Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
The Discovery of Fluorine in Cool Extreme Helium Stars
Neutral fluorine (F I) lines are identified in the optical spectra ofcool extreme helium (EHe) stars. These are the first F I linesidentified in a star's spectrum, and they provide the first measurementof fluorine abundances in EHe stars. The results show that fluorine isoverabundant in EHe stars. The overabundance of fluorine provides uswith evidence for the synthesis of fluorine in these stars, which isdiscussed in light of asymptotic giant branch evolution and theexpectation from accretion of an He white dwarf by a C-O white dwarf.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| BI Lyncis: A hydrogen-deficient binary consisting of two low-mass giants of spectral types early-B and G
BI Lyn has previously been misclassified as anevolved binary system containing either a hot subdwarf or white dwarfand a thick accretion disk. New intermediate dispersion spectra arepresented which demonstrate clearly that the hot component is a luminouslow-mass helium star and the cool companion is a rapidly rotating G-typegiant. Techniques of spectrum synthesis have been used to establish thedimensions of both components. Although the orbital period of the systemremains unknown, other phenomena are entirely consistent with theseobserved dimensions. A ~ 0.34 d periodic photometric variation could beexplained by pulsations in the B-type primary and, by analogy with otherH-deficient binaries, it is suggested that the variable absorption inHα may be due to a gas stream flowing through the inner Lagrangianpoint. Based on observations obtained with the Isaac Newton and WilliamHerschel Telescopes, and on INES data from the IUE satellite.
| The R Coronae Borealis stars - atmospheres and abundances
An abundance analysis of the H-deficient and He- and C-rich R CoronaeBorealis (R CrB) stars has been undertaken to examine the ancestry ofthe stars. The investigation is based on high-resolution spectra andline-blanketed H-deficient model atmospheres. The models successfullyreproduce the flux distributions and all spectral features, bothmolecular bands and high-excitation transitions, with one importantexception, the C i lines. Since photoionization of C i dominates thecontinuous opacity, the line strengths of C i are essentiallyindependent of the adopted carbon abundance and stellar parameters. Allpredicted C i lines are, however, much too strong compared withobservations, with a discrepancy in abundance corresponding to 0.6 dexwith little star-to-star scatter. Various solutions of this ``carbonproblem'' have been investigated. A possible solution is that classicalmodel atmospheres are far from adequate descriptions of supergiants suchas the R CrB stars. We can also not exclude completely, however, thepossibility that the gf-values for the C i lines are in error. This issupported by the fact that the C ii, [C i] and C_2 lines are reproducedby the models with no apparent complications. In spite of the carbonproblem, various tests suggest that abundance ratios are little affectedby the uncertainties. Judging by chemical composition, the R CrB starscan be divided into a homogeneous majority group and a diverse minority,which is characterized by extreme abundance ratios, in particular asregards Si/Fe and S/Fe. All stars show evidence of H- and He-burning indifferent episodes as well as mild s-process enhancements. Four of themajority members are Li-rich, while overabundances of Na, Al, Si and Sare attributes of all stars. An anti-correlation found between the H andFe abundances of H-deficient stars remains unexplained. These enigmaticstars are believed to be born-again giants, formed either through afinal He-shell flash in a post-AGB star or through a merger of two whitedwarfs. Owing to a lack of theoretical predictions of the resultingchemical compositions, identification of the majority and minoritygroups with the two scenarios is unfortunately only preliminary.Furthermore, Sakurai's object and V854Cen exhibit aspects of both majority and minority groups,which may suggest that the division into two groups is too simplistic.
| Catalogue of H-alpha emission stars in the Northern Milky Way
The ``Catalogue of Stars in the Northern Milky Way Having H-alpha inEmission" appears in Abhandlungen aus der Hamburger Sternwarte, Band XIin the year 1997. It contains 4174 stars, range {32degr <= l() II< 214degr , -10degr < b() II < +10degr } having the Hαline in emission. HBH stars and stars of further 99 lists taken from theliterature till the end of 1994 were included in the catalogue. We givethe cross-identification of stars from all lists used. The catalogue isalso available in the Centre de Données, Strasbourg ftp130.79.128.5 or http://cdsweb.u-strasbg.fr and at the HamburgObservatory via internet.
| From OAO2 to HST. A quarter of a century of ultraviolet astronomy.
Not Available
| A series of theoretical line profiles formed in the irradiated expanding atmospheres of close binary components
We studied the effects of irradiation on the line formation in theexpanding atmospheres of the components of close binary systems. Weconsidered a two-level atom approximation in Non-LTE situation. Thethickness of