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Variable Star Designations for Extreme Helium Stars
We discuss the classifications used by the GCVS for pulsatinghydrogen-deficient supergiants. We argue that the current PV Teldesignation is misleading, and that it should be replaced by three newclasses -- V652 Her, V2076 Oph and FQ Aqr. The characteristics andmembership of each class are well defined.
| Variability and evolution in various classes of post-AGB stars
We aim to compare properties of early-type post-asymptotic giant-branch(post-AGB) stars, including normal first-time B-type post-AGB stars, andextreme helium stars (EHes). Hipparcos photometry for 12 post-AGB starsand 7 EHe stars has been analyzed; 5 post-AGB stars are clearlyvariable. The Hipparcos data are not sufficiently sensitive to detectvariability in any of the EHes.
| An evolutionary catalogue of galactic post-AGB and related objects
Aims.With the ongoing AKARI infrared sky survey, of much greatersensitivity than IRAS, a wealth of post-AGB objects may be discovered.It is thus time to organize our present knowledge of known post-AGBstars in the galaxy with a view to using it to search for new post-AGBobjects among AKARI sources. Methods: We searched the literatureavailable on the NASA Astrophysics Data System up to 1 October 2006, anddefined criteria for classifying sources into three categories: verylikely, possible and disqualified post-AGB objects. The category of verylikely post-AGB objects is made up of several classes. Results: We havecreated an evolutionary, on-line catalogue of Galactic post-AGB objects,to be referred to as the Toruń catalogue of Galactic post-AGB andrelated objects. The present version of the catalogue contains 326 verylikely, 107 possible and 64 disqualified objects. For the very likelypost-AGB objects, the catalogue gives the available optical and infraredphotometry, infrared spectroscopy and spectral types, and links tofinding charts and bibliography.A stable version of the catalogue is available at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/799
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| On the angular momentum evolution of merged white dwarfs
We study the angular momentum evolution of binaries containing two whitedwarfs (WDs) which merge and become cool helium-rich supergiants. Ourobject is to compare predicted rotation velocities with observations ofhighly evolved stars believed to have formed from such a merger, whichinclude the RCrB and extreme He stars.The principal case study involves a short-period binary containing a0.6-Msolar carbon-oxygen (CO) WD, and a 0.3-MsolarHe WD. The initial condition for the angular momentum distribution isdefined by the orbital configuration where the secondary fills its Rochelobe.Since mass transfer from the secondary is unstable, the WD breaks up ona dynamical time-scale. After accreting some mass, the primary isassumed to ignite helium and evolve to become a yellow supergiant with aHe-rich surface. We assume conservation of angular momentum to computethe initial angular momentum distribution in a collisionless disc andsubsequently in the giant envelope. At the end of shell-helium burning,the giant contracts to form a WD. We derive the surface rotationvelocity during this contraction.The calculation is repeated for a range of initial mass ratios, and alsofor the case of mergers between two helium (He) WDs; the latter willcontract to the helium main sequence rather than the WD sequence.Assuming complete conservation of angular momentum, we predictacceptable angular rotation rates for cool giants and during the initialsubsequent contraction. However, such stars will only survive spin-up toreach the WD sequence (CO+He merger) if the initial mass ratio is closeto unity. He+He merger products must lose angular momentum in order toreach the helium main sequence.Minimum observed rotation velocities in extreme helium stars are lowerthan our predictions by at least one-half, indicating that CO+He mergersmust lose at least one-half of their angular momentum, possibly througha wind during shell-helium burning, but more likely from the disc,following secondary disruption.
| A photometric and spectroscopic study of the hottest pulsating extreme helium star, V2076 Oph (HD 160641)
We present results from a three-site photometric and high-resolutionspectroscopic campaign on the hottest known extreme helium star V2076Oph (HD 160641). A core programme of intensive observations covered twoweeks and a much lower sampling rate extended over another two months.Despite the fact that the data seem to indicate periodicity near half aday (though the light curves are clearly not formed by a singleperiodicity), conventional Fourier analysis of the data fails to revealcoherent frequencies. Furthermore, we are unable to recover frequencieswhich were apparently clear in an earlier campaign on the star. Evidenceof monotonic pulsation amplitude changes is seen at the higherfrequencies from a wavelet analysis, but more data are needed beforethis study can be extended to lower frequencies. The application oflinear stochastic differential equation (LSDE) methods indicates thatthe observed light variations could be a result of random variationsgiving the appearance of intermittent periodicity. High-resolutionspectroscopic observations were obtained during the campaign andadditional observations were made three years later. Complex lineprofile variations were observed. It is proposed that the differentbehaviour of the emission line studied may indicate it is associatedwith a stellar wind or resident circumstellar material. The frequenciesthat are extracted from the velocity data do not conform to thoseidentified in the current or previous photometric campaigns.
