Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
FUSE Observations of Germanium, Zirconium and Lead in Sdb Stars
We report the detection of the Ge III, Zr IV, Pb III and Pb IV resonancelines in Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of hotsubdwarf B stars (sdB). We analyze 18 stars that cover the effectivetemperature domain for this class of stars. We carry out an abundanceanalysis and demonstrate that Ge, Zr and Pb abundances are higher thanthe ones observed in the Sun's photosphere in almost every star. Weperform radiative levitation calculations on Ge, Zr and Pb, and showthat the theory predicts higher Ge and Zr abundances than theobservations. Moreover, the large scatter of observed abundances cannotbe explained by the radiative levitation alone. This suggests that othermechanisms must be taken into account for explaining the abundances ofGe, Zr and Pb in the atmospheres of sdB stars.
| Atmospheric Parameters for Subdwarf B Stars: a Consistency Check Between the Balmer Lines and the Far-Ultraviolet Spectrum
The use of Balmer line profiles to determine atmospheric parameters ofsubdwarf B stars is a well-established method that relies on acomparison of spectroscopic observations with synthetic spectragenerated from model atmospheres. This method allows us to determineparameters such as the effective temperature, surface gravity andphotospheric helium abundance. The self-consistency of these parameterscan be investigated by examining the Lyman lines covered by currentfar-ultraviolet observations. We discuss the results of a preliminaryanalysis of a sample of ten subdwarf B stars for which both optical andfar-ultraviolet spectra were secured. At temperatures below 30 000 K wefind good consistency between optical and ultraviolet spectra when someallowance is made for the metal-line blanketing present in theultraviolet region. At higher effective temperatures, however, theconsistency is not as satisfactory. Possible solutions to this puzzleare considered.
| 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.
| Beyond the iron group: Heavy metals in hot subdwarfs
We report the discovery of strong photospheric resonance lines of GaIII, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdBand sdOB stars at temperatures ranging from 22 000 K to 40 000 K. Linesof other heavy elements are also detected, however in these cases moreatomic data are needed. Based on these discoveries, we present ahypothesis to explain the apparent lack of silicon in sdB stars hotterthan 32 000 K. The existence of triply ionised Ge, Sn, and Pbsuggests that rather than silicon sinking deep into the photosphere, itis removed from the star in a fractionated stellar wind. This hypothesisprovides a challenge to diffusion models of sdB stars.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. These observations areassociated with program #8635 and #5319.
| Studying the populations of our Galaxy using the kinematics of sdB stars
We have analysed the kinematics of a sample of 114 hot subdwarf stars.For 2/3 of the stars, new proper motions, spectroscopic and photometricdata are presented. The vast majority of the stars show a kinematicbehaviour that is similar to that of Thick Disk stars. Some stars havevelocities rather fitting to solar, i.e. Thin Disk, kinematics. About˜15 objects have orbital velocities which differ considerably fromthose of Disk stars. These are members of the Galactic Halo. Weinvestigated the velocity dispersions and calculated the orbits. Moststars feature orbits with disk character (eccentricity of less than0.5), a few reach far above the Galactic plane and have very eccentricorbits (eccentricity of more than 0.7). The intermediate eccentricityrange is poorly populated. This seems to indicate that the (Thick) Diskand the Halo are kinematically disjunct. Plotting a histogram of theorbit data points along z leads to the z-distance probabilitydistribution of the star; doing this for the whole sample leads to thez-distance probability distribution of the sample. The logarithmichistogram shows two slopes, each representing the scale height of apopulation. The disk component has a scale height of 0.9 (±0.1)kpc, which is consistent with earlier results and is similar to that ofthe Thick Disk. The other slope represents a component with a scaleheight ˜7 kpc, a much flatter gradient than for the diskcomponent. This shows that the vast majority of the sdBs are disk stars,but a Halo minority is present, too. The kinematic history andpopulation membership of the sdB stars on the whole is different fromthat of the cooler HBA stars, which are predominantly or evenexclusively Halo objects. This leads to the question, whether the HalosdB stars are of similar origin as the HBA stars, or whether theirkinematical behaviour possibly represents another origin, such asinfalling stellar aggregates or inner disk events.Based on data obtained at the 1.54 m telescope of the European SouthernObservatory, runs 58.D-0478, 60.D-0306, 62.L-0220, 64.L-0077,64.H-0023(A), 65.H-0250, 65.H-0341(A) and 68.D-0192(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.
