Spectropolarimetric Survey of Hydrogen-rich White Dwarf Stars
We have conducted a survey of 61 southern white dwarfs searching formagnetic fields using Zeeman spectropolarimetry. Our objective is toobtain a magnetic field distribution for these objects and, inparticular, to find white dwarfs with weak fields. We found one possiblecandidate (WD 0310-688) that may have a weak magnetic field of-6.1+/-2.2 kG. Next, we determine the fraction and distribution ofmagnetic white dwarfs in the solar neighborhood and investigate theprobability of finding more of these objects based on the currentincidence of magnetism in white dwarfs within 20 pc of the Sun. We havealso analyzed the spectra of the white dwarfs to obtain effectivetemperatures and surface gravities.
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Discovery of magnetic fields in hot subdwarfs
We present polarisation measurements of sdB and sdO stars using FORS1 onthe VLT. The observations were made as part of a project to determinewhether magnetic fields in two super-metal-rich stars can explain theirextreme abundance peculiarities. Field strengths of up to ~1.5 kG rangehave been measured at varying levels of significance in each of our sixtargets, however no clear evidence was found between apparently normalsubdwarfs and the metal-rich objects. The origin of the magnetic fieldsis unknown. We also discuss the implications of our measurements formagnetic flux conservation in late stages of stellar evolution.
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Rotation Velocities of Red and Blue Field Horizontal-Branch Stars
We present measurements of the projected stellar rotation velocities(vsini) of a sample of 45 candidate field horizontal-branch (HB) starsspanning a wide range of effective temperatures, from red HB stars withTeff~=5000K to blue HB stars with Teff of 17,000K.Among the cooler blue HB stars (Teff=7500-11500 K), weconfirm prior studies showing that, although a majority of stars rotateat vsini<15kms-1, there exists a subset of ``fastrotators'' with vsini as high as 30-35 km s-1. All but one ofthe red HB stars in our sample have vsini<10kms-1, and noanalogous rotation bimodality is evident. We also identify anarrow-lined hot star (Teff~=16,000K) with enhancedphotospheric metal abundances and helium depletion, similar to theabundance patterns found among hot BHB stars in globular clusters, andfour other stars that may also belong in this category. We discussdetails of the spectral line fitting procedure that we use to deducevsini and explore how measurements of field HB star rotation may shedlight on the issue of HB star rotation in globular clusters.
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Five Peculiar Evolved B-stars
We present the results of quantitative spectral analyses of fiveapparently normal B stars. These objects seem to be highly evolvedlow-mass stars. Abundance anomalies (He, C, N enrichment), slow rotationand/or high gravities suggest that these programme stars are evolvedlow-mass B stars. In order to trace their evolutionary status severalscenarios are discussed. Post-AGB evolution can be ruled out. PG0229+064 and PG 1400+389 could be horizontal branch (HB) stars, while HD76431, SB 939 and PG 0823+499 already evolved away from the extreme HB(EHB). The low helium abundance of HD 76431 is consistent with post-EHBevolution. The observed abundance anomalies of the other four stars,however, are difficult to reconcile with this scenario.
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Early type stars at high galactic latitudes. II. Four evolved B-type stars of unusual chemical composition
We present the result of differential spectral analyses of a furtherfour apparently normal B-type stars. Abundance anomalies (e.g. He, C, Nenrichment), slow rotation and/or high gravities suggest that theprogramme stars are evolved low-mass B-type stars. In order to tracetheir evolutionary status several scenarios are discussed. Post-AGBevolution can be ruled out. PG 0229+064 and PG 1400+389 could behorizontal branch (HB) stars, while HD 76431 and SB 939 have alreadyevolved away from the extreme HB (EHB). The low helium abundance of HD76431 is consistent with post-EHB evolution. The enrichment in helium,carbon and nitrogen can be explained either by deep mixing of nuclearlyprocessed material to the surface or by diffusion processes modified bymagnetic fields and/or stellar winds. A kinematic study of theirgalactic orbits indicates that the stars belong to an old diskpopulation. Based on observations obtained at the W. M. KeckObservatory, which is operated by the Californian Association forResearch in Astronomy for the California Institute of Technology and theUniversity of California. Based on observations collected at theGerman-Spanish Astronomical Center (DSAZ), Calar Alto, operated by theMax-Planck-Institut für Astronomie Heidelberg jointly with theSpanish National Commission for Astronomy. Based on observationscollected at the European Southern Observatory (ESO proposal No.65.H-0341(A)).
