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Surface abundances of light elements for a large sample of early B-type stars - IV. The magnesium abundance in 52 stars - a test of metallicity
From high-resolution spectra a non-local thermodynamic equilibriumanalysis of the MgII 4481.2-Åfeature is implemented for 52 earlyand medium local B stars on the main sequence (MS). The influence of theneighbouring line AlIII 4479.9-Åis considered. The magnesiumabundance is determined; it is found that logɛ(Mg) = 7.67 +/-0.21 on average. It is shown that uncertainties in the microturbulentparameter Vt are the main source of errors inlogɛ(Mg). When using 36 stars with the most reliableVt values derived from OII and NII lines, we obtain the meanabundance logɛ(Mg) = 7.59 +/- 0.15. The latter value isprecisely confirmed for several hot B stars from an analysis of the MgII7877-Åweak line. The derived abundance logɛ(Mg) = 7.59 +/-0.15 is in excellent agreement with the solar magnesium abundancelogɛsolar (Mg) = 7.55 +/- 0.02, as well as with theproto-Sun abundance logɛps(Mg) = 7.62 +/- 0.02. Thus,it is confirmed that the Sun and the B-type MS stars in ourneighbourhood have the same metallicity.
| 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.
| An empirical temperature calibration for the Δ a photometric system . I. The B-type stars
We establish an empirical effective temperature calibration of mainsequence, luminosity class V to III B-type stars for the Δ aphotometric system which was originally developed to detect magneticchemically peculiar objects of the upper main sequence (early B-type toearly F-type) at 5200 Å. However, this system provides the index(g_1-y) which shows an excellent correlation with (B-V) as well as (b-y)and can be used as an indicator of the effective temperature. This issupplemented by a very accurate color-magnitude diagram, y or V versus(g_1-y), which can be used, for example, to determine the reddening,distance and age of an open cluster. This makes the Δ aphotometric system an excellent tool to investigate theHertzsprung-Russell-Diagram (HRD) in more detail. Using thereddening-free parameters and already established calibrations withinthe Strömgren uvbyβ, Geneva 7-color and Johnson UBV systems, apolynomial fit of third degree for the averaged effective temperaturesto the individual (g_1-y)0 values was derived. For thispurpose, data from the literature as well as new observations were takenresulting in 225 suitable bright normal B-type objects. The statisticalmean of the error for this sample is 238 K which is sufficient toinvestigate the HRD of distant galactic open clusters as well asextragalactic aggregates in the future.
| 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.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| 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.
| Boron Abundances in B-Type Stars: A Test of Rotational Depletion during Main-Sequence Evolution
Boron abundances have been derived for seven main-sequence B-type starsfrom Hubble Space Telescope STIS spectra around the B III λ2066line. In two stars, boron appears to be undepleted with respect to thepresumed initial abundance. In one star, boron is detectable but isclearly depleted. In the other four stars, boron is undetectable,implying depletions of 1-2 dex. Three of these four stars are nitrogenenriched, but the fourth shows no enrichment of nitrogen. Onlyrotationally induced mixing predicts that boron depletions areunaccompanied by nitrogen enrichments. The inferred rate of borondepletion from our observations is in good agreement with thesepredictions. Other boron-depleted nitrogen-normal stars are identifiedfrom the literature. In addition, several boron-depleted nitrogen-richstars are identified, and while all fall on the boron-nitrogen trendpredicted by rotationally induced mixing, a majority have nitrogenenrichments that are not uniquely explained by rotation. The spectrahave also been used to determine iron group (Cr, Mn, Fe, and Ni)abundances. The seven B-type stars have near-solar iron groupabundances, as expected for young stars in the solar neighborhood. Wehave also analyzed the halo B-type star PG 0832+676. We find[Fe/H]=-0.88+/-0.10, and the absence of the B III line gives the upperlimit [B/H]<-2.5. These and other published abundances are used toinfer the star's evolutionary status as a post-asymptotic giant branchstar. Based on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555. These observations are associatedwith proposal GO 07400.
| Boron Abundances in Early B Stars: Results from the B III Resonance Line in IUE Data
We have used archival International Ultraviolet Explorer high-dispersionSWP spectra to study the B III resonance line at 2065.8 Å in 44early B stars. We find a median boron abundance about half that of solarsystem meteoritic material, consistent with the values found fromprevious studies of boron in early B stars. About one-third of the starsstudied appear to have boron abundances that are a factor of 4 or morelower than this median. Many of these are stars with enhanced nitrogenabundances, confirming the belief that deep envelope mixing can occur inmain-sequence B stars. A few stars with low boron abundances have normalN/C ratios. It is unclear whether all of these can be explained as starswhere mixing depleted the boron but did not go deep enough to bringCN-processed material to the surface or if some stars were actuallyformed with an unusually low boron abundance.
| 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.
| Age variation of helium abundance in He-rich stars.
