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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.
| The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars
We review the observed properties of extremely hot, hydrogen-deficientpost-asymptotic giant branch (AGB) stars of spectral type [WC] andPG1159. Their H deficiency is probably caused by a (very) latehelium-shell flash or an AGB final thermal pulse, laying bare interiorstellar regions that are usually kept hidden below the hydrogenenvelope. Thus, the photospheric elemental abundances of these starsallow us to draw conclusions about details of nuclear burning and mixingprocesses in the precursor AGB stars. We summarize the state of the artof stellar evolution models that simulate AGB evolution and theoccurrence of a late He-shell flash. We compare predicted elementalabundances to those determined by quantitative spectral analysesperformed with advanced non-LTE model atmospheres. Good qualitative andquantitative agreement is found. Future work can contribute to an evenmore complete picture of the nuclear processes in AGB stars.
| 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.
| 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.
| 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.
| Secular contraction in extreme helium stars and the future of V4334 Sgr
Speculative connections have been made between Sakurai's Object andother hydrogen-deficient stars, principally the RCB stars and [WC]central stars of planetary nebulae. RCBs have also been postulated asthe precursors of extreme helium stars (EHes). The question arises towhether Sakurai's Object will evolve down the [WC]-PG1159 evolutiontrack, or the RCB-EHe-HesdO track. From a number of IUE observations, wehave measured the secular contraction rates and pulsation masses ofseveral EHes. These are inconsistent with the predicted contractionrates for helium-shell burning giants produced by a final helium-shellflash (the [WC]-PG1159 track). Although there may be some similaritiesbetween Sakurai's Object and some RCBs, if the RCB-EHe conection isvalid then these must be superficial rather than structural.
| H I Spectra and Column Densities toward HVC and IVC Probes
We show 21 cm line profiles in the direction of stars and extragalacticobjects, lying projected on high- and intermediate-velocity clouds (HVCsand IVCs). About half of these are from new data obtained with theEffelsberg 100 m telescope, about a quarter are extracted from theLeiden-Dwingeloo Survey (LDS), and the remaining quarter were observedwith other single-dish telescopes. H I column densities were determinedfor each HVC/IVC. Paper I of this series uses these in combination withoptical and ultraviolet high-resolution measurements to deriveabundances. Here an analysis is given of the difference and ratio of N(HI) as observed with a 9' versus a 35' beam. For HVCs and IVCs the ratioN(H I-9')/N(H I-35') lies in the range 0.2-2.5. For low-velocity gasthis ratio ranges from 0.75 to 1.3 (the observed ratio is 0.85-1.4, butit appears that the correction for stray radiation is slightly off). Thesmaller range for the low-velocity gas may be caused by confusion in theline of sight, so that a low ratio in one component can be compensatedby a high ratio in another-for 11 low-velocity clouds fitted by onecomponent the distribution of ratios has a larger dispersion. Comparisonwith higher angular resolution data is possible for 16 sight lines.Eight sight lines with H I data at 1'-2' resolution show a range of0.75-1.25 for N(H I-2')/N(H I-9'), while in eight other sight lines N(HI-Lyα)/N(H I-9') ranges from 0.74 to 0.98.
| Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption linemeasurements for the high- and intermediate-velocity clouds (HVCs andIVCs). New values are derived for N(H I) in the direction of observedprobes, in order to arrive at reliable abundances and abundance limits(the H I data are described in Paper II). Distances to stellar probesare revisited and calculated consistently, in order to derive distancebrackets or limits for many of the clouds, taking care to properlyinterpret nondetections. The main conclusions are the following. (1)Absolute abundances have been measured using lines of S II, N I, and OI, with the following resulting values: ~0.1 solar for one HVC (complexC), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC,and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally,approximate values in the range 0.5-2 solar are found for three moreIVCs. (2) Depletion patterns in IVCs are like those in warm disk or halogas. (3) Most distance limits are based on strong UV lines of C II, SiII, and Mg II, a few on Ca II. Distance limits for major HVCs aregreater than 5 kpc, while distance brackets for several IVCs are in therange 0.5-2 kpc. (4) Mass limits for major IVCs are0.5-8×105 Msolar, but for major HVCs theyare more than 106 Msolar. (5) The Ca II/H I ratiovaries by up to a factor 2-5 within a single cloud, somewhat morebetween clouds. (6) The Na I/H I ratio varies by a factor of more than10 within a cloud, and even more between clouds. Thus, Ca II can beuseful for determining both lower and upper distance limits, but Na Ionly yields upper limits.
| 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.
| Properties of atmospheres and winds of H-deficient central stars and related objects
According to latest results presented at this workshop, it appears thatthe surface chemistry of hydrogen-deficient post-AGB stars can now beexplained by evolutionary calculations involving a late He-shell flash.A quantitative comparison of theoretical predictions with observedelement abundances can eventually constrain details of this evolutionaryscenario. I will emphasize here the diverse chemical compositionsobserved in H-deficient post-AGB stars, which call for an explanation.The abundance patterns vary from almost pure helium atmospheres tohydrogen- and helium-free atmospheres, with a rich choice ofcompositions in between, involving rather different mixtures of H, He,and CNO.
| 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
| 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
| A search for variability in the helium-rich subdwarf HD 144941
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| 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.
