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New γ Cassiopeiae-like objects: X-ray and optical observations of SAO 49725 and HD 161103
A growing number of early Be stars exhibit X-ray luminosities that areintermediate between those typical of early type stars and those emittedby most Be/X-ray binaries in quiescence. We report on XMM-Newtonobservations of two such Be stars, SAO 49725 andHD 161103, which were originally discovered in asystematic cross-correlation between the ROSAT all-sky survey andSIMBAD. The new observations confirm the X-ray luminosity excess(LX ~ 1032-33 erg s-1) and the hardnessof their X-ray spectra, which are both unusual for normal early typestars. An iron Kα complex is clearly detected in HD161103 in which the H-like, He-like, and fluorescentcomponents are resolved, while strong evidences also exist for thepresence of similar features in SAO 49725. X-rayspectra can be equally well-fitted by a thermal plasma (mekal) with T ~108 K and solar abundances or by a power law + iron linemodel with photon index ~1.5-1.8, both with a soft thermal componentwith T ~ 107 K. The intensity of the fluorescence 6.4 keVline reflects the presence of large amounts of cold material close tothe X-ray sources and strongly argues against accretion onto a companionneutron star in a large orbit. On the other hand, the probable thermalorigin of the X-ray emission, as supported by the ionised iron lines,disagrees with those observed in all known Be/X-ray binaries, in which anon-thermal component is always required. Remarkably, the X-ray featuresare similar to those of white dwarves in several cataclysmic variables.There is no evidence of high frequency pulsations in neither of the twosystems. However, a large oscillation in the light curve of HD161103 with P ~ 3200 s is readily detected. The X-ray lightcurve of SAO 49725 exhibits clear variability by ~80%on time scales as short as ~1000 s. New optical observations provideupdated spectral types (B0.5 III-Ve) and disclose a dense, large, andapparently stable circumstellar disc for both stars. The nature of theexcess X-ray emission is discussed in light of the models proposed forγ Cas, magnetic disc-star interaction, oraccretion onto a compact companion object - whether neutron star orwhite dwarf. These two new objects, added to similar cases discovered inXMM-Newton surveys, point to the emergence of a new class ofγ Cas analogs.
| 3D mapping of the dense interstellar gas around the Local Bubble
We present intermediate results from a long-term program of mapping theneutral absorption characteristics of the local interstellar medium,motivated by the availability of accurate and consistent parallaxes fromthe Hipparcos satellite. Equivalent widths of the interstellar NaID-line doublet at 5890 Å are presented for the lines-of-sighttowards some 311 new target stars lying within ~ 350 pc of the Sun.Using these data, together with NaI absorption measurements towards afurther ~ 240 nearby targets published in the literature (for many ofthem, in the directions of molecular clouds), and the ~ 450lines-of-sight already presented by (Sfeir et al. \cite{sfeir99}), weshow 3D absorption maps of the local distribution of neutral gas towards1005 sight-lines with Hipparcos distances as viewed from a variety ofdifferent galactic projections.The data are synthesized by means of two complementary methods, (i) bymapping of iso-equivalent width contours, and (ii) by densitydistribution calculation from the inversion of column-densities, amethod devised by Vergely et al. (\cite{vergely01}). Our present dataconfirms the view that the local cavity is deficient in cold and neutralinterstellar gas. The closest dense and cold gas ``wall'', in the firstquadrant, is at ~ 55-60 pc. There are a few isolated clouds at closerdistance, if the detected absorption is not produced by circumstellarmaterial.The maps reveal narrow or wide ``interstellar tunnels'' which connectthe Local Bubble to surrounding cavities, as predicted by the model ofCox & Smith (1974). In particular, one of these tunnels, defined bystars at 300 to 600 pc from the Sun showing negligible sodiumabsorption, connects the well known CMa void (Gry et