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
| Boron Abundances of B Stars of the Orion Association
The boron abundance of four B-type stars from the Orion association hasbeen determined from Goddard High Resolution Spectrograph (GHRS)/HubbleSpace Telescope (HST) spectra covering the B II 1362 Angstroms resonanceline. After correction for large non-LTE effects, the mean boronabundance is similar to the meteoritic abundance, but a factor of 4 (0.6dex) greater than published abundances of boron in interstellar gas.This difference in boron abundance between young stars and gas isunexplained. The Orion stars were selected by their oxygen abundances:two are enriched in oxygen by about 60% relative to the others. Analysisof the GHRS spectra shows that the O-rich stars are deficient in boronby about 250% (0.4 dex) relative to the O-poor stars. Then, if thedifferences in oxygen abundance reflect differing levels ofcontamination of the natal clouds with (O-rich) ejecta of local Type IIsupernovae, the ejecta were not enriched in boron synthesized by the nu-process.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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 the Orion cometary cloud L1616
With its cometary appearance and a reflection nebula near its edgefacing some bright Orion stars, Lynds' cloud L1616 shows ample evidenceof being affected by one or more of these massive stars. To estimate itsmass and star formation efficiency, as well as to determine if it isgravitationally bound, we mapped this cloud in J=1-->0 transitions of^12CO and ^13CO. It is found that the distribution of the emission inthe line wings shows clear evidence for substantial mass motions. Also,the `virial' mass of the cloud is found to be five times the actualcloud mass determined from the ^13CO column density map. It is arguedthat this cloud has abnormally high star formation efficiency and ispossibly disintegrating. The morphology and the location of the cloudindicate that it is being affected by the star epsilon Orionis, which isalso (possibly) responsible for the cloud's unusual star formationefficiency. Over a range of values of the relevant parameters, the staris found to satisfy quantitatively the requirements of being the causeof the observed characteristics of the cloud.
| The threshold for stellar winds in hot main-sequence stars
The profiles of ultraviolet resonance lines of C IV were surveyed in asample of 29 cluster and association members in the spectral type rangeO9-B2 III-V, together with a few field stars of interest. Thetemperatures and gravities of the stars were taken from the modelatmosphere analysis by Grigsby, Morrison, & Anderson (1992), and theluminosities were estimated on the basis of cluster and associationdistances from the recent literature. A parameter Pw wasdefined in order to describe the degree and assymetry of the C IVprofile. This parameter, together with total C IV equivalent width, wasfound to be well correlated with stellar luminosity and temperature. Afew anomalous stars were noted: tau Sco, HD 66665, HD 13621, and the ONstars HD12323 and HD 201345. The results suggest a sudden onset ofobservable mass loss at T(effective) = 27,500 +/- 500 K, log (L/solarluminosity) = 4.4 +/- 0.12, in agreement with the previous study byPrinja (1989). At T(effective) = 28,000 K and log g = 4, our non-LTEmodel atmospheres show an enhancement in the ground-state population ofC(+3) in their topmost layer, which could be responsible for initiationof the winds via radiation pressure on the C(+3) ions, or for the onsetof visibility of C(+3) ions in the wind because of an increase in theoptical depth in the C IV lines in the outermost layers.
| 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.
| A new list of effective temperatures of chemically peculiar stars. II.
Not Available
| An atlas of ultraviolet P Cygni profiles
We have selected spectra of 232 stars from the International UltravioletExplorer (IUE) archives for inclusion in an atlas intended for varioususes but tailored especially for the study of stellar winds. The atlascovers the range in spectral types from O3 to F8. The full atlas coversthe reduced and normalized high resolution spectra from the IUE long-and short-wavelength spectrographs. Here we discuss the selection of thestars and the data reduction, and we present in velocity units theprofiles of lines formed in the stellar winds. The selected lines covera wide range of ionizations, allowing a comparison of the profiles fromdifferent ions in the wind of each star and a comparison of thedifferent wind lines as a function spectral type and luminosity. We alsopresent the basic data on the program stars to facilitate study of thedependence of wind features on stellar parameters such as luminosity,temperature, escape velocity, and v sin i. We provide an overview of thecharacteristic behavior of the wind lines in the H-R diagram. Thecomplete spectra are available in digital form through the NASAAstrophysics Data System (ADS). We offer a description of the electronicdatabase that is available through the ADS and guidelines for obtainingaccess to that database.
| 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.
