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Hot subdwarf stars in close-up view. II. Rotational properties of single and wide binary subdwarf B stars
Subluminous B stars (sdBs) form the extremely hot end of the horizontalbranch and are therefore related to the blue horizontal branch (BHB)stars. While the rotational properties of BHB stars have beeninvestigated extensively, studies of sdB stars have concentrated onclose binaries that are influenced by tidal interactions between theircomponents. Here we present a study of 105 sdB stars, which are eithersingle stars or in wide binaries where tidal effects become negligible.The projected rotational velocities have been determined by measuringthe broadening of metal lines using high-resolution optical spectra. Allstars in our sample are slow rotators (vrotsini < 10 kms-1). Furthermore, the vrotsini-distributions ofsingle sdBs are similar to those of hot subdwarfs in wide binaries withmain-sequence companions as well as close binary systems with unseencompanions and periods exceeding ?1.2 d. We show that bluehorizontal and extreme horizontal branch stars are also related in termsof surface rotation and angular momentum. Hot BHB stars (Teff> 11 500 K) with diffusion-dominated atmospheres are slow rotatorslike the hot subdwarf stars located on the extreme horizontal branch,which lost more envelope and therefore angular momentum in the red-giantphase. The uniform rotation distributions of single and wide binary sdBspose a challenge to our understanding of hot subdwarf formation.Especially the high fraction of helium white dwarf mergers predicted bytheory seems to be inconsistent with the results presented here.Based on observations at the Paranal Observatory of the EuropeanSouthern Observatory for programmes number 165.H-0588(A), 167.D-0407(A),071.D-0380(A) and 072.D-0487(A). Based on observations at the La SillaObservatory of the European Southern Observatory for programmes number073.D-0495(A), 074.B-0455(A), 076.D-0355(A), 077.D-0515(A) and078.D-0098(A). Based on observations collected at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto, operatedjointly by the Max-Planck Institut für Astronomie and the Institutode Astrofísica de Andalucía (CSIC). Some of the datapresented here were obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation. Based on dataobtained with the Hobby-Eberly Telescope (HET), which is a joint projectof the University of Texas at Austin, the Pennsylvania State University,Stanford University, Ludwig-Maximilians-Universität München,and Georg-August-Universität Göttingen.
| Accurate Estimations of Stellar and Interstellar Transition Lines of Triply Ionized Germanium
In this paper, we report on weighted oscillator strengths of E1transitions and transition probabilities of E2 transitions amongdifferent low-lying states of triply ionized germanium using highlycorrelated relativistic coupled cluster (RCC) method. Due to theabundance of Ge IV in the solar system, planetary nebulae, white dwarfstars, etc., the study of such transitions is important from anastrophysical point of view. The weighted oscillator strengths of E1transitions are presented in length and velocity gauge forms to checkthe accuracy of the calculations. We find excellent agreement betweencalculated and experimental excitation energies. Oscillator strengths offew transitions, wherever studied in the literature via othertheoretical and experimental approaches, are compared with our RCCcalculations.
| 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.
| A survey for pulsating subdwarf B stars with the Nordic Optical Telescope
Context. A search programme for pulsating subdwarf B stars was conductedwith the Nordic Optical Telescope on La Palma over 59 nights between1999 and 2009. Aims: The purpose of the programme was tosignificantly extend the number of rapidly pulsating sdB stars to betterunderstand the properties of this new group of variable compact stars. Methods: Candidates were selected initially from the HS and HEsurveys, but were supplemented with additional objects from othersurveys. Short sequences of time-series photometry were made on thecandidates to determine the presence of rapid pulsations. Results: In total twenty new pulsators were found in this survey, mostof which have already been published and some extensively studied. Wepresent four new short period pulsators, bringing the total of suchpulsators up to 49. We also give limits on pulsation amplitudes for 285objects with no obvious periodic variations, summarise the results ofthe survey, and provide improved physical parameters on the compositepulsators for which only preliminary estimates were published earlier.Full Table 5 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/513/A6
| Beyond the iron group: Heavy metals in hot subdwarfs
We report the discovery of strong photospheric resonance lines of GaIII, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdBand sdOB stars at temperatures ranging from 22 000 K to 40 000 K. Linesof other heavy elements are also detected, however in these cases moreatomic data are needed. Based on these discoveries, we present ahypothesis to explain the apparent lack of silicon in sdB stars hotterthan 32 000 K. The existence of triply ionised Ge, Sn, and Pbsuggests that rather than silicon sinking deep into the photosphere, itis removed from the star in a fractionated stellar wind. This hypothesisprovides a challenge to diffusion models of sdB stars.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635 and #5319.