al. \cite{gry85}),which is part of the Local Bubble, with the supershell GSH 238+00+09(Heiles \cite{heiles98}). High latitude lines-of-sight with the smallestabsorption are found in two ``chimneys'', whose directions areperpendicular to the Gould belt plane. The maps show that the LocalBubble is ``squeezed'' by surrounding shells in a complicated patternand suggest that its pressure is smaller than in those expandingregions.We discuss the locations of several HI and molecular clouds. Usingcomparisons between NaI and HI or CO velocities, in some cases we areable to improve the constraints on their distances. According to thevelocity criteria, MBM 33-37, MBM 16-18, UT 3-7, and MBM 54-55 arecloser than ~ 100 pc, and MBM 40 is closer than 80 pc. Dense HI cloudsare seen at less than 90 pc and 85 pc in the directions of the MBM 12and MBM 41-43 clouds respectively, but the molecular clouds themselvesmay be far beyond. The above closest molecular clouds are located at theneutral boundary of the Bubble. Only one translucent cloud, G192-67, isclearly embedded within the LB and well isolated.These maps of the distribution of local neutral interstellar NaI gas arealso briefly compared with the distribution of both interstellar dustand neutral HI gas within 300 pc.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/411/447
| WIYN Open Cluster Study. VII. NGC 2451A and the Hipparcos Distance Scale
We provide new evidence that NGC 2451A is undoubtedly a young opencluster, although sparsely populated. New cluster membership has beenderived from relative proper motions of 5868 stars. In total, 136 starsdown to V~15 have membership probability Pμ>=2%. NewCCD BV photometry indicates that about 70 stars are indeed main-sequencestars of NGC 2451A. This is also supported by our measurements of radialvelocities. A total of 34 very likely cluster members yield a meanheliocentric radial velocity for NGC 2451A equal to +22.9 kms-1. The high quality of our BV photometry, a confirmation ofcluster membership from proper motions and radial velocities, and arecently obtained metallicity estimate for several cluster stars allowus to perform a precise isochrone fit. The Yale isochrones, updated bythe latest available input physics, have been fitted to the cluster'scolor-magnitude diagram, yielding a distance modulusV0-MV in the range 6.35 to 6.38, which is inexcellent agreement with the distance modulus (m-M)0=6.38derived from the Hipparcos data recently by van Leeuwen and Robichon etal. For NGC 2451A the isochrone fit yields an age of 60+/-20 Myr; hence,the cluster appears to be somewhat younger than the Pleiades. We alsopresent alternative evidence suggesting that the cluster could beslightly older than the Pleiades. The most important result of thisstudy is an excellent match between the main-sequence fitting andHipparcos distances to NGC 2451A. If we consider noticeable similaritiesbetween NGC 2451A and Pleiades, then the Hipparcos distance anomaly forPleiades may not require an explanation of astrophysical nature.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Astrophysics in 1999
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
| Extreme Ultraviolet Astronomy
Astronomical studies in the extreme ultraviolet (EUV) band of thespectrum were dismissed during the early years of space astronomy asimpossible, primarily because of the mistaken view that radiation inthis band would be absorbed by the interstellar medium. Observations inthe 1980s from sounding rockets and limited duration orbital spacecraftbegan to show the potential of this field and led to the deployment oftwo spacecraft devoted to EUV astronomy: the UK Wide Field Camera andthe Extreme Ultraviolet Explorer. The instrumentation in these missions,although quite limited in comparison with instrumentation in otherfields of space astronomy, provided unique and far-reaching results.These included new information on solar system topics, stellarchromospheres and corona, white dwarf astrophysics, cataclysmicvariables, the interstellar medium, galaxies, and clusters of galaxies.We summarize these findings herein.
| What Is the Nature of the Spectroscopic Companion of the Early B Star λ Scorpii?
