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Fe-bump instability: the excitation of pulsations in subdwarf B and other low-mass stars
We consider the excitation of radial and non-radial oscillations inlow-mass B stars by the iron-bump opacity mechanism. The results aresignificant for the interpretation of pulsations in subdwarf B stars,helium-rich subdwarfs and extreme helium stars, including the EC14026and PG1716 variables.We demonstrate that, for radial oscillations, the driving mechanismbecomes effective by increasing the contrast between the iron-bumpopacity and the opacity from other sources. This can be achieved eitherby increasing the iron abundance or by decreasing the hydrogenabundance. The location of the iron-bump instability boundary is foundto depend on the mean molecular weight in the envelope and also on theradial order of the oscillation. A bluer instability boundary isprovided by increasing the iron abundance alone, rather than the entiremetal component, and is required to explain the observed EC14026variables. A bluer instability boundary is also provided by higherradial order oscillations. Using data for observed and theoreticalperiod ranges, we show that the coolest EC14026 variables may vary inthe fundamental radial mode, but the hottest variables must vary inmodes of higher radial order.In considering non-radial oscillations, we demonstrate that g-modes ofhigh radial order and low spherical degree (l < 4) may be excited insome blue horizontal branch stars with near-normal composition (Z =0.02). Additional iron enhancement extends the g-mode instability zoneto higher effective temperatures and also creates a p-mode instabilityzone. The latter is essentially contiguous with the radial instabilityzone. With sufficient iron, the p- and g-mode instability zones overlap,allowing a small region where the EC14026- and PG1716-type variabilitycan be excited simultaneously. The overlap zone is principally afunction of effective temperature and only weakly a function ofluminosity. However, its location is roughly 5000 K, too low comparedwith the observed boundary between EC14026 and PG1716 variables. Thediscrepancy cannot be resolved by simply increasing the iron abundance.
| 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.
| Abundance analysis of the cool extreme helium star LSS3378
Abundance analysis of the cool extreme helium (EHe) star LSS3378 ispresented. The abundance analysis is done using local thermodynamicequilibrium (LTE) line formation and LTE model atmospheres constructedfor EHe stars.The atmosphere of LSS3378 shows evidence of H-burning, He-burning, ands-process nucleosynthesis. The derived abundances of iron peak andα-elements indicate the absence of selective fractionation or anyother processes that can distort chemical composition of these elements.Hence, the Fe abundance [log ɛ(Fe) = 6.1] is adopted as aninitial metallicity indicator. The measured abundances of LSS3378 arecompared with those of R Coronae Borealis (RCB) stars and with rest ofthe EHe stars as a group.
| The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. II. Kinematics and Full Sample Analysis
This paper continues the analysis of faint high-latitude B stars fromMartin. Here we analyze the kinematics of the stars and combine themwith the abundance information from the first paper to classify eachone. The sample contains 31 Population I runaways, 15 old evolved stars(including 5 blue horizontal-branch [BHB] stars, 3 post-HB stars, 1pulsating helium dwarf, and 6 stars of ambiguous classification), 1 Fdwarf, and 2 stars that do not easily fit in one of the othercategories. No star in the sample unambiguously shows thecharacteristics of a young massive star formed in situ in the halo. Thetwo unclassified stars are probably extreme Population I runaways. Thelow binary frequency and rotational velocity distribution of thePopulation I runaways imply that most were ejected from dense starclusters by the dynamic ejection scenario. However, we remain puzzled bythe lack of runaway Be stars. We also confirm that PB 166 and HIP 41979are both nearby solar-metallicity BHB stars.Based on observations made at the 2.1 m Otto Struve Telescope ofMcDonald Observatory, operated by the University of Texas at Austin.
| 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.
| Non-Lte Metal Abundances in V652 HER and HD 144941
Two evolutionary scenarios are proposed for the formation of extremehelium stars: a post-AGB star suffering from a late thermal pulse, orthe merger of two white dwarfs. An identification of the evolutionarychannel for individual objects has to rely on surface abundances. Wepresent preliminary results from a non-LTE analysis of CNO, Mg and S fortwo unique objects, V652 Her and HD 144941. Non-LTE abundancecorrections for these elements range from negligible values to 0.7dex. Non-LTE effects typically lead to systematic shifts in theabundances relative to LTE and reduce the uncertainties.
