Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Hot Subdwarf Stars
Hot subdwarf stars (sdBs, sdOs) are core helium-burning stars at theblue end of the horizontal branch or have evolved even beyond thatstage. They are found in all Galactic stellar populations and aresufficiently common to account for the UV-upturn of early-type galaxies.About half of the sdBs reside in close binaries; companions are whitedwarfs or low-mass main-sequence stars. Binary population-synthesismodels explain naturally the actual sdB binary fractions of field andglobular cluster stars as well as of He-sdOs if white-dwarf mergers areconsidered. Hot helium flashes explain the chemical composition ofHe-sdOs. Asteroseismology of a dozen pulsating sdB stars alloweddetermination of their masses and detection of a planet to V391 Peg. Thediscoveries of an sdO star unbound to the Galaxy, potential SN Iaprogenitors and probably a hidden population of neutron stars or blackhole companions have great impact on astrophysics at large.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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).
| UBVI and Hα Photometry of the Young Open Cluster NGC 2244
New UBVI and Hα photometry has been performed for the young opencluster NGC 2244. We classified 30 OB stars as being members of thecluster using proper-motion data and spectral types from previousinvestigators, along with photometric diagrams obtained in this study.We measured Hα emission strength of the stars by Hαphotometry and set up a selection criterion to select pre-main-sequence(PMS) stars with Hα emission. Fourteen PMS stars and seven PMScandidates were found using the criterion. In addition, six stars foundnear the positions of ROSAT HRI X-ray sources were assumed to be PMSstars and the optical counterparts of these X-ray sources. We determineda reddening of =0.47+/-0.04 for the cluster and atotal-to-selective extinction ratio of RV=3.1+/-0.2. Thedistance modulus derived was V0-MV=11.1. Bycomparing our photometric results with theoretical evolution models, wederived a main-sequence turnoff age of 1.9 Myr and a PMS age spread ofabout 6 Myr. The slope of the initial mass function, Γ, calculatedin the mass range 0.5<=logm<=2.0 could be flat(Γ=-0.7+/-0.1).
| 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.
| 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).
| 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
| 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.
| Spectral analysis of the binary sdB star Feige 36.
Not Available
| Calibrating horizontal-branch stars with Hipparcos.
Not Available
| The distribution of neutral hydrogen in the interstellar medium. 1: The data
We compile, from the existing literature, the largest sample to date(842 data points) of hydrogen column density measurements, N(H I), ofthe gas in the interstellar medium. We include only results obtainedfrom absorption measurements toward individual stars (594 in our sample)in an effort to construct a three-dimensional picture of theinterstellar gas. We derive hydrogen column densities toward a fractionof the stars in the sample from published column density measurements ofmetal ions. A three-dimensional physical model derived from this dataset will be presented in a companion paper. The observed stars spandistances from a few parsecs to a few thousand parsecs, and more thanhalf of the sample serves to describe the local interstellar mediumwithin a few hundred parsecs of the Sun. Hydrogen column densities rangefrom 1017 to 1022/sq cm. We describe here thevarious observational methods used to estimate the hydrogen columndensities and present the table with the stellar and hydrogen columndensity data. The provided table is intended as a global reference work,not to introduce new results.
| Ultraviolet and radio observations of Milky Way halo gas
Interstellar-absorption-line and 21-cm emission-line data for sightlines to 56 stars are combined in order to study the kinematics andspatial distribution of the gas that is at great distances from theGalactic plane. Measurements of the interstellar velocities and H Icolumn densities from the 21-cm emission and Ly-alpha absorption areincluded. The problem of contamination of the interstellar Ly-alphaabsorption line by stellar Ly-alpha absorption is analyzed, and thisinformation is used to reevaluate the vertical distribution of H I. Anew method for determining lower limits on the vertical distribution ofgas by including information on the velocity structure in the gas ispresented. The data for individual sight lines are discussed.
| 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.
| 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.
| 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.
| 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.
| The OB-type subdwarf HD 149382 and the nature of the subdwarf B stars
Results are reported for analysis of high-dispersion spectra andmonochromatic emission measures of the OB-type subdwarf HD 149382. Aneffective temperature of 40,000 K is determined together with a surfacegravity (log g) of 5.8. It is found that helium is deficient by a factorof 10 as compared with Population I stars, sulfur is overabundant by afactor of 10, and all other heavy elements are probably underabundant.This star is shown to be an intermediate case between the O and Bsubdwarf classes. Suggestions are made as to the nature of Bp stars,B-type subdwarfs, and DA white dwarfs. The evolutionary status of Bsubdwarfs is considered.
| Spectroscopically peculiar stars near the south galactic pole.
Abstract image available at:http://adsabs.harvard.edu/abs/1973AJ.....78..295G
| A finding list of early-type stars near the south galactic pole.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1971AJ.....76..338S&db_key=AST
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Sculptor |
| Ascensió Recta: | 00h42m58.31s |
| Declinació: | -38°07'37.3" |
| Magnitud Aparent: | 10.254 |
| Moviment propi RA: | 43.7 |
| Moviment propi Dec: | -7.5 |
| B-T magnitude: | 10.171 |
| V-T magnitude: | 10.248 |
Catàlegs i designacions:
|