The B star λ Sco is known to be a spectroscopic binary system.The companion, which is in a short periodic orbit with the B star, is sofar unknown. X-ray observations with ROSAT by Berghöfer et al. in1997 have shown a super-soft X-ray excess, which is unusual for B starsof spectral type B1.5 IV. Here we present an analysis of our longExtreme Ultraviolet Explorer (EUVE) observation of λ Sco. Basedon these data and all available X-ray observations of this star, weconstrain the physical parameters of the companion. As long as no otherexplanation is available for the EUV/soft X-ray excess, thespectroscopic companion of λ Sco is most likely an ultramassivewhite dwarf. The primary B star is thus the most massive star known tohave a white dwarf companion. Such a stellar system can have evolvedonly by mass transfer. Stellar evolution scenarios predict the existenceof such binary systems, which are expected to be precursors of theultrasoft X-ray sources and which finally explode in a supernova TypeIa. The EUV light curve of λ Sco shows significant short-termvariations on a 20% level. A period-folding search carried out tofurther investigate the EUV light curve of λ Sco does not provideclear evidence for any periodicity present in the data. It is worthwhileto mention that the analysis of variance periodogram shows a 2 σfeature at 4.7 cycles days-1, which is close to the mainpulsation frequency of the β Cep-type B star. Further observationshave to confirm the existence of such a periodicity in the EUV lightcurve. Furthermore, when folded with the orbital period, the EUV lightcurve of λ Sco shows two broader dips of 30% intensity loss atphases φ=0.56 and 0.11. At these two phases, the stars pass eachother in the line of sight. We discuss these features in the EUV lightcurve of λ Sco in terms of orbital dependent changes in theabsorption column of the primary's wind along the line of sight towardthe white dwarf companion.
| The Three-dimensional Structure of the Warm Local Interstellar Medium. I. Methodology
In this first in a series of papers, we develop a methodology forconstructing three-dimensional models of the local interstellar cloud(LIC) and adjacent warm clouds in the local interstellar medium (LISM).Our models are based on the column density of neutral hydrogen gas(NHI) inferred primarily from measurements of the deuteriumcolumn density toward nearby stars obtained from the analysis of HubbleSpace Telescope ultraviolet spectra. We also use values ofNHI inferred from spectra of hot white dwarfs and B-typestars obtained by the Extreme Ultraviolet Explorer satellite. These verydifferent methods give consistent results for the three white dwarfstars in common. We assume that along each line of sight allinterstellar gas moving with a speed consistent with the LIC velocityvector has a constant density, NHI=0.10 cm-3, andextends from the heliosphere to an edge determined by the value ofNHI moving at this speed. A number of stars have velocitiesand/or depletions that indicate absorption by other warm clouds in theirlines of sight. On this basis α Cen A and B and probably alsoɛ Ind lie inside the Galactic center (G) cloud, HZ 43 and 31 Comlie inside what we call the north Galactic pole cloud, and β Cet islocated inside what we call the south Galactic pole cloud. We show thelocations of these clouds in Galactic coordinates. The Sun is locatedvery close to the edge of the LIC toward the Galactic center and thenorth Galactic pole. The absence of Mg II absorption at the LIC velocitytoward α Cen indicates that the distance to the edge of the LIC inthis direction is <=0.05 pc and the Sun should leave the LIC and perhapsenter the G cloud in less than 3000 yr. Comparison of LIC and totalvalues of NHI toward pairs of stars with separations between0.9d and 20 deg reveals a pattern of good agreement so long as bothstars lie within 60 pc of the Sun. Thus the LIC and perhaps also othernearby warm clouds have shapes that are smooth on these angular scales.In our second paper we will therefore fit the shape of the LIC with aset of smooth basis functions (spherical harmonics).
| Spectroscopic confirmation of a white dwarf companion to the B star 16 Dra
Using an Extreme Ultraviolet Explorer (EUVE) spectrum, we confirm theidentification of a white dwarf companion to the B9.5V star 16 Dra(HD150100), and constrain its surface temperature to lie between 29,000K and 35,000 K. This is the third B star + white dwarf non-interactingSirius-type binary to be confirmed, after y Pup (HR2875, HD59635) andtheta Hya (HR3665, HD79469). 16 Dra and its white dwarf companion aremembers of a larger resolved proper motion system including the B9V star17 Dra A (HD150117). The white dwarf must have evolved from a progenitormore massive than this star, M_MS~3.7M_sun. White dwarf companions to Bstars are important since they set an observational limit on the maximummass for white dwarf progenitors, and can potentially be used toinvestigate the high mass ends of the initial-final mass relation andthe white dwarf mass-radius relation.
