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The Clusters Ages Experiment (CASE). I. V209 ω Cen: An Eclipsing Post-Common-Envelope Binary in the Globular Cluster ω Cen
We use photometric and spectroscopic observations of the detachedeclipsing binary V209 ω Cen to derive the masses, radii, andluminosities of the component stars. The system exhibits total eclipsesand, based on the measured systemic velocity and the derived distance,is a member of the globular cluster ω Cen. We obtain 0.945+/-0.043Msolar, 0.983+/-0.015 Rsolar, and 6.68+/-0.88Lsolar for the cooler but larger and more luminous primarycomponent. The secondary component has 0.144+/-0.008 Msolar,0.425+/-0.008 Rsolar, and 2.26+/-0.28 Lsolar. Theeffective temperatures are estimated at 9370 K for the primary and at10,866 K for the secondary. On the color-magnitude diagram of thecluster, the primary component occupies a position between the tip ofthe blue straggler region and the extended horizontal branch, while thesecondary component is located close to the red border of the areaoccupied by hot subdwarfs. However, its radius is too large and itseffective temperature is too low for it to be an sdB star. We propose ascenario leading to the formation of a system with such unusualproperties, with the primary component ``reborn'' from a former whitedwarf that accreted a new envelope through mass transfer from itscompanion. The secondary star has lost most of its envelope whilestarting its ascent onto the subgiant branch. It failed to ignite heliumin its core and is currently powered by a hydrogen-burning shell.This paper uses data obtained with the 6.5 m Magellan Telescopes locatedat Las Campanas Observatory, Chile.
| The Lowest Mass White Dwarf
Extremely low mass white dwarfs are very rare objects likely formed incompact binary systems. We present MMT optical spectroscopy of 42low-mass white dwarf candidates serendipitously discovered in a surveyfor hypervelocity B-type stars. One of these objects, SDSS J0917+46, hasTeff=11,288+/-72 K and logg=5.48+/-0.03 with an estimatedmass of 0.17 Msolar, it is the lowest gravity/mass whitedwarf currently known. However, 40 of the low-mass candidates are normalDA white dwarfs with apparently inaccurate SDSS g magnitudes. We revisitthe identification of low-mass white dwarf candidates previously foundin the SDSS and conclude that four objects have M<0.2Msolar. None of these white dwarfs show excess emission froma binary companion, and radial velocity searches will be necessary toconstrain the nature of the unseen companions.
| Hot subdwarfs from the ESO supernova Ia progenitor survey. II. Atmospheric parameters of subdwarf O stars
Aims. We address the origin and evolutionary status of hot subdwarfstars by studying the optical spectral properties of 58 subdwarf O (sdO)stars. Combining them with the results of our previously studiedsubdwarf B (sdB) stars, we aim at investigating possible evolutionarylinks. Methods: We analyse high-resolution (R > 18 000),high-quality optical spectra of sdO stars obtained with the ESO VLT UVESechelle spectrograph in the course of the ESO Supernova Ia ProgenitorSurvey (SPY). Effective temperatures, surface gravities, andphotospheric helium abundances are determined simultaneously by fittingthe profiles of hydrogen and helium lines using dedicated syntheticspectra calculated from an extensive grid of NLTE model atmospheres. Results: We find spectroscopic or photometric evidence for coolcompanions to eight sdO stars, as well as a binary consisting of two sdOstars. A clear correlation between helium abundances and the presence ofcarbon and/or nitrogen lines is found: below solar helium abundance, nosdO star shows C or N lines. In contrast, C and/or N lines are presentin the spectra of all sdO stars with supersolar helium abundance. Wethus use the solar helium abundance to divide our sample intohelium-deficient and helium-enriched sdO stars. While helium-deficientsdO stars are scattered in a wide range of the {T_eff}-log(g)-diagram,most of the helium-enriched sdO stars cluster in a narrow region attemperatures between 40 000 and 50 000 K and gravities between log g=5.5and 6.0. Conclusions: . An evolutionary link between sdB stars and sdOstars appears plausible only for the helium-deficient sdO stars. Theyprobably have evolved away from the extreme horizontal branch; i.e.,they are the likely successors to sdB stars. In contrast, theatmospheric properties of helium-enriched sdO stars cannot be explainedwith canonical single-star evolutionary models. Alternative scenariosfor both single-star (late hot flasher) and binary evolution(white-dwarf merger; post-RGB evolution) fail to reproduce the observedproperties of helium-enriched sdO stars in detail. While we regard thepost-RGB scenario as inappropriate, the white-dwarf merger and the latehot-flasher scenarios remain viable to explain the origin ofhelium-enriched sdO stars.Basedon observations collected at the Paranal Observatory of the EuropeanSouthern Observatory for programme No. 165.H-0588(A) and 167.D-0407(A).
