Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Discovery of magnetic fields in the βCephei star ξ1 CMa and in several slowly pulsating B stars*
We present the results of a magnetic survey of a sample of eightβCephei stars and 26 slowly pulsating B (SPBs) stars with the FOcalReducer low dispersion Spectrograph at the Very Large Telescope. A weakmean longitudinal magnetic field of the order of a few hundred Gauss isdetected in the βCephei star ξ1CMa and in 13 SPBstars. The star ξ1CMa becomes the third magnetic staramong the βCephei stars. Before our study, the star ζCas wasthe only known magnetic SPB star. All magnetic SPB stars for which wegathered several magnetic field measurements show a field that varies intime. We do not find a relation between the evolution of the magneticfield with stellar age in our small sample. Our observations imply thatβCephei and SPB stars can no longer be considered as classes ofnon-magnetic pulsators, but the effect of the fields on the oscillationproperties remains to be studied.
| Abundance analysis of prime B-type targets for asteroseismology. I. Nitrogen excess in slowly-rotating β Cephei stars
Seismic modelling of the β Cephei stars promises major advances inour understanding of the physics of early B-type stars on (or close to)the main sequence. However, a precise knowledge of their physicalparameters and metallicity is a prerequisite for correct modeidentification and inferences regarding their internal structure. Herewe present the results of a detailed NLTE abundance study of nine primetargets for theoretical modelling: γ Peg,δ Cet, ν Eri,β CMa, ξ1 CMa,V836 Cen, V2052 Oph,β Cep and DD (12) Lac (hereafter 12Lac). The following chemical elements are considered: He, C,N, O, Mg, Al, Si, S and Fe. Our curve-of-growth abundance analysis isbased on a large number of time-resolved, high-resolution opticalspectra covering in most cases the entire oscillation cycle of thestars. Nitrogen is found to be enhanced by up to 0.6 dex in four stars,three of which have severe constraints on their equatorial rotationalvelocity, Ω R, from seismic or line-profile variation studies:β Cep (Ω R ~ 26 km s-1),V2052 Oph (Ω R ~ 56 km s-1),δ Cet (Ω R < 28 km s-1) andξ1 CMa (Ω R sin i 10 kms-1). The existence of core-processed material at the surfaceof such largely unevolved, slowly-rotating objects is not predicted bycurrent evolutionary models including rotation. We draw attention to thefact that three stars in this subsample have a detected magnetic fieldand briefly discuss recent theoretical work pointing to the occurrenceof diffusion effects in β Cephei stars possibly capable of alteringthe nitrogen surface abundance. On the other hand, the abundances of allthe other chemical elements considered are, within the errors,indistinguishable from the values found for OB dwarfs in the solarneighbourhood. Despite the mild nitrogen excess observed in someobjects, we thus find no evidence for a significantly higherphotospheric metal content in the studied β Cephei stars comparedto non-pulsating B-type stars of similar characteristics.
| A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i 100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.
| UVBLUE: A New High-Resolution Theoretical Library of Ultraviolet Stellar Spectra
We present an extended ultraviolet-blue (850-4700 Å) library oftheoretical stellar spectral energy distributions computed at highresolution, λ/Δλ=50,000. The UVBLUE grid, as wenamed the library, is based on LTE calculations carried out with ATLAS9and SYNTHE codes developed by R. L. Kurucz and consists of nearly 1800entries that cover a large volume of the parameter space. It spans arange in Teff from 3000 to 50,000 K, the surface gravityranges from logg=0.0 to 5.0 with Δlogg=0.5 dex, while sevenchemical compositions are considered:[M/H]=-2.0,-1.5,-1.0,-0.5,+0.0,+0.3, and +0.5 dex. For its coverageacross the Hertzsprung-Russell diagram, this library is the mostcomprehensive one ever computed at high resolution in theshort-wavelength spectral range, and useful application can be foreseenfor both the study of single stars and in population synthesis models ofgalaxies and other stellar systems. We briefly discuss some relevantissues for a safe application of the theoretical output to ultravioletobservations, and a comparison of our LTE models with the non-LTE (NLTE)ones from the TLUSTY code is also carried out. NLTE spectra are found,on average, to be slightly ``redder'' compared to the LTE ones for thesame value of Teff, while a larger difference could bedetected for weak lines, which are nearly wiped out by the enhanced coreemission component in case of NLTE atmospheres. These effects seem to bemagnified at low metallicity (typically [M/H]<~-1). A match with aworking sample of 111 stars from the IUE atlas, with availableatmosphere parameters from the literature, shows that UVBLUE modelsprovide an accurate description of the main mid- and low-resolutionspectral features for stars along the whole sequence from the B to ~G5type. The comparison sensibly degrades for later spectral types, withsupergiant stars that are in general more poorly reproduced than dwarfs.As a possible explanation of this overall trend, we partly invoke theuncertainty in the input atmosphere parameters to compute thetheoretical spectra. In addition, one should also consider the importantcontamination of the IUE stellar sample, where the presence of binaryand variable stars certainly works in the sense of artificiallyworsening the match between theory and observations.