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| 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.
| High-resolution optical spectroscopy of the hot R CrB star V348 Sagittarii
High resolution optical spectra of the unique hot R CrB star V348 Sgrare presented and more than 500 spectral lines of 14 elements areidentified. The spectrum is very complex and four types of line profilescan be distinguished: a) pure absorption lines, b) P-Cygni lines, c)pure emission lines and d) P-Cygni (HeI) or emission (CII) profiles withcentral absorption reversal. The spectrum is interpreted as that of anexpanding atmosphere of small optical thickness with wind velocitiesbetween 150 and 200 km/s. Hydrogen is definitely present in theexpanding atmosphere. The absorption line spectrum is very similar tothat of the extreme helium stars DY Cen and LSE 78.
| Spectral analysis o9f LSE 78: an extreme helium star similar to BD - 9 deg 4395 and DY Centauri
LSE 78 is a hydrogen-deficient early-B supergiant, spectroscopicallysimilar to the pulsating extreme helium star BD - 9 deg 4395 and the hotR CrB star DY Cen. The photospheric parameters of LSE 78 have beenderived from optical high-resolution spectra by the method of fineanalysis. The principal results are Teff = 18000 K, log g =2.00, nH/nHe is less than 0.0001, n(sucC)/nHe = 0.01; hydrogen is extremely deficient. The effectivetemperature is consistent with broadband visual and ultravioletspectrophotometry and an extinction EB - V approximately0.25, subject to peculiar extinction in the UV (lambda less than 2000A).Assuming a He-shell burning model, a mass approximately 0.8 solar massmay be estimated for LSE 78. Its previous evolution is reflected in thechemistry of the atmosphere, which contains enriched nitrogen fromCNO-cycle hydrogen burning, carbon from 3 alpha helium burning, andoxygen from alpha-capture reactions. The latter makes LSE 78 the mostoxygen-rich extreme helium star analysed so far, withnC/nO approximately 3. This low ratio indicatesthat the atmospheric carbon and oxygen are the products of near completehelium burning. The iron abundance is sim 0.2 times solar. Weak andvariable emission lines due to He I and C II are detected, but are neveras strong as those detected in BD-9 deg 4395 or DY Cen. By analogy, theyprovide evidence for a recombination shell, or extended atmosphere, witha temperature slightly below Teff, around LSE 78.
| Spectral analysis of DY Centauri, a hot R Coronae Borealis star with unusually high hydrogen content
The photospheric parameters of DY Cen, an H-deficient early-B supergiantthat is spectroscopically similar to the pulsating, extreme-He star BD-9deg 4395, have been derived from high-resolution spectra via atomic dataand line-blanketed model atmospheres. DY Cen is shown to be a metal-poorEHe star whose composition supports an anticorrelation between H and Feabundances in EHe stars. Its previous evolution is reflected in itsN-enriched atmospheric chemistry.
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Ultraviolet and radio observations of Milky Way halo gas
Interstellar-absorption-line and 21-cm emission-line data for sightlines to 56 stars are combined in order to study the kinematics andspatial distribution of the gas that is at great distances from theGalactic plane. Measurements of the interstellar velocities and H Icolumn densities from the 21-cm emission and Ly-alpha absorption areincluded. The problem of contamination of the interstellar Ly-alphaabsorption line by stellar Ly-alpha absorption is analyzed, and thisinformation is used to reevaluate the vertical distribution of H I. Anew method for determining lower limits on the vertical distribution ofgas by including information on the velocity structure in the gas ispresented. The data for individual sight lines are discussed.
| South African Astronomical Observatory. Research.
Not Available
| DY CEN and the hot R Coronae Borealis stars
The first observations of the short-term photometric behavior of the hotR CrB star DY Cen along with spectroscopic data are presented. Analysisof the latter yields nH less than 0.9 nHe and T(eff) = (14,000 + or -1500) K, while the star has been found to exhibit low-amplitude,short-period (3.8-5.5 d) photometric variations superimposed on one ofmuch longer duration (longer than 25 d). Provided DY Cen is extremelyhydrogen-deficient, and these variations may be interpreted as radialpulsations, then L of about 13,000-18,000 solar luminosities and M ofabout 0.71-0.78 solar mass are implied. The long-term variation cannotbe interpreted within the framework of the period-temperature law forhydrogen-deficient stars. The classification criteria and evolutionarystatus of the hot R CrB stars are reviewed and, in this context, it isshown that DY Cen displays marked differences compared to the othermembers, indicating a different evolutionary status for this object. Thestar's physical properties are intermediate between the Extreme Heliumstars and the classical R CrB stars and, hence, may represent a uniquetransition object between these groups.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | אריה |
| התרוממות ימנית: | 10h38m55.23s |
| סירוב: | +10°03'48.5" |
| גודל גלוי: | 9.907 |
| תנועה נכונה: | -12 |
| תנועה נכונה: | -4.9 |
| B-T magnitude: | 9.725 |
| V-T magnitude: | 9.892 |
קטלוגים וכינוים:
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INHABITED SKY