| Orbital elements of the double B star HR 2875
Echelle spectroscopy has been obtained at Lick Observatory and at MountStromlo Observatory which reveals that the B star plus white dwarfsystem HR 2875 is actually a triple system comprised of the B3.5primary, a B6 secondary and the white dwarf. Orbital parameters for thetwo B stars have been determined and indicate a period P=15.0811 daysand an eccentricity e=0.68. The location of the white dwarf relative tothe pair of B stars is unknown but may be sufficiently close to perturbtheir orbit. Further studies of the triple system are planned.
| Astrophysics in 1998
From Alpha (Orionis and the parameter in mixing-length theory) to Omega(Centauri and the density of the universe), the Greeks had a letter forit. In between, we look at the Sun and planets, some very distantgalaxies and nearby stars, neutrinos, gamma rays, and some of theanomalies that arise in a very large universe being studied by roughlyone astronomer per 10^7 Galactic stars.
| Optical spectroscopy of the candidate luminous white dwarf in the young Large Magellanic Cloud cluster NGC 1818
An optical spectrum of the Elson et al. candidate luminous white dwarfin the young LMC cluster NGC 1818 shows conclusively that it is not adegenerate star. A model atmosphere fit gives Teff~31500K andlogg~4.4, typical of a normal main-sequence B star. However, if it is atrue LMC member then the star is underluminous by almost 3mag. Itsposition in the cluster colour-magnitude diagram also rules out thepossibility that this is an ordinary B star. The luminosity is, however,consistent with a ~0.5-Msolar post-asymptotic giant branch orpost-extended horizontal branch object, although if it has evolved viasingle-star evolution from a high-mass (7.6-9.0Msolar)progenitor then we might expect it to have a much higher mass,~0.9Msolar. Alternatively, it may have evolved in a closebinary. In this case the object offers no implications for the maximummass for white dwarf progenitors, or the initial-final mass relation.Finally, we suggest that it could in fact be an evolved member of theLMC disc, and merely projected by chance on to NGC 1818.Spectroscopically, though, we cannot distinguish between theseevolutionary states without higher resolution (echelle) data.
| Theta Hya: spectroscopic identification of a second B star+white dwarf binary
We report the identification, in an Extreme Ultraviolet Explorer (EUVE)spectrum, of a hot white dwarf companion to the 3rd magnitude late-Bstar theta Hya (HR3665, HD79469). This is the second B star+white dwarfbinary to be conclusively identified; Vennes et al. (1997), and Burleigh& Barstow (1998) had previously reported the spectroscopic discoveryof a hot white dwarf companion to the B5V star y Pup (HR2875). Sincethese two degenerate stars must have evolved from main sequenceprogenitors more massive than their B star companions, they can be usedto place observational lower limits on the maximum mass for white dwarfprogenitors, and to investigate the upper end of the initial-final massrelation. Assuming a pure hydrogen composition, we constrain thetemperature of the white dwarf companion to theta Hya to lie between25,000K and 31,000K. We also predict that a third bright B star, 16 Dra(B9.5V), might also be hiding an unresolved hot white dwarf companion.
| A search for hidden white dwarfs in theROSATEUV survey - II. Discovery of a distant DA+F6/7V binary system in a direction of low-density neutral hydrogen
The ROSAT Wide Field Camera (WFC) survey of the extreme ultraviolet(EUV) has provided us with evidence for the existence of a previouslyunidentified sample of hot white dwarfs in unresolved, detached binarysystems. These stars are invisible at optical wavelengths due to theclose proximity of their much more luminous companions (spectral type Kor earlier). However, for companions of spectral type ~A5 or later thewhite dwarfs are easily visible at far-ultraviolet wavelengths, and canbe identified in spectra taken by IUE. 16 such systems have beendiscovered in this way through ROSATEUVEIUE observations, including fouridentified by us in Paper I. In the present paper we report the resultsof our continuing search during the final year of IUE operations. Onenew system, RE J0500-364 (DA+F6/7V), has been identified. This starappears to lie at a distance of ~500-1000 pc, making it one of the mostdistant white dwarfs, if not the most distant, to be detected in the EUVsurveys. The very low line-of-sight neutral hydrogen volume density tothis object could place a lower limit on the length of the beta CMainterstellar tunnel of diffuse gas, which stretches away from the LocalBubble in a similar direction to RE J0500-364. In this paper we alsoanalyse a number of the stars observed where no white dwarf companionwas found. Some of these objects show evidence for chromospheric andcoronal activity. Finally, we present an analysis of the previouslyknown WD+active F6V binary HD 27483 (Bohm-Vitense 1993), and show that,at T~22000K, the white dwarf may be contributing significantly to theobserved EUV flux. If so, it is one of the coolest such stars to bedetected in the EUV surveys.
