目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| Interstellar 12C/13C ratios through CH^+λλ 3957,4232 absorption in local clouds: incomplete mixing in the ISM
The 12C/13C isotope ratio is a tracer of stellaryields and the efficiency of mixing in the ISM.12CH+/13CH+ is not affectedby interstellar chemistry, and is the most secure way of measuring12C/13C in the diffuse ISM.R=12C/13C is 90 in the solar system. Previousmeasurements of 12CH+λλ3957.7,4232.3and 13CH+λλ3958.2,4232.0 absorptiontoward nearby stars indicate some variations in12C/13C, with values ranging from 40 to 90suggesting inefficient mixing. Except for the cloud toward ζOph,these R values are strongly affected by noise. With UVES on the VLT wehave improved on the previous interstellar 12C/13Cmeasurements. The weighted 12C/13C ratio in thelocal ISM is 78.27 ± 1.83, while the weighted dispersion of ourmeasurements is 12.7, giving a 6.9σ scatter. Thus we report on a6.9σ detection of 16.2% root-mean-square variations in the carbonisotopic ratio on scales of ~100 pc: R= 74.7 ± 2.3 in theζOph cloud, while R = 88.6 ± 3.0 toward HD 152235 in theLupus clouds, R = 62.2 ± 5.3 towards HD 110432 in the Coalsack,and R = 98.9 ± 10.1 toward HD 170740. The observed variations in13C/12C are the first significant detection ofchemical heterogeneity in the local ISM.
| 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.
| NLTE models of line-driven stellar winds. I. Method of calculation and first results for O stars
New numerical models of line-driven stellar winds of late O stars arepresented. Statistical equilibrium (NLTE) equations of the most abundantelements are solved. Properly obtained occupation numbers are used tocalculate consistent radiative force and radiative heating terms. Winddensity, velocity and temperature are calculated as a solution of modelhydrodynamical equations. Contrary to other published models we accountfor a multicomponent wind nature and do not simplify the calculation ofthe radiative force (e.g. using force multipliers). We discuss theconvergence behaviour of our models. The ability of our models topredict correct values of mass-loss rates and terminal velocities ofselected late O stars (mainly giants and supergiants) is demonstrated.The systematic difference between predicted and observed terminalvelocities reported in the literature has been removed. Moreover, wefound good agreement between the theoretical wind momentum-luminosityrelationship and the observed one for Cyg OB2 supergiants.Appendices A, B and C are only 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.
| Improved Hipparcos Parallaxes of Coma Berenices and NGC 6231
A method to reprocess the Hipparcos Intermediate Astrometry Data thatreduces the propagation of the along-scan spacecraft attitude errors wasdeveloped and successfully used to obtain a more consistent parallax ofthe Pleiades (Makarov, published in 2002). The same technique is usednow to correct the Hipparcos parallaxes of the Coma Berenices and NGC6231 open clusters, which are also in error. The new mean parallax ofComa is 12.40+/-0.17 mas (against previously 11.43 mas) and of NGC 62311.7+/-0.4 mas (against previously -0.8 mas). The new data for Coma arein excellent agreement with the pre-Hipparcos main-sequence fittingestimates. All six members of NGC 6231 that have negative parallaxes inHipparcos obtain positive parallaxes. These results suggest that themain source of astrometric error has been correctly identified, and thata more accurate Hipparcos catalog can be computed.
| A Method for Simultaneous Determination of AV and R and Applications
A method for the simultaneous determination of the interstellarextinction (AV) and of the ratio of total to selectiveextinction (R), derived from the 1989 Cardelli, Clayton, & Mathisfitting of the interstellar extinction law, is presented and applied toa set of 1900 color excesses derived from observations of stars inUBVRIJHKL. The method is used to study the stability of AVand R within selected regions in Perseus, Scorpius, Monoceros, Orion,Sagittarius, Ophiuchus, Carina, and Serpens. Analysis shows that R isapproximately constant and peculiar to each sector, with mean valuesthat vary from 3.2 in Perseus to 5.6 in Ophiuchus. These results aresimilar to published values by Aiello et al., He et al., Vrba &Rydgren, O'Donnell, and Cardelli, Clayton, & Mathis.