| Improved Helium Line Formation for Extreme Helium Stars
Quantitative analyses of extreme helium stars to date face thedifficulty that theory fails to reproduce the observed helium lines intheir entirety, wings and line cores. Here, we demonstrate how theissues can be resolved using state-of-the-art non-LTE line formation forthese chemically peculiar objects. Two unique B-type objects arediscussed in detail, the pulsating variable V652 Her and the metal-poorstar HD 144941. The improved non-LTE computations for helium show thatanalyses assuming LTE or based on older non-LTE model atoms can predictequivalent widths, for the He I 10 830 Å transition in particular,in error by up to a factor 3. Our modeling approach also succeedsin largely resolving the general mismatch for effective temperatures ofEHe stars derived from ionization equilibria and from spectral energydistributions.
| Extreme helium stars: non-LTE matters. Helium and hydrogen spectra of the unique objects V652 Her and HD 144941
Quantitative analyses of low-mass hydrogen-deficient (super-)giant stars- so-called extreme helium stars - to date face two major difficulties.First, theory fails to reproduce the observed helium lines in theirentirety, wings and line cores. Second, a general mismatch exists foreffective temperatures derived from ionization equilibria and fromspectral energy distributions. Here, we demonstrate how the issue can beresolved using state-of-the-art non-LTE line-formation for thesechemically peculiar objects. Two unique high-gravity B-type objects arediscussed in detail, the pulsating variable V652 Herand the metal-poor star HD 144941. In the first caseatmospheric parameters from published LTE analyses are largelyrecovered, in the other a systematic offset is found. Hydrogenabundances are systematically smaller than previously reported, by up toa factor ~2. Extreme helium stars turn out to be important testbeds fornon-LTE model atoms for helium. Improved non-LTE computations show thatanalyses assuming LTE or based on older non-LTE model atoms can predictequivalent widths, for the He I 10 830 Å transition in particular,in error by up to a factor ~3.
| 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.
| Calculated spectra for HeH+ and its effect on the opacity of cool metal-poor stars
The wavelength and Einstein A coefficient are calculated for allrotation-vibration transitions of4He1H+,3He1H+,4He2H+ and 3He2H+, giving a complete line list and thepartition function for 4HeH+ and itsisotopologues. This opacity is included in the calculation of the totalopacity of low-metallicity stars and its effect is analysed fordifferent conditions of temperature, density and hydrogen numberfraction. For a low helium number fraction (as in the Sun), it is foundthat HeH+ has a visible but small effect for very lowdensities (ρ<= 10-10 g cm-3), attemperatures around 3500 K. However, for high helium number fraction,the effect of HeH+ becomes important for higher densities(ρ<= 10-6 g cm-3), its effect being mostimportant for a temperature around 3500 K. Synthetic spectra for avariety of different conditions are presented.
| The Origins and Evolutionary Status of B Stars Found Far from the Galactic Plane. I. Composition and Spectral Features
The existence of faint blue stars far above the Galactic plane that havespectra that are similar to nearby Population I B stars presents severalinteresting questions. Among them are the following: Can a Population IB star travel from the disk to a position many kiloparsecs above theplane in a relatively short main-sequence lifetime? Is it possible thatsingle massive star formation is occurring far from the Galactic plane?Are these objects something else masquerading as main-sequence B stars?This paper (the first of two) analyzes the abundances of a sample ofthese stars and reveals several that are chemically similar to nearbyPopulation I B stars, whereas others clearly have abundance patternsmore like those expected in blue horizontal-branch (BHB) orpost-asymptotic giant branch stars. Several of those with old evolvedstar abundances also have interesting features of note in their spectra.We also consider why this sample does not have any classical Be starsand identify at least two nearby solar-metallicity BHB stars.Based on observations made at the 2.1 m Otto Struve Telescope ofMcDonald Observatory operated by the University of Texas at Austin.