| 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.
| A non-LTE analysis of the spectra of two narrow lined main sequence stars in the SMC
An analysis of high-resolution VLT/UVES spectra of two B-type mainsequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud(SMC), has been undertaken, using the non-LTE tlusty model atmospheresto derive the stellar parameters and chemical compositions of each star.The chemical compositions of the two stars are in reasonable agreement.Moreover, our stellar analysis agrees well with earlier analyses of H IIregions. The results derived here should be representative of thecurrent base-line chemical composition of the SMC interstellar medium asderived from B-type stars.
| Metallicity of mono- and multiperiodic β Cephei stars
Analyzing IUE ultraviolet spectra of β Cep pulsating stars wenoticed that multiperiodic variables have a larger mean metal abundancein the photosphere, [ m/H] , than monoperiodic ones. We applystatistical tests to verify this dichotomy. We obtain that, with a largeprobability, the multiperiodic β Cep stars have greater values of [m/H] . This result is consistent with the linear non-adiabatic theory ofpulsation of early B-type stars.
| Metallicities of the β Cephei stars from low-resolution ultraviolet spectra
We derive basic stellar parameters (angular diameters, effectivetemperatures, metallicities) and interstellar reddening for all βCephei stars observed during the IUE satellite mission, including thosebelonging to three open clusters. The parameters are derived by means ofan algorithmic procedure of fitting theoretical flux distributions tothe low-resolution IUE spectra and ground-based spectrophotometricobservations. Since the metallicity has a special importance forpulsating B-type stars, we focus our attention in particular on thisparameter.Tables 1, 2, 4 and 5 are only available in electronic form athttp://www.edpsciences.org
| Beyond the iron group: Heavy metals in hot subdwarfs
We report the discovery of strong photospheric resonance lines of GaIII, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdBand sdOB stars at temperatures ranging from 22 000 K to 40 000 K. Linesof other heavy elements are also detected, however in these cases moreatomic data are needed. Based on these discoveries, we present ahypothesis to explain the apparent lack of silicon in sdB stars hotterthan 32 000 K. The existence of triply ionised Ge, Sn, and Pbsuggests that rather than silicon sinking deep into the photosphere, itis removed from the star in a fractionated stellar wind. This hypothesisprovides a challenge to diffusion models of sdB stars.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635 and #5319.