| Post-AGB stars as testbeds of nucleosynthesis in AGB stars
We construct a data base of 125 post-AGB objects (including R CrB andextreme helium stars) with published photospheric parameters (effectivetemperature and gravity) and chemical composition. We estimate themasses of the post-AGB stars by comparing their position in the (logT{eff}, log g) plane with theoretical evolutionary tracks ofdifferent masses. We construct various diagrams, with the aim of findingclues to AGB nucleosynthesis. This is the first time that a large sampleof post-AGB stars has been used in a systematic way for such a purposeand we argue that, in several respects, post-AGB stars should be morepowerful than planetary nebulae to test AGB nucleosynthesis. Our mainfindings are that: the vast majority of objects which do not showevidence of N production from primary C have a low stellar mass(Mstar < 0.56 Mȯ); there is no evidencethat objects which did not experience 3rd dredge-up have a differentstellar mass distribution than objects that did; there is clear evidencethat 3rd dredge-up is more efficient at low metallicity. The sample ofknown post-AGB stars is likely to increase significantly in the nearfuture thanks to the ASTRO-F and follow-up observations, making theseobjects even more promising as testbeds for AGB nucleosynthesis.
| An Analysis of Ultraviolet Spectra of Extreme Helium Stars and New Clues to Their Origins
Abundances of about 18 elements including the heavy elements Y and Zrare determined from Hubble Space Telescope Space Telescope ImagingSpectrograph ultraviolet spectra of seven extreme helium stars (EHes):LSE 78, BD +10 2179, V1920 Cyg, HD 124448, PV Tel, LS IV-1 2, and FQAqr. New optical spectra of BD +10 2179, V1920 Cyg, and HD 124448 wereanalyzed, and published line lists of LSE 78, HD 124448, and PV Tel wereanalyzed afresh. The abundance analyses are done using LTE lineformation and LTE model atmospheres especially constructed for theseEHes. The stellar parameters derived from an EHe's UV spectrum are insatisfactory agreement with those derived from its optical spectrum.Adopted abundances for the seven EHes are from a combination of the UVand optical analyses. Published results for an additional 10 EHesprovide abundances obtained in a nearly uniform manner for a total of 17EHes, the largest sample on record.The initial metallicity of an EHe is indicated by the abundance ofelements from Al to Ni; Fe is adopted to be the representative ofinitial metallicity. Iron abundances range from approximately solar toabout 1/100 solar. Clues to EHe evolution are contained within the H,He, C, N, O, Y, and Zr abundances. Two novel results are (1) the Oabundance for some stars is close to the predicted initial abundance yetthe N abundance indicates almost complete conversion of initial C, N,and O to N by the CNO cycles; and (2) three of the seven stars with UVspectra show a strong enhancement of Y and Zr attributable to ans-process.The observed compositions are discussed in light of expectations fromaccretion of an He white dwarf by a C-O white dwarf. Qualitativeagreement seems likely except that a problem may be presented by thosestars in which the O abundance is close to the initial O abundance.Based on observations obtained with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.
| New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
| Period changes in the pulsating extreme helium stars V652 Her and BX Cir
Thirty-four new timings of maxima of the radially pulsating extremehelium (EHe) star V652 Her (= BD+13° 3224) obtained during 1996-2004are reported. The quartic ephemeris (for the pulsation period decrease)is extended and the higher-order terms are defined more accurately. Wealso report 45 new timings of maxima for the closely similar star BX Cir(= LSS 3184) over the same baseline and present the first evidence thatthis star also shows a decreasing period. The data obtained thus far canbe represented by a quadratic ephemeris - a linearly decreasing periodin BX Cir - though the decrease rate (dP/dn~ 3.7 ×10-10 d) is much smaller than in V652 Her.
