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Stellar winds and mass loss from extreme helium stars
Extreme helium stars are very rare low-mass supergiants in a late stageof evolution. They are probably contracting to become white dwarfsfollowing a violent phase of evolution which caused them to becomehydrogen-deficient giants, possibly R CrB stars. Using the latestgeneration of models for spherically expanding stellar atmospheres, weset out to measure mass-loss rates for a representative fraction ofthese stars. We have used high-resolution ultraviolet and opticalspectra, and ultraviolet, optical and near-infrared photometry from avariety of archives. Overall atmospheric parameters have mostly beentaken from previous analyses and checked for consistency. Mass-lossrates were measured by fitting the P Cygni and asymmetric profiles of C,N and Si ultraviolet resonance lines and lie in the range of10-10-10-7 Msolar yr-1.These rates follow a Castor-type () relation marking a lower limit forthe mass loss from hot stars of all kinds. The mass-loss rates of thestars studied also show a strong correlation with their proximity to theEddington limit. There is no firm evidence for variability in thestellar wind, although photospheric pulsations have been reported inmany cases.
| Variable Star Designations for Extreme Helium Stars
We discuss the classifications used by the GCVS for pulsatinghydrogen-deficient supergiants. We argue that the current PV Teldesignation is misleading, and that it should be replaced by three newclasses -- V652 Her, V2076 Oph and FQ Aqr. The characteristics andmembership of each class are well defined.
| Variability and evolution in various classes of post-AGB stars
We aim to compare properties of early-type post-asymptotic giant-branch(post-AGB) stars, including normal first-time B-type post-AGB stars, andextreme helium stars (EHes). Hipparcos photometry for 12 post-AGB starsand 7 EHe stars has been analyzed; 5 post-AGB stars are clearlyvariable. The Hipparcos data are not sufficiently sensitive to detectvariability in any of the EHes.
| The Subdwarf Database: Released
The work on the Subdwarf Database, presented at the previous meeting,has been completed, and the tool is now publicly available. The firstrelease contains data from close to 240 different literature sources,but more still awaits entry. The database interface includes advancedsearch capabilities in coordinate, magnitude and color space. Outputtables can be generated in HTML with hyperlinks to automaticallygenerated finding charts, the Aladin viewer and a detailed data sheetthat displays all registered data for each target, including physicaldata such as temperature, gravity and helium abundance, together with afinding chart. Search results can be visualized automatically asinteractive position, magnitude or color diagrams.
| 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.
| ORFEUS II echelle spectra: H_2 towards the northern Galactic pole
We examine the distribution of molecular hydrogen in the localinterstellar medium towards the northern galactic pole at latitudesb>45circ. High resolution UV echelle spectra of the nearbysdO stars BD +37 1977 (l=187.1, b=+45.6) and HD 128220 (l=19.9, b=+64.9)observed during the ORFEUS II mission are analysed. Distances for thesestars are calculated to be 1030 pc and 440 pc, respectively, with anerror of about 30%. We determine column densities for H_2 rotationalexcitation states up to J=7. For BD +37 1977 we find a low total H_2column density of (2.1^{+0.4}_{-0.2})?1015 cm^{-2}.Probably, the molecular gas is excited by UV pumping to an equivalentexcitation temperature of T=395+/-11 K. For HD 128220 the total H_2column density is (6.5^{+3.3}_{-4.1})?1017 cm^{-2} intwo unresolved absorption components. The Boltzmann excitationtemperature has a value of T=77+/-10 K. The equivalent temperature ofthe stages normally excited by UV pumping (J=4 to 7), which are onlyobserved in one of the components, is T<1500 K. We further obtaincolumn densities for H i from Lyman profile fitting and for S and Si.Only 10% or less of the H i seen in absorption is found in H i 21 cmspectra. This indicates either gas concentrated in a few (or only one)dense, possibly circumstellar clumps, or extended flocculent moleculargas in the northern polar disk region of our Galaxy. Assuming pressureequilibrium, a layer of flocculent molecular gas at a height z = 80 pccould be possible. For that, an analysis of further lines of sight isneccessary.
