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A systematic study of variability among OB-stars based on HIPPARCOS photometry
Context: Variability is a key factor for understanding the nature of themost massive stars, the OB stars. Such stars lie closest to the unstableupper limit of star formation. Aims: In terms of statistics, thedata from the HIPPARCOS satellite are unique because of time coverageand uniformity. They are ideal to study variability in this large,uniform sample of OB stars. Methods: We used statisticaltechniques to determine an independant threshold of variabilitycorresponding to our sample of OB stars, and then applied an automaticalgorithm to search for periods in the data of stars that are locatedabove this threshold. We separated the sample stars into 4 maincategories of variability: 3 intrinsic and 1 extrinsic. The intrinsiccategories are: OB main sequence stars (~2/3 of the sample), OBe stars(~10%) and OB Supergiant stars (~1/4).The extrinsic category refers toeclipsing binaries. Results: We classified about 30% of the wholesample as variable, although the fraction depends on magnitude level dueto instrumental limitations. OBe stars tend to be much more variable(≈80%) than the average sample star, while OBMS stars are belowaverage and OBSG stars are average. Types of variables include αCyg, β Cep, slowly pulsating stars and other types from the generalcatalog of variable stars. As for eclipsing binaries, there arerelatively more contact than detached systems among the OBMS and OBestars, and about equal numbers among OBSG stars.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Stellar and wind properties of LMC WC4 stars. A metallicity dependence for Wolf-Rayet mass-loss rates
We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based(2.3 m MSSSO, NTT) spectroscopy to determine the physical parameters ofsix WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellarparameters are revised significantly relative to Gräfener et al.(\cite{Grafener1998}) based on improved observations and moresophisticated model atmosphere codes, which account for line blanketingand clumping. We find that stellar luminosities are revised upwards byup to 0.4 dex, with surface abundances spanning a lower range of 0.1 leC/He le 0.35 (20-45% carbon by mass) and O/He le 0.06 (<=10% oxygenby mass). Relative to Galactic WC5-8 stars at known distance, andanalysed in a similar manner, LMC WC4 stars possess systematicallyhigher stellar luminosities, ~ 0.2 dex lower wind densities, yet asimilar range of surface chemistries. We illustrate how theclassification C III lambda 5696 line is extremely sensitive to winddensity, such that this is the principal difference between the subtypedistribution of LMC and Galactic early-type WC stars. Temperaturedifferences do play a role, but carbon abundance does not affect WCspectral types. We illustrate the effect of varying temperature andmass-loss rate on the WC spectral type for HD 32257 (WC4, LMC) and HD156385 (WC7, Galaxy) which possess similar abundances and luminosities.Using the latest evolutionary models, pre-supernova stellar masses inthe range 11-19 Msun are anticipated for LMC WC4 stars, with7-14 Msun for Galactic WC stars with known distances. Thesevalues are consistent with pre-cursors of bright type-Ic supernovae suchas SN 1998bw (alias GRB 980425) for which a minimum total mass of C andO of 14 Msun has been independently derived. Based onobservations made with the NASA-CNES-CSA Far Ultraviolet SpectroscopicExplorer, and NASA-ESA Hubble Space Telescope. Also based onobservations collected at the European Southern Observatory in program63.H-0683, and at the Australian National University Siding SpringObservatory.
| The 74th Special Name-list of Variable Stars
We present the Name-list introducing GCVS names for 3153 variable starsdiscovered by the Hipparcos mission.
| Supplementary southern standards for UBV(RI)c photometry
We present UBV(RI)c photometry for 80 southern red and blue stars foruse as additional standards. The data are tied to the Johnson UBV andCousins (RI)c systems and extend the range of the available stars forcolor equation determination, especially in (U-B) for blue stars and(V-R) and (V-I) for red stars. Comparisons with published data are madeand particularly good agreement is found with Bessell for the red(Gliese) stars.
| An Einstein Observatory SAO-based catalog of B-type stars
About 4000 X-ray images obtained with the Einstein Observatory are usedto measure the 0.16-4.0 keV emission from 1545 B-type SAO stars fallingin the about 10 percent of the sky surveyed with the IPC. Seventy-fourdetected X-ray sources with B-type stars are identified, and it isestimated that no more than 15 can be misidentified. Upper limits to theX-ray emission of the remaining stars are presented. In addition tosummarizing the X-ray measurements and giving other relevant opticaldata, the present extensive catalog discusses the reduction process andanalyzes selection effects associated with both SAO catalog completenessand IPC target selection procedures. It is concluded that X-rayemission, at the level of Lx not less than 10 exp 30 ergs/s, is quitecommon in B stars of early spectral types (B0-B3), regardless ofluminosity class, but that emission, at the same level, becomes lesscommon, or nonexistent, in later B-type stars.
| The EXOSAT high Galactic latitude survey
This study presents a survey of serendipitous sources performed in thevery soft X-ray band (0.05-2.0 keV) using the Exosat imaging telescopes.It covers 783 sq deg of high Galactic latitude sky and includes 210serendipitous sources which define a complete (flux-limited) sample. Twohundred of the 210 detected sources are identified via extensive opticaland radio observations together with cross-correlations with catalogs ofknown objects. The log N-log S relation was found to be consistent withthat of the Einstein extended Medium-Sensitivity Survey (EMSS) and withthe 'Euclidean' value of 1.5. The normalization of the relation is astrong function of the assumed spectral slope of AGN. It is inferredfrom the consistency with the EMSS results that the average (energy)slope of extragalactic sources in the soft X-ray band is very steep(approximately 1.5). An analysis of the association between AGNdetection and Galactic N(H) also shows that the average slope is steepand inconsistent with the canonical value of 0.7.
| Broad-band photometry of selected southern ultraviolet-bright stars.
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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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