| Abundances of Neutron-Capture Elements in the Hot Extreme Helium Stars V1920 Cygni and HD 124448
Analysis of Hubble Space Telescope Space Telescope Imaging Spectrographultraviolet spectra of two hot extreme helium (EHe) stars, V1920 Cyg andHD 124448, provides the first measurements of abundances ofneutron-capture elements for EHe stars. Although the two stars havesimilar abundances for elements up through the iron group, they differstrikingly in their abundances of heavier elements: V1920 Cyg isenriched by a factor of 30 in light neutron-capture elements (Y/Fe andZr/Fe) relative to HD 124448. These differences in abundances ofneutron-capture elements among EHe stars are exhibited by the R CrBstars and are evidence supporting the view that there is an evolutionaryconnection between these two groups of hydrogen-deficient stars. Also,the abundances of Y and Zr in V1920 Cyg provide evidence that at leastone EHe star went through an s-process synthesis episode in its earlierevolution.Based on observations obtained with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
| Merged binary white dwarf evolution: rapidly accreting carbon-oxygen white dwarfs and the progeny of extreme helium stars
We have examined the evolution of merged low-mass double white dwarfsthat become luminous helium stars. We have approximated the mergingprocess by the rapid accretion of matter, consisting mostly of helium,on to a carbon-oxygen (CO) white dwarf. After a certain mass isaccumulated, a helium shell flash occurs, the radius and luminosityincrease and the star becomes a yellow giant. Mass accretion is stoppedartificially when the total mass reaches a pre-determined value. Whenthe mass above the helium-burning shell becomes small enough, the starevolves blueward almost horizontally in the Hertzsprung-Russell diagram.The theoretical models for the merger of a 0.6-Msolar COwhite dwarf with a 0.3-Msolar He white dwarf agree very wellwith the observed locations of extreme helium stars in thelogTeff-logg diagram, with their observed rates of bluewardevolution, and with luminosities and masses obtained from theirpulsations. Together with predicted merger rates for CO+He white dwarfpairs, the evolutionary time-scales are roughly consistent with theobserved numbers of extreme helium stars. Predicted surface carbon andoxygen abundances can be consistent with the observed values if carbonand oxygen produced in the helium shell during a previous asymptoticgiant branch phase are assumed to exist in the helium zone of theinitial CO white dwarfs. These results establish the CO+He white dwarfmerger as the best, if not only, viable model for the creation ofextreme helium stars and, by association, the majority of R CoronaeBorealis stars.
| What Are the Hot R Coronae Borealis Stars?
We investigate the evolutionary status of four stars: V348 Sgr, DY Cen,and MV Sgr in the Galaxy and HV 2671 in the LMC. These stars have incommon random deep declines in visual brightness, which arecharacteristic of R Coronae Borealis (RCB) stars. RCB stars aretypically cool hydrogen-deficient supergiants. The four stars studied inthis paper are hotter (Teff=15-20 kK) than the majority ofRCB stars (Teff=5000-7000 K). Although these are commonlygrouped together as the hot RCB stars they do not necessarily share acommon evolutionary history. We present new observational data and anextensive collection of archival and previously published data that isreassessed to ensure internal consistency. We find temporal variationsof various properties on different timescales that will eventually helpus to uncover the evolutionary history of these objects. DY Cen and MVSgr have typical RCB helium abundances, which exclude any currentlyknown post-asymptotic giant branch (post-AGB) evolutionary models.Moreover, their carbon and nitrogen abundances present us with furtherproblems for their interpretation. V348 Sgr and HV 2671 are in generalagreement with a born-again post-AGB evolution, and their abundances aresimilar to Wolf-Rayet central stars of planetary nebulae (PNs). Thethree Galactic stars in the sample have circumstellar nebulae, whichproduce forbidden line radiation (for HV 2671 we have no information).V348 Sgr and DY Cen have low-density, low-expansion velocity nebulae(resolved in the case of V348 Sgr), while MV Sgr has a higher density,higher expansion velocity nebula. All three stars, on the other hand,have split emission lines, which indicate the presence of an equatorialbulge but not of a Keplerian disk. In addition, the historical lightcurves for the three Galactic hot RCB stars show evidence for asignificant fading in their maximum-light brightnesses of ~1 mag overthe last 70 yr. From this we deduce that their effective temperaturesincreased by a few thousand degrees. If V348 Sgr is a born-again star,as we presume, this means that the star is returning from the born-againAGB phase to the phase of a central star of PN. Spectroscopically, nodramatic change is observed over the last 50 years for V348 Sgr and MVSgr. However, there is some evidence that the winds of V348 Sgr and DYCen have increased in strength in the last decade. HV 2671, located inthe LMC, has not been analyzed in detail but at 5 Å resolution isalmost identical to V348 Sgr. Through the bolometric correction derivedfor V348 Sgr and the known distance, we can estimate the absolute Vmagnitude of HV 2671 (MV=-3.0 mag) and its bolometricluminosity (~6000 Lsolar).