| Infrared flux excesses from hot subdwarfs. II. 72 more objects
In our search, started in February, 1994, for JHK excess fluxes amongthe hot subdwarf population as an indicator for the presence of binarycompanions, results for 72 more hot objects (=63 hot subdwarfs + 1Horizontal Branch B star + 7 white dwarfs + 1 non-subdwarf object)observed with the Carlos Sanchez CVF IR photometer (in June and October,1994), are presented. The exact number of binary hot subdwarfs hasgained renewed importance after the recent discovery of pulsators withG-F companions. The total number of candidates we propose may help toset some constraints; for example, out of 41 objects with excesses, 13may have G-type binary companions. From our new sample, 14 discoveriesof binary candidates have been found: BD+25 4655, Feige 108, HD 4539, HD149382, HD 216135, KPD 2109+440, LSI+63 198, LSIV+10 9, LSV+22 38, PG0011+221, PG 0116+242, PG 0314+103, PG 2151+100 and TON 139. Besides, 2more from reanalysis of February, 1994, data - BD+37 1977 and BD+481777, may now be found to be IR excess candidates. Two suspectedbinaries, PB 8555 and SB 7, are also confirmed. By fitting \cite[Kurucz(1993)]{kur93} model spectra and assuming zero-age main sequencecompanions, we find upper limits on the subdwarf gravities. Thedistributions of upper limits on log(g), mostly between about 5.25 and6.5, are nearly identical for both sdBs and sdOs. Figure~1 is onlyavailable in the electronic version of the paper(http://www.edpsciences.com)
| Infrared flux excesses from hot subdwarfs.
Observations of 27 hot subdwarf stars and white dwarfs have beenperformed with an IR photometer in a search for excess fluxes in the JHKbands. 22 of the objects are observed for the first time in theinfrared. In 11 of the stars a significant excess flux was found, 2 ofwhich are new discoveries: GD 274 and GD 299. We confirm the suspicionof Moehler et al. (1990) that PG0232+095 is a composite system, and wereinforce the suggestion of Thejll et al. (1991, 1994a) that Feige 34 isa composite system. The excesses are most likely due to companion stars,mainly of spectral types A - K. We estimate the subdwarf gravities,assuming zero-age main sequence companions, and thus find upper limitson log(g). For the hot sdO Feige 34 we estimate from the publishedtrigonometric parallaxes an absolute magnitude M_v_^F34^=5.6^+0.9^_-1.6_and surface gravity log(gpi_)=6.8^+0.3^_-0.7_.
| UBVRI photometry of G, K, M HIPPARCOS stars. II
As indicated by Grenon (1989), the data of the present series of reportson the UBVRI photometry of late-type stars in the Hipparcos InputCatalog are to be employed in computations of Hipparcos observing time,as well as in evaluating the observability of faint stars by thesatellite. Attention is here given to late type stars in the V = 8-12range, including distant red giants in the Galactic plane (Hipparcosproposal 189), as well as high proper motion stars included in the G,LTT, LP, and MCC catalogs.
| The kinematics of Milky Way halo gas. I - Observations of low-ionization species
Ultraviolet interstellar line day observed with the IUE toward 70 halostars and four extragalactic sight lines are analyzed in a study of thelarge-scale kinematic properties of the Milky Way halo gas. The motionsof the low-ionization gas is focused on. Large systematic velocities arefound, and a pronounced asymmetry in the absorption characteristics ofhalo gas toward the Galactic poles is indicated. In the north,substantial amounts of material are falling toward the disk atvelocities up to about 120 km/s in the most extreme case. Toward thesouth, low-ionization material shows no extreme or systematic motions.
| Model stars with degenerate dwarf cores and helium-burning shells - A stationary-burning approximation
The characteristics of model stars consisting of a degenerate dwarf coreand an envelope which is burning a nuclear fuel or fuels in its interiorare explored. The models are relevant to stars which are accretingmatter from a companion, to single stars in late stages of evolution, tostripped noninteracting remnants of binary star evolution, and tomerging and merged degenerate dwarfs. For any given mass and choice ofnuclear fuels, a sequence of models is constructed which differ withrespect to the mass of the degenerate core and the envelopecharacteristics. Each sequence has at least three distinct branches: adegenerate dwarf branch along which envelope mass increases withdecreasing luminosity, a plateau branch characterized by a very smallenvelope mass and by a nearly constant luminosity which reaches themaximum achievable value for the sequence, and an asymptotic giantbranch which is at the lowest temperatures achievable and along whichenvelope mass decreases with increasing luminosity.