| The central star of the planetary nebula N 66 in the Large Magellanic Cloud: A detailed analysis of its dramatic evolution 1983-2000
The central star of the planetary nebula N 66 (alias WS 35, SMP 83 andHV 5967) in the Large Magellanic Cloud enhanced its brightnessdramatically in 1993 and 1994. Within the subsequent four years itreturned to the previous level. Its spectrum resembles that of aWolf-Rayet star of the nitrogen sequence (WN4.5). We monitored theobject intensively from ground and with the Hubble Space Telescope. Nowwe present the complete set of spectroscopic observations from thedifferent epochs before, during and after the brightness outburst of N66. The stellar spectra from the different epochs are analyzed in detailby means of most advanced non-LTE models for expanding stellaratmospheres. The main results are: the luminosity, log L/Lsun= 4.6, before and after the outburst is exceptionally high for a centralstar of a planetary nebula. During the outburst in 1994, it even climbedup to log L/L_sun = 5.4 for about one year. The effective temperatureof about 112 kK remained roughly constant, i.e. the luminosity mainlyincreased because of a larger effective stellar radius. The mass lossrate increased from 10-5.7 Msun yr-1 inthe quiet state to 10-5.0 Msun yr-1during the outburst. The chemical composition of the stellar atmosphereis that of incompletely CNO-processed matter: it is dominated by heliumwith a rest of hydrogen, nitrogen being slightly enhanced and carbonstrongly depleted. We extensively discuss possible scenarios for thenature and evolutionary origin of N 66, which should explain theexceptional stellar parameters, the atmospheric composition, theoutburst mechanism, and the existence of the bipolar nebula which wasejected only a few thousand years ago and contains about 0.6 solarmasses of hydrogen-rich matter. If being a single star, N 66 might be(i) a low-mass star after the Asymptotic Giant Branch, as usuallyadopted for central stars of planetary nebulae, (ii) a massive, i.e.non-degenerate star, or (iii) a merger produced from two white dwarfs.Although there are no direct indications for binarity, we alternativelydiscuss whether N 66 might be (iv) a massive star which lost itshydrogen envelope in a recent common-envelope phase with a less massivecompanion, or (v) a white dwarf accreting mass from a companion with ahigh rate. None of the scenarios is free of any contradiction to atleast one of the observational facts. However, the binary scenarios poseless severe problems. If N 66 is a white dwarf accreting matter in aclose-binary system, its present accretion rate would bring it to theChandrasekhar limit within a few hundred thousand years. Thus N 66 mightbe a candidate for a future type Ia supernova explosion in our cosmicneighborhood.Partly based on observations with the NASA/ESA Hubble Space Telescopeperformed at the STScI which is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contract NAS5-26555.Partly based on observations with the International Ultraviolet Explorer(IUE).
| Radial Pulsations of Helium Stars with Masses from 1 to 10 M_solar
We present the results of our hydrodynamic calculations of radialpulsations in helium stars with masses 1 M_solar < M < 10 M_solar,luminosity-to-mass ratios 1 x 10^3 L_solar/M_solar < L/M < 2 x10^4 L_solar/M_solar, and effective temperatures 2 x 10^4 K < T_eff< 10^5 K for mass fractions of helium Y=0.98 and heavy elementsZ=0.02. We show that the lower boundary of the pulsation-instabilityregion corresponds to L/M ~ 10^3 L_solar/M_solar and that theinstability region for L/M < 5 x 10^3 L_solar/M_solar is bounded byeffective temperatures T_eff < 3 x 10^4 K. As the luminosity rises,the instability boundary moves into the left part of theHertzsprung-Russell diagram and radial pulsations can arise in starswith effective temperatures T_eff < 10^5 K at L/M > 7 x 10^3L_solar/M_solar. The velocity amplitude for the outer boundary of thehydrodynamic model increases with L/M and lies within the range 200
| Astrophysics in 2002
This has been the Year of the Baryon. Some low temperature ones wereseen at high redshift, some high temperature ones were seen at lowredshift, and some cooling ones were (probably) reheated. Astronomerssaw the back of the Sun (which is also made of baryons), a possiblesolution to the problem of ejection of material by Type II supernovae(in which neutrinos push out baryons), the production of R CoronaeBorealis stars (previously-owned baryons), and perhaps found the missingsatellite galaxies (whose failing is that they have no baryons). A fewquestions were left unanswered for next year, and an attempt is made todiscuss these as well.
