Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Bright Metal-poor Stars from the Hamburg/ESO Survey. I. Selection and Follow-up Observations from 329 Fields
We present a sample of 1777 bright (91.0) metal-poor([Fe/H]<-2.0) giants of 9%+/-2%, which is lower than previouslyreported. However, the frequency rises to similar (>20%) and highervalues with increasing distance from the Galactic plane. Although thenumbers of stars at low metallicity are falling rapidly at the lowestmetallicities, there is evidence that the fraction of carbon-enhancedmetal-poor stars is increasing rapidly as a function of decliningmetallicity. For ~60 objects, high-resolution data have already beenobtained; one of these, HE 1327-2326, is the new record holder for themost iron-deficient star known.
| 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.
| NLTE Analyses of Sdb Stars: Progress and Prospects
We report on preliminary results of a hybrid non-LTE analysis ofhigh-resolution, high-S/N spectra of the helium-rich subdwarf B starFeige 49 and the helium-poor sdB HD 205805. Non-LTE effects are found tohave a notable impact on the stellar parameter and abundancedetermination. In particular the He I lines show significant deviationsfrom detailed balance, with the computed equivalent widths strengthenedby up to ˜35 %. Non-LTE abundance corrections for the metals (C,N, O, Mg, S) are of the order ˜0.05 -- 0.25 dex on the mean, whilecorrections of up to ˜0.7 dex are derived for individualtransitions. The non-LTE approach reduces systematic trends and thestatistical uncertainties in the abundance determination. Consequently,non-LTE analyses of a larger sample of objects have the potential to putmuch tighter constraints on the formation history of the different sdBpopulations than currently discussed.
| 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.
| New runaway OB stars with HIPPARCOS
A Monte Carlo method for detection of runaway OB stars fromobservational data is proposed. 61 runaway OB star candidates have beendetected by an analysis of Hipparcos proper motions. The peculiartangential and total transverse velocities have been determined forthese stars. A list of the detected runaway star candidates ispresented. The evidence of a discrepancy between photometric andparallactic distances of runaway OB star candidates is presented.
| 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.
| Kinematical trends among the field horizontal branch stars
Horizontal branch (HB) stars in the field of the Milky Way can be usedas tracers for the study of early stages of the evolution of our galaxy.Since the age of individual HB stars is not known a priori, we havestudied the kinematics of a sample of field HB stars measured withHipparcos to look for signs of age and population nature. Our samplecomprises 14 HBA, 2 HBB and 5 sdB/O stars. We found that the kinematicsof the HBA stars is very different from that of the sdB/O stars(including those from an earlier study). The HBA stars have low orbitalvelocities, some are even on retrograde orbits. Their orbits have largeeccentricities and in many cases reach large distances above thegalactic plane. In contrast, the sdB/O stars show disk-like orbitalcharacteristics. The few HBB stars (with T_eff> 10,000 K) in oursample seem to have kinematics similar to that of the sdB/O stars. Inorder to see if there is a trend among the HB stars in their kinematics,we investigated also RR Lyrae stars measured with Hipparcos. Here wefound a mixed kinematical behaviour, which was already known fromprevious studies. Some RR Lyrae stars have disk-like orbits (most ofthese being metal rich) but the majority has halo-like orbits, verysimilar to those of our HBA stars. Since the atmospheres of most typesof HB stars do not reflect original metallicities any more thekinematics is the only aspect left to study the origin and populationmembership of these stars. Thus, the clear trend found in kinematics ofstars along the HB, which is also a sequence in stellar mass, shows thatthe different kinds of field HB stars arose from stars having differentorigins in age and, e.g., metallicity or mass loss rate. Based in parton HIPPARCOS data
| Voyager Far-Ultraviolet Observations of Globular Clusters
We report on the observations of nine globular clusters carried out bythe Voyager ultraviolet spectrometers (UVSs). Three of the observedclusters, M13, NGC 6752, and M70, exhibit an intense far-ultraviolet(FUV) spectrum down to the Lyman limit. However, the spectrum obtainedfor NGC 6752 is heavily contaminated by the nearby star HD 177999 forwavelengths longward of 1200 Å. For M70 the Voyager spectrum iscompletely dominated by the B3 star HD 172535. The FUV spectral energydistribution of M13 confirms results from the Ultraviolet ImagingTelescope that the main contributors to the FUV emission are hot sdBstars. M13's integrated spectrum resembles that of an sdB star ofT_eff~28,000 K. For two clusters, M92 and M5, UVS detected a weaksignal, making it difficult to reach any conclusion on the underlyinghot components of the systems. Four clusters, M15, NGC 2298, NGC 6656,and NGC 6793, were observed but not detected. In some cases it waspossible to identify a weak stellar spectrum, which, however, verylikely originates from foreground FUV emitters not related to thesystems. For the nondetected clusters we provide upper limits on the FUVflux.
