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Ýlgili Makaleler
Progenitors of type Ia supernovae: Binary stars with white dwarf companions
Type Ia SNe (SNe Ia) are thought to come from carbon oxygen white dwarfsthat accrete mass from binary companions until they approach theChandrasekhar limit, ignite carbon, and undergo complete thermonucleardisruption. A survey of the observed types of binaries that containwhite dwarfs is presented. We propose that certain systems that seemmost promising as SN Ia progenitors should be more intensively observedand modeled, to determine whether the white dwarfs in these systems willbe able to reach the Chandrasekhar limit. In view of the number ofpromising single-degenerate systems and the dearth of promisingdouble-degenerate systems, we suspect that single-degenerates producemost or perhaps all SNe Ia, while double-degenerates produce some orperhaps none.
| Iron in Hot DA White Dwarfs
We present a study of the iron abundance pattern in hot, hydrogen-rich(DA) white dwarfs. The study is based on new and archivalfar-ultraviolet spectroscopy of a sample of white dwarfs in thetemperature range 30,000 K<~Teff<~64,000 K. The spectraobtained with the Far Ultraviolet Spectroscopic Explorer, along withspectra obtained with the Hubble Space Telescope Imaging Spectrographand the International Ultraviolet Explorer, sample Fe III-Fe VIabsorption lines, enabling a detailed iron abundance analysis over awider range of effective temperatures than previously afforded. Themeasurements reveal abundance variations in excess of 2 orders ofmagnitude between the highest and the lowest temperatures probed, butalso show considerable variations (over 1 order of magnitude) betweenobjects with similar temperatures and surface gravities. Such variationsin cooler objects may be imputed to accretion from unseen companions orso-called circumstellar debris, although the effect of residual massloss and selective radiation pressure in the hottest objects in thesample remain dominant.
| An Archive of IUE Low-Dispersion Spectra of the White Dwarf Stars
We have produced an archive of the ultraviolet low-dispersion spectrafor the full set of white dwarf stars observed with the InternationalUltraviolet Explorer (IUE) satellite over the course of its 18 yrmission. This archive contains the spectra of 322 individual degeneratestars which have been processed to optimize the signal-to-noise for eachstar. In particular, all spectra have been corrected for residualtemporal and thermal effects and placed on the Hubble Space TelescopeFaint Object Spectrograph absolute flux scale using procedures describedby Massa & Fitzpatrick. Wherever possible, multiple observations ofindividual stars have been co-added to further enhance signal-to-noiseand have been combined into a single spectrum including the full 1150 to3150 Å wavelength region observed by IUE. The contents of thisspectral archive are described and the details of data reductionprocedures are provided, along with the url for access to the electronicfiles of the processed spectra.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/247
| Resolving Sirius-like binaries with the Hubble Space Telescope
We present initial results from a Hubble Space Telescope ultravioletimaging survey of stars known to have hot white dwarf companions whichare unresolved from the ground. The hot companions, discovered throughtheir EUV or UV emission, are hidden by the overwhelming brightnesses ofthe primary stars at visible wavelengths. Out of 17 targets observed, wehave resolved eight of them with the Wide Field Planetary Camera 2,using various ultraviolet filters. Most of the implied orbital periodsfor the resolved systems are hundreds to thousands of years, but in atleast three cases (56 Persei, ζ Cygni and RE J1925-566) it shouldbe possible to detect the orbital motions within the next few years, andthey may eventually yield new dynamically determined masses for thewhite dwarf components. The 56 Persei and 14 Aurigae systems are foundto be quadruple and quintuple, respectively, including the known opticalcomponents as well as the newly resolved white dwarf companions. Themild barium star ζ Cygni, known to have an 18-year spectroscopicperiod, is marginally resolved. All of these newly resolved Sirius-typebinaries will be useful in determining gravitational redshifts andmasses of the white dwarf components.