| Time-series spectroscopy of pulsating sdB stars - III. Line indices of PG 1605+072
We present the detection and analysis of line index variations in thepulsating sdB star PG 1605+072. We have found a strong dependence ofline index amplitude on Balmer line order, with high-order Balmer lineamplitudes up to 10 times larger than Hβ. Using a simple model, wehave found that the line index may not only be dependent on temperature,as is usually assumed for oscillating stars, but also on surfacegravity. This information will provide another set of observables thatcan be used for mode identification of sdBs.
| Physical parameters of helium-rich subdwarf B stars from medium resolution optical spectroscopy
Most subdwarf B (sdB) stars have spectra that show helium (He)abundances depleted to a lesser or greater extent. This has beenattributed to diffusion and gravitational settling. However a small butsignificant number of stars with similar temperatures and gravities showrelatively strong helium spectra. The question is how these helium-richsdB (He-sdB) stars are related to their He-deficient counterparts. Wepresent physical parameters for sample of He-sdB stars usingline-blanketed LTE models. From the analysis we show that the He-sdBstars are quite distinct from the He-poor sdB. We also explore the ideaof a possible link between some extreme helium (EHe) stars and He-richsubdwarf O (He-sdO) stars.Based on observations made with the William Herschel and Isaac NewtonTelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias.
| Radial Pulsations of Helium Stars and a Model for BX CIR
The results of hydrodynamical calculations of radially pulsating heliumstars with masses 0.5M ȯ≤M≤0.9M ȯ, bolometricluminosities 600L ȯ≤5×103 L ȯ, and effectivetemperatures 1.5×104 K≤T eff≤3.5×104 K arepresented. The pulsation instability of these stars is due to theeffects of ionization of iron-group elements in layers with temperaturesT2×105 K. The calculations were carried out using opacitiesfor the relative mass abundances of hydrogen and heavy elements X=0 andZ=0.01, 0.015, and 0.02. Approximate formulas for the pulsation constantQ over the entire range of pulsation instability of the hot helium starsin terms of the mass M, radius R, effective temperature T eff, andheavy-element abundance Z are derived. The instability of BX Cir toradial pulsations with the observed period Π=0.1066 d occurs only fora mass M≥0.55M ȯ, effective temperature T eff≥23000 K, andheavy-element abundance Z≥0.015. The allowed mass of BX Cir is inthe range 0.55M ȯ≤M≤0.8M ȯ, which corresponds toluminosities 800L ȯ≤M≤1400L ȯ and mean radii 1.7Rȯ≲R≲2.1R ȯ.
| Temperature and gravity of the pulsating extreme helium star LSS 3184 (BX Cir) through its pulsation cycle
We report the analysis of optical spectra of the extreme helium star LSS3184 (BX Cir) to determine its effective temperature and gravitythroughout its pulsation cycle. The spectra were also used to measureits chemical abundances. We report rest gravity, log g = 3.38 +/- 0.02,and a chemical abundance mixture consistent with those reported earlierin a study using an optical spectrum with lower spectral resolution anda lower signal to noise ratio. Our analysis decreases the upper limitfor the H abundance to H < 6.0 (mass fraction < 7.1 x10-7 ). Our gravity corresponds to stellar mass M = 0.47 +/-0.03 Msun. We find that the effective log g varies throughthe pulsation cycle with an amplitude of 0.28 dex. The effective gravityis smaller than the rest gravity except when the star is very near itsminimum radius. The change in effective gravity is primarily caused byacceleration of the stellar surface. Based on the optical spectra, wefind the temperature varies with an amplitude of 3450 K. We find a timeaveraged mean temperature, 23 390 +/- 90 K, consistent with that foundin the earlier optical spectrum study. The mean temperature is 1750 Khotter than that found using combined ultraviolet spectra and V and Rphotometry and the variation amplitude is larger. This discrepancy issimilar to that found for the extreme helium star V652Her. Based on observations obtained at the Anglo-AustralianTelescope, Coonabarabran, NSW, Australia.
| 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.