| IUE studies of mass loss phenomena in hot stars
The determination of mass loss rates and wind velocities in a largesample of hot stars with IUE; the discovery of mass loss and winds inSdO stars; UV studies of mass loss in OB stars in the Magellanic Cloudsand beyond; stimuli to the development of improved radiation pressurewind models; and the discovery of substantial stellar wind variability,epitomized by the occurrence and variation of wind discrete absorptioncomponents in a wide range of stellar types are discussed. This progressresults from the combination of high spectral sensitivity, wavelengthmultiplexing, observatory nature, and longevity of IUE, important infacilitating detailed variability studies of individual sources over awide range of timescales.
| A catalogue of spectroscopically identified hot subdwarf stars.
Not Available
| Extreme hydrogen-deficient stars
Some properties and problems of the extreme hydrogen-deficient stars arediscussed. These stars comprise the R Coronae Borealis variables, thehydrogen-deficient carbon stars and the extreme helium stars. Alsodiscussed are the hydrogen-deficient binaries and the pulsating heliumstar V652 Her.
| A radial velocity survey of extremely hydrogen-deficient stars
A radial velocity survey of hot extremely hydrogen-deficient starsconfirms HDE 320156 to be a single-lined spectroscopichydrogen-deficient binary. The three hydrogen-deficient stars detectedall show weak C-lines. Little evidence is found for the small-amplituderadial velocity variations known to be present among the extreme-helium(EHe) stars. The heliocentric radial velocity distributions andkinematics of the known EHe stars are found to be consistent with an oldstellar population which is strongly concentrated towards the Galacticcenter, and evidence suggests that the EHe stars are all single.
| A comparative study of the ultraviolet spectrum of hot subdwarfs
The high and low resolution IUE spectra of four O subdwarfs (BD+75°325, BD +48°1777, BD +37°1977, BD +37°442) areanalyzed. Line identification is given for the most abundant ions in thefar ultraviolet. Fe V is by far the ion with the largest number oflines. Broad P Cygni profiles of the N V and C IV resonance doublets areobserved in BD +37°442 and BD +37°1977, with terminal velocitiesof 2200 and 2300 km s-1 respectively. The N IV 1718 Åline presents an extended blue wing also arising in the winds of thesestars. A low interstellar extinction (E(B-V) ? 0.05) is derived fromthe weakness of the 2200 Å i.s. band, and from theLyα profile. Mean radial velocities are given forthe photospheric and interstellar atomic species, and a high stellarvelocity is found for the four stars, in agreement with the PopulationII nature of these objects.
| A survey of ultraviolet objects
An all-sky survey of ultraviolet objects is presented together with astatistical analysis that leads to the conclusion that there is asignificantly higher population of hot subdwarfs lying below themain-sequence than hitherto thought. The distribution of all ultravioletobjects, main sequence ultraviolet objects, and MK unclassifiedultraviolet objects are shown in galactic coordinates, and the absolutemagnitudes and color-color diagrams for these groups are presented.Scale heights are derived, giving values similar to planetary nebulaefor the hottest groups.
| Highly-evolved stars
The ways in which the IUE has proved useful in studying highly evolvedstars are reviewed. The importance of high dispersion spectra forabundance analyses of the sd0 stars and for studies of the wind from thecentral star of NGC 6543 and the wind from the 0 type component of VelaX-1 is shown. Low dispersion spectra are used for absolutespectrophotometry of the dwarf nova, Ex Hya. Angular resolution isimportant for detecting and locating UV sources in globular clusters.
| Spectral atlas of helium-rich stars
An atlas of coude spectra of 33 helium-rich O- and B-stars and 11comparison stars in the 3700-4600 A spectral range is presented. Theatlas comprises stars with temperatures from 10,000 to 60,000 deg K inwhich He lines are visible in the spectra, and includes variable Hestars; stars with spectral types O8 to B5 and magnitudes (m(v)) lessthan or equal to 11.0 are considered. The purpose of the atlas is togive a sample of spectra in which, from the observational point of view,the differences caused by certain physical parameters includingeffective temperature, logarithm of the gravity and observed rotationalvelocity, can be judged by eye. Mechanisms for He enrichment in theatmosphere are also discussed.