| Abundance analyses of cool extreme helium stars
Extreme helium stars (EHes) with effective temperatures from 8000 to13000K are among the coolest EHes and overlap the hotter R CrB stars ineffective temperature. The cool EHes may represent an evolutionary linkbetween the hot EHes and the R CrB stars. Abundance analyses of fourcool EHes, BD+1°4381 (FQ Aqr), LS IV -14°109, BD -1°3438 (NOSer) and LS IV -1°002 (V2244 Oph), are presented. All these starsshow evidence of H- and He-burning at earlier stages of their evolution.To test for an evolutionary connection, the chemical compositions ofcool EHes are compared with those of hot EHes and R CrB stars. Relativeto Fe, the N abundance of these stars is intermediate between those ofhot EHes and R CrB stars. For the R CrB stars, the metallicity M derivedfrom the mean of Si and S appears to be more consistent with thekinematics than that derived from Fe. When metallicity M derived from Siand S replaces Fe, the observed N abundances of EHes and R CrB starsfall at or below the upper limit corresponding to thorough conversion ofinitial C and O to N. There is an apparent difference between thecomposition of R CrB stars and EHes, the former having systematicallyhigher [N/M] ratios. The material present in the atmospheres of many RCrB stars is heavily CN- and ON-cycled. Most of the EHes have onlyCN-cycled material in their atmospheres. There is an indication that theCN- and ON-cycled N in EHes was partially converted to Ne byα-captures. If EHes are to evolve to R CrB stars, fresh C in EHeshas to be converted to N; the atmospheres of EHes have just sufficienthydrogen to raise the N abundance to the level of R CrB stars. If Ne isfound to be normal in R CrB stars, the proposal that EHes evolve to RCrB stars fails. The idea that R CrB stars evolve to EHes is ruled out;the N abundance in R CrB stars has to be reduced to the level of EHes,as the C/He, which is observed to be uniform across EHes, has to bemaintained. Hence the inferred [N/M], C/He and [Ne/M] ratios, and theH-abundances of these two groups indicate that the EHes and the R CrBstars may not be on the same evolutionary path. The atmospheres ofH-deficient stars probably consist of three ingredients: a residue ofnormal H-rich material, substantial amounts of H-poor CN(O)-cycledmaterial, and C- (and O-) rich material from gas exposed to He-burning.This composition could be a result of final He-shell flash in a singlepost-AGB star (FF scenario), or a merger of two white dwarfs (DDscenario). Although the FF scenario accounts for Sakurai's object andother stars (e.g., the H-poor central stars of planetary nebulae),present theoretical calculations imply higher C/He and O/He ratios thanare observed in EHes and R CrB stars. Quantitative predictions arelacking for the DD scenario.