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| Multicomponent radiatively driven stellar winds. II. Gayley-Owocki heating in multitemperature winds of OB stars
We show that the so-called Gayley-Owocki (Doppler) heating is importantfor the temperature structure of the wind of main sequence stars coolerthan the spectral type O6. The formula for Gayley-Owocki heating isderived directly from the Boltzmann equation as a direct consequence ofthe dependence of the driving force on the velocity gradient. SinceGayley-Owocki heating deposits heat directly on the absorbing ions, wealso investigated the possibility that individual components of theradiatively driven stellar wind have different temperatures. This effectis negligible in the wind of O stars, whereas a significant temperaturedifference takes place in the winds of main sequence B stars for starscooler than B2. Typical temperature differences between absorbing ionsand other flow components for such stars is of the order 103K. However, in the case when the passive component falls back onto thestar, the absorbing component reaches temperatures of order106 K, which allows for emission of X-rays. Moreover, wecompare our computed terminal velocities with the observed ones. Wefound quite good agreement between predicted and observed terminalvelocities. The systematic difference coming from the using of the socalled ``cooking formula'' has been removed.
| The origin of the runaway high-mass X-ray binary HD 153919/4U1700-37
Based on its Hipparcos proper motion, we propose that the high-massX-ray binary HD 153919/4U1700-37 originates in the OB association ScoOB1. At a distance of 1.9 kpc the space velocity of 4U1700-37 withrespect to Sco OB1 is 75 km s-1. This runaway velocityindicates that the progenitor of the compact X-ray source lost about 7Msun during the (assumed symmetric) supernova explosion. Thesystem's kinematical age is about 2 +/- 0.5 million years which marksthe date of the supernova explosion forming the compact object. Thepresent age of Sco OB1 is la 8 Myr; its suggested core, NGC 6231, seemsto be somewhat younger ( ~ 5 Myr). If HD 153919/4U1700-37 was born as amember of Sco OB1, this implies that the initially most massive star inthe system terminated its evolution within la 6 million years,corresponding to an initial mass ga 30 Msun. With theseparameters the evolution of the binary system can be constrained. Basedon data obtained with ESA's astrometric satellite Hipparcos.
| 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
| 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
| 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.
| UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
| ICCD speckle observations of binary stars. XIX - an astrometric/spectroscopic survey of O stars
We present the results of a speckle interferometric survey made with theCHARA speckle camera and 4 m class telescopes of Galactic O-type starswith V less than 8. We can detect with the speckle camera binaries inthe angular separation range 0.035-1.5 arcsec with delta M less than 3,and we have discovered 15 binaries among 227 O-type systems. We combinedour results on visual binaries with measurements of wider pairs from theWashington Double Star Catalog and fainter pairs from the HipparcosCatalog, and we made a literature survey of the spectroscopic binariesamong the sample. We then investigated the overall binary frequency ofthe sample and the orbital characteristics of the known binaries.Binaries are common among O stars in clusters and associations but lessso among field and especially runaway stars. There are many triplesystems among the speckle binaries, and we discuss their possible rolein the ejection of stars from clusters. The period distribution of thebinaries is bimodal in log P, but we suggest that binaries with periodsof years and decades may eventually be found to fill the gap. The massratio distribution of the visual binaries increases toward lower massratios, but low mass ratio companions are rare among close,spectroscopic binaries. We present distributions of the eccentricity andlongitude of periastron for spectroscopic binaries with ellipticalorbits, and we find strong evidence of a bias in the longitude ofperiastron distribution.
| 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.
| Cyclicities in the light variations of LBVs. I. The multi-periodic behaviour of the LBV candidate ζ^1^ Sco.
Differential Stroemgren uvby photometry of ζ^1^ Sco collectedduring the time interval 1982-1995 is analysed together with new VBLUWphotometry with a complementary discussion of unpublished JHKLM data. Wereport the discovery of a ~1750-2200d regular oscillation of amplitude~0.01mag in all bands, and pulsation-like oscillations with at least onefrequency f_1_=0.03155 (P~32days) and a possible second frequencyf_2_~5/4 f_1_ ~0.03891 (P~25.7 days). Most likely, all such starsexhibit multi-periodic light variability. A strong noise componentremaining in the prewhitened signal leads us to postulate that thedriver for LBV-like eruptions is stochastic resonance. This hypothesisis supported by the historic light curve of ζ^1^ Sco which showsthat two centuries ago the star was about 2 magnitudes brighter, while amillenium ago it was only 1 magnitude brighter than today, an indicationthat ζ^1^ Sco should be regarded as a candidate Luminous BlueVariable.
| A Survey for H alpha Emission in Massive Binaries: The Search for Colliding Wind Candidates
I report the results of the first all-sky survey of H alpha emission inthe spectra of O-type binaries. The survey includes 26 systems, of which10 have emission that extends clearly above the continuum. This is thefirst report of emission for four of these. An additional three systemsshow small distortions in the H alpha profile that may result from weakemission. I compare the distribution of emission systems in H-R diagramsfor both binary and single stars, using a survey of single O-type starsdone by Conti (1974). Emission in main-sequence systems is extremelyrare and is completely absent in my sample of binary stars. Among binarystars, 78% of the systems containing giants show some emission, while nosingle giants in Conti's sample do. In the case of supergiants, 78% ofsingle stars show emission, while all supergiant binaries show strongemission. H alpha emission may come from a variety sources, but the factthat binaries have a higher incidence and strength of emission inpost--main-sequence stages may indicate that wind interactions are acommon source of emission in massive binaries. To ascertain whether ornot colliding winds have been observed, it will be necessary to studythe H alpha line profile throughout several orbits of each candidatecolliding wind system and look for recurring orbital-phase--relatedvariations. Such a study is underway.