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Strömgren uvby photometry of B-type stars from the Palomar-Green Survey
We present Strömgren uvby photometry for a sample of 31 highGalactic latitude stars selected from the Palomar-Green Survey. The datainclude photometric magnitudes accurate to <= 0.01 mag in most cases,plus colours and the reddening free [c_1] and [u-b] indices, whichpossess a precision of better than 0.02 and 0.04 mag, respectively. Thelatter should be suitable for the reliable determination of stellarphotometric temperatures.
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A search for magnetic stars in late stages of stellar evolution
No abstract submitted
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The Edinburgh-Cape Blue Object Survey - I. Description of the survey
The Edinburgh-Cape Blue Object Survey is a major survey to discover bluestellar objects brighter than B~18 in the southern sky. It is planned tocover an area of sky of 10000 deg^2 with --b-->30 deg and delta<0deg. The blue stellar objects are selected by automatic techniques fromU and B pairs of UK Schmidt Telescope plates scanned with the COSMOSmeasuring machine. Follow-up photometry and spectroscopy are beingobtained with the SAAO telescopes to classify objects brighter thanB=16.5. This paper describes the survey, the techniques used to extractthe blue stellar objects, the photometric methods and accuracy, thespectroscopic classification, and the limits and completeness of thesurvey.
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A Large-Scale Spectroscopic Survey of Early-Type Stars at High Galactic Latitudes
We present new model atmosphere analyses of optical spectroscopy of alarge sample of B-type stellar candidates. Of a total of 298 objects,the largest sample of its kind to date, 205 were drawn from the PalomarGreen Survey of high Galactic latitude ultraviolet-excess stellarobjects and comprise a complete magnitude-limited sample. Effectivetemperatures, surface gravities, and helium abundances for the hotsubdwarf (high-gravity) component of the sample are derived from adetailed line profile analysis of the hydrogen and helium absorptionlines in intermediate-resolution (3-5 A FWHM) optical spectra. Aseparate analysis of the lower gravity component is made using a newlycalculated grid of synthetic spectra. Additional estimates of theeffective temperatures are made from wide- and intermediate-bandphotometry taken from the literature. We are currently undertaking twofollow-up programs. (1) Detailed abundance analyses of high-resolutionechelle spectra of the lower gravity component of the survey usingmodern model atmosphere and synthetic spectrum techniques willdifferentiate between massive Population I main-sequence B stars andlow-mass, lower luminosity Population II blue horizontal branch starsand post-asymptotic giant branch stars. (2) The derived atmosphericparameters for the higher gravity component, the field extendedhorizontal branch stars, will be combined with radial velocitymeasurements to determine their spatial and kinematic distributions,which will distinguish between competing evolutionary scenarios for thishot, evolved stellar population.
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A search for stars with strong magnetic fields among horizontal-branch stars and hot subdwarfs
Observations obtained with the 6-m BTA telescope for a few A, Bhorizontal-branch stars and hot subdwarfs have been used to measuretheir effective magnetic fields. No magnetic fields similar to those ofAp, Bp main-sequence stars were found.
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A search for stars with strong magnetic fields among horizontal-branch stars and hot subdwarfs
Not Available
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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_.
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Broad-band photometry of selected southern ultraviolet-bright 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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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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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.
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Optical observations of ultraviolet objects. I - Spectral classification of 103 stars /l = 200-275 deg/
Results are presented of a program of spectral classification of 103stars originally selected as ultraviolet objects from TD-1 satellitephotometry with the S2/68 experiment. Most of the objects appear to bespectroscopically normal stars; the method of selection yielded a sampleof relatively unreddened B stars at distances up to about 2 kpc. Thisresult is compared with recent studies of the spatial distribution ofinterstellar extinction in the same regions of the sky.
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A sample of new hot subluminous stars taken from the list of ultraviolet objects detected by the S2/68 Sky Survey Experiment
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1980A&A....85..367B&db_key=AST
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NEWS@SKY (Science&Space News)