| Studying the populations of our Galaxy using the kinematics of sdB stars
We have analysed the kinematics of a sample of 114 hot subdwarf stars.For 2/3 of the stars, new proper motions, spectroscopic and photometricdata are presented. The vast majority of the stars show a kinematicbehaviour that is similar to that of Thick Disk stars. Some stars havevelocities rather fitting to solar, i.e. Thin Disk, kinematics. About˜15 objects have orbital velocities which differ considerably fromthose of Disk stars. These are members of the Galactic Halo. Weinvestigated the velocity dispersions and calculated the orbits. Moststars feature orbits with disk character (eccentricity of less than0.5), a few reach far above the Galactic plane and have very eccentricorbits (eccentricity of more than 0.7). The intermediate eccentricityrange is poorly populated. This seems to indicate that the (Thick) Diskand the Halo are kinematically disjunct. Plotting a histogram of theorbit data points along z leads to the z-distance probabilitydistribution of the star; doing this for the whole sample leads to thez-distance probability distribution of the sample. The logarithmichistogram shows two slopes, each representing the scale height of apopulation. The disk component has a scale height of 0.9 (±0.1)kpc, which is consistent with earlier results and is similar to that ofthe Thick Disk. The other slope represents a component with a scaleheight ˜7 kpc, a much flatter gradient than for the diskcomponent. This shows that the vast majority of the sdBs are disk stars,but a Halo minority is present, too. The kinematic history andpopulation membership of the sdB stars on the whole is different fromthat of the cooler HBA stars, which are predominantly or evenexclusively Halo objects. This leads to the question, whether the HalosdB stars are of similar origin as the HBA stars, or whether theirkinematical behaviour possibly represents another origin, such asinfalling stellar aggregates or inner disk events.Based on data obtained at the 1.54 m telescope of the European SouthernObservatory, runs 58.D-0478, 60.D-0306, 62.L-0220, 64.L-0077,64.H-0023(A), 65.H-0250, 65.H-0341(A) and 68.D-0192(A).
| A Survey for Pulsating Hot B Subdwarfs in the Northern Hemisphere
We present the final results of a survey to search for pulsating hot Bsubdwarf (sdB) stars. We picked our candidates on the basis of theirvalues of Teff and logg, as estimated from availablequantitative optical spectroscopy. With the help of a three-channelphotometer, we gathered ``white-light'' light curves using a uniformsampling time of 10 s for a total of 74 stars. Our survey had a yield of~5.4%, leading to the discovery of detectable luminosity variations infour objects: PG 1047+003, KPD 2109+4401, KPD 1930+2752, and PG0014+067. For the light curves of the other 70 stars, we derive upperlimits on the amplitudes of possible periodic signals (in the periodwindow 20-1000 s) that range from ~0.05% of the mean brightness of thestar at an apparent magnitude of y~12 to ~0.20% at y~15. We use oursurvey results as well as those of others to discuss the location andextent of the empirical instability ``strip'' for pulsating sdB stars inthe logg-Teff diagram. We also address the question of thecoexistence of pulsators and nonvariable stars in the same region ofthat diagram. Based in part on observations gathered at theCanada-France-Hawaii Telescope, operated by the National ResearchCouncil of Canada, the Centre National de la Recherche Scientifique deFrance, and by the University of Hawaii.
| Metal abundances of sdB stars
The surface abundance patterns of 13 sdB stars have been determined fromhigh resolution, high S/N, optical spectra. As typical in early B typestars, the metal lines are few and very weak. The metal abundancepatterns are compared with the atmospheric parameters to search forpossible trends, which are predicted by diffusion theory. However, it isremarkable that some metals (e.g. N, Mg, Fe) have similar abundancesirrespective of the stellar parameters, whereas other metals (e.g. C, O)and the helium abundances vary considerably from star to star. Threepeculiar sdB stars were found which show an enormous enrichment of irongroup elements. An isotopic anomaly of helium has been found in two sdBstars. In these stars 4He is largely replaced by the isotope3He. We discovered additionally four single lined, shortperiod radial velocity variable sdB stars and measured their radialvelocity curves.