| SB 21, an extremely helium-rich subdwarf O-star
Spectral observations of the subdwarf O star SB 21 are reported. Theobservations were made at a dispersion of 29 A/mm in the spectral range4250-4800 A using a 3.6-m telescope and an image dissector scanner. TheH gamma line is found to be absent in SB 21, while the He I lines at4388, 4471 and 4713 A and the He II lines at 4686 and possibly 4542 aredetected. A detailed nonlocal thermodynamic equilibrium model analysisof the data indicates an effective temperature of 35,000 K, log g of 5.5and a He/H density ratio greater than 10. Differences between theextremely helium rich objects SB 21, CPD-31 1701 and BD + 48 177 andother extremely helium-rich objects such as BD +37 1877 and BD +37 442are pointed out.
| Atmospheres for hot, high-gravity stars. I - Pure hydrogen models
An extensive grid of pure hydrogen model atmospheres for hot,high-gravity stars is presented. The models are intended to aid theanalysis of visual, ultraviolet, and soft X-ray spectra of hot DA whitedwarfs and EUV sources. The grid extends from log g equals 4.0(1.0)9.0and T(eff) ranges from 20,000 K up to the Eddington limit for mostsurface gravities. Most of the models are LTE unblanketed calculations,but selected NLTE models and blanketed LTE models have also beencomputed in order to assess the importance of these effects. For eachmodel, continuum fluxes are tabulated covering the entire range ofwavelengths for which there is significant flux. Stromgren colors, UBVcolors, and bolometric corrections are also given. Profiles andequivalent widths of the L-alpha, L-beta, L-gamma, H-alpha, H-beta,H-gamma, and H-delta lines are given as well.
| Analysis of the UV spectrum of three hot subdwarfs
International Ultraviolet Explorer high resolution observations of threeO subdwarfs (BD + 37 deg 442, BD + 32 deg 1977, and BD + 48 deg 1777)were taken. The spectra are characterized by very high excitationphotospheric lines. Intense interstellar lines are present, whosepossible circumstellar origin is suggested in view of the absence of the2200 A interstellar band.
| The degenerate stars with hydrogen atmospheres. I
Statistical results are presented for an analysis of 162 DA degeneratestars (those with hydrogen atmospheres). It is shown thatmultichannel-spectrometer colors for groups of similar stars lie closeto those predicted from pure hydrogen atmospheres for certain models andthat the temperatures obtained from fitting energy distributions tomodels agree with those from the G-R and U-V colors. Statisticallytreated equivalent widths are found to agree with theoreticalpredictions. These widths suggest that mean log g must exceed 7.0 andprobably lies above or near 8.0; the colors are consistent with log gnear or above 8.0. The space frequency of stars in the sample is derivedfrom the number of stars within limits of effective temperature.
| Spectral classification of ultraviolet objects.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1978A&A....66L...9B&db_key=AST
| A spectrophotometric study of ultraviolet and X-ray sources with objective prism plates
Several objective prism spectra of objects known to have largeultraviolet or X-ray fluxes are discussed. A marked Balmer excess and Halpha emission are found in X Per(= 3U 0352 + 30) and in the similar Bestar HD 200775. No Balmer discontinuity is detectable in HD 226868 (CygX-1) during the low X-ray states, in HZ 43 (MX 1313 + 29), and in thehot subluminous star BD + 37 deg 1977. Comparison with the farultraviolet spectrum of this star (Carnochan et al., 1975) leads to theconclusion that the star is slightly reddened (EB-V = 0.15plus or minus 0.05) with an optical-ultraviolet energy distributionclose to a 100,000 K black-body.
| BD + 37 deg 1977 - A very hot subdwarf
BD + 37 deg 1977, one of the hottest objects identified by observationswith the ESRO TD 1A satellite, is shown to be a subluminous O-type star.Helium line profiles of the star are illustrated.
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