| High-resolution spectroscopy of globular cluster post-Asymptotic Giant Branch stars
We present model atmosphere analyses of high resolution Keck and VLToptical spectra for three evolved stars in globular clusters, viz. ZNG-1in M 10, ZNG-1 in M 15 and ZNG-1 in NGC 6712. The derived atmosphericparameters and chemical compositions confirm the programme stars to bein the post-Asymptotic Giant Branch (post-AGB) evolutionary phase.Differential abundance analyses reveal CNO abundance patterns in M 10ZNG-1, and possibly M 15 ZNG-1, which suggest that both objects may haveevolved off the AGB before the third dredge-up occurred. The abundancepattern of these stars is similar to the third class of optically,bright post-AGB objects discussed by van Winckel (\cite{van97}).Furthermore, M 10 ZNG-1 exhibits a large C underabundance (with\bigtriangleup[C/O] -1.6 dex), typical of other hot post-AGBobjects. Differential \bigtriangleup[α/Fe] abundance ratios inboth M 10 ZNG-1 and NGC 6712 ZNG-1 are found to be approximately 0.0dex, with the Fe abundance of the former being in disagreement with thecluster metallicity of M 10. Given that the Fe absorption features inboth M 10 ZNG-1 and NGC 6712 ZNG-1 are well observed and reliablymodelled, we believe that these differential Fe abundance estimates tobe secure. However, our Fe abundance is difficult to explain in terms ofprevious evolutionary processes that occur on both the Horizontal Branchand the AGB.Equivalent width data table is 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/cgi-bin/qcat?J/A+A/419/1123
| A chemical analysis of five hot stars towards the Galactic centre
High resolution echelle spectroscopy is presented for thirteen starslying in the direction of the Galactic centre which, on the basis ofphotographic photometry and low dispersion spectroscopy, have beenclassified as early-B-type. Eight of these stars have large rotationalvelocities which preclude a detailed analysis. The five stars withmoderate to low projected rotational velocities have been analysed usingmodel atmosphere techniques to determine atmospheric parameters andchemical compositions. Two of these stars appear to be evolved bluehorizontal branch objects on the basis of their chemical compositionsand small projected rotational velocity. The evolutionary status of athird is ambiguous but it is probably a post-asymptotic-giant branchstar. The remaining two objects are probably young massive stars andshow enhanced abundances of N, C, Mg and Si, consistent with theirformation in the inner part of the Galactic disk. However their Oabundances are normal, confirming results found previously for otherearly-type stars, which would imply a flat abundance gradient for thiselement in the inner region of our Galaxy.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.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/411/447
| 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
| The present-day chemical composition of the SMC from UVES spectra of the sharp-lined, B-type dwarf AV 304
High-resolution spectroscopic VLT/UVES observations are presented forthe B-type main-sequence star, AV 304, in the Small Magellanic Cloud(SMC). These spectra have been analysed using LTE model-atmospheretechniques, to derive stellar atmospheric parameters and chemicalcompositions. As AV 304 is located within the hydrogen burningmain-sequence band, its chemical composition should reflect that of theSMC interstellar medium (ISM). A detailed line-by-line differentialanalysis has been undertaken relative to a Galactic comparison star. Ageneral metal deficiency for the alpha -process elements O, Si & Sof -0.43+/-0.05 dex is found for AV 304, with iron having a similarunderabundance. Oxygen may be relatively overabundant by ~ 0.1 dex andcarbon and aluminium underabundant by ~ 0.2 dex. A large nitrogenunderabundance (of -1.2 dex relative to hydrogen and -0.7 dex relativeto iron) is found. This is interpreted in terms of the CNO bi-cyclehaving been suppressed in the SMC. Furthermore, the large nitrogendeficiency is in excellent agreement with that found for SMC H IIregions. Indeed, this represents a first for stellar astrophysics -confirming the low base-line nitrogen composition of the SMC ISM (viz.12+log(N/H) ~ 6.66 +/- 0.10 dex).
| More nitrogen rich B-type stars in the SMC cluster, NGC 330
High resolution spectra of seven early B-type giant/supergiant stars inthe SMC cluster NGC 330 are analysed to obtain their chemicalcompositions relative to SMC field and Galactic B-type stars. It isfound that all seven stars are nitrogen rich with an abundanceapproximately 1.3 dex higher than an SMC main-sequence field B-typestar, AV304. They also display evidence for deficiencies in carbon, butother metals have abundances typical of the SMC. Given the number ofB-type stars with low projected rotational velocities in NGC 330 (allour targets have v sin i < 50 km s-1), we suggest that itis unlikely that the stars in our sample are seen almost pole-on, butrather that they are intrinsically slow rotators. Furthermore, none ofour objects displays any evidence of significant Balmer emissionexcluding the possibility that these are Be stars observed pole-on.Comparing these results with the predictions of stellar evolution modelsincluding the effects of rotationally induced mixing, we conclude thatwhile the abundance patterns may indeed be reproduced by these models,serious discrepancies exist. Most importantly, models including theeffects of initially large rotational velocities do not reproduce theobserved range of effective temperatures of our sample, nor thecurrently observed rotational velocities. Binary models may be able toproduce stars in the observed temperature range but again may beincapable of producing suitable analogues with low rotationalvelocities. We also discuss the clear need for stellar evolutioncalculations employing the correct chemical mix of carbon, nitrogen andoxygen for the SMC.
