the Galaxy, LMC and SMC areexplained in terms of different abundances. Furthermore, for almost allobjects with dense winds (mostly supergiants), the commonly usedvelocity field exponent β could be derived, indicating a typicalvalue of β=~1. A comparison with the improved theory of radiationdriven winds (as presented recently by Pauldrach et al. 1994) shows thatthe observed wind momentum-luminosity relationship can be understoodqualitatively in terms of the theory. However, there exist significantsystematic discrepancies as a function of effective temperature,luminosity class and wind performance numberη=˙(M)vinfinity_c/L. We stress that thesediscrepancies would not have been detected with previous simplifiedHalpha_ -approaches. The deficiencies of the theory arediscussed and suggestions for future improvements are made.
| Observational manifestations of early mixing in B and O type stars.
Not Available
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| An IUE survey of interstellar H I LY alpha absorption. 1: Column densities
We measure Galactic interstellar neutral hydrogen column densities byanalyzing archival interstellar Ly alpha absorption line data toward 554B2 and hotter stars observed at high resolution with the IUE satellite.This study more than doubles the number of lines of sight with measuresof N(H I) based on Ly alpha. We have included the scattered lightbackground correction algorithm of Bianchi and Bohlin in our datareduction. We use the correlation between the Balmer discontinuity(c1) index and the stellar Ly alpha absorption in order toassess the effects of stellar Ly alpha contamination. Approximately 40%of the B stars with measured (c1) index, exhibit seriousstellar Ly alpha contamination. One table contains the derived values ofthe interstellar N(H I) for 393 stars with at most small amounts ofstellar contamination. Another lists the observed values of total N(H I)for 161 stars with suspected stellar Ly alpha contamination and/oruncertain stellar parameters.
| Evolution of initially mixed massive main sequence stars
The main sequence evolution of massive stars is calculated under theassumption that an unspecified mixing process were able to keep themodels homogeneous during some fraction of the core hydrogen burningphase. The mixing could be due to rotation. The results are used todiscuss discrepancies in stellar parameters between recent spectralanalyses of OB stars and standard stellar evolution theory. The observedsurface helium abundances serve as input to determine the duration ofthe mixing. It is shown that mixing on the main sequence alone cannotexpain the derived values. although the agreement becomes much betterwhen using the tracks presented here.
| Synthetic UV Lines of Si IV, C IV, and He II from a Population of Massive Stars in Starburst Galaxies
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...418..749R&db_key=AST
| Intrinsic parameters of galactic luminous OB stars
We present the results of a spectroscopic analysis of 25 galacticluminous OB stars. The gravities are in general lower than previouslyassumed, especially for giants. The helium abundances are also higherthan normal in many cases. Using data from the spectroscopic analysis,radii, luminosities and masses are derived. The observed wind terminalvelocities are used with the analytic approximation for the theory ofradiatively driven winds (Kudritzki et al., 1989) to obtain masses andmass loss rates. The masses derived from the spectroscopic analysis andthose derived independently from the wind theory agree, except for asmall difference attributed to deficiencies in the physics of the modelatmospheres used. A comparison with evolutionary tracks indicates that(1) evolution theory cannot explain the observed helium abundancepattern and that (2) the masses derived from the spectroscopic analysis(or the wind theory) are systematically lower than those derived fromevolutionary calculations.
| Photospheric Absorption Lines in the Ultraviolet Spectra of O-Stars and B-Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990MNRAS.246..392P&db_key=AST
| Terminal velocities for a large sample of O stars, B supergiants, and Wolf-Rayet stars
It is argued that easily measured, reliable estimates of terminalvelocities for early-type stars are provided by the central velocityasymptotically approached by narrow absorption features and by theviolet limit of zero residual intensity in saturated P Cygni profiles.These estimators are used to determine terminal velocities, v(infinity),for 181 O stars, 70 early B supergiants, and 35 Wolf-Rayet stars. For OBstars, the values are typically 15-20 percent smaller than the extremeviolet edge velocities, v(edge), while for WR stars v(infinity) = 0.76v(edge) on average. New mass-loss rates for WR stars which are thermalradio emitters are given, taking into account the new terminalvelocities and recent revisions to estimates of distances and to themean nuclear mass per electron. The relationships between v(infinity),the surface escape velocities, and effective temperatures are examined.
| Spectroscopy of luminous blue stars in M31 and M33
Spectra have been obtained for classification of 42 candidatesupergiants and 12 probable OB stars in M31 and eight early-type starsin M33. Twenty-six of those in M31 and six in M33 are confirmed asapparent single members with spectral types ranging from O8 to F8. Theirinterstellar extinction and luminosities are derived from publishedphotographic and CCD photometry for all of the confirmed members. Thepreliminary and still incomplete HR diagram obtained for M31 shows anapparent lack of the most massive stars, stars with initial massesgreater than 60 solar masses. The effects of incompleteness andobservational selection on the interpretation of this HR diagram arediscussed.
| Empirical temperature calibrations for early-type stars
Three temperature calibrations of suitable photometric quantities havebeen derived for O and B stars. A sample of 120 stars with reliableT(eff.) determinations has been used for establishing each calibration.The different calibrations have been critically discussed and compared.Temperature determinations for 1009 program stars have been obtainedwith an accuracy of the order of 10 percent.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Kugu |
| Sað Açýklýk: | 20h08m07.11s |
| Yükselim: | +42°36'22.0" |
| Görünürdeki Parlaklýk: | 8.055 |
| Uzaklýk: | 2173.913 parsek |
| özdevim Sað Açýklýk: | -6.4 |
| özdevim Yükselim: | -2.3 |
| B-T magnitude: | 8.171 |
| V-T magnitude: | 8.065 |
Kataloglar ve belirtme:
|