| A New Association of Post-T Tauri Stars near the Sun
Observing ROSAT sources in an area 20°×25° centered at thehigh-latitude (b=-59°) active star ER Eri, we found evidences for anearby association, that we call the Horologium association (HorA),formed by at least 10 very young stars, some of them being bona fidepost-T Tauri stars. We suggest other six stars as possible members ofthis proposed association. We examine several requirements thatcharacterize a young stellar association. Although no one of them,isolated, gives an undisputed prove of the existence of the HorA, alltogether practically create a strong evidence for it. In fact, the Liline intensities are between those of the older classical T Tauri starsand the ones of the Local Association stars. The space velocitycomponents of the HorA relative to the Sun (U=-9.5+/-1.0, V=-20.9+/-1.1,W=-2.1+/-1.9) are not far from those of the Local Association, so thatit could be one of its last episodes of star formation. In this regionof the sky there are some hotter and non-X-ray active stars, withsimilar space velocities, that could be the massive members of the HorA,among them, the nearby Be star Achernar. The maximum of the massdistribution function of its probable members is around 0.7-0.9Msolar. We estimate its distance as ~60 pc and its size as~50 pc. If spherical, this size would be larger than the surveyed area,and many other members could have been missed. ER Eri itself was foundto be not a member, but a background RS CVn-like system. We alsoobserved three control regions, two at northern and southern Galacticlatitudes and a third one in the known TW Hya association (TWA), and theproperties and distribution of their young stars strengthen the realityof the HorA. Contrary to the TWA, the only known binaries in the HorAare two very wide systems. The HorA is much more isolated from cloudsand older (~30 Myr) than the TWA and could give some clues about thelifetime of the disks around T Tauri stars. Actually, none of theproposed members is an IRAS source indicating an advanced stage of theevolution of their primitive accreting disks. Based on observations madeunder the Observatório Nacional-ESO agreement for the jointoperation of the 1.52 m ESO telescope and at the Observatório doPico dos Dias, operated by MCT/Laboratório Nacional deAstrofísica, Brazil
| 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).
| 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.
| The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| Evidence for an ionization gradient in the local interstellar medium: EUVE observations of white dwarfs
We present an enlarged and homogeneous set of interstellar hydrogen andhelium column densities obtained through continuum and ionization edgesfitting of nearby hot white dwarfs observed with the Extreme UltravioletExplorer (EUVE). We compare the HI columns towards the targets and theirlocations relative to the soft X-ray cavity contours drawn by Snowden etal. (1998). We then use the inferred ionization degrees of hydrogen andhelium (available towards 17 objects) as a database for a search ofsystematic trends in the local interstellar medium. We find that theionization degree of helium does not show any trend, with a constantvalue around 40%. At variance with helium, hydrogen ionization varieswithin the Local Bubble: there is an ionization increase along thegeneral direction of the Canis Major cavity. These results areconsistent with hydrogen being ionized by the Canis Major hot starsAdara and Mirzam. However, the co-existence of fully neutral hydrogenand strongly ionized helium along some l-o-s remains to be explained.
| A Catalog of Spectroscopically Identified White Dwarfs
A catalog of 2249 white dwarfs which have been identifiedspectroscopically is presented complete through 1996 April. Thiscompilation is the fourth edition of the Villanova Catalog ofSpectroscopically Identified White Dwarfs. For each degenerate star, thefollowing data entries with references are provided: (1) a catalogcoordinate designation or WD number, in order of right ascension; (2)the right ascension and declination for epoch 1950.0; (3) the spectraltype based upon the new system; (4) a catalog symbol denoting binarymembership; (5) a list of most names known to exist for a given star;(6) proper motion and position angle; (7) broadband UBV photometry, V,B-V, U-B (8) multichannel spectrophotometry, v(MC), g-r (9)Strömgren narrowband photometry, y, b-y, u-b (10) an absolutevisual magnitude based upon the best available color-magnitudecalibration or trigonometric parallax; (11) the observed radial velocityuncorrected for gravitational redshift or solar motion; and (12) thetrigonometric parallax with mean error when available. Notes for unusualor peculiar stars and a coded Reference Key alphabetized by the firstauthor's last name are presented, as well as an expanded tablecross-referencing all names to the catalog WD number. An introductionand full descriptions of the entries are provided in the text.
| 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.