| Cyclic and secular variation in the temperatures and radii of extreme helium stars
The ultraviolet properties of 17 extreme helium stars have been examinedusing 150 IUE spectra. Combining short-wave and long-wave image pairsand using a grid of hydrogen-deficient model atmospheres and aχ2 minimization procedure, 70 measurements of effectivetemperature (Teff), angular diameters (θ) andinterstellar extinction (EB_V) were obtained. In most cases,these were in good agreement with previous measurements, but there aresome ambiguities in the case of the hotter stars, where the solutionsfor Teff and EB_V become degenerate, and in thecase of the cooler stars with large EB_V, where the totalflux is no longer dominated by the ultraviolet. The behaviour of 12helium stars was examined over an interval exceeding 10yr. The surfacesof four stars (HD 168476, HD 160641, BD -9°4395 and BD -1°3438)were found to be heating at rates between 20 and 120Kyr-1, inremarkable agreement with theoretical predictions. This result providesthe first direct evidence that extreme helium stars are heliumshell-burning stars of up to ~0.9Msolar contracting towardsthe white dwarf sequence. Low-luminosity helium stars do not show adetectable contraction, also in agreement with theory, although one, BD+10°2179, may be expanding. The short-term behaviour of threevariable helium stars (PV Tel variables: HD 168476, BD +1°4381, LSIV-1°2) was examined over a short interval in 1995. All three showedchanges in Teff and θ on periods consistent withprevious observations. Near-simultaneous radial velocity (v)measurements were used to establish the total change in radius, withsome reservations concerning the adopted periods. Subsequently,measurements of the stellar radii and distances could be derived. WithTeff and surface gravities established previously, stellarluminosities and masses were thus obtained directly from observation. Inthe case of HD 168476, the mass is 0.94 ± 0.68 M\odot.Assuming a similar gravity for LSIV -1°2 based on its neutral heliumline profiles, its mass becomes 0.79 ± 0.46 M\odot.The θ amplitude for BD +1°4381 appears to be overestimated bythe IUE measurements and leads to a nonsensical result. These firstdirect measurements of luminous extreme helium star masses agree wellwith previous estimates from stellar structure and pulsation theory.
| The chemical composition of the pulsating helium star V652 HER
We present an analysis of an optical blue spectrum of the pulsatinghelium star V652 Her (=BD+13(deg) 3224) in order to determine itseffective temperature, surface gravity and chemical composition. Byfitting synthetic spectra to the observations we find that for ourspectrum T_eff =24 550+/-500 K, log g=3.68+/-0.05 (cgs) and v_t=5+/-5 kms^{-1}. The surface gravity, together with a previous measurement of thestellar radius, indicates the mass of V652 Her to beM=0.69(+0.15}_{-0.12) M_{\odot}. The surface composition ischaracterised by abundances of n_H=0.009, n_He=0.988, n_C ~= 0.000040,n_N=0.0025 and n_O=0.00010 (number fractions). These abundancesrepresent a mixture of some hydrogen-rich material (0.2% by mass) withpredominantly CNO-processed helium (99.8% by mass). The metallicity ofV652 Her, represented by the N abundance as a sum of primordial C+N+Oabundances, by the iron abundance, and by other metals, corresponds to anear-solar mixture, with [Fe/H]=-0.10+/-0.15. Such a metallicitysupports the contention that Z-bump opacities drive pulsations inmetal-rich helium stars in an instability finger that extends to lowluminosities for stars with T_eff ~20,000 K. There is no evidence forthe products of any nuclear processes other than the CNO cycle on thestellar surface. If V652 Her was formed by the merger of two whitedwarfs, its surface composition demands that they should both be heliumwhite dwarfs. Conversely, if it is the product of single-star evolution,it is more likely to be a post-giant branch star. In either case it isprobably evolving onto the helium main-sequence, with importantconsequences for understanding the origin of hot subluminous stars. Asmall discrepancy remains between T_eff and log g measured from theaverage blue-visual spectrum in this paper, and that measured fromUV-optical spectrophotometry previously. Further work will be necessaryto resolve this, and to make progress in determining the mass of V652Her. Based on observations obtained at the Anglo-Australian Telescope,Coonabarabran, NSW, Australia.
| Spectral analysis of the low-gravity extreme helium stars LSS 4357, LS II+33.5 deg and LSS 99
We have carried out quantitative analyses of three very low surfacegravity extreme helium stars with very similar spectra. Their effectivetemperatures of ~ 16 000K fill a gap in a nearly continuous sequence ofextreme helium stars all having similar luminosity-to-mass ratios, butextending from effective temperatures around 12 000 K to more than 20000 K. Because of the low surface gravities and extremely rich linespectra, the model atmosphere calculations have been reviewed, andlarge-scale spectral synthesis techniques have been introduced to theanalyses for the first time. In addition to the high carbon and nitrogenabundances usually seen in extreme helium stars, two of the programmestars have extremely high oxygen abundances, comparable with or greaterthan their carbon abundances. Based on observations obtained at theEuropean Southern Observatory, La Silla, Chile.