| 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.
| The present-day chemical composition of the LMC
High-resolution observations of five OB-type main-sequence stars in theLarge Magellanic Cloud (LMC) have been obtained with the UCLéchelle spectrograph on the 3.9-m Anglo-Australian Telescope.These spectra have been analysed using LTE model-atmosphere techniques,to derive stellar atmospheric parameters and chemical compositions. Asthese stars are located within the hydrogen burning main-sequence band,their surface abundances should reflect those of the present-dayinterstellar medium. Detailed line-by-line differential analyses havebeen undertaken relative to Galactic comparison stars. We conclude thatthere exists a general metal deficiency of -0.31+/-0.04 dex within theLMC, and find no significant abundance variations between cluster andfield stars. There is also tentative evidence to suggest a lower oxygento iron abundance ratio, and an over-deficiency of magnesium relative tothe other alpha -elements. These are discussed in terms of previousabundance analyses and models of discontinuous (or bursting) starformation within the LMC. Finally, there is some evidence to suggest agreater chemical enrichment of material within the H Ii region LH 104.
| A detailed abundance analysis of the hot post-AGB star ZNG-1 in M10
We present a model-atmosphere analysis for the bright (V~13) star ZNG-1,in the globular cluster M10. From high-resolution (R~40000) opticalspectra we confirm ZNG-1 to be a post-asymptotic giant branch (post-AGB)star. The derived atmospheric parameters areTeff=26500+/-1000K and logg=3.6+/-0.2dex. A differentialabundance analysis reveals a chemical composition typical of hotpost-AGB objects, with ZNG-1 being generally metal poor, although heliumis approximately solar. The most interesting feature is the large carbonunderabundance of more than 1.3dex. This carbon deficiency, along withan observed nitrogen enhancement relative to other elements, may suggestthat ZNG-1 evolved off the AGB before the third dredge-up occurred.Also, iron depletions observed in other similar stars suggest thatgas-dust fractionation in the AGB progenitor could be responsible forthe observed composition of these objects. However, we need not invokeeither scenario since the chemical composition of ZNG-1 is in goodagreement with abundances found for a Population II star of the samemetallicity.
| Model atmosphere and kinematical analyses of early-type stars from the Edinburgh-Cape Survey
We present high-resolution spectroscopic observations of 21 B-typestars, selected from the Edinburgh-Cape Blue Object Survey. Modelatmosphere analyses confirm that 14 of these stars are young,main-sequence B-type objects with Population I chemical compositions.The remaining seven are found to be evolved objects, includingsubdwarfs, horizontal branch and post-AGB objects. A kinematicalanalysis shows that all 14 young main-sequence stars could have formedin the disc and subsequently been ejected into the halo. These resultsare combined with the analysis of a previous subsample of stars takenfrom the Survey. Of the complete sample, 31 have been found to be young,main-sequence objects, with formation in the disc, and subsequentejection into the halo, again being found to be a plausible scenario.
| Lead, Tin, and Germanium in the Small Magellanic Cloud Main-Sequence B Star AV 304
We have determined the lead, tin, and germanium abundances of the earlyB main-sequence star AV 304 in the Small Magellanic Cloud (SMC) by usingarchival STIS/HST G140M spectral data to measure the Ge IV 1229.8Å, Sn IV 1313.5 Å, and Pb IV 1313.1 Å resonance lines.We find [Ge/H]~-1.8, [Sn/H]~-1, and [Pb/H]~-0.15. We argue that findinga nearly solar system lead abundance in a metal-poor SMC star isconsistent with both previously measured trends in SMC s-processelements and recent theoretical s-process stellar and chemical evolutionmodels that predict that a large fraction of the lead in our own Galaxywas produced by the s-process in low-mass stars of metallicity [Fe/H]between -1.5 and -1. We have also estimated the isotopic and hyperfinesplitting of the Pb IV 1313.1 line and conclude that any uncertaintiesin the isotopic mixture do not significantly affect the derived leadabundance.
| 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.
| Dating Ptolemy's star catalogue through proper motions : the Hipparchan epoch.