| A Catalog of Spectroscopically Confirmed White Dwarfs from the Sloan Digital Sky Survey Data Release 4
We present a catalog of 9316 spectroscopically confirmed white dwarfsfrom the Sloan Digital Sky Survey Data Release 4. We have selected thestars through photometric cuts and spectroscopic modeling, backed up bya set of visual inspections. About 6000 of the stars are newdiscoveries, roughly doubling the number of spectroscopically confirmedwhite dwarfs. We analyze the stars by performing temperature and surfacegravity fits to grids of pure hydrogen and helium atmospheres. Among therare outliers are a set of presumed helium-core DA white dwarfs withestimated masses below 0.3 Msolar, including two candidatesthat may be the lowest-mass yet found. We also present a list of 928 hotsubdwarfs.
| The lack of close binaries among hot horizontal branch stars in NGC 6752
Aims.We present the results of a spectroscopic search for close binariesamong horizontal branch (HB) stars in NGC 6752.Methods: .We used the ESO VLT-FORS2 instrument to obtain mediumresolution (R=4100) spectra of 51 hot HB stars with 8 000 K ≤ T_eff≤ 32 000 K during four consecutive nights. Eighteen of our targetsare extreme horizontal branch (EHB) stars with {T_eff}≥ 22 000 K.Radial velocity variations were measured with cross-correlationtechniques, previously evaluated the statistical and systematic errorsassociated with them. Results: .No close binary system has beendetected among our 51 targets. The data corrected for instrumentaleffects indicate that the radial velocity variations are always below≈15 km s-1 (3σ level). From a statistical analysisof our results, we conclude that (at 95% confidence level) the fractionof binaries with a 0.5 {Mȯ} companion among EHB starsin NGC 6752 is smaller than 20%. Conclusions:.This empirical evidence sharply contrasts with what has been found forhot subdwarfs in the field, and opens new questions about the formationof EHB stars in globular clusters (and possibly in the field as well).
| The Subdwarf Database: Released
The work on the Subdwarf Database, presented at the previous meeting,has been completed, and the tool is now publicly available. The firstrelease contains data from close to 240 different literature sources,but more still awaits entry. The database interface includes advancedsearch capabilities in coordinate, magnitude and color space. Outputtables can be generated in HTML with hyperlinks to automaticallygenerated finding charts, the Aladin viewer and a detailed data sheetthat displays all registered data for each target, including physicaldata such as temperature, gravity and helium abundance, together with afinding chart. Search results can be visualized automatically asinteractive position, magnitude or color diagrams.
| NGC 6121-V46: a Low-Mass Double Degenerate Ellipsoidal Variable in a Globular Cluster
The variable sdB known as V46 in the globular cluster M 4 has remainedenigmatic since its discovery almost 10 years ago. We present hereradial velocity measurements obtained from medium-resolution VLT/FORS2spectra that show variations at twice the period of the luminositychanges. This implies that the system is an ellipsoidal variable. Unlikethe other sdB binaries of this nature, the fundamental parameters ofthis star we derive suggest that it lies below the Zero-Age ExtremeHorizontal Branch. From the cluster distance and the gravity wedetermine the mass of V46 to be 0.19 Mȯ. This istoo low to sustain core helium burning. From the mass function we derivea lower limit for the companion of only 0.26 Mȯ. Wediscuss the star's origin in the context of close binary evolution inthe field and globular clusters.