| HR 2875 - Spectroscopic discovery of the first B star + white dwarf binary
We report the discovery, in an Extreme Ultraviolet Explorer (EUVE)shortwavelength spectrum, of an unresolved hot white dwarf companion tothe 5th magnitude B5Vp star HR 2875. This is the first time that anoninteracting white dwarf + B star binary has been discovered;previously, the earliest type of star known with a white dwarf companionwas Sirius (A1V). As the white dwarf must have evolved from amain-sequence progenitor with a mass greater than that of a B5V star(6.0 solar masses or greater), this places a lower limit on the maximummass for white dwarf progenitors, with important implications for ourknowledge of the initial-final mass relation. Assuming a pure-hydrogenatmospheric composition, we constrain the temperature of the white dwarfto be between 39,000 and 49,000 K. We also argue that this degeneratestar is likely to have a mass significantly greater than the mean massfor white dwarf stars (0.55 solar mass). Finally, we suggest that otherbright B stars (e.g., Theta Hya) detected in the extreme ultravioletsurveys of the ROSAT Wide Field Camera and EUVE may also be hiding hotwhite dwarf companions.
| Hot White Dwarfs in the Extreme-Ultraviolet Explorer Survey. IV. DA White Dwarfs with Bright Companions
We present an analysis of optical, ultraviolet, and X-ray spectralproperties of a sample of 13 hot hydrogen-rich (DA) white dwarfs, eachpaired with a luminous unresolved companion. Using low-dispersionInternational Ultraviolet Explorer spectra, ROSAT photometry, andExtreme-Ultraviolet Explorer photometry and spectroscopy, we estimatethe effective temperature, mass, and distance of the white dwarfs.Additionally, we examine the question of their atmospheric composition.We establish orbital properties for most binaries by means ofhigh-dispersion optical spectroscopy obtained with the Hamilton echellespectrograph at Lick Observatory; the same data help uncover evidence ofactivity in some of the secondary stars that is also notable in ROSATX-ray measurements. In particular, we find high-amplitude (>20 kms-1) velocity variations in only two stars (HD 33959C and HR 8210),low-amplitude variations in four additional objects (HD 18131, HR 1608,theta Hya, and BD +27 deg1888), and no variations (<2 km s-1) in theremainder. We have observed Ca H and K in emission in four (BD +08deg102, HD 18131, HR 1608, and EUVE J0702+129) of the six objects thatwere also detected in the 0.52-2.01 keV ROSAT PSPC band, while thesource of the hard X-ray emission in HD 33959C remains unknown; otherinvestigators have noted some evidence of activity in the remaining0.52-2.01 keV detection, HD 217411. Properties of the white dwarfs arealso investigated; EUV spectroscopy shows the effect of a low heavyelement abundance in the atmosphere of the white dwarf in HD 33959C andof a high heavy element abundance in HD 223816; measurements of allother objects are apparently consistent with emission from pure-hydrogenatmospheres. However, current data do not constrain well the white dwarfparameters, and, to remedy the situation, we propose to obtainspectroscopy of the complete H Lyman line series.
