The central star of the planetary nebula PB 8: a Wolf-Rayet-type wind of an unusual WN/WC chemical composition
A considerable fraction of the central stars of planetary nebulæ(CSPNe) are hydrogen-deficient. As a rule, these CSPNe exhibit achemical composition of helium, carbon, and oxygen with the majorityshowing Wolf-Rayet-like emission line spectra. These stars areclassified as CSPNe of a spectral type [WC]. We perform a spectralanalysis of CSPN PB 8 with the Potsdam Wolf-Rayet (PoWR) models forexpanding atmospheres. The source PB 8 displays wind-broadened emissionlines from strong mass loss. Most strikingly, we find that its surfacecomposition is hydrogen-deficient, but not carbon-rich. With massfractions of 55% helium, 40% hydrogen, 1.3% carbon, 2% nitrogen, and1.3% oxygen, it differs greatly from the 30-50% of carbon which aretypically seen in [WC]-type central stars. The atmospheric mixture in PB8 has an analogy in the WN/WC transition type among the massiveWolf-Rayet stars. Therefore we suggest to introduce a new spectral type[WN/WC] for CSPNe, with PB 8 as its first member. The central star of PB8 has a relatively low temperature of T* = 52 kK, as expected forcentral stars in their early evolutionary stages. Its surrounding nebulais less than 3000 years old, i.e. relatively young. Existingcalculations for the post-AGB evolution can produce hydrogen-deficientstars of the [WC] type, but do not predict the composition found in PB8. We discuss various scenarios that might explain the origin of thisunique object.This paper includes data gathered with the 6.5-m Magellan Telescopeslocated at Las Campanas Observatory, Chile.Some of the data presented inthis paper were obtained from the Multimission Archive at the SpaceTelescope Science Institute (MAST). STScI is operated by the AURA, Inc.,under NASA contract NAS5-26555. Support for MAST for non-HST data isprovided mainly by the NASA Office of Space Science via grant NAG5-7584.Based on INES data from the IUE satellite.
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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.
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New Estimates of the Solar-Neighborhood Massive Star Birthrate and the Galactic Supernova Rate
The birthrate of stars of masses >=10 Msolar is estimatedfrom a sample of just over 400 O3-B2 dwarfs within 1.5 kpc of the Sunand the result extrapolated to estimate the Galactic supernova ratecontributed by such stars. The solar-neighborhood Galactic-plane massivestar birthrate is estimated at ~176 stars kpc-3Myr-1. On the basis of a model in which the Galactic stellardensity distribution comprises a ``disk+central hole'' like that of thedust infrared emission (as proposed by Drimmel and Spergel), theGalactic supernova rate is estimated at probably not less than ~1 normore than ~2 per century and the number of O3-B2 dwarfs within the solarcircle at ~200,000.
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Catalog of Galactic OB Stars
An all-sky catalog of Galactic OB stars has been created by extendingthe Case-Hamburg Galactic plane luminous-stars surveys to include 5500additional objects drawn from the literature. This work brings the totalnumber of known or reasonably suspected OB stars to over 16,000.Companion databases of UBVβ photometry and MK classifications forthese objects include nearly 30,000 and 20,000 entries, respectively.
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Version 2000 of the Catalogue of Galactic Planetary Nebulae
The ``Catalogue of Galactic Planetary Nebulae (Version 2000)'' appearsin Abhandlungen aus der Hamburger Sternwarte, Band XII in the year 2001.It is a continuation of CGPN(1967) and contains 1510 objects classifiedas galactic PNe up to the end of 1999. The lists of possible pre-PNe andpossible post-PNe are also given. The catalogue is restricted only tothe data belonging to the location and identification of the objects. Itgives identification charts of PNe discovered since 1965 (published inthe supplements to CGPN) and those charts of objects discovered earlier,which have wrong or uncertain identification. The question ``what is aplanetary nebula'' is discussed and the typical values of PNe and oftheir central stars are summarized. Short statistics about thediscoveries of PNe are given. The catalogue is also available in theCentre de Données, Strasbourg and at Hamburg Observatory viainternet. The Catalogue is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/378/843
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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.
