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B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| Surface abundances of light elements for a large sample of early B-type stars - III. An analysis of helium lines in spectra of 102 stars
Non-local thermodynamic equilibrium analysis of He I lines in spectraof 102 B stars is implemented in order to derive the helium abundanceHe/H, the microturbulent parameter Vt and the projectedrotation velocity v sini. A simultaneous determination of He/H andVt for the stars is effected by analysing equivalent widthsof the 4471- and 4922-Å lines primarily as indicators of He/H andthe 4713-, 5016-, 5876- and 6678-Å lines primarily as indicatorsof Vt. The rotation velocities v sini are found from profilesof the same lines. It is shown that, when Vt > 7 kms-1, the Vt(He I) values determined from He Ilines are systematically overestimated as compared with theVt(OII, NII) values derived from OII and NII lines. Thisdiscrepancy is especially appreciable for hot evolved B giants withVt(He I) = 16-23 km s-1 and may indicate a failureof classical model atmospheres to represent the strong He I lines forthese stars.Two programme stars, HR 1512 and 7651, are found to be helium-weakstars. The remaining 100 stars are divided into three groups accordingto their masses M. The microturbulent parameter Vt(He I) islow for all stars of group A (M= 4.1-6.9 Msolar) and for allstars with the relative ages t/tMS < 0.8 of group B (M=7.0-11.2 Msolar). Their Vt(He I) values are withinthe 0 to 5 km s-1 range, as a rule; the mean value isVt= 1.7 km s-1. Only evolved giants of group B,which are close to the termination of the main-sequence (MS)evolutionary phase (t/tMS > 0.8), show Vt(He I)up to 11 km s-1. The helium abundance He/H is correlated withthe relative age t/tMS in both groups; the averaged He/Henhancement during the MS phase is 26 per cent. For group C, containingthe most massive stars (M= 12.4-18.8 Msolar), theVt(He I) values display a correlation with t/tMS,varying from 4 to 23 km s-1. The He/H determination for hotevolved B giants of the group with Vt(He I) > 15 kms-1 depends on a choice between the Vt(He I) andVt(OII, NII) scales. The mean He/H enrichment by 67 per centduring the MS phase is found, if the abundances He/H are based on theVt(OII, NII) scale; however, two evolved giants withespecially high v sini, HR 7446 and 7993, show the He/H enhancement byabout a factor of 2.5. When using the same Vt scale, we founda trend of He/H with projected rotational velocities v sini a largedispersion for v sini > 150 km s-1 can result fromdifferences in masses M.A comparison with the stellar model computations with rotationallyinduced mixing shows that the observed helium enrichment during the MSphase can be explained by rotation with initial velocities 250-400 kms-1. The He/H distribution on M and v sini based on theVt(OII, NII) scale seems to be in better agreement with thetheory than one based on the Vt(He I) scale. The mean valueHe/H = 0.10 derived for stars in the zero age main sequence (ZAMS)vicinity can be adopted as the typical initial helium abundance forearly B stars in the solar neighbourhood.
| The Angular Momentum Evolution of 0.1-10 Msolar Stars from the Birth Line to the Main Sequence
Projected rotational velocities (vsini) have been measured for a sampleof 145 stars with masses between 0.4 and greater than 10Msolar (median mass 2.1 Msolar) located in theOrion star-forming complex. These measurements have been supplementedwith data from the literature for Orion stars with masses as low as 0.1Msolar. The primary finding from analysis of these data isthat the upper envelope of the observed values of angular momentum perunit mass (J/M) varies as M0.25 for stars on convectivetracks having masses in the range ~0.1 to ~3 Msolar. Thispower law extends smoothly into the domain of more massive stars (3-10Msolar), which in Orion are already on the zero-age mainsequence. This result stands in sharp contrast to the properties ofmain-sequence stars, which show a break in the power law and a sharpdecline in J/M with decreasing mass for stars with M<2Msolar. A second result of our study is that this break isseen already among the pre-main-sequence stars in our Orion sample thatare on radiative tracks, even though these stars are only a few millionyears old. A comparison of rotation rates seen for stars on either sideof the convective-radiative boundary shows that stars do not rotate assolid bodies during the transition from convective to radiative tracks.As a preliminary demonstration of how observations can be used toconstrain the processes that control early stellar angular momentum, weshow that the broad trends in the data can be accounted for by simplemodels that posit that stars (1) lose angular momentum before they aredeposited on the birth line, plausibly through star-disk interactions;(2) undergo additional braking as they evolve down their convectivetracks; and (3) are subject to core-envelope decoupling during theconvective-radiative transition.