| An extensive Delta a-photometric survey of southern B and A type bright stars
Photoelectric photometry of 803 southern BS objects in the Deltaa-system as detection tool for magnetic chemically peculiar (=CP2) starshas been carried out and compared to published spectral types. Thestatistical yield of such objects detected by both techniques ispractically the same. We show that there are several factors whichcontaminate the search for these stars, but this contamination is onlyof the order of 10% in both techniques. We find a smooth transition fromnormal to peculiar stars. Our sample exhibits the largest fraction ofCP2 stars at their bluest colour interval, i.e. 10% of all stars in thecolour range -0.19 <= B-V < -0.10 or -0.10 <= b-y < -0.05.No peculiar stars based on the Delta a-criterion were found at bluercolours. Towards the red side the fraction of CP2 stars drops to about3% for positive values of B-V or b-y with red limits roughlycorresponding to normal stars of spectral type A5. The photometricbehaviour of other peculiar stars: Am, HgMn, delta Del, lambda Boo, Heabnormal stars, as well as Be/shell stars and supergiants shows someslight, but definite deviations from normal stars. Spectroscopic andvisual binaries are not distinguished from normal stars in their Delta abehaviour. The results of this work justify larger statistical work(e.g. in open clusters) employing more time-saving photometric methods(CCD). \newpage Based on observations obtained at the European SouthernObservatory, La Silla, Chile. This research has made use of the Simbaddatabase, operated at CDS, Strasbourg, France. Table 2 is only availablein electronic form via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| On the nature of Be/X-ray binaries
It has been suggested that most Be/X-ray binaries are low X-rayluminosity nearby objects, containing white dwarfs (Chevalier &Ilovaisky 1998). We show that existing evidence indicates that all knownBe/X-ray binaries are relatively bright X-ray sources containing neutronstars and that the spectral distribution of this group differsconsiderably from that of isolated Be stars. We suggest that thedifferent X-ray properties of the systems can be explained by the sizesof the orbits of the neutron stars. Systems with close orbits are brighttransients which show no quiescent emission as a consequence ofcentrifugal inhibition of accretion. Systems with wide orbits arepersistent sources and display no large outbursts. Systems withintermediate orbits present a mixture of both behaviours.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| An Optical Atlas of Extreme Ultraviolet Explorer (EUVE) Sources
The Extreme Ultraviolet Explorer (EUVE) has been detecting EUV sourcessince its launch in June 1992. Positions of 540 sources have been madeavailable to the community by the EUVE team. We have extracted 7' X 7'images centered on these 540 EUVE sources from the Space TelescopeScience Institute digitized sky archives. We present these images asmosaic finder charts to aid observers trying to identify EUVE sources,or to characterize known sources. (SECTION: Atlases)
| A search for hidden white dwarfs in the ROSAT extreme ultraviolet survey
The ROSAT Wide Field Camera survey has provided us with evidence for theexistence of a previously unidentified sample of hot white dwarfs (WDs)in non- interacting binary systems, through the detection of extremeultraviolet (EUV) and soft X-ray emission. These stars are hidden atoptical wavelengths because of their close proximity to much moreluminous main- sequence (MS) companions (spectral type K or earlier).However, for companions of spectral type ~A5 or later, the white dwarfsare easily visible at far-UV wavelengths, and can be identified inspectra taken by the International Ultraviolet Explorer (IUE). Eleven WDbinary systems have previously been found in this way from ROSAT,Extreme Ultraviolet Explorer (EUVE) and IUE observations. In this paperwe report the discovery of three more such systems through ourprogrammes in recent episodes of IUE. The new binaries are HD 2133, REJ0357+283 (the existence of which was predicted by Jeffries, Burleigh& Robb in 1996), and BD+27 deg1888. In addition, we haveindependently identified a fourth new WD+MS binary, RE J1027+322, whichhas also been reported in the literature by Genova et al., bringing thetotal number of such systems discovered as a result of the EUV surveysto 15. We also discuss here six stars which were observed as part of theprogramme, but for which no white dwarf companion was found. Four ofthese are coronally active. Finally, we present an analysis of theWD+K0IV binary HD 18131, which includes the ROSAT PSPC X-ray data.
| H(alpha) Emission from Late Type Be Stars
We show here that the H(alpha) flux from late type Be stars can beexplained as emission from an HII regions formed in the gas envelopearound the Be star, by the UV flux emitted by a helium star binarycompanion. We also discuss the observability of the helium starcompanions.