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The distribution of bright OB stars in the Canis Major-Puppis-Vela region of the Milky Way
The picture of the young stellar groups in the Canis Major-Puppis-Vela(215 deg
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Spectroscopic investigation of old planetaries. IV. Model atmosphere analysis
The results of a NLTE model atmosphere analysis of 27 hydrogen-richcentral stars of old planetary nebulae (PN) are reported. These starswere selected from a previous paper in this series, where we gaveclassifications for a total of 38 central stars. Most of the analyzedcentral stars fill a previously reported gap in the hydrogen-richevolutionary sequence. Our observations imply the existence of twoseparated spectral evolutionary sequences for hydrogen-rich and -poorcentral stars/white dwarfs. This is in line with theoreticalevolutionary calculations, which predict that most post-AGB stars reachthe white dwarf domain with a thick hydrogen envelope of ~ 10(-4) M_sun.We determine stellar masses from the comparison with evolutionary tracksand derive a mass distribution for the hydrogen-rich central stars ofold PNe. The peak mass and the general shape of the distribution is inagreement with recent determinations of the white dwarf massdistribution. The properties of most analyzed stars are well explainedby standard post-AGB evolution. However, for eight stars of the sampleother scenarios have to be invoked. The properties of three of them areprobably best explained by born again post-AGB evolution. Two of theseare hybrid CSPN (hydrogen-rich PG 1159 stars), but surprisingly thethird star doesn't show any signs of chemical enrichment in itsatmosphere. The parameters of five stars are not in accordance withpost-AGB evolution. We discuss alternative scenarios such as thestripping of the hydrogen-rich envelope by a companion during the firstred giant phase or the formation of a common envelope with a possiblemerging of both components. Two stars (HDW 4 andHaWe 5) remain mysterious after all. They resembleordinary hot DA white dwarfs, but due to very large evolutionary agesthe presence of a PN cannot be explained. We speculate that the nebulaemay be shells produced by ancient nova outbursts. A wide spread ofhelium abundances is observed in the photospheres of central stars ofold PNe. It is shown that a good correlation between helium abundancesand luminosity is present. It is inferred that when the stars'luminosities fall below L~ 300 L_sun depletion starts and the heliumabundance steadily decreases with decreasing luminosity. The existenceof this correlation is in qualitative agreement with recent theoreticalcalculations of gravitational settling in the presence of a stellarwind.
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The Optical Spectrum of the O-Type Subdwarf BD +28 deg4211
High-resolution spectra of the O subdwarf BD +28 deg4211 in the opticalregion show emission lines of H and He II within the absorption profileof H?. Central emission is also present in H? and in the HeII lines at 4686 and 5411 Å. Weaker emission lines of N V, C IV,and Si IV are also present, as are a large number of narrow absorptionlines of O IV, O V, Si IV, and N IV, from which an upper limit on vsinican be estimated.
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Spectral analysis of O(He)-type post-AGB stars
We present an analysis of two recently discovered, very hothydrogen-deficient post-AGB stars of spectral type O(He), namely HS1522+6615 and HS 2209+8229, by means of state-of-the-art NLTE modelatmospheres based on new optical, UV, and X-ray observations. In thespectra of HS 1522+6615 we discovered a variability of the\Ionw{O}{6}{5290} line complex on a time scale of few days. We alsoreport on the discovery of a ring structure in the planetary nebula K1-27 which has an O(He)-type central star. We discuss how the group ofO(He) stars fits in our picture of hydrogen-deficient post-AGB stellarevolution. Based on observations obtained at the German-SpanishAstronomical Center, Calar Alto, operated by the Max-Planck-Institutfür Astronomie Heidelberg jointly with the Spanish NationalCommission for Astronomy; collected at the European SouthernObservatory, La Silla, Chile; on observations made with ROSAT, retrievedfrom the archive; and on observations made with the InternationalUltraviolet Explorer (IUE) and retrieved from the IUE Final Archive;this research has made use of the SIMBAD database, operated at CDS,Strasbourg, France
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UBV beta Database for Case-Hamburg Northern and Southern Luminous Stars
A database of photoelectric UBV beta photometry for stars listed in theCase-Hamburg northern and southern Milky Way luminous stars surveys hasbeen compiled from the original research literature. Consisting of over16,000 observations of some 7300 stars from over 500 sources, thisdatabase constitutes the most complete compilation of such photometryavailable for intrinsically luminous stars around the Galactic plane.Over 5000 stars listed in the Case-Hamburg surveys still lackfundamental photometric data.
