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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.
| 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.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| 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.
| 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.
| Variability in the extreme helium star LSS 5121
We report a photometric and spectroscopic study of the hot extremehelium star LSS 5121. We found photometricvariability, but no period was evident in its periodogram. This isconsistent with the previous proposal, based on spectral linevariations, that LSS 5121 is a non-radial pulsator similar to other hotextreme helium stars. Based on observations made at the University ofTexas McDonald Observatory, Fort Davis, Texas and the Jacobus KapteynTelescope operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofisica de Canarias.
| 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.
| 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.
| 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.
| Variability in intermediate- and high-temperature extreme helium stars
Not Available
| Spectral analyses of late-type [WC] central stars of planetary nebulae: more empirical constraints for their evolutionary status
The optical spectra of the five recently observed late-type Wolf-Rayetcentral stars He2-459 ([WC8]), M2-43 ([WC8]), SwSt1 ([WC9]), PM1-188 andIRAS21282+5050 (both [WC11]) are analyzed by means of sphericallyexpanding model atmospheres. The stellar parameters T_* (effectivetemperature), v_infty (final velocity of the wind), R_* (stellar radius)and dot {M} (mass loss rate) are determined by NLTE simulations whichaccount for the elements hydrogen, helium, carbon and oxygen. With twoexceptions (SwSt1 and IRAS21282) the results presented here fit into thesample of already examined [WCL]-type objects. Altogether 13 out of 17known [WCL]-CSPN have been analyzed so far. The presence of hydrogen inthe atmospheres of [WC11] and [WC12] stars becomes more and moreevident. In five out of seven analyzed objects of these subtypeshydrogen emission features of stellar origin can be identified. Thespectra of the latest subtypes ([WC11], [WC12]) show rather narrow linesand thus allow to detect features of nitrogen (N II, N III), neon (Ne I)and silicon (Si III, Si IV). For the first time we present modelcalculations accounting for these elements and perform abundanceestimates for the eight narrow-lined stars (all [WC11] and [WC12] plusSwSt1). The obtained surface compositions are discussed in the light ofrecent evolutionary calculations which account for diffuse mixing duringthermal pulses on the Asymptotic Giant Branch. Partly based onobservations obtained at the German-Spanish Astronomical Center, CalarAlto, Spain
| 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
| Continuous opacity from Ne^-
Free-free absorption coefficients of the negative neon ion arecalculated by the phase-shift approximation based on multiconfigurationHartree-Fock continuum wave functions. These wave functions accuratelyaccount for electron-neon correlation and polarization, and yieldscattering cross-sections in excellent agreement with the latestexperimental values. The coefficients are expected to give the bestcurrent estimates of Ne^- continuous absorption. We find that Ne^- makesonly a small contribution (less than 0.3 per cent) to stellar opacities,including hydrogen-deficient stars with enhanced Ne abundances.
| 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.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| 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.
| The extreme helium star BD-9 deg 4395
Simultaneous optical and UV spectroscopy is used to investigate both thephotospheric variations and emission-line behavior of the extreme heliumstar BD-9 deg 4395. The photospheric parameters of the star arerederived from the optical spectra by the method of fine analysis, onthe basis of up-to-date atomic data and new line-blanketed modelatmospheres. The principal results are: T(eff) = 22,700 K, log g = 2.55,n(H)/n(He) = 0.0015, and n(C)/n(He) = 0.0004. Assuming an He-shellmodel, a mass of about 0.7 solar mass is obtained. The emission-linespectrum which principally contains lines due to He I, C II, and Si II,is probably due to recombination in a shell, or extended atmosphere,with a temperature slightly below T(eff). Both nonvariable and variablecomponents may be identified, and suggest substantial dynamical activityin the shell.
| 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.
| The UV spectrum of the WC 11 star CPD -56 deg 8032
UV observations of the WC 11 central star of the compact planetarynebula CPD -56 deg 8032 show that the continuum flux is variable. Thisvariability appears to be caused by a change in the circumstellarextinction properties. The UV energy distribution can be fitted withhelium star models with T(eff) about 25000 K, after correcting for twotypes of reddening: (1) E(B-V) about 0.4 using Seaton's (1979) reddeningcurve applicable to diffuse interstellar medium; (2) E(B-V) about 0.25to 0.30 using the reddening curve defined by the laboratory extinctionproperties of amorphous carbon grains of Bussolletti et al. (1987). Thestellar wind mass loss is estimated from the P-Cygni profile of C II1760.6 from the high resolutions IUE spectrum. Similarities with otherWC 11 stars are discussed.
| The analysis of astronomical spectra
Efforts to develop and improve computer analysis techniques forastronomical spectra in SERC Collaborative Computational Project 7(CCP7) during the period 1985-1988 are reviewed. Consideration is givento the organizational structure and personnel of CCP7, advancedcomputers for LTE and non-LTE stellar-atmosphere models, the networksystems STARLINK and JANET, the CCP7 software library, andCCP7-sponsored meetings and newsletters. The studies underway at 11observatories and university centers in the UK and the FRG are listedand briefly characterized.
| A universal formula for determining the stellar mass loss rate
The quiet outflow of stellar mass, which occurs as stellar wind, isconsidered. Mass loss rates (M) for 400 stars were used to derive aformula expressing M as a function of four stellar parameters: L,T(eff), Y, and Z.
| Mass loss from stars : the universal formula for mass loss rate
Not Available
| A new excitation mechanism for stellar pulsations in HdC stars.
Not Available
| Beta Cephei pulsation and the helium-strong stars
Six He-strong stars are identified which may be good candidates insearches for Beta Cephei pulsation based on their location in a plot ofbeta vs (u-b). The significance of detection of pulsational variabilityamong the He-peculiar stars in the study of excitation mechanisms forBeta Cephei stars is discussed.
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