| Identification of soft high galactic latitude RASS X-ray sources. I. A complete count-rate limited sample
We present a summary of spectroscopic identifications for a completesample of bright soft high galactic latitude X-ray sources drawn fromthe ROSAT All-Sky Survey which have PSPC count-rates CR > 0.5 {ctss}(-1) and hardness ratios HR1 < 0. Of a total of 397 sources, 270had catalogued counterparts although most of these were not previouslyknown as X-ray sources; of the remaining 127 sources neither X-ray noroptical properties were previously known. Of the whole sample of verysoft X-ray sources 155 were also discovered by the Wide-Field-Camera onboard ROSAT. We present spectroscopic identifications of 108 sources andother identifications for further 18 sources; 1 source remainsunidentified so far. In practically all cases a unique opticalcounterpart exists facilitating identification. The largest sourceclasses are AGN, magnetic cataclysmic variables, and hot white dwarfs.Based in part on observations with the ESO/MPI 2.2m telescope at LaSilla, Chile
| The Extreme Ultraviolet Explorer Stellar Spectral Atlas
We present an atlas of extreme ultraviolet (EUV) spectra of 95 brightstellar sources observed between 1992 July and 1996 June with theExtreme Ultraviolet Explorer (EUVE) spectrometers. These data are takenin the short- (SW; 70-190 Angstroms), medium- (MW; 140-380 Angstroms),and long-wavelength bandpasses (LW; 280-760 Angstroms) at roughly 0.5,1, and 2 Angstroms resolution, respectively. We describe thespectrometers and detail the procedure used to reduce the observationaldata to spectra. The atlas is grouped by the type of source: O-A stars,F-M stars, white dwarfs, and cataclysmic variables. We present a briefoverview of the general nature and EUV spectral distribution of eachgroup and present accompanying notes and individual spectra for eachsource. We show selected F-M sources in more detail with identificationsof the brightest spectral lines illustrating the characteristics of theEUV spectra of stars of various temperatures. The current study is themost complete compilation to date of aggregate spectra of bright EUVstellar sources.
| 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)
| 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.
| Discovery of a White Dwarf Companion (MS0354.6-3650 = EUVE J0356-366) to a G2V Star
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996AJ....112..258C&db_key=AST
| Optical and X-Ray Characteristics of Stars Detected in the Einstein Slew Survey
We detect X-rays for the first time from 63 cool (types AM) stars. Thesestars are part of the 229 total stellar X-ray sources identified to datein the Einstein Slew Survey (hereafter Slew). We also list new X-raydata on one A star that may have a corona, five OB stars, and reportdiscoveries of two new T Tauri stars and two new cataclysmic variables.The stellar content of the Slew high-latitude subset (currently 93%identified for |b_II_| > 20^deg^) is 26%. This agrees well with theEinstein Extended Medium Sensitivity Survey (EMSS) stellar sample, whichonly considered high latitudes. Because of the large solid angle coveredby the Slew, and its shallow limiting flux, the sample will better probethe bright end of the stellar X-ray luminosity function (L_X_ >10^30^ ergs s^-1^). Presently (based on the 221 Slew stars with knownspectral types), the sample is dominated by late-type systems (coolerthan F; 68%). These include dMe's, BY Dra, RS CVn, and FK Comae systems.Based on the limiting magnitudes of catalogs searched to date, the Slewis assessed to be complete for spectral types earlier than K. Hence, Kand M systems will be prominent in the ~50 as yet unidentified stars. Wehave embarked on an extensive program to (1) confirm the X-rayidentifications with the optical counterparts; (2) search for Ca II Hand K and Balmer line emission as activity signatures; and (3) searchfor supporting evidence of magnetic activity by measuring rotationalvelocities and relating them to X-ray luminosity level. Of 64 proposedcoronal systems observed to date, we have confirmed 44 active stars,while in the remaining 20 we have been unable to find definitiveactivity. We have confirmed an additional 19 active stars from stellardatabase searches, and reclassified nine systems as having nonstellaroptical counterparts from optical follow-up identification work andextragalactic database searches. We discuss notable new discoveries.From the sample of single active F7-MS Slew stars with measured ν sini-values, we find a strong (99.9% confidence level) linear correlationof X-ray luminosity with ν sin i and with stellar radius (R).However, L_X_ is uncorrelated with angular rotation speed at the 99%level. For the combined Slew and EMSS single star F7-MS sample, we findthe same 99.9% ν sin i-L_X_ and R-L_X_ correlations. The L_X_-νsin i relation for the combined sample appears to flatten with respectto the quadratic behavior seen for optically selected stellar samples atrotational velocities in excess of ~16 km s^-1^. For the unevolvedsubset of the Slew single star sample, we also find a correlationbetween L_X_ and Rossby number (R_0_; more than 99% confidence). Aleast-squares fit gives L_X_ ~ R_0_^-0.4^, which is similarly flatterthan the quadratic dependence seen in optical samples. Using the stellarsurface X-ray flux F_X_ versus B - V diagram, we interpret these resultsas saturation of the stellar surface by active regions at F_X_/F_bol_ ~10^-3^.
| 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.
| A ROSAT XUV pointed phase source catalogue.