| Atmospheric parameters of field subdwarf B stars
High signal-to-noise ratio optical spectrophotometry of a sample offield subluminous B stars drawn largely from the Palomar Greenultraviolet-excess survey is analyzed with a new grid of modelatmospheres and synthetic spectra. The stellar effective temperatures,surface gravities, and photospheric helium abundances are determinedsimultaneously from a detailed analysis of hydrogen and heliumabsorption line profiles. The derived temperatures and gravities placethe subluminous B stars in the theoroetical Hertzsprung-Russell (H-R)diagram along and bounded below by theoretical sequences of the zero-ageextended horizontal branch, lending strong support to the hypothesisthat these stars are composed of helium-burning cores of approximately0.5 solar mass overlain by very thin layers of hydrogen (approximatelyless than 0.02 solar mass). Various scenarios for their pastevolutionary history are examined in the context of their probablefuture evolution into white dwarfs of lower than average mass.
| A catalogue of spectroscopically identified hot subdwarf stars.
Not Available
| Spectral Analysis of Hot Subdwarfs (Fieldversus Globular Cluster Population)
Not Available
| Studies of hot B subdwarfs. III - Carbon, nitrogen, and silicon abundances in three sdB stars
Optical and high-dispersion IUE observations of three hot B subdwarfs(UV 1758 + 36, Ton S-227, and Feige 65) are presented. These data areanalyzed with model atmosphere techniques, and element abundances for C,N, and Si are derived. The abundances are either near (N) or below (C,Si) the solar value; large variations (1) in the extent ofunderabundances of carbon and silicon among the objects, as well as (2)in the abundances (with respect to the solar values) characterizing eachstar are observed. A preliminary interpretation of the observedvariations in these and other hot subdwarfs in terms of radiative forcesdisrupting the downward diffusion of heavy elements is presented.
| Studies of hot B subdwarfs. II - Energy distributions of three bright sdB/sdOB stars in the 950-5500 A range
Voyager ultraviolet spectrometer observations of the subdwarf B or OBstars HD 205805, UV 1758+36 and Feige 66 are presented. All threeobjects display the H I Layman series in absorption. These observationsare combined with low dispersion IUE spectrophotometry and withStromgren photometry to construct virtually complete energydistributions, which extend over the range 950-5500 angstroms. Effectivetemperatures based on model atmosphere calculations for high gravity,hygrogen rich stars are determined. Our analyses yield Te28,200 + or - 1300 K for HD 205805, Te 31, 800 + or - 1100 Kfor UV 1758+36, and Te 35,700 + or 1500 K for Feige 66. Theimportance of far ultraviolet observations below Lalpha inreducing the uncertainties associated with the interstellar extinctionand the degradation of the IUE sensitivity is emphasized.
| Analysis of high-depression IUE spectra of subdwarf B stars
High-dispersion IUE spectra of the subdwarf B stars UV 1758+36 and TonS-227 are presented. These spectra are characterized by the presence ofa large number of photospheric low- and medium-excitation lines fromnumerous ions. The lines of C, N, and Si are used in conjunction withlocal thermodynamic equilibrium (LTE) metal line calculations to derivepreliminary abundances for these elements. These results are comparedand contrasted with those obtained or the hotter OB and O subdwarfs.
| Spectral Analysis of High-Dispersion IUE Data of Two Hot Subdwarfs B Stars
Not Available
| 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.
| 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.
| UV 1758 + 36, a hot subluminous B star
A new subdwarf B star has been discovered by the ultraviolet sky surveytelescope (S2/68). Its ultraviolet photometry corresponds to an almostunreddened early B star with V = 11.5 mag. The visible spectrum isdominated by strong broad Balmer lines and is characteristic of thesubdwarf B stars. A preliminary analysis of available data yieldsatmospheric parameters: effective temperature of 32,500 K, log (g) =5.25, and a helium/hydrogen abundance ratio of 0.017. The radialvelocity may be variable.
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