| An All-Sky Catalog of Faint Extreme Ultraviolet Sources
We present a list of 534 objects detected jointly in the ExtremeUltraviolet Explorer (EUVE) 100 Angstroms all-sky survey and in theROSAT X-Ray Telescope 0.25 keV band. The joint selection criterionpermits use of a low count rate threshold in each survey. This lowthreshold is roughly 60% of the threshold used in the previous EUVEall-sky surveys, and 166 of the objects listed here are new EUV sources,appearing in neither the Second EUVE Source Catalog nor the ROSAT WideField Camera Second Catalog. The spatial distribution of this all-skycatalog shows three features: an enhanced concentration of objects inUrsa Major, where the Galactic integrated H I column reaches its globalminimum; an enhanced concentration in the third quadrant of the Galaxy(lII from 180 deg to 270 deg) including the Canis Major tunnel, whereparticularly low H I columns are found to distances beyond 200 pc; and aparticularly low number of faint objects in the direction of the fourthquadrant of the Galaxy, where nearby intervening H I columns areappreciable. Of particular interest is the composition of the 166detections not previously reported in any EUV catalog. We offerpreliminary identifications for 105 of these sources. By far the mostnumerous (81) of the identifications are late-type stars (F, G, K, M),while 18 are other stellar types, only five are white dwarfs (WDs), andnone are extragalactic. The paucity of WDs and extragalactic objects maybe explained by a strong horizon effect wherein interstellar absorptionstrongly limits the effective new-source search volume and, thereby,selectively favors low-luminosity nearby sources over more luminous butdistant objects.
| A White Dwarf Companion to the B5 V Star HR 2875 (y Puppis)
An extreme-ultraviolet source in the EUVE and ROSAT Wide Field Cameracatalogs, also identified with a very soft ROSAT PSPC X-ray source, iscoincident with the B5 V star HR 2875 (y Pup). We present an analysis ofPSPC and EUVE spectra revealing the presence of a hot white dwarfcompanion to HR 2875, which then constitute the first confirmed B starplus white dwarf system. The B star shows variable Si II, He I, and H Iline profiles in high-dispersion spectra, possibly related to its binarynature, but our measurements limit the radial velocity semiamplitude ofthe B star to <=2 km s-1. We speculate that the white dwarf is mostlikely massive, and we propose some observational tests of the model.
| New massive X-ray binary candidates from the ROSAT Galactic Plane Survey. I. Results from a cross-correlation with OB star catalogues.
We report the discovery of several new OB/X-ray accreting binarycandidates. These massive systems were found by cross-correlating inposition SIMBAD OB star catalogues with the part of the ROSAT all-skysurvey located at low galactic latitudes (|b|<=20°) and selectingthe early type stars which apparently displayed the most significantexcess of X-ray emission over the `normal' stellar level. The presentsearch is restricted to stars earlier than B6 and X-ray luminosities>=10^31^erg/cm^2^/s. Follow-up optical and X-ray observations allowedto remove misidentified OB stars and spurious matches with interloperX-ray emitters (mostly active coronae) leaving five very likely newmassive X-ray binaries: the O7 star LS 5039 and the Be stars BSD 24-491, LS 992, LS 1698 and LS I +61 235. This latter source was alreadymentioned in an earlier paper. LS 1698 is the probable opticalcounterpart of the hard X-ray transient 4U 1036-56. These new candidateshave 0.1-2.4keV un-absorbed luminosities >=2x10^33^erg/cm^2^/sindicating an accreting neutron star or black hole. On the average theirsoft X-ray luminosities are comparable to those observed from hard X-raytransients in quiescence or from persistent low luminosity Be/X-raysources. The four Be stars have Balmer emission slightly less intensethan previously known systems showing strong outbursts. This suggeststhat the relative weakness of the circumstellar envelope may explain thelow luminosities to some extent. Two additional X-ray binary candidates,HD 161103 and SAO 49725 require further confirmation of their X-rayexcess. Their lower soft X-ray luminosities (1-5x10^32^erg/cm^2^/s)could qualify them as Be + accreting white dwarf systems. Four other Bstars in the Orion and Canis Major OB associations, HD 38087, HD 38023,HD 36262 and HD 53339 exhibit X-ray flux excesses in the range2-7x10^31^erg/cm^2^/s whose origin is unclear. Finally very soft X-rayemission was detected from HR 2875 suggesting the presence of anon-accreting white dwarf companion to the B5 star.