| Surface abundances of light elements for a large sample of early B-type stars - II. Basic parameters of 107 stars
Effective temperatures Teff, surface gravities logg andinterstellar extinctions AV are found for 107 B stars.Distances d of the stars, which are based on the derivedTeff, logg and AV values, show good agreement withthose obtained from the Hipparcos parallaxes. Comparing theTeff and logg values with evolutionary computations, we infermasses, radii, luminosities, ages and relative ages of the stars.Empirical relations between the Teff and logg parameters, onthe one hand, and the photometric indices Q, [c1] and β,on the other hand, are constructed; these relations give a fast methodfor the Teff and logg estimation of early and medium B stars.Inclusion of the infrared J, H and K colours into the Teff,logg and AV determination shows that (i) the Teffand logg parameters are altered only slightly; (ii) the AVvalue is rather sensitive to these colours, so an accuracy better than0.05mag in the JHK data is necessary for precise AVevaluation.
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Multiplicity of Massive Stars
We discuss the observed multiplicity of massive stars and implicationson theories of massive star formation. After a short summary of theliterature on massive star multiplicity, we focus on the O- and B-typestars in the Orion Nebula Cluster, which constitute a homogenous sampleof very young massive stars. 13 of these stars have recently been thetargets of a bispectrum speckle interferometry survey for companions.Considering the visual and also the known spectroscopic companions ofthese stars, the total number of companions is at least 14.Extrapolation with correction for the unresolved systems suggests thatthere are at least 1.5 and perhaps as much as 4 companions per primarystar on average. This number is clearly higher than the mean number of~0.5 companions per primary star found for the low-mass stars in thegeneral field population and also in the Orion Nebula cluster. Thissuggests that a different mechanism is at work in the formation ofhigh-mass multiple systems in the dense Orion Nebula cluster than forlow-mass stars.
| 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
| Surface abundances of light elements for a large sample of early B-type stars - I. Spectral observations of 123 stars; measurements of hydrogen and helium lines; infrared photometry
High-resolution spectral observations of 123 B0-B5 stars in the mainsequence evolutionary phase were obtained at two observatories, namelythe McDonald Observatory (McDO) and the Crimean AstrophysicalObservatory (CrAO). Accurate equivalent widths W of two Balmer lines,Hβ and Hγ, and ten Hei lines were obtained for all the stars,as well as of the Heiiλ4686 line for the hottest ones. A carefulanalysis of the measured equivalent widths was performed. It is shownthat there is a very good agreement between the W values derived fromthe McDO and CrAO spectra for 14 common stars. A comparison withpublished data leads to the conclusion that the W values measuredearlier by some authors for strong Hei lines are very likely to beunderestimated. Infrared photometric observations in the J, H, and Kbands were performed for 70 programme stars. All these data will be usedin other papers: in particular for the Teff and loggdetermination and for the He, C, N and O abundance analyses.
| Multiplicity of the massive stars in the Orion Nebula cluster
We present bispectrum speckle interferometry observations of 13 brightOrion Nebula cluster member stars of spectral type O or B.Diffraction-limited images with a resolution lambda/D of 75 mas in theK'-band were obtained with the SAO 6 m telescope. In our speckle imageswe find 8 visual companions in total. Using the flux ratios of theresolved systems to estimate the masses of the companions, we find thatthe systems generally have mass ratios below 1/2. The distribution ofmass ratios seems to be consistent with a companion mass functionsimilar to the field IMF. Considering both, the visual and thespectroscopic companions of the 13 target stars, the total number ofcompanions is at least 14. Extrapolation with correction for theunresolved systems suggests that there are at least 1.5 companions perprimary star on average. This number is clearly higher than the meannumber of ~ 0.5 companions per primary star found for the low-mass starsin the Orion Nebula cluster as well as in the field population. Thissuggests that a different mechanism is at work in the formation ofhigh-mass multiple systems in the dense Orion Nebula cluster than forlow-mass stars.
| Dust shells around certain early-type stars with emission lines.