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New and misclassified Planetary Nebulae
Altogether 86 objects discovered in the period 1991-1994 have beenaccepted by the author as new PN (Table 1). In Appendices A and B tothis table the lists of possible pre-PN as well as of possible post-PN,respectively, are given. Table 2 presents 6 misclassified objects whichwe suggest removing from CGPN.
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Spectral analysis of the multiple-shell planetary nebula LoTr4 and its very hot hydrogen-deficient central star.
We present a spectral analysis of the planetary nebula LoTr4 and itsexciting star and report the discovery of a multiple-shell nebulastructure. Two clearly separated shells can be detected around thecentral star with radii of 7.5" and 13.5" with a strong decrease ofbrightness from south-east to north-west. Relative to these shells thecentral star appears to be off-centered by about 2" to the south-east. Afragment of a third shell is found with a radius of 22.5" south-east ofthe central star. This and the high degree of asymmetry indicate thatthe nebula ejection was a complicated process. LoTr4 is of highexcitation with a geometrical radius of r=0.6^+0.2^_-0.3_pc and a massof M=0.29Msun_. Its helium abundance is definitely solar(+/-0.05dex), while the trace elements could also have slightlydifferent abundances (+/-0.1dex). A stratification of the He/H abundanceratio in the nebula can be excluded. The central star had been analyzedby means of NLTE model atmospheres. From line profile fits we derive aneffective temperature of T_eff_=120+/-15kK and a surface gravity oflogg=5.5+/-0.3. The photosphere is dominated by helium and hydrogen(n_H_/n_He_=0.5 by number). We find a solar nitrogen abundance(n_N_/n_He_=0.001). Neither carbon nor oxygen lines are exhibited in itsspectrum which sets the upper limits to n_C_/n_He_<0.004 andn_O_/n_He_<0.008 which are the solar abundance ratios. Theseabundances indicate that the central star of LoTr4 can be a "born-againpost-AGB star". This scenario has also been invoked for the evolution ofPG1159 stars. In this picture the O(He) stars, LoTr4 and K1-27, mighteither be direct progenitors of the PG1159 stars or they haveexperienced less drastic mass-loss events.
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Metal abundances in subdwarf O stars.
We describe a non-LTE analysis of seven subdwarf O stars and fourcentral stars of planetary nebulae. The analysis yields metal abundances(or upper limits to these) of C, N, O, Mg, Al, and Si for most of thesestars. Also effective temperature, surface gravity and helium abundancewere determined for those programme stars for which no reliableestimates of these parameters were available. The derived abundancepatterns prove to be disparate. We also compare the sdO stars in oursample with other sdO analyses (18 object in total) and find that thisstellar class is not homogeneous with respect to metal abundances. Atentative subclassification according to helium content andluminosity-to-mass ratio indicates that the compact, high-gravity sdOstars have similar metal abundances with a strong signature of hydrogenburning in the CNO cycle. They only differ in the amounts of helium andcarbon present in the photosphere, probably caused by varyingefficiencies of mixing or surface erosion. Luminous sdOs, on the otherhand, show markedly different metallicities, suggesting differentorigins. The star CPD-31 1701 is identified as a peculiar object. Ascompared with other O subdwarfs with nearly identical parameters, itsHe^44^Sc II line profiles appear shallow and rounded, lacking a welldefined core. They can also not be fitted by theoretical profiles. Noexplanation for this discrepancy could be found yet. Our analysis yieldsmicroturbulence velocities in the range 0-10km/s for all stars with asufficient number of metal lines. No indication was found for highervalues like the 20km/s reported by Hubeny et al. (1991) for the sdO starBD+75 325.