| The First 50 Years at Palomar, 1949-1999 Another View: Instruments, Spectroscopy and Spectrophotometry and the Infrared
We review the research on a wide variety of topics using data obtainedwith the 200-inch Hale telescope. Using state-of-the-art spectrographs,photometers, spectrometers and infrared detectors, the Palomarastronomers investigated the spectra of stars, interstellar matter, AGNsand quasars in great detail. Spectral resolutions ranged from 1000 A forbroad-band photometry to 0.04 A using interferometric techniques.
| Infrared flux excesses from hot subdwarfs. II. 72 more objects
In our search, started in February, 1994, for JHK excess fluxes amongthe hot subdwarf population as an indicator for the presence of binarycompanions, results for 72 more hot objects (=63 hot subdwarfs + 1Horizontal Branch B star + 7 white dwarfs + 1 non-subdwarf object)observed with the Carlos Sanchez CVF IR photometer (in June and October,1994), are presented. The exact number of binary hot subdwarfs hasgained renewed importance after the recent discovery of pulsators withG-F companions. The total number of candidates we propose may help toset some constraints; for example, out of 41 objects with excesses, 13may have G-type binary companions. From our new sample, 14 discoveriesof binary candidates have been found: BD+25 4655, Feige 108, HD 4539, HD149382, HD 216135, KPD 2109+440, LSI+63 198, LSIV+10 9, LSV+22 38, PG0011+221, PG 0116+242, PG 0314+103, PG 2151+100 and TON 139. Besides, 2more from reanalysis of February, 1994, data - BD+37 1977 and BD+481777, may now be found to be IR excess candidates. Two suspectedbinaries, PB 8555 and SB 7, are also confirmed. By fitting \cite[Kurucz(1993)]{kur93} model spectra and assuming zero-age main sequencecompanions, we find upper limits on the subdwarf gravities. Thedistributions of upper limits on log(g), mostly between about 5.25 and6.5, are nearly identical for both sdBs and sdOs. Figure~1 is onlyavailable in the electronic version of the paper(http://www.edpsciences.com)
| A search for magnetic stars in late stages of stellar evolution
No abstract submitted
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Calibrating horizontal-branch stars with Hipparcos.
Not Available
| On the nature of the UV turnup in early-type galaxies.
We study the UV turnup in early-type galaxies from coadded IUE spectrareaching an unprecedented signal to noise ratio. Some spectral groupsresulted with strong or moderate UV turnup, while in others it is veryweak or absent. We study the relationships of the UV turnup intensitywith absolute magnitude, X-ray and Hα luminosities. Galaxies inthe strong UV turnup groups are systematically bright, and have highX-ray and Hα emission luminosities; however, some other galaxieswith the latter properties do not exhibit a significant UV turnup. Thespectral groups with an important intermediate age component are far-UVweak. The contribution from an intermediate age population with varyingstrength might explain why some red stellar population early-typegalaxies present the UV turnup while others do not. The availablespectral groups have been further coadded into one with strong UV turnupand another one without it, and we analyse their difference. We comparethe spectral slope of the isolated UV turnup with IUE spectra of varioushot sources which had been proposed in previous studies to explain it.We conclude that only very hot stars like sdO and sdB subdwarfs, somenuclei of planetary nebulae or DO white dwarfs, have the proper slope toexplain the UV turnup. The UV turnup as detected through the IUEaperture is not featureless: it presents absorptions similar to thoseobserved in the galaxies with weak far-UV flux. These features appear tobe the λ1400A and λ1600A ones, characteristic ofmoderately cool white dwarfs (DA5), which indicates that these stars aredominant flux contributors between λ1300-2000A in the far-UV weakgroups. The cooling time of DA5 stars together with the evolutionarytime since they left the main sequence, imply that they evolved fromlow-mass stars possibly associated with the initial burst of starformation in early-type galaxies and/or merger events at intermediateages. On the other hand, two possible scenarii are discussed for theorigin of the hot component which causes the UV turnup, one related tolate stages of normal evolution of low-mass stars, and another relatedto past nuclear activity events and jets which might have blown away theatmospheres of red giants in the central parts of the galaxies, exposingthe hot stellar cores and mimicking a spectral distribution like that ofthe hottest stars observed.
