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Discovery of a Compact Companion to the Hot Subdwarf Star BD +37° 442
We report the results of the first X-ray observation of the luminous andhelium-rich O-type subdwarf BD +37° 442 carried out with theXMM-Newton satellite in 2011 August. X-ray emission is detected with aflux of about 3× 10-14 erg cm-2s-1 (0.2-1 keV) and a very soft spectrum, well fit bythe sum of a blackbody with temperature kT BB =45+11 - 9 eV, and a power law with a poorlyconstrained photon index. Significant pulsations with a period of 19.2 sare detected, indicating that the X-ray emission originates in a whitedwarf or neutron star companion, most likely powered by accretion fromthe wind of BD +37° 442.
| A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| 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.
| Catalogue of Ap, HgMn and Am stars
We present a catalogue of 8205 known or suspected Ap, HgMn and Am stars.This paper is a major update of our first edition of the catalog of Apand Am stars and includes revised identifications, additional stars andrevised information obtained from the literature.Catalogue (full Table 1) 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/498/961
| 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.
| 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.
| 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.
| 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.
| Open problems in pulsating hot subdwarfs after high speed photometry in Loiano
A sample of 37 hot subdwarf stars has been observed with the two headsfast photometer at the Loiano Observatory; among them the sdO star HD128220 shows certainly pulsations, the sdB star PG 0856+121 is veryprobably pulsating, while the claim of variability of BD +37°442requires additional observations. The occurrence of pulsations in a sdOstar, far from the temperature strip corresponding to the EC 14026stars, could suggest the existence of a new class of pulsating stars.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| 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_.
| 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.
| Observations of Variable Stars with the R-Coronae Phenomenon and Other Unique Objects
Not Available
| General catalogue of AP and AM stars
A catalog of all the known Ap and Am stars, containing a total of 6684objects, was prepared. The few stars that were wrongly considered as Apor Am star are noted as well as some others for which their identity asAp or Am stars is not yet well established. The catalog gives theidentificators and the main observational data for each star, includingthe magnitude, color, spectral type, duplicity, periodicity, magneticfield, and projected rotational velocity.
| The studies of proper motions in the regions of open clusters. II - NGC 752
Relative proper motions of 1777 stars in the region of the open clusterNGC 752 have been determined from the PDS measurements of eleven Pulkovonormal astrograph plates. In a 110-arcmin-diameter circle around thecluster, 175 stars have membership probabilities P greater than 0percent. The total number of cluster stars, obtained by weighting themwith membership probabilities, reaches 130. The catalog of propermotions is complete down to m(pg) = 150 mag, the limiting value is 154mag. The average standard error of the catalog proper motions variesfrom + or - 000075 arcsec/yr to + or - 00011 arcsec/yr for the rest. Theluminosity function for cluster stars shows a deficiency of faint stars.
| 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
| NLTE Analysis and Chemical Composition of Hot Low-Mass Stars
Spectroscopic analyses of hot pre-white dwarfs, i.e. subluminous O and Bstars, are presented. In the B-type stars, the resulting abundancepatterns are indicative of atmospheric diffusion. Amongst the O-typesubdwarfs, a new group of comparatively luminous stars is identified.Their position in the HR-diagram suggests that, unlike the "classical"sdOs, they are in a post-AGB stage of evolution. Spectroscopic evidenceis presented showing that the born-again post-AGB star scenario of Ibenet al. (1983) can explain their origin.
| 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.
| Optical observations of ultraviolet objects. II - Classification and photometry /l = 0 to 145 deg/
UBV photometry and MK spectral classification are presented for a sampleof stars originally selected as UV objects from satellite photometrywith the S2/68 experiment. Although most stars of the sample arespectroscopically normal, a number of hot, subluminous stars were alsofound. Radial velocities are given for some of the stars observed. It isproposed that the majority of the apparently normal early B stars athigh galactic latitudes which are not runaway stars are subluminousdisc-population analogues of horizontal branch B stars of Population II.
| BD+37° 442 or BD+37° 443?
We correct the correction brought by Bartoliniet al. (1982) to theidentification of the star BD+37° 442.
| List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry
A list is presented of 333 stars, excluded from the GCVS and itssupplements, whose probability of variability ranges from high tocertain. The standard deviations observed in the V magnitude togetherwith the known spectral types, however, only allow speculation as to thetype of variable in question pending supplementary observations whichreveal the individual characteristics of these stars.
| Search for variability in hydrogen-poor stars. I - Preliminary results of photoelectric observations for six stars
The evolutionary status of hydrogen-poor stars is assessed.Photoelectric observations of six objects are reported as a first stepin a long-term project devoted to a search for variability of a largesample of hydrogen-poor stars. The observed stars exhibit phenomena ofmicrovariability with an amplitude of the order of 0.1m or less. Twoextreme helium stars are examined: a period in the range of0.162d-0.164d is found for BD + 10 deg 2179 with P = 0.1079962d for BD +13 deg 3324. It is noted that the mass-losing O subdwarf (sdO) BD = + 37deg 443 exhibits short-term fluctuations with a time-scale of severalminutes and long-term variations on a scale of months.
| 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.
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