| Physical parameters for subdwarf B stars with composite spectra
New intermediate-resolution spectra have been obtained for a number ofsubdwarf B stars having both single and composite spectra. Physicalparameters have been determined for the sdB stars and, incomposite-spectrum systems, their cool companions. For these binaries,we have developed a method which uses the blue-optical spectrum todetermine the effective temperatures of both stars, the surface gravityof the hot stars and the radius ratio of the system. The surface gravityof the cool star is measured using the infrared calcium triplet. Thesurface gravities of these cool companions identify them asmain-sequence stars with masses in the range 0.8-1.2 Msun,confirming a previous energy distribution analysis. There is alsoevidence that the composite-spectrum sdBs are more helium-poor thansingle-spectrum sdBs. Based on observations made with the Isaac Newtonand William Herschel Telescopes.
| Non-linear radial pulsation models for the early-type helium stars V652 Her and BX Cir
We report new non-linear pulsation models of the helium starsV652 Her and BX Cir. Linear theoryhas previously shown their pulsations to be due to iron-group bumpinstability. Recent high-resolution spectroscopic observations haveprovided high-precision measurements of their radial velocity curves andof their radii. Their masses remain less well determined. A hydrodynamiccode including recent OPAL opacity data has been used to construct themodels. These are compared with the observational data. In particular,we attempt to reproduce accurately the observed radial velocity andluminosity curves. The results impose additional constraints on thosestellar dimensions, including mass, which remain poorly determined byobservation. Final results show a model for V652 Herwhich reproduce the observed velocity and luminosity curves with 0.7Msun, and 23 400 K. For BX Cir, the mass must lie between0.50 and 0.38 Msun, if the temperature is in the range 22400-24 000 K. However, the luminosity of the models is smaller than thatmeasured directly by a factor of two.
| Time-resolved spectral analysis of the pulsating helium star V652 Her
A series of 59 moderate-resolution high signal-to-noise spectra of thepulsating helium star V652 Her covering 1.06pulsation cycles was obtained with the William Herschel Telescope. Thesehave been supplemented by archival ultraviolet and visualspectrophotometry and used to make a time-dependent study of theproperties of V652 Her throughout the pulsationcycle. This study includes the following features: the most preciseradial velocity curve for V652 Her measured so far,new software for the automatic measurement of effective temperature,surface gravity and projected rotation velocities frommoderate-resolution spectra, self-consistent high-precision measurementsof effective temperature and surface gravity around the pulsation cycle,a demonstration of excessive line-broadening at minimum radius andevidence for a pulsation-driven shock front, a new method for the directmeasurement of the radius of a pulsating star using radial velocity andsurface gravity measurements alone, new software for the automaticmeasurement of chemical abundances and microturbulent velocity, updatedchemical abundances for V652 Her compared withprevious work (\cite{Jef99}), a reanalysis of the total flux variations(cf. \cite{Lyn84}) in good agreement with previous work, and revisedmeasurements of the stellar mass and radius which are similar to recentresults for another pulsating helium star, BX Cir.Masses measured without reference to the ultraviolet fluxes turn out tobe unphysically low (~0.18 M{\odot}). The best estimate forthe dimensions of V652 Her averaged over thepulsation cycle is given by: lt; Teff >=22 930+/-10 K and< log g > =3.46+/-0.05 (ionization equilibrium), =20 950+/-70 K (total flux method), =2.31+/-0.02 R{\odot}, < L>=919+/-14L{\odot}, M=0.59+/-0.18 M{\odot} and d=1.70+/-0.02kpc. Two significant problems were encountered. The line-blanketedhydrogen-deficient model atmospheres used yield effective temperaturesfrom the optical spectrum (ionization equilibrium) and visual and UVphotometry (bolometric flux) that are inconsistent. Secondly, the IUEspectra are poorly distributed in phase and have low signal-to-noise.These problems may introduce systematic errors of up to 0.1M{\odot}. Based on observations obtained with the WilliamHerschel Telescope, the United Kingdom Infrared Telescope, and on INESdata from the IUE satellite.
| Projection effects in the spectra of early-type pulsating stars
The relation between the surface velocity of a pulsating star in thestar's inertial frame and the apparent expansion velocity measured by aremote observer has been classically formulated in terms of a projectionfactor p. This factor depends on the relative limb darkening incontinuum and spectral lines and may therefore be a function ofcomposition. We have computed synthetic spectra to investigate thebehaviour of p and, indeed, of the apparent line profile for radiallypulsating stars over a range of temperature and gravity withhydrogen-rich and hydrogen-poor atmospheres. We have subsequentlyderived values of p suitable for the interpretation of velocitymeasurements of pulsating early type stars. Effective temperature hasthe most important influence on p: a change of 10 000 K, in T_effintroduces a change of 2-3 per cent with important consequences formeasurements of stellar radii to an accuracy <1%.