Not Available
| Intermediate-Mass Star Models with Different Helium and Metal Contents
We present a comprehensive theoretical investigation of the evolutionaryproperties of intermediate-mass stars. The evolutionary sequences werecomputed from the zero-age main sequence up to the central He exhaustionand often up to the phases that precede the carbon ignition or to thereignition of the H-shell, which marks the beginning of the thermalpulse phase. The evolutionary tracks were constructed by adopting a widerange of stellar masses (3<=M/Msolar <=15) and chemicalcompositions. In order to account for current uncertainties on the He toheavy elements enrichment ratio (ΔY/ΔZ), the stellar modelswere computed by adopting at Z=0.02 two different He contents (Y=0.27,0.289) and at Z=0.04 three different He contents (Y=0.29, 0.34, and0.37). Moreover, to supply a homogeneous evolutionary scenario thataccounts for young Magellanic stellar systems the calculations were alsoextended toward lower metallicities (Z=0.004, Z=0.01), by adoptingdifferent initial He abundances. We evaluated for both solar (Z=0.02)and super-metal-rich (SMR, Z=0.04) models the transition massMup between the stellar structures igniting carbon and thosethat develop a full electron degeneracy inside the carbon-oxygen core.We found that Mup is of the order of 7.7+/-0.5Msolar for solar composition, while for SMR structures anincrease in the He content causes a decrease in Mup, andindeed it changes from 9.5+/-0.5 Msolar at Y=0.29, to8.7+/-0.2 Msolar at Y=0.34, and to 7.7+/-0.2Msolar at Y=0.37. We also show that Mup presents anonlinear behavior with metallicity, and indeed it decreases when movingfrom Z=0.04 to Z~0.001 and increases at lower metal contents. Thisfinding confirms the predictions by Cassisi & Castellani and morerecently by Umeda et al. and suggests that the rate of SNe type Iadepends on the chemical composition of the parent stellar population.This approach allows us to investigate in detail the evolutionaryproperties of classical Cepheids. In particular, we find that the rangeof stellar masses that perform the blue loop during the centralHe-burning phase narrows when moving toward metal-rich and SMRstructures. This evidence and the substantial decrease in theevolutionary time spent by these structures inside the instability stripbring out that the probability of detecting long-period Cepheids in SMRstellar systems is substantially smaller than in more metal-poorsystems. Moreover, and even more important, we find that the time spentby Cepheids along the subsequent crossings of the instability strip alsodepends on the stellar mass. In fact, our models suggest that low-mass,metal-poor Cepheids spend a substantial portion of their lifetime alongthe blueward excursion of the blue loop, while at higher masses(M/Msolar>=8) the time spent along the redward excursionbecomes longer. Models at solar chemical composition present an oppositebehavior, i.e., the time spent along the redward excursion is longerthan the blueward excursion among low-mass Cepheids and vice versa forhigh-mass Cepheids. Oddly enough, the time spent along the bluewardexcursion is for models at Z=0.01 longer than the redward excursion overthe entire mass range. This suggests a nonlinear dependence of crossingtimes on metallicity. The time spent along the first crossing of theinstability strip is generally negligible with the exception ofhigh-mass, metal-poor stellar structures for which it becomes of theorder of 15%-20% of the total crossing time.
| Long-term period and amplitude variations in beta Cephei stars.
Not Available
| High-resolution stellar and interstellar spectra of HD 100340
We present new, high-resolution, high signal-to-noise ratio, optical and21-cm HI observations of the faint blue halo star HD 100340. Adifferential abundance analysis with respect to the Galactic disc starHR 2387 shows no significant peculiarities. We find that HD 100340 liesat a distance of 3 kpc, and a kinematical analysis strongly supports a`runaway star' origin. We also present improved cloud models for threeintermediate velocity clouds seen in Ca II K or H I towards HD 100340.