| HIPPARCOS observations of hydrogen-deficient carbon stars
Parallax measurements for 21 hydrogen-deficient carbon stars have beenmade by the Hipparcos satellite. These stars include most of thebrighter R Coronae Borealis (RCB) variables, other coolhydrogen-deficient carbon (HdC) stars, and several higher-temperatureextreme helium (eHe) stars. Most of these stars have either negative orstatistically insignificant parallaxes, indicating that they lie beyondthe detection capability of Hipparcos. Although the distances to thegalactic hydrogen-deficient carbon stars remain unknown, at least theHipparcos observations do confirm that these objects must have highluminosity like the LMC RCB stars, for which M_bol = -4 to -5. Basedupon Hipparcos proper motions, we derive UVW velocities for the RCB andHdC stars, assuming M_bol = -3 and -5. The UW-velocity dispersion of theRCB/HdC stars is similar to that already reported for the eHe stars,further supporting that these groups of stars have predominantly bulgedistributions. However, UW Cen may be a second example of a halo RCBstar currently seen transitting the galactic plane.
| Spectral analysis of the high-gravity extreme helium star LS IV+6 deg 2
The optical spectrum of the early B hydrogen-deficient star LS IV+6 deg2 has been analyzed. It is hottest high-gravity extreme helium star(EHe) yet studied. The He I spectrum shows all predicted permitted andforbidden transitions in absorption. LS IV+6 deg 2 is a comparativelymetal-rich EHe star; abundances of C, N, O, Ne, Mg, Al, and P aretypical of other EHes, whilst Si and S are somewhat deficient. With thesurface parameters given, LS IV+6 deg 2 lies close to the boundary ofthe helium star pulsation instability finger near Teff of about 27,000K. Available data indicate that the radial velocity is variable, butgive no indication of amplitude or period.
| Spectral analysis of the extreme helium star LSS3184
LSS3184 is a hydrogen-deficient, early B-type giant, recently found topulsate with a period of 2.5 hours. Its photospheric parameters havebeen derived from optical high-resolution spectra by the method of fineanalysis. The principal results are T_eff=23300 K, log g=3.35,{n_H}/{n_He}simle0.00015, {n_C}/{n_He}=0.003, {n_N}/{n_He}=0.0005, and{n_O}/{n_He}=0.0003. Hydrogen is extremely deficient. The effectivetemperature is consistent with broad-band visual and ultravioletspectrophotometry and an extinction E_B-V ~ 0.27. Its previous evolutionis reflected in the chemistry of the atmosphere, which contains enrichednitrogen from CNO-cycle hydrogen burning, and carbon from 3alpha heliumburning. Thus LSS3184 is a true extreme helium star with a compositionsimilar to \bd{-9}{4395}. With T_eff, log g, and pulsation propertiesvery similar to the C-poor and N-rich helium star V652Her, evolutionarymechanisms which can result in very different surface compositions forthe two stars must be examined. Based on observations obtained at theEuropean Southern Observatory, La Silla, Chile, and with the IUEsatellite retrieved from the IUE archive at the World Data Centre,Rutherford Appleton Laboratory, UK
| Age variation of helium abundance in He-rich stars.
Not Available
| 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
| Properties of He-rich stars. I. Their evolutionary state and helium abundance.
A determination of the surface gravity and an abundance analysis ofhelium in a sample of 17 He-rich and 5 normal, reference stars ispresented. These results are derived from low resolution CCD spectra,but each star was measured at least 6 times in order to obtain asignificant average spectrum for the spectroscopic variables. The heliumabundances derived from the models used are very close to 0.1 fornormal, reference stars and are larger for the others, clearlyindicating the He-rich phenomenon in them. NLTE effects, errors on themicroturbulence value or on the surface gravity do not influence theestimated helium abundances. Nevertheless, synthesized Geneva coloursare affected by the He-rich peculiarity, especially the [U-B] indexwhich systematically changes by -0.025 mag per 0.1 of He abundance forthe coolest stars in the sample. We cannot confirm the correlationbetween the evolutionary state and the helium abundance reportedpreviously (Zboril et al. 1994, International Conference on CP andMagnetic Stars.", Tatranska Lomnica, eds. J. Zverko and J. Ziznovsky, p.105), although we used a more reliable technique of log(g)determination. All He-rich objects lie within the main sequence: theirsurface gravities are all inside the range 4.1
| The emission-line spectrum of the hot R Coronae Borealis star MV SGR
Spectra of MV Sgr obtained at CTIO in 1992 May show many emission linesin the red. Most of the emission lines show two components with aminimum centred at the stellar velocity. Species represented include H,HeI, TiII and FeII. Of special interest is emission in the LiI 6707-Aresonance doublet. [OI] and [NII] emission indicate the presence of anebula around the star. The radial velocity of the absorption lines(AlIII, SiIII, NII and CII) and FeII emission lines agrees with theradial velocity given by Jeffery et al.