| Spectroscopic Search for Binaries among Ehb Stars in Globular Clusters
We performed a spectroscopic search for binaries among hot horizontalbranch stars in globular clusters. We present final results for a sampleof 51 stars in NGC 6752 and preliminary results for the first 15 starsanalyzed in M 80. The observed stars are distributed along all the HBsin the range 8000 ≤ Teff ≤ 32 000 K, and have beenobserved during four nights. Radial velocity variations were measuredwith the cross-correlation technique. We analyzed the statistical andsystematic errors associated with the measurements in order to evaluatethe statistical significance of the observed variations. No close binarysystem has been detected, neither among cooler stars nor among thesample of hot EHB stars (18 stars with Teff≥ 22 000 K inNGC 6752). The data corrected for instrumental effects indicate that theradial velocity variations are always below the 3σ level of 15 km s-1. These results are in sharp contrast with thosefound for field hot subdwarfs, and open new questions about theformation of EHB stars in globular clusters and possibly of the fieldsubdwarfs.
| High resolution spectroscopy of bright subdwarf B stars. I. Radial velocity variables
Radial velocity curves for 15 bright subdwarf B binary systems have beenmeasured using high precision radial velocity measurements from high S/Noptical high-resolution spectra. In addition, two bright sdB stars arediscovered to be radial velocity variable but the period could not yetbe determined. The companions for all systems are unseen. The periodsrange from about 0.18 days up to more than ten days. The radial velocitysemi amplitudes are found to lie between 15 and 130 {km s-1}.Using the mass functions, the masses of the unseen companions have beenconstrained to lower limits of 0.03 up to 0.55 Mȯ, andmost probable values of 0.03 up to 0.81 Mȯ. Theinvisible companions for three of our program stars are undoubtedlywhite dwarfs. In the other cases they could be either white dwarfs ormain sequence stars. For two stars the secondaries could possibly bebrown dwarfs. As expected, the orbits are circular for most of thesystems. However, for one third of the program stars we find slightlyeccentric orbits with small eccentricities of ɛ≈ 0.02 0.06.This is the first time that non-circular orbits have been found in sdBbinaries. No correlation with the orbital period can be found.
| Binaries discovered by the SPY project. IV. Five single-lined DA double white dwarfs
We present results from our ongoing follow-up observations of doublewhite dwarf binaries detected in the ESO SN Ia Progenitor SurveY (SPY).We discuss our observing strategy and data analysis and present theorbital solutions of five close double white dwarf binaries:HE0320-1917, HE1511-0448, WD0326-273, WD1013-010 and WD1210+140. Theirperiods range from 0.44 to 3.22 days. In none of these systems we findany spectral lines originating from the companion. This rules out mainsequence companions and indicates that the companion white dwarfs aresignificantly older and cooler than the bright component. Infraredphotometry suggests the presence of a cool, helium-rich white dwarfcompanion in the binary WD 0326-273. We briefly discuss the consequencesof our findings for our understanding of the formation and evolution ofdouble white dwarfs.