Not Available
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| A calibration of Geneva photometry for B to G stars in terms of Teff, log G and [M/H]
We have used recent Kurucz models and numerous standard stars to improvethe calibration of the Geneva photometric system proposed a few yearsago. A new photometric diagram for the classification of intermediatestars (8500 <= Teff <= 11000 K) is proposed and fills agap that the previous calibration had left open. Evidence is given for aclear inadequacy of the new Kurucz models in the region of the parameterspace where convection begins to take over radiation in the star'satmosphere. This problem makes the determination of the surface gravitydifficult, but leaves that of the other parameters apparentlyunaffected. The determination of metallicity is considerably improved,thanks to the homogeneous spectroscopic data published recently by\cite[Edvardsson et al. (1993)]{ref23}. Instead of showing thetraditional diagrams, we chose to publish the diagrams of the physicalparameters with the inverted grids inside, i.e. the lines of constantphotometric parameters.
| Proper motions of stars in the region of the Orion Nebula cluster (C 0532-054).
Relative proper motions and membership probabilities for 333 starswithin an area of 1.6deg by 1.8deg centred on the Orion Nebula M 42 aredetermined using plates taken over a period of 83 years with the doubleastrograph of Shanghai Observatory (scale of 30"/mm). The plates weremeasured with the ASTROSCAN automatic plate-measuring machine of LeidenObservatory. The average proper motion accuracy obtained for stars inthe photographic magnitude range 7 to 14 is 0.3mas/yr. Errors aresomewhat larger towards fainter and brighter magnitude, but the majoritylie well below 1mas/yr. 64% of the stars have been measured successfullyon at least 13 out of 18 plates. The number of stars with membershipprobabilities higher than 0.7 is 184. It is shown by a detaileddiscussion that the proper motions and membership probabilities of thestars determined in this paper are in good agreement with the resultspresented recently by other authors. Although there is a clearconcentration in the proper motion diagram, both the remainingdispersion of the internal motions and the distribution of "members" asprojected on the sky indicate that the stars in this region are notbound as one system, but do have a common origin. A similar conclusioncan be drawn from a comparison with spectroscopic and radial velocitydata.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| A study of some stars with circumstellar dust envelopes. I
We present the results of a study of circumstellar dust envelopes of 36stars of early(O-B-A) types in the directions of the associations CasOB1, Cas OB2, Per OB1, and Ori OB1. We determine the absorption at 1640Å, the linear radius of the dust envelopes, the mean value of thecoefficient k, and the masses of the envelopes. They differsignificantly from one another.
| 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.
| Machine-readable version of the Parenago catalogue of stars in the area of the Orion nebula
Not Available
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| Spectroscopic binaries in the Orion OB1 association
A radial velocity study of the brighter members of the Ori OB2association is reported. The radial velocity variables, new preliminaryorbital elements for six spectroscopic binaries, and the projected axialrotation in stars for which the orbital elements are lacking arereported. The correlation between the proportion of binaries and theaverage axial rotation for the subgroups of the association isdiscussed. It is found that the Orion main-sequence members rotate atalmost the same rate as field stars of the same types. The averageradial velocity for the whole association is 23 km/s. The percentage ofspectroscopic binaries with periods shorter than 100 days plus themagnetic Ap-Bp among the main-sequence members of the association is 32percent.
| Spectroscopic binaries in the Orion Nebula cluster
A search was made for spectroscopic binaries among the 26 brightestmembers of this very young cluster. Orbital elements are given for fourbinaries (three known binaries, one newly discovered), and one star is aprobable binary. In agreement with published predictions from n-bodysimulations for binary formation by capture, it is found that thespectroscopic binaries have long periods (median of 20 days), havelow-mass companions, are not rotating synchronously with their orbitalmotions even for a period of 6.5 days, and are probably not concentratedtoward the cluster center more than are the massive single stars. Thebinary frequency is probably normal for a cluster with a high meanrotational velocity.
| The Orion complex according to observations of the space telescope Glazar.