We present a catalogue of XUV sources from observations by the WideField Camera (WFC) on ROSAT during the pointed phase. The ROSAT WFC is atelescope sensitive in the extreme UV range (17-210eV) which observes inparallel with the ROSAT X-Ray Telescope (XRT). The 5916 pointedobservations processed are from the calibration and verification phasein June 1990 and from the period 9 Feb. 1991 to 15 July 1994. Thecatalogue contains 1022 independent source detections which correspondto 328 individual sources, many of which have been observed repeatedly.Each observation was done with one of four filters S_1_, S_2_, P_1_ andP_2_. Of the 328 sources 113 are new sources (they are not listed in the"2RE" catalogue) and 274 have been identified with optical counterparts.The catalogue contains coordinates, observed count rates, normalizedsource count rates and the proposed optical counterpart with itsspectral class. For observations with filters S_1_ and S_2_ an in-flightcalibration has been applied using data of White Dwarfs which wereassumed to have a constant flux and which were observed repeatedly (inpointed observations and/or during the All Sky Survey). In this way, itwas possible to correct for the time dependant degradation of thedetector efficiency and to normalize the count rates to those valid atthe beginning of the mission.
| 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.
| Lithium, X-ray activity and rotation in an X-ray selected sample of solar-type stars
We present an analysis of the connection between X-ray activity level,photospheric abundance of lithium and surface rotation in late typeactive main sequence stars (G and K), using the ratio between opticaland X-ray luminosity fx/fv as an uniform activity indicator. We performthis analysis for a sample of X-ray selected sources from Einstein-basedsurveys compared to stars from the Pleiades open cluster and to a sampleof active binary stars. We show that these parameters show differentdegrees of statistical correlation in the three samples. In particular,the Pleiades sample shows a significant correlation between all threequantities, while in the X-ray selected sample lithium and rotation aresignificantly correlated with each other but neither is correlated withthe activity level. No correlation is evident for the three quantitiesstudied in the active binary sample. We show how the behavior of theX-ray selected sample can be used to discriminate among differenthypothesis about the nature of the so-called `yellow star excess'observed in X-ray flux-limited surveys, showing that this is composed bya population of young, near Zero-Age Main Sequence (ZAMS) stars withcharacteristics similar to the Pleiades.
| Extreme Ultraviolet Spectrophotometry of HD16538 and HR:8210 Ik-Pegasi
The main-sequence stars HR 8210 (1K Peg) and HD 15638 have both beenfound to have DA white dwarf companions by observations with the lUEsatellite, following their discovery as bright EUV sources by the ROSA TWide Field Camera. Accurate estimates of the temperature and surfacegravity of each white dwarf, based on the lUE data, were oniy possiblewhen external constraints such as the distance to the primary wereapplied. We present Extreme Ultraviolet Explorer observations of each ofthese systems. From the resulting spectra we are able to measure thetemperature and surface gravity of each white dwarf with much greateraccuracy than possible with the earlier data. We demonstrate that thewhite dwarf atmospheres have H layers sufficiently thick that theyappear as pure H DAs. There is no direct evidence for the presence ofany additional material in the photo spheres that might have come frominteractions with the primaries. However, an inconsistency between thetemperature of the HD 15638 white dwarf obtained from the EUVE spectrum,with a pure H model atmosphere, and that derived from the UV data mightbe explained by very small traces of heavy-element pollution. Thedistance we estimate for the white dwarf in HR 8210 is consistent withthat to the A star, indicating that the A star has not departed fromsingle-star evolution, nor has it evolved away from the main sequence.Measurements of the interstellar column densities yield He i/H I ratiosconsistent with cosmic abundances. Key words: stars: atmospheres -binaries: general - stars: individual: HD 15638 - stars: individual: HR8210 - white dwarfs - ultraviolet: stars.