| A search for stars with strong magnetic fields among horizontal-branch stars and hot subdwarfs
Observations obtained with the 6-m BTA telescope for a few A, Bhorizontal-branch stars and hot subdwarfs have been used to measuretheir effective magnetic fields. No magnetic fields similar to those ofAp, Bp main-sequence stars were found.
| A search for stars with strong magnetic fields among horizontal-branch stars and hot subdwarfs
Not Available
| Studies of Hot B Subdwarfs. X. The Distribution and Space Density of Hot, Hydrogen-rich Subdwarfs Determined from the Palomar-Green Survey
Abstract image available at:http://adsabs.harvard.edu/abs/1995ApJ...446..646V
| Atmospheric parameters of field subdwarf B stars
High signal-to-noise ratio optical spectrophotometry of a sample offield subluminous B stars drawn largely from the Palomar Greenultraviolet-excess survey is analyzed with a new grid of modelatmospheres and synthetic spectra. The stellar effective temperatures,surface gravities, and photospheric helium abundances are determinedsimultaneously from a detailed analysis of hydrogen and heliumabsorption line profiles. The derived temperatures and gravities placethe subluminous B stars in the theoroetical Hertzsprung-Russell (H-R)diagram along and bounded below by theoretical sequences of the zero-ageextended horizontal branch, lending strong support to the hypothesisthat these stars are composed of helium-burning cores of approximately0.5 solar mass overlain by very thin layers of hydrogen (approximatelyless than 0.02 solar mass). Various scenarios for their pastevolutionary history are examined in the context of their probablefuture evolution into white dwarfs of lower than average mass.
| Studies of hot B subdwarfs. 9: Cousins BVRI photometry and the binary fraction of hot, hydrogen-rich subdwarfs in the Palomar-Green survey
Cousins BVRI photometry is presented for a sample of 100 hot,hydrogen-rich subdwarf candidates selected principally from thePalomar-Green (PG) and Kitt Peak-Downes colorimetric surveys. Thecombination of these data with existing Stroemgren photometry permitsthe construction of energy distributions in the range lambda lambda3500-7900. With the help of flux-ratio diagrams, we have isolated 31objects with flat energy distributions indicative of a composite nature.Approximate effective temperatures and spectral types are derived forthe primary and secondary stars, respectively. We present evidence,based on the absolute visual magnitudes determined for the primarycomponents, that the secondary stars in most systems are likely to beoverluminous compared to normal main-sequence stars. Because ofselection effects, the spectral types of the secondary stars arerestricted to the range late G to early M. When allowance is made forthese selection effects, the fraction of hot hydrogen-rich subdwarfswith main-sequence companions in the PG survey can be inferred, and isestimated to be approximately 54%-66%.
| A catalogue of Fe/H determinations - 1991 edition
A revised version of the catalog of Fe/H determinations published by G.Cayrel et al. (1985) is presented. The catalog contains 3252 Fe/Hdeterminations for 1676 stars. The literature is complete up to December1990. The catalog includes only Fe/H determinations obtained from highresolution spectroscopic observations based on detailed spectroscopicanalyses, most of them carried out with model atmospheres. The catalogcontains a good number of Fe/H determinations for stars from open andglobular clusters and for some supergiants in the Magellanic Clouds.
| Atmospheres and Abundances of Blue Horizontal Branch Stars and Related Objects
Not Available
| Broad-band photometry of selected southern ultraviolet-bright stars.
Not Available
| The kinematics of Milky Way halo gas. I - Observations of low-ionization species
Ultraviolet interstellar line day observed with the IUE toward 70 halostars and four extragalactic sight lines are analyzed in a study of thelarge-scale kinematic properties of the Milky Way halo gas. The motionsof the low-ionization gas is focused on. Large systematic velocities arefound, and a pronounced asymmetry in the absorption characteristics ofhalo gas toward the Galactic poles is indicated. In the north,substantial amounts of material are falling toward the disk atvelocities up to about 120 km/s in the most extreme case. Toward thesouth, low-ionization material shows no extreme or systematic motions.
| A catalogue of spectroscopically identified hot subdwarf stars.