| 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.
| Hot subdwarf stars: galactic orbits and distribution perpendicular to the plane.
The spatial distribution and the population nature of subdwarf B typestars in the galaxy is investigated based on the kinematics of thesestars. With new and available proper motions, radial velocities, anddistances, the orbits of 41 stars have been calculated using a galacticmass model. The orbits are well behaved and 10 stars reach to|z|>=2kpc. Many orbits are very eccentric, reaching in to just 2kpcfrom the galactic centre, or veering out to beyond 20kpc. None of thestars can be identified uniquely as classical Population II objects. Theaverage eccentricity ecc of the orbits of our sample is 0.24, theaverage normalised z-extent nze of the orbits is 0.16, and theasymmetric drift of our sample is -36km/s. This suggests that our sampleof sdB stars is part of a population of thick disk stars. A statisticalanalysis of the orbits shows that the subdwarf stars have a spatialdistribution in z compatible with an exponential one with a scale heighth_z_ of about 1.0kpc. However, since only few stars reach to large z thespatial distribution is only well defined to z =~2kpc. The distributionin z shows a relative minimum near z=0pc and has maxima near z=300pc.This reflects the smaller probability to find the stars in the disk thanaway from the disk, as expected for any orbit reaching to larger z.Scale height studies based on limited samples of stars in specifieddirections can therefore easily be flawed when they do not reach tolarge enough distances to overcome this aspect of the z-distribution.
| 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
| The Spacelab-1 Very Wide Field Survey of UV-excess objects. IV. The performance of the instrument in combination with optical photometry as a means of identifying stars with peculiar properties.
UV (195 nm) and Stroemgren uvby photometry of a 110 square degree fieldat high southern galactic latitudes are analyzed through a comparison ofi) UV magnitudes for 57 stars of various types common to the publishedTD1 catalogue and the Very Wide Field Camera (VWFC); and ii) observedand theoretical two-colour diagrams. The higher sensitivity of the VWFC(=~0.5magnitude) and its more complete survey are exemplified by thedetection and UV measurement of a series of objects with moderateUV-excess in addition to detection of some very blue objects of variousnature down to fainter than 12th magnitude in the optical domain. Adeeper survey with a VWFC-type instrument could provide a completesample for studies of the group properties of faint blue stars. Duringthe uvby reductions it was found that the usual procedure of plottingresiduals as functions of declination, hour angle and airmass can be apowerful and diagnostic test of photometer rigidity.
| Early type high-velocity stars in the solar neighborhood. IV - Four-color and H-beta photometry
Results are presented from photometric obaservations in the Stromgrenuvby four-color and H-beta systems of early-type high-velocity stars inthe solar neighborhood. Several types of photometrically peculiar starsare selected on the basis of their Stromgren indices and areprovisionally identified as peculiar A stars, field horizontal-branchstars, metal-poor stars near the Population II and old-disk turnoffs,metal-poor blue stragglers, or metallic-line A stars. Numerousphotometrically normal stars were also found.
| 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
| 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.
| Studies of hot B subdwarfs. II - Energy distributions of three bright sdB/sdOB stars in the 950-5500 A range
Voyager ultraviolet spectrometer observations of the subdwarf B or OBstars HD 205805, UV 1758+36 and Feige 66 are presented. All threeobjects display the H I Layman series in absorption. These observationsare combined with low dispersion IUE spectrophotometry and withStromgren photometry to construct virtually complete energydistributions, which extend over the range 950-5500 angstroms. Effectivetemperatures based on model atmosphere calculations for high gravity,hygrogen rich stars are determined. Our analyses yield Te28,200 + or - 1300 K for HD 205805, Te 31, 800 + or - 1100 Kfor UV 1758+36, and Te 35,700 + or 1500 K for Feige 66. Theimportance of far ultraviolet observations below Lalpha inreducing the uncertainties associated with the interstellar extinctionand the degradation of the IUE sensitivity is emphasized.