| Six detached white-dwarf close binaries
We determine the orbits of four double-degenerate systems (DDs),composed of two white dwarfs and two white-dwarf-M-dwarf binaries. Thefour DDs, WD1022+050, WD1428+373, WD1824+040 and WD2032+188, showorbital periods of 1.157155(5), 1.15674(2), 6.26602(6) and 5.0846(3) d,respectively. These periods combined with estimates for the masses ofthe brighter component, based on their effective temperatures, allow usto constrain the masses of the unseen companions. We estimate that theupper limit for the contribution of the unseen companions to the totalluminosity in the four DDs ranges between 10 and 20 per cent. In thecase of the two white-dwarf-M-dwarf binaries, WD1042-690 and WD2009+622,we calculate the orbital parameters by fitting simultaneously theabsorption line from the white dwarf and the emission core from the Mdwarf. Their orbital periods are 0.337083(1) and 0.741226(2) d,respectively. We find signatures of irradiation on the inner face of thecompanion to WD2009+622. We calculate the masses of both components fromthe gravitational redshift and the mass-radius relationship for whitedwarfs and find masses of 0.75-0.78 and 0.61-0.64Msolar forWD1042-690 and WD2009+622, respectively. This indicates that the starsprobably reached the asymptotic giant branch in their evolution beforeentering a common envelope phase. These two white-dwarf-M-dwarf binarieswill become cataclysmic variables, although not within a Hubble time,with orbital periods below the period gap.
| HS 2333+3927: A new sdB+dM binary with a large reflection effect
We have discovered periodic light variations (P = 0.1718023 d) in thesdB star HS 2333+3927 in the BVR bands withamplitudes of 0.21, 0.28 and 0.33 mag, respectively. Sinusoidal radialvelocity variations at the same period were detected with asemi-amplitude of K1 = 89.6 km s-1, indicatingthat it is binary system and that the light variations are caused by thereflection effect with no eclipses. A mass function of f(m)= 0.0128Mȯ has been determined. The analysis of the light curvedid not yield a unique solution, mainly because the albedo of thesecondary is poorly constrained. Two solutions of equal quality with ahigh (A2= 1.0) and a low (A2= 0.39) albedo wereconsidered further. Variability of the Balmer line profiles, mostnotably for Hα, was discovered, probably also caused by thereflection effect. A spectroscopic analysis results in Teff =36 500 K, log g = 5.70, and log (nHe/nH) = -2.15.These characteristics are typical for sdB stars. Mass-radius relationsare derived from the results of the analysis of light andradial-velocity curves. Comparison with the observed mass-radiusrelation of the sdB star and with that of lower main sequence stars forthe companion allows us to discard the high albedo solution, because theresulting mass of the primary and the radius of the secondary would beunreasonably low. From a discussion of evolutionary models we constrainthe plausible mass of the sdB to the range between 0.29Mȯ and 0.47 Mȯ. Accordingly, the mass ofthe secondary is between 0.24 Mȯ and 0.32Mȯ, indicating a spectral type of M3 to M4. HS 2333+3927is only the sixth sdB+dM system discovered so far. An improvedmeasurement of the gravity and the projected rotational velocity of thesdB star is required to further constrain the masses and to identify theevolutionary state of the sdB star uniquely.Based on observations obtained at the German-Spanish Astronomical Center(DSAZ) at Calar Alto, the Nordic Optical Telescope (NOT) and the JacobusKapteyn Telescope (JKT) on La Palma, the Hoher List Observatory inGermany and the 80 cm telescope of the Instituto de Astrofisica deCanarias (IAC80) on Tenerife.
| Discovery of a spectroscopic binary comprising two hot helium-rich subdwarfs
Helium-rich subdwarf B (He-sdB) stars form a very small fraction of thepopulation of hot subdwarf stars in our Galaxy. They have been foundboth in the field of our Galaxy as well as in globular clusters. Theevolution of these rare stars has recently been the subject of muchdebate involving both binary and single star evolution models. We reportour serendipitous discovery that the prototype - PG1544+488 - is a binary containing two low-mass helium-rich hotsubdwarfs. This discovery challenges existing evolutionary models forHe-sdB stars and suggests that they may be formed through close binaryevolution and following the ejection of a common envelope.Based on observations made with the William Herschel and Isaac NewtonTelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias and with the NASA-CNES-CSA FarUltraviolet Spectroscopic Explorer.