| 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.
| White Dwarf Mergers and the Progeny of Extreme Helium Stars
We have examined the evolution of merged low-mass double white dwarfswhich become luminous helium stars. We have approximated the mergingprocess by the rapid accretion of matter, consisting mostly of helium,onto, first, a helium (He) white dwarf and, recently, a carbon-oxygen(CO) white dwarf. After a certain mass is accumulated, a helium shellflash occurs, the radius and luminosity increase and the star becomes ayellow giant. Mass accretion is stopped artificially when the total massreaches a pre-determined value. Subsequent evolution depends on theaccretor composition. The He-WD accretor contracts towards the heliummain-sequence where it will become a subdwarf B star. Before that, itbriefly acquires properties identical to those of the extreme heliumstar V652 Her. The CO-WD accretor evolves blueward almost horizontallyin the HR diagram. Specific models agree very well with the observedlocations of extreme helium stars in the log Teff- log gdiagram, and with recent measurements of their secular evolution rates,luminosities and masses. Together with predicted merger rates, theevolutionary timescales are roughly consistent with observed numbers ofextreme helium stars. The models are conceptually consistent withobserved surface abundances. The white dwarf merger model remains aviable explanation for the creation of extreme helium stars and, byassociation, the majority of R Coronae Borealis stars.
| Physical properties of the pulsating hydrogen-deficient star LSS 3184 (BX Cir)
We report new determinations of the radius and mass of the pulsatinghelium-rich, hydrogen-deficient star LSS 3184(BX Cir) using measurements of radial velocity andangular radius throughout its pulsation cycle. Measurements of radialvelocity, and thus changes in stellar radius (Delta R_star ), were madeusing Anglo-Australian Telescope echelle spectra. Hubble Space Telescopeultraviolet spectra and ground-based BV photometry were used to findtemperatures and fluxes throughout the pulsation cycle. The temperaturesand fluxes were used to find the angular radius of the star (alpha ).The alpha , Delta alpha , and Delta R values thus found were used tocalculate the mean stellar radius < R_star > = 2.31 +/- 0.10 R_sun. If we use the previously determined log g = 3.35 +/- 0.1 forLSS 3184 and our radius estimate, we find its mass tobe M_star = 0.42 +/- 0.12 M_sun. Based on observations obtained with theNASA/ESA Hubble Space Telescope, which is operated by STScI for theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555. Based on observations obtained at theAnglo-Australian Telescope, Coonabarabran, NSW, Australia
| The evolution of a rapidly accreting helium white dwarf to become a low-luminosity helium star
We have examined the evolution of merged low-mass double white dwarfswhich become low-luminosity (or high-gravity) extreme helium stars. Wehave approximated the merging process by the rapid accretion of matter,consisting mostly of helium, on to a helium white dwarf. After a certainmass is accumulated, a helium shell flash occurs, the radius andluminosity increase and the star becomes a yellow giant. Mass accretionis stopped artificially when the total mass reaches a pre-determinedvalue. As the helium-burning shell moves inwards with repeating shellflashes, the effective temperature gradually increases as the starevolves towards the helium main sequence. When the mass interior to thehelium-burning shell is approximately 0.25Msolar, the starenters a regime where it is pulsationally unstable. We have obtainedradial pulsation periods for these models. These models have propertiesvery similar to those of the pulsating helium star V652 Her. We havecompared the rate of period change of the theoretical models with thatobserved in V652 Her, as well as with its position on theHertzsprung-Russell diagram. We conclude that the merger between twohelium white dwarfs can produce a star with properties remarkablysimilar to those observed in at least one extreme helium star, and is aviable model for their evolutionary origin. Such helium stars willevolve to become hot subdwarfs close to the helium main sequence. Wealso discuss the number of low-luminosity helium stars in the Galaxyexpected for our evolution scenario.
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