| Early-type high-velocity stars in the solar neighborhood. III - Radial velocities, rotation indices, and line-strength indices for southern candidates
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1983AJ.....88.1349S&db_key=AST
| 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.
| Spectral analysis of the OB subdwarf HD 149 382
The ultraviolet and blue spectrum of the OB subdwarf HD 149 382 isanalyzed in an investigation of the star's evolutionary status. Aneffective temperature of 35,000 + or - 2000 K is derived from fittingthe observed continuum, hydrogen lines and helium lines to fluxes ofunblanketed non-LTE models. LTE analyses of heavy-element lines yielddeficiencies of carbon and oxygen and normal nitrogen abundance comparedto the sun, which indicates the CNO cycle's possession of the material.Calcium is overabundant, phosphorus and sulfur are normal, and aluminumand silicon are deficient. A tentative explanation of the abundancepattern in terms of the diffusion hypothesis is proposed, and the C/N/Oabundance ratios indicate that the CNO cycle operated on a mixture whichoriginally may have been poorer in the heavier elements than the solarmixture.
| Early-type high-velocity stars in the solar neighborhood. I - List of candidates
Suspected nearby high-velocity stars of spectral types B and A areidentified by their proper motions, which are generally too small forthe stars to have been included in previous catalogs of high-velocitystars. The same selection process is then extended to spectral type Fstars, in order to both provide a companion sample and investigatewhether the hypothetical high-velocity, metal-rich population continuesto later spectral types. Of the presented list of 371 stars withconfirmed proper motions, 168 are of A and B spectral types. Photometricand spectroscopic observations of these stars are required to eliminatethose with misclassified spectra and convert proper motions into spacevelocities. The identification of type of stars is important forunderstanding apparent exceptions to the correlation between stellarages, metal abundances, and kinematic properties.
| On the apparent normality of O and B stars far from the galactic plane
Results of a search for subluminosity or other peculiarities in thespectra of apparently normal OB stars located at large distances fromthe galactic plane are presented. Photographic spectrograms werecompared for stars of type B5 and earlier located within 0.5 kpc of andfurther than 1.5 kpc from the galactic plane, and for three subdwarfstars. Values of the projected axial rotation, surface gravity, absolutemagnitude and the strengths of certain He I and Balmer lines determinedfor both groups of OB stars are found to be similar, and distinct fromthe spectral characteristics of the subdwarfs. The results suggest that,unless the normality of the distant OB stars is illusory, either OBstars can be formed outside the galactic plane, or a mechanism mustexist to eject them from the plane with large velocities, unless theirmain sequence lifetimes are substantially longer than currentlybelieved.
| Highly-evolved stars
The ways in which the IUE has proved useful in studying highly evolvedstars are reviewed. The importance of high dispersion spectra forabundance analyses of the sd0 stars and for studies of the wind from thecentral star of NGC 6543 and the wind from the 0 type component of VelaX-1 is shown. Low dispersion spectra are used for absolutespectrophotometry of the dwarf nova, Ex Hya. Angular resolution isimportant for detecting and locating UV sources in globular clusters.
| Four-colour and H beta photometry of southern B stars at high galactic latitudes
Four-color and H beta photometry has been obtained for 105 early-type HDstars with galactic latitudes less than -45 deg. They are mostly late Bstars of luminosity class V to III. Two new Am stars and several Ap orBp stars are detected photometrically. Absolute magnitudes aredetermined from the photometry and from MK types where available. Theinterstellar reddening of many of the more distant stars is very small,suggesting either the existence of undetected peculiar stars in thesample or that there are areas of effectively zero reddening at highsouthern galactic latitudes.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Журавль |
| Прямое восхождение: | 21h39m10.62s |
| Склонение: | -46°05'51.5" |
| Видимая звёздная величина: | 10.232 |
| Собственное движение RA: | 77.9 |
| Собственное движение Dec: | -12.1 |
| B-T magnitude: | 9.908 |
| V-T magnitude: | 10.206 |
Каталоги и обозначения:
|