| Discovery of a helium-core white dwarf progenitor
We discovered that HD 188112, a bright (V = \magpt{10}{2}), nearby (80pc) B-type star, is a unique subluminous B (sdB) star. SdB stars areusually identified with models of core helium burning ExtremeHorizontal-Branch (EHB) stars of half a solar mass. A spectral analysisof the hydrogen and helium lines resulted in T_eff = 21 500 +/- 500 K,log g = 5.66 +/- 0.05 placing the star below the EHB. HD 188112 wasfound to be radial velocity (RV) variable and the RV curve has beenmeasured to be perfectly sinusoidal with a period of 0.606585 days and asemi-amplitude of 188.3 km s-1 indicating that it is a closebinary system. From the atmospheric parameters and the Hipparcosparallax we conclude that the sdB star is of low mass (0.24Msun). The mass of the sdB is too low to sustain core heliumburning and it is now evolving into a helium core white dwarf. A lowerlimit to the mass of its unseen companion of 0.73 Msun isderived from the mass function. Because the companion does notcontribute to the spectral energy distribution from the UV to theinfrared it cannot be a main sequence star but must be a white dwarf(WD), a neutron star (NS) or a black hole. The system would qualify aspre-supernova Ia candidate (sdB+WD) if its total mass is above theChandrasekhar limit (1.4 Msun), or as post-supernova(sdB+NS) if the companion mass is above that limit, requiring theinclination angle to be lower than 51deg or 48deg,respectively.Based on observations collected at the German-Spanish AstronomicalCenter (DSAZ), Calar Alto, operated by the Max-Planck-Institut fürAstronomie, Heidelberg jointly with the Spanish National Commission forAstronomy.Based on observations collected at the European Southern Observatory(ESO proposal No. 65.H-0341(A) and 69.D-0016(A)).
| Metal abundances of sdB stars
The surface abundance patterns of 13 sdB stars have been determined fromhigh resolution, high S/N, optical spectra. As typical in early B typestars, the metal lines are few and very weak. The metal abundancepatterns are compared with the atmospheric parameters to search forpossible trends, which are predicted by diffusion theory. However, it isremarkable that some metals (e.g. N, Mg, Fe) have similar abundancesirrespective of the stellar parameters, whereas other metals (e.g. C, O)and the helium abundances vary considerably from star to star. Threepeculiar sdB stars were found which show an enormous enrichment of irongroup elements. An isotopic anomaly of helium has been found in two sdBstars. In these stars 4He is largely replaced by the isotope3He. We discovered additionally four single lined, shortperiod radial velocity variable sdB stars and measured their radialvelocity curves.
| Intense accretion and mass loss of a very low mass young stellar object
We present visible and near-infrared photometry and spectroscopy ofLS-RCrA 1, a faint, very late-type object (M 6.5-M 7) seen in thedirection of the R Coronae Australis star forming complex. While itsemission spectrum shows prominent features of accretion and mass losstypical of young stellar objects, its underlying continuum andphotometric properties are puzzling when trying to derive a mass and agebased on pre-main sequence evolutionary tracks: the object appears to befar too faint for a young member of the R Coronae Australis complex ofits spectral type. We speculate that this may be due to either itsevolution along pre-main sequence tracks being substantially altered bythe intense accretion, or to a combination of partial blocking andscattering of the light of the object by a nearly edge-on circumstellardisk. The rich emission line spectrum superimposed on the stellarcontinuum is well explained by an intense accretion process: theHalpha , CaII infrared triplet, and HeI 6678 lines showequivalent widths typical of very active classical T Tauri stars. Thenear-infrared observations show anomalously weak spectral features andno significant excess emission in the K band, which we tentativelyinterpret as indicating line filling due to emission in a magneticaccretion funnel flow. At the same time, numerous, strong forbiddenoptical lines ([OI], [NII] and [SII]) and H2 emission at 2.12mu m suggest that the object is simultaneously undergoing mass loss,providing another example that shows that mass loss and accretion areclosely related processes. Such an intense accretion and mass lossactivity is observed for the first time in a young stellar object in thetransition region between low mass stars and brown dwarfs, and providesa valuable observational test on the effects of accretion on theevolution of objects with such low masses. Based on observationscollected at the European Southern Observatory in La Silla and CerroParanal (Chile), in programs 59.E-0679, 63.I-0546, 264.I-5723,64.L-0049, 65.H-0492, and 67.C-0109.