| A Radial Velocity Database for Stephenson-Sanduleak Southern Luminous Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....113..823R&db_key=AST
| Cross-correlation characteristics of OB stars from IUE spectroscopy
We present a catalogue of homogeneous measures of the linewidthparameter, v_esin i, for 373 O-type stars and early B supergiants(including the separate components of 25 binary and three triplesystems), produced by cross-correlating high-resolution,short-wavelength IUE spectra against a `template' spectrum of tauSco. Wealso tabulate terminal velocities. There are no O supergiants in oursample with v_esin i<65 km s^-1, and only one supergiant earlier thanB5 has v_esin i<50 km s^-1, confirming that an important linebroadening mechanism in addition to rotation must be present in theseobjects. A calibration of the area under the cross-correlation peakagainst spectral type is used to obtain estimates of continuum intensityratios of the components in 28 spectroscopically binary or multiplesystems. At least seven SB2 systems show evidence for the `Struve-Sahadeeffect', a systematic variation in relative line strength as a functionof orbital phase. The stellar wind profiles of the most rapid rotator inour sample, the O9III:n* star HD 191423 (v_esin i=436km s^-1), show itto have a `wind-compressed disc' similar to that of HD 93521; this starand other rapid rotators are good candidates for studies of non-radialpulsation.
| A Spectral Atlas of Hot, Luminous Stars at 2 Microns
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..107..281H&db_key=AST
| 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
| Far-Ultraviolet Stellar Photometry: Fields Centered on rho Ophiuchi and the Galactic Center
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..104..101S&db_key=AST
| Absorption Line Profile Variations among the O Stars. I. The Incidence of Variability
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJS..103..475F&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.
| Terminal Velocities and the Bistability of Stellar Winds
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...455..269L&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The ^12^CH^+^/^13^CH^+^ ratio in the Coalsack.
We have measured the ^12^C/^13^C interstellar ratio in the Coalsack, aprominent dark nebula in the southern sky, by means of high resolution(λ/δλ=6x10^4^) observations of the λ4232ACH^+^ line in the direction of the star HD110432 (d=~0.31kpc). Thecollection of a total of 53 spectra of this star and of a referencebright star (α Vir) has allowed us to obtain a mean spectrum withsignal-to-noise ratio =~1500 at 1 σ level. We have detected theweak ^13^CH^+^ absorption well resolved from the ^12^CH^+^ line andderived a column density ratio =71+/-11 by means of line profile fittingwith Voigt profiles. This result is in very good agreement with the fewexisting determinations of the column density ratio in other 4 clouds atd <~0.5kpc. The isotopic ratio is remarkably constant along differentdirections of the local interstellar medium and has a mean value=67+/-3. This value is in good agreement with the most recent estimatesof the ^12^C/^13^C ratio in the solar vicinity derived from CO andH_2_CO emission lines observed at radio frequencies.
| A spectroscopic database for Stephenson-Sanduleak Southern Luminous Stars
A database of published spectral classifications for objects in theStepenson-Sanduleak Luminous Stars in the Southern Milky Way catalog hasbeen compiled from the literature. A total of 6182 classifications for2562 stars from 139 sources are incorporated.
| Far-ultraviolet stellar photometry: Fields in Sagittarius and Scorpius
Far-ultraviolet photometry for 741 objects in a field in Sagittariuscentered near M8 and 541 objects in a field centered near sigma Scorpiiis presented. These data were extracted from electographic imagesobtained with two cameras during a shuttle flight in 1991 April/May. Thecameras provided band passes with lambdaeff = 1375 A andlambdaeff = 1781 A. Synthetic colors show that these bandsare sensitive to effective temperature for hot stars. Our measurementswere placed on a quantitative far-ultraviolet magnitude scale byconvolving the spectra of stars observed by IUE with our cameras'spectral response functions. Fifty-eight percent of the ultravioletobjects were identified with visible stars using the SIMBAD databasewhile another 40% of the objects are blends of early type stars tooclose together to separate with our resolution. Our photometry iscompared with that from the TD-1, OAO 2, and ANS satellites and the S201(Apollo 16) far-ultraviolet camera and found to agree at the level of afew tenths of a magnitude. Unlike previous studies, almost half of theidentified visual counterparts to the ultraviolet objects are early Bstars. A plot of distance modulus against ultraviolet color excessreveals a significant population of stars with strong ultravioletexcess.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