| The Second Extreme-Ultraviolet Explorer Source Catalog
We present the second catalog of extreme-ultraviolet objects detected bythe Extreme-Ultraviolet Explorer. The data include (1) all-sky surveydetections from the initial 6 month scanner-survey phase, (2) additionalscanner detections made subsequently during specially programmedobservations designed to fill in low-exposure sky areas of the initialsurvey, (3) sources detected with deep-survey-telescope observationsalong the ecliptic, (4) objects detected by the scanner telescopesduring targeted spectroscopy observations, and ( 3) other observations.We adopt an innovative source detection method that separates the usuallikelihood function into two parts: an intensity diagnostic and aprofile diagnostic. These diagnostics allow each candidate detection tobe tested separately for both signal-to-noise ratio and conformance withthe known instrumental point-spread function. We discuss the dependenceof the false-alarm rate and the survey's completeness on the survey'ssensitivity threshold. We provide three lists of the EUV sourcesdetected: the all-sky survey detections, the deep-survey detections, andsources detected during other phases of the mission. Each list givespositions and intensities in each wave band. The total number of objectslisted is 734. For approximately 65% of these we also provide plausibleoptical, UV, radio, and/or X-ray identifications.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| The ROSAT Wide Field Camera all-sky survey of extreme-ultraviolet sources - II. The 2RE Source Catalogue
During 1990-1991 the Wide Field Camera (WFC) on the ROSAT satelliteperformed the first all-sky survey at EUV wavelengths. The survey wasconducted in two `colours' using broad-band filters to define wavebandscovering the ranges 60-140 A and 112-200 A. It was fully imaging, witheffective spatial resolution of about 3 arcmin FWHM, and point sourcelocation accuracy of typically better than 1 arcmin. From an initialanalysis, Pounds et al. published the WFC Bright Source Catalogue (BSC)of 383 sources. In this paper we report results from reprocessing of thecomplete survey database; the resulting list of sources is the `2RE'Catalogue. It contains 479 sources, of which 387 are detected in bothsurvey wavebands, a significant advance on the BSC (80 per cent versus60 per cent). Improvements over the original BSC include: (i) betterrejection of poor aspect periods, and smaller random errors in theaspect reconstruction; (ii) improved background screening; (iii)improved methods for source detection; (iv) inclusion of atime-variability test for each source; (v) more extensive investigationof the survey sensitivity. We define the catalogue selection criteria,and present the catalogue contents in terms of tables and sky maps. Wealso discuss the sky coverage, source number-flux relations, opticalidentifications and source variability.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The first Extreme Ultraviolet Explorer source catalog
The Extreme Ultraviolet Explorer (EUVE) has conducted an all-sky surveyto locate and identify point sources of emission in four extremeultraviolet wavelength bands centered at approximately 100, 200, 400,and 600 A. A companion deep survey of a strip along half the eclipticplane was simultaneously conducted. In this catalog we report thesources found in these surveys using rigorously defined criteriauniformly applied to the data set. These are the first surveys to bemade in the three longer wavelength bands, and a substantial number ofsources were detected in these bands. We present a number of statisticaldiagnostics of the surveys, including their source counts, theirsensitivites, and their positional error distributions. We provide aseparate list of those sources reported in the EUVE Bright Source Listwhich did not meet our criteria for inclusion in our primary list. Wealso provide improved count rate and position estimates for a majorityof these sources based on the improved methodology used in this paper.In total, this catalog lists a total of 410 point sources, of which 372have plausible optical ultraviolet, or X-ray identifications, which arealso listed.
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| Сазвежђа: | Крма |
| Ректацензија: | 07h29m05.70s |
| Deклинација: | -38°48'44.0" |
| Apparent магнитуда: | 5.43 |
| Даљина: | 170.648 parsecs |
| Proper motion RA: | -20.7 |
| Proper motion Dec: | 15.4 |
| B-T magnitude: | 5.204 |
| V-T magnitude: | 5.37 |
Каталог и designations:
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INHABITED SKY