| Chemical evolution of the Orion association. 2: The carbon, nitrogen, oxygen, silicon, and iron abundances of main-sequence B stars
Carbon, nitrogen, oxygen, and silicon abundances are presented fromlocal thermodynamic equilibrium (LTE) and non-LTE analyses of C II, NII, and Si III lines in the spectra of 18 main-sequence B stars from thefour subgroups comprising the Orion association. Iron LTE abundancesfrom Fe III lines are also presented. The C, N, and Fe abundances showno significant variations across the subgroups, but the O and Siabundances are found to be higher for some of the youngest stars thatare collocated on the sky and at a common distance. The O and Siabundances are correlated. Although such a correlation may in partreflect measurement errors, it is suggested that the enrichment of youngstars in O and Si arose because they were formed from regions of themolecular cloud enriched with the ejecta of Type II supernovae, whichare predicted to be rich in O and Si but not in C and N. With theexception of one star, we see no evidence for CN-cycled material on thestars' surfaces. The stellar abundances agree, within the expecteduncertainties, with published nebular analyses that show Orion to beslightly underabundant in C, N, and O relative to the Sun.
| An abundance comparison between B stars and nebula in the Orion association.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993RMxAA..27..111C&db_key=AST
| Far-ultraviolet stellar photometry: A field in Orion
Far-ultraviolet photometry for 625 objects in Orion is presented. Thesedata were extracted from electrographic camera images obtained duringsounding rocket flights in 1975 and 1982. The 1975 images were centeredclose to the belt of Orion while the 1982 images were centeredapproximately 9 deg further north. One hundred and fifty stars fell inthe overlapping region and were observed with both cameras. Sixty-eightpercent of the objects were tentatively identified with known starsusing the SIMBAD database while another 24% are blends of objects tooclose together to separate with our resolution. As in previous studies,the majority of the identified ultraviolet sources are early-type stars.However, there are a significant number for which no such identificationwas possible, and we suggest that these are interesting objects whichshould be further investigated. Seven stars were found which were brightin the ultraviolet but faint in the visible. We suggest that some ofthese are nearby white dwarfs.
| Machine-readable version of the Parenago catalogue of stars in the area of the Orion nebula
Not Available
| Non-LTE, line-blanketed model atmospheres for late O- and early B-type stars
The use of non-LTE line-blanketed model atmospheres to analyze thespectra of hot stars is reported. The stars analyzed are members ofclusters and associations, have spectral types in the range O9-B2 andluminosity classes in the range III-IV, have slow to moderate rotation,and are photometrically constant. Sampled line opacities of iron-groupelements were incorporated in the radiative transfer solution; solarabundances were assumed. Good to excellent agreement is obtained betweenthe computed profiles and essentially all the line profiles used to fixthe model, and reliable stellar parameters are derived. The synthetic MII 5581 equivalent widths agree well with the observed ones at the lowend of the temperature range studied, but, above 25,000 K, the syntheticline is generally stronger than the observed line. The behavior of theobserved equivalent widths of N II, N III, C II and C III lines as afunction of Teff is studied. Most of the lines show much scatter, withno consistent trend that could indicate abundance differences from starto star.
| Chemical evolution of the Orion association. I - The oxygen abundance of main-sequence B stars
Oxygen abundances are presented from LTE and non-LTE analyses of O IIlines in spectra of 18 B mainsequence stars of the four subgroupscomprising the Orion association. The lowest abundances are found in theoldest subgroups (Ia and Ib). The youngest subgroup Id, the Trapeziumcluster, and some stars of the slightly older Ic subgroup seem to havean abundance that is up to about 40 percent higher. It is suggested thatthis enrichment resulted from the mixing of supernovae ejecta from theIc subgroup with those adjacent portions of the dense molecular cloudthat subsequently collapsed to form the Trapezium cluster. These stellaranalyses confirm published abundance analyses of the H II region'semission lines that show Orion to have an oxygen abundance that is lessthan that of the Sun: the mean LTE/NLTE abundances are log epsilon(O) =8.67/8.65 +/- 0.12 for the 18 stars whereas the solar abundance is logepsilon(O) = 8.93.
| Average Profiles and Equivalent Widths of Hydrogen Lines of 76 Normal and Peculiar Stars - Masses and Radii of Studied Stars
Not Available
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Membre dels grups següents:
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Dades d'Observació i Astrometria
| Constel·lació: | Orion |
| Ascensió Recta: | 05h31m20.90s |
| Declinació: | -06°42'30.0" |
| Magnitud Aparent: | 6.22 |
| Distancia: | 540.541 parsecs |
| Moviment propi RA: | 2.2 |
| Moviment propi Dec: | 1.6 |
| B-T magnitude: | 5.995 |
| V-T magnitude: | 6.18 |
Catàlegs i designacions:
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