Not Available
| Broad-band photometry of selected southern ultraviolet-bright stars.
Not Available
| Luminosities, masses, and ages of B-type stars
Measurements of H-gamma lines and photometry of the Balmer discontinuityare combined with models of stellar atmospheres and interiors to derivevalues of T(eff) log g, mass, and M(bol) for main-sequence B-type starsin the Alpha Per and the Pleiades clusters and in the Orion, Sco-Gen,and I Lac associations. The masses as a function of T(eff) agree withthe analyses of eclipsing binary orbits. The luminosities yielddistances for the observed clusters and associations that are in goodagreement with independent determinations. The observed main sequence isbroader than predicted by standard models and suggests that mixing mayplay a role in extending the main-sequence lifetime. Stars within eachcluster and association show a range of ages, a result that suggeststhat star formation occurred over a period of at least 1-3 x 10 to the7th yr.
| Stars of Early Types in the Region of Stellar Association OB1 Observed with the Glazar Telescope
Not Available
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| A new calibration of Geneva photometry in terms of Te, log G and mass for B stars
A new, semiempirical calibration of the two reddening-free parameters Xand Y of Geneva photometry in terms of effective temperature and gravityis proposed. The calibration is based on Kurucz's grid of atmospheremodels and on a set of fundamental stars; it is valid for main sequencestars with Te equal to or greater than 10,000 K. Theoreticalevolutionary tracks were then used to calibrate the Y vs X diagram interms of stellar mass, for M equal to or greater than 2.5 - 3.0 solarmasses.
| Influence of rotational darkening on ultraviolet interstellar extinction
The influence of rotational darkening on ultraviolet stellar fluxes offast rotating stars has been rediscussed. For this aim ANSfilter-photometric magnitudes of weakly reddened B stars were correctedfor interstellar extinction. The distribution of the stars in therotational velocity-color index diagrams displays a scatter exceedingthe photometric errors. The distributions of the stars agreequalitatively with the theoretically calculated color indices. Theeffect of rotational darkening should be taken into account as apossible source of individual deviations of the interstellar extinctionfrom the average law.
| The velocity dispersion of the Orion Nebula cluster.
Proper motions of 73 stars have been determined from an analysis of PDSmeasurements of 138 plates (obtained over 77 years) and are used toinvestigate the kinematical state of the Orion Nebula cluster.Forty-nine of the stars are found to be highly probable members. Anaverage proper-motion dispersion of + or - 0.70 + or - 0.06 mas/yr isobtained for Orion Nebula cluster stars brighter than V of about 12.5that lie within 0.5 deg of the Trapezium cluster. An average ratio ofthe radial to the tangential dispersions of 1.34 + or - 0.24 is alsofound. Results suggest that the two brightest stars in Trapezium may bein the process of being ejected from the cluster.
| A complex investigation of the star formation region in the Orion constellation
The proper motions of 54 young stars in the Ori OB1 cluster are refined.Cluster contraction with a characteristic time of 3 x 10 to the 6th yris found as well as cluster rotation with a period of 6 x 10 to the 6thyr. A cluster velocity of -10 km/s is determined from the spatialvelocities of 26 stars, indicating that about 1.4 x 10 to the 7th yrago, the cluster was situated in the Galactic symmetry plane; thiscoincides with the age of the oldest subcluster of the association.
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| קבוצת-כוכבים: | אוריון |
| התרוממות ימנית: | 05h36m15.00s |
| סירוב: | -05°38'53.0" |
| גודל גלוי: | 6.54 |
| מרחק: | 543.478 פארסק |
| תנועה נכונה: | -0.4 |
| תנועה נכונה: | 0 |
| B-T magnitude: | 6.299 |
| V-T magnitude: | 6.505 |
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