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The spectrum of the central star of the planetary nebula in M22
New optical observations of the central star of the planetary nebulaIRAS 18333-2357 in the globular cluster M22 show lines of H I, He II, CIV, N IV, and N V. The spectrum closely resembles the sdO star KS 292,which has surface abundances enhanced by products of hydrogen shellburning and helium burning, and an effective temperature of 75,000 K.The lines of C in IRAS 18333-2357 seem somewhat stronger than in KS 292,and the lines of N are considerably stronger. The presence ofsubstantial hydrogen is surprising in view of the hydrogen-poor natureof the nebular ejecta. If IRAS 18333-2357 is as hot as its analog, itsluminosity is about 14,000 lunar luminosity. This value is higher thanthat theoretically expected for single-star evolution of M22 clusterstars.
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Spectroscopic investigation of old planetaries. II - Detection of a 'hybrid' central star
By means of a spectroscopic survey of old planetary nebulae itdefinitely confirms the planetary nebula status of S 68 (PK 030 + 06 1)and detects the 'hybrid' nature of its nucleus: the spectrogram showsstrong C IV and He II lines similar to those of the PG 1159 stars, andstrong Balmer lines, indicating a quite unusual composition. The nucleusof S 68 constitutes obviously a new class of hot central stars which isin conflict with present evolutionary scenarios.
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UBV photometry of OB+ stars in the southern Milky Way
One thousand two hundred and twenty six new observations are combinedwith previously published results of the author to yield an internalyconsistent set of magnitudes and colors on the international UBV systemfor 666 stars classified as OB+ in the Stephenson-Sanduleak OB starsurvey. The U - B, B - V diagram indicates that these stars consistprimarily of O-type stars and early B-type supergiants, reddened by upto E(B - V) = 2.1 mag.
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NLTE-analysis of subluminous O stars - KS 292
High-resolution spectra of the southern Galactic sdO star KS 292 areanalyzed using modern NLTE model atmospheres in order to determine theatmospheric parameters. The results indicate that KS 292 probablybelongs to a newly discovered group of post-AGB stars which arecharacterized by effective temperatures and gravities typical forcentral stars of planetary nebulae but which apparently lackcircumstellar nebulae.
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Atmospheres and Abundances of Blue Horizontal Branch Stars and Related Objects
Not Available
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Non-LTE calculation of nitrogen lines: a sensitive temperature indicator for very hot sdO stars.
Not Available
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Radial velocities and spectral types for a sample of faint blue stars.
Not Available
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The carbon abundance of the subluminous O-star KS 292.
Not Available
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A catalogue of spectroscopically identified hot subdwarf stars.
Not Available
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Photometry of faint blue stars. VII - More southern stars
Photoelectric uvby photometry is presented for 103 southern stars,mostly from the LB and PB catalogs. Using photometric criteria, thestars appear to be a mixture of hot subdwarfs, horizontal-branch andpopulation II objects. Four high-latitude hot stars which are too redfor their c1 indices are suggested to be possible binary systems.Observations of metal-weak secondary standards indicate that the uvbyphotometry of the population II stars is close to the standard system.
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UBV Photoelectric Photometry Catalogue (1986). III Errors and Problems on DM and HD Stars
Not Available
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UBV photometry for southern OB stars
New UBV photometry of 1227 OB stars in the southern Milky Way ispresented. For 1113 of these stars, MK spectral types have been reportedpreviously in a comprehensive survey to B = 10.0 mag.
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On the 4430 A interstellar band - A visual classification
A system of visual classification of the strength of the interstellarband 4430 A has been developed, based on spectra taken at spectralclassification dispersion. The intensity of the band was divided into 8main classes, defined by a group of selected standard stars. The systemwas applied to 1,111 southern OB stars contained in the catalog ofspectral types by Garrison, Hiltner, and Schild (1977). The mean errorin the classification was estimated to be plus or minus 1 class.Equivalent widths were measured for 100 stars; an excellent correlationwith the visual estimations was obtained. A calibration between thesetwo parameters is given.
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MK spectral classifications for southern OB stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977ApJS...35..111G&db_key=AST
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The spatial distribution of young stars in VELA (l=250 to 284).
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977A&AS...27..343D&db_key=AST
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INHABITED SKY