| Infrared spectrum and proper motion of the brown dwarf companion of HR 7329 in Tucanae
Up to now only four brown dwarf companions to normal stars have beenfound and confirmed by both spectroscopy and proper motion (namelyGl 229 B, G 196-3 B, Gl570 D, and CoD-33 deg 7795 B). On the basis of anoptical spectrum taken with HST/STIS Lowrance et al. (2000) recentlypointed out another possible candidate companion. The companioncandidate is located at a distance of 4{' '} from the A0-starHR 7329, which is considered as a member of a movinggroup of young stars in Tucanae located at a distance of only ~ 48 pc.In order to confirm or disregard the companion nature of the candidate,we have determined the proper motion of the brown dwarf candidate withan epoch difference of 1.8 years, and found that it is consistent with aco-moving companion of HR 7329. Additional to theproper motion measurement, we have also taken an H-band spectrum usingISAAC on the ESO-VLT. From this spectrum, we conclude that the companioncandidate has spectral type M 7 to M 8, which is in agreement with theoptical spectrum. We thus conclude that HR 7329 B ismost likely a brown dwarf companion. The mass ratio of this pair (A0 toM 7-8, i.e. ~ 100:1) is the largest known among brown dwarf companions,which is relevant for studying the formation of brown dwarfs ascompanions. Based on observations obtained at the European SouthernObservatory on Cerro Paranal and La Silla in program\break 65.L-0144.
| Standard stars: CCD photometry, transformations and comparisons
We discuss variations of the atmospheric extinction coefficients andtransformation equations to the standard UBVRI system based onobservations of standard stars during 1996-97 at Siding SpringObservatory using a thinned SITe CCD and coloured glass filters. In thetransformation from the initial natural system to the Landolt version ofthe standard system, a large nonlinear term related to the Balmerdiscontinuity was required for the U transformation. We then modifiedthe U filter, and the subsequent transformation to the SAAO version ofthe standard UBVRI system had only small nonlinear correction terms forU, B, and I. The correction terms relating to U and B are evidently dueto the Balmer discontinuity, while that relating to I seems to be due tothe Paschen discontinuity at a wavelegth of approximately 8200 Å.We also compared the results with Landolt's observations, and confirmedthe difference between the two sets of standard stars (SAAO andLandolt).
| 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.
| 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.
| Broad-band photometry of selected southern ultraviolet-bright stars.
Not Available
| Radial velocities and spectral types for a sample of faint blue stars.
Not Available
| A catalogue of spectroscopically identified hot subdwarf stars.
Not Available
| 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.
| 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.
| Photometry of faint blue stars. V - A third list of southern stars
Stromgren photometry is presented for 37 faint blue stars, six of whichhave been identified by ultraviolet observations made with the ESRO TD-1satellite. The stars are classified using their observed photometricindices.
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Datos observacionales y astrométricos
| Constelación: | Sagitario |
| Ascensión Recta: | 19h54m31.42s |
| Declinación: | -28°20'20.7" |
| Magnitud Aparente: | 10.207 |
| Movimiento Propio en Ascensión Recta: | 32.5 |
| Movimiento Propio en Declinación: | 18.3 |
| B-T magnitude: | 9.946 |
| V-T magnitude: | 10.186 |
Catálogos y designaciones:
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