Not Available
| Spectral Analysis of Hot Subdwarfs (Fieldversus Globular Cluster Population)
Not Available
| Analysis of faint blue stars at high galactic latitudes
Ultraviolet and visual spectra of 16 faint B-type stars at high galacticlatitudes are analyzed for effective temperature, gravity, and heliumabundance by means of model atmospheres. Nine stars are helium poorwhile six have normal helium abundance; SB 939 is found to be anintermediate helium star; SB 357 appears to be a rapidly rotating Bestar; and PHL 346 is probably metal rich. From the (g,Teff)-diagram itis concluded that the helium poor stars are horizontal branch stars.Four of them are sdB stars and belong to the extended horizontal branch.The helium normal stars are main sequence stars with masses between 3and 8 solar masses, which are far away from the star forming regions inthe galactic plane.
| Studies of hot B subdwarfs. III - Carbon, nitrogen, and silicon abundances in three sdB stars
Optical and high-dispersion IUE observations of three hot B subdwarfs(UV 1758 + 36, Ton S-227, and Feige 65) are presented. These data areanalyzed with model atmosphere techniques, and element abundances for C,N, and Si are derived. The abundances are either near (N) or below (C,Si) the solar value; large variations (1) in the extent ofunderabundances of carbon and silicon among the objects, as well as (2)in the abundances (with respect to the solar values) characterizing eachstar are observed. A preliminary interpretation of the observedvariations in these and other hot subdwarfs in terms of radiative forcesdisrupting the downward diffusion of heavy elements is presented.
| A catalogue of Fe/H determinations, 1984 edition
The present version of the Cayrel de Strobel et al. (1981) catalog ofFe/H abundance ratio determinations contains 1921 values for 1035 stars,which represents an augmentation over the previous publication of 48 and47 percent, respectively. In addition, the literature search conductedis complete up to December, 1983. Stellar metal abundance, effectivetemperature, spectroscopic gravity, spectral type, and photometricindices are covered.
| Photometry of faint blue stars. IV - Some 'very definitely blue' PHL stars
Photometry on the uvby system is presented for 56 faint blue stars,mostly from the Palomar-Haro-Luyten 'very definitely blue' list. Themajority of these stars are hot subdwarfs.
| Studies of hot B subdwarfs. I - Stromgren photometry of the Greenstein-Sargent-sample
Stromgren colors for a sample of 25 hot B subdwarfs are presented. Thesestars form a well-defined sequence in a two-color diagram, the existenceof which is shown to be consistent with the evolutionary scenario forsdB stars proposed by Wesemael et al. (1982). A comparison of thephotometric data with appropriate model atmosphere calculations suggeststhat hot B subdwarfs have effective temperatures ranging from 25,000 Kto upward of 40,000 K, and surface gravities generally above log g =5.0.
| The atmosphere of subluminous B stars
The spectra of eight high galactic latitude B stars, suspected to besubluminous, are analyzed for effective temperature, gravity, and heliumto hydrogen ratio. Model atmospheres include (LTE) UV line blanketing(for T/eff/ less than or equal to 30,000 K) and NLTE effects (for T/eff/greater than 30,000 K). Among the eight confirmed subdwarfs, six havetemperatures around 26,000 K. Two objects are considerably hotter(34,000 K and 41,000 K) and belong to the sdOB subclass. Gravity rangesfrom log g = 5.0 to 5.5 for the sdB's, the sdOB's having log g = 5.7 and6.0, respectively. Helium is strongly depleted in the sdB's and sdOB's:from He I 4471 (if present) the number fraction obtained is 0.0016n(He)/n(H) = 0.0016-0.016. From the g, T(eff)-diagram it is concludedthat the subluminous B stars and sdOB stars are (generalized) heliummain sequence stars with approximately 0.5 solar mass. The heliumdeficiency of the envelope is brought about by diffusion.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Baliklar |
| Sað Açýklýk: | 00h47m29.22s |
| Yükselim: | +09°58'55.6" |
| Görünürdeki Parlaklýk: | 10.252 |
| özdevim Sað Açýklýk: | 2.4 |
| özdevim Yükselim: | 21 |
| B-T magnitude: | 10.053 |
| V-T magnitude: | 10.236 |
Kataloglar ve belirtme:
|