| A ROSAT Survey of Hot DA White Dwarfs in Non-Interacting Binary Systems
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.270..499B&db_key=AST
| 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.
| Extreme Ultraviolet Explorer Bright Source List
Initial results from the analysis of the Extreme Ultraviolet Explorer(EUVE) all-sky survey (58-740 A) and deep survey (67-364 A) arepresented through the EUVE Bright Source List (BSL). The BSL contains356 confirmed extreme ultraviolet (EUV) point sources with supportinginformation, including positions, observed EUV count rates, and theidentification of possible optical counterparts. One-hundred twenty-sixsources have been detected longward of 200 A.
| The hot white-dwarf companions of HR 1608, HR 8210, and HD 15638
We have obtained low-dispersion IUE spectra of the late-type stars HD15638 (F3 V), HR 1608 (=63 Eridani, KO IV), and HR 8210 (A8m). Each ofthese stars had been detected as a strong EUV source with the Wide FieldCamera aboard the ROSAT satellite. The short-wavelength IUE spectrum ofeach star reveals the presence of a hot white-dwarf companion. We havefit the Lyman-alpha profile and UV continuum of each white dwarf usingpure hydrogen models. The excellent fit of the data to the modelsprovides confirmation of the Finley and Koester absolute calibration ofthe SWP camera of IUE. The UV data alone are insufficient to constrainthe model gravity, but an additional constraint is provided by thephotometric distance to the late-type primary. The most interesting ofthe three white dwarfs is the companion to HR 8210 for which our resultsimply a mass of 1.15 +0.05/-0.15 solar mass. This result is in goodagreement with the lower limit on the mass derived from thespectroscopic orbit (greater than 1.1 solar mass), provided that theinclination is close to 90 deg.
| An optical Atlas of ROSAT Wide Field Camera EUV sources
The ROSAT Wide Field Camera has been detecting EUV sources since itslaunch in June 1990. A preliminary list of 384 bright sources has beensupplied by the Wide Field Camera team to the EUVE Guest ObserverCenter, and to the astronomical community. We have extracted 5.4 x 5.4arcmin images of all 384 WFC sources from the Space Telescope ScienceInstitute digitized sky archives. These images are presented asmosaicked finder charts for observers trying either to identify WFCsources or to characterize known sources.
| The ROSAT Wide Field Camera all-sky survey of extreme-ultraviolet sources. I - The Bright Source Catalogue
First comprehensive results from an initial processing of the ROSAT WideField Camera all-sky survey for cosmic sources of extreme-ultravioletradiation are presented. The reduction of the survey data has yielded acatalog of 383 relatively bright EUV sources, forming the WFC BrightSource Catalogue. Details of the EUV source positions and count ratesare given, as are optical identifications where known. It is found thatthe log N-log S distributions are unusually flat for the white dwarfstars, but almost Euclidean for the nearby main-sequence late-typestars. The sky distribution of identified white dwarfs is highlynonuniform, suggesting gross variations in the opacity of theinterstellar medium within about 100 pc.
| The Einstein Slew Survey
A catalog of 819 sources detected in the Einstein IPC Slew Survey of theX-ray sky is presented; 313 of the sources were not previously known asX-ray sources. Typical count rates are 0.1 IPC count/s, roughlyequivalent to a flux of 3 x 10 exp -12 ergs/sq cm s. The sources havepositional uncertainties of 1.2 arcmin (90 percent confidence) radius,based on a subset of 452 sources identified with previously knownpointlike X-ray sources (i.e., extent less than 3 arcmin).Identifications based on a number of existing catalogs of X-ray andoptical objects are proposed for 637 of the sources, 78 percent of thesurvey (within a 3-arcmin error radius) including 133 identifications ofnew X-ray sources. A public identification data base for the Slew Surveysources will be maintained at CfA, and contributions to this data baseare invited.
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Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Saat |
| Sað Açýklýk: | 02h28m19.02s |
| Yükselim: | -61°18'19.4" |
| Görünürdeki Parlaklýk: | 8.89 |
| Uzaklýk: | 199.203 parsek |
| özdevim Sað Açýklýk: | -7.8 |
| özdevim Yükselim: | 7.4 |
| B-T magnitude: | 9.354 |
| V-T magnitude: | 8.929 |
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