| The Second Extreme Ultraviolet Explorer Right Angle Program Catalog
We present the detection of 235 extreme ultraviolet sources, of which169 are new detections, using the Extreme Ultraviolet Explorer's (EUVE)Right Angle Program (RAP) data. This catalog includes observations sincethe first EUVE RAP catalog (1994 January) and covers 17% of the sky. TheEUVE RAP uses the all-sky survey telescopes (also known as``scanners''), mounted at right angles to the Deep Survey andspectrometer instruments, to obtain photometric data in four wavelengthbands centered at ~100 Å (Lexan/B), ~200 Å (Al/Ti/C), ~400Å (Ti/Sb/Al), and ~550 Å (Sn/SiO). This allows the RAP toaccumulate data serendipitously during pointed spectroscopicobservations. The long exposure times possible with RAP observationsprovide much greater sensitivity than the all-sky survey. We presentEUVE source count rates and probable source identifications from theavailable catalogs and literature. The source distribution is similar toprevious extreme ultraviolet (EUV) catalogs with 2% early-type stars,45% late-type stars, 8% white dwarfs, 6% extragalactic, 24% with no firmclassification, and 15% with no optical identification. We also present36 detections of early-type stars that are probably the result ofnon-EUV radiation. We have detected stellar flares from approximately 12sources, including: EUVE J0008+208, M4 star G32-6 (EUVE J0016+198), anew source EUVE J0202+105, EUVE J0213+368, RS CVn V711 Tau (EUVEJ0336+005), BY Draconis type variable V837 Tau (EUVE J0336+259), the newK5 binary EUVE J0725-004, EUVE J1147+050, EUVE J1148-374, EUVE J1334-083(EQ Vir), EUVE J1438-432 (WT 486/487), EUVE J1808+297, and the M5.5estar G208-45 (EUVE J1953+444). We present sample light curves for thebrighter sources.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| The chemical composition of the young, Inter-Cloud population
High-resolution AAT spectroscopy and lower resolution spectrophotometryare presented for three early B-type stars that are members of theyoung, Inter-Cloud population between the Magellanic Clouds. Thesespectra have been analyzed using LTE model-atmosphere techniques, toderive the stellar atmospheric parameters and photospheric chemicalcompositions. The latter should reflect that of the present-dayinterstellar medium (ISM) within the Inter-Cloud Region (ICR). From adifferential analysis, the three ICR stars appear to have a mean metalabundance of ~ 1.1 dex lower than their Population i Galactic analogues,and 0.5 dex lower than the SMC star (AV 304). Hence, the ICR gas doesnot reflect the present-day composition of either the SMC (or LMC) ISM.Age (and distance) estimates were obtained using the theoreticalisochrones of Bertelli et al. (\cite{ber94}); these imply that theyoung, Inter-Cloud population has an age dispersion of at least 10-40Myr, and provide evidence for a distance gradient across the ICR. Wediscuss our results within the context of recent numerical simulationsof the gravitational interactions between the Galaxy-LMC-SMC, thatpredict that the ICR was tidally disrupted from the SMC some 200 Myrago. If the SMC was chemically homogeneous, a comparison of the ICRabundance determinations with the SMC age-metallicity relationship wouldthen imply that the formation of the ICR must have occurred ~ 8.5 Gyrago. Alternatively and more plausible, we postulate that the ICR gasformed from a mixture of SMC gas and an unenriched component. This isconsistent with model-predictions that both a halo and disc componentshould have contributed to the material within the ICR during the tidaldisruption.
| EUVE Pointed Observations of the beta Canis Majoris Interstellar Tunnel
We present results from extended pointings of the EUVE scannertelescopes in the direction of the rarefied interstellar tunnel towardthe star beta CMa. Five sources were detected, each of whichsubsequently proved to be caused by ultraviolet leaks in the EUVEshort-wavelength filters. Our initial expectation was to detect many newEUV sources in the 80 deg^2 region of very low neutral interstellarhydrogen column density covered by these extended pointings. However, noEUV sources were detected. This result is consistent with a source ofshort-wavelength EUV opacity, providing indirect evidence for thepresence of highly ionized (coronal) gas at distances of more than 5 pcin the line of sight toward this region of anomalous interstellarabsorption.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Canis Major |
| Ascensió Recta: | 06h31m51.30s |
| Declinació: | -23°25'06.0" |
| Magnitud Aparent: | 4.33 |
| Distancia: | 628.931 parsecs |
| Moviment propi RA: | 0 |
| Moviment propi Dec: | 0 |
| B-T magnitude: | 4.03 |
| V-T magnitude: | 4.286 |
Catàlegs i designacions:
|