| A search for variability in the helium-rich subdwarf HD 144941
Not Available
| Non-linear radial pulsation models for extreme helium stars: application to V652 HER (BD+13 deg3224)
Opacity Project (OP) and OPAL opacities were used to calculatenon-linear hydrodynamic models of 0.7-M_ radially pulsating extremehelium stars, having mean (averaged over a pulsation cycle) effectivetemperatures and luminosities in the ranges10_4<=<=5x10^4 K and 1282<=/L_<=8091respectively. Separated helium and metal ionizing layers cause localmaxima (or bumps) in the depth-dependent Rosseland mean opacity, whichwere found to be responsible for two distinct regions of pulsationinstability; they will be referred to as the helium instability region(HeIR) and Z-bump instability region (ZBIR). At <3000 L_ thecooler HeIR and hotter ZBIR are separated by a region of stability whereradial pulsations are either unexcited or characterized by a very smallradial displacement amplitude (DeltaR/R~10^-3). At =1282 L_ forexample, the stability region is bounded by _red~=12000 Kand _blue~=17500 K; it becomes narrower with increasingluminosity, so that both instability regions merge at ~=3000 L_where _red~=blue~=15000 K. AllZBIR models represent fundamental mode radial pulsators; for these casesan approximate formula was derived to express the pulsation constant (Q)in terms of and the mean mass-radius ratio. BD+13 deg3224(V652 Her) remains the only hot extreme helium star known to be a radialpulsator; published photometric and spectroscopic data were used to testnon-linear radial pulsation models and, indirectly, new opacities uponwhich they were based. The best, and remarkably good, agreement betweenobserved and theoretical (radial velocity and luminosity) curves wasobtained with M=0.72 M_, =23500 K and =1062 L_when OP opacities (with mixture X=0.0015, Y=0.98287 and Z=0.01563) wereadopted. As a consequence, BD+13 deg3224 was identified as a ZBIRfundamental mode radial pulsator having a mean surface gravity of logg=3.7.
| A linear analysis of the radial pulsations of H-deficient stars
This paper presents results of a linear analysis of the radialpulsations (oscillations) of H-deficient stars, in which recentlypublished OPAL opacity data were used. The classical kappa mechanismassociated with the Fe opacity peak at T~2x10^5 K extends theinstability region downward (like a `finger') around log T_eff~4.3 inthe Hertzsprung-Russell (HR) diagram. The pulsations of V652 Her and LSS3184, relatively less luminous helium stars, are attributed to theclassical kappa mechanism. The strange modes caused by the opacity peakmake envelopes with T_eff as high as 10^5K overstable if the luminosityis sufficiently high. Pulsating luminous H-deficient stars are locatedin the strange-mode instability region in the HR diagram. Their periodsare more or less consistent with theoretical ones of overstablelow-order radial modes for M~1 to ~0.7 M_solar. The pulsation period ofthe hydrogen-deficient binary upsilon Sgr is found to be consistent withmodels having a mass of ~3 M_solar, although its radius is larger thanthose predicted by evolutionary models for pure helium stars. The massis high enough to be a progenitor of Type Ib supernovae.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| An Extension of the Case-Hamburg OB Star Surveys
We have extended the Case-Hamburg OB star surveys to b = +- 30 degreesfor l = +- 60 degres using the Curtis Schmidt telescope and 4 degreeobjective prism at the Cerro Tololo Inter-American Observatory. Acatalog of 234 OB stars and other objects with peculiar spectra ispresented along with finding charts for those objects too faint to beincluded on the BD or CD charts. (SECTION: Stars)
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| Сазвежђа: | Кентаур |
| Ректацензија: | 14h14m58.63s |
| Deклинација: | -46°17'19.3" |
| Apparent магнитуда: | 10.007 |
| Proper motion RA: | -7.3 |
| Proper motion Dec: | -0.7 |
| B-T magnitude: | 9.853 |
| V-T magnitude: | 9.995 |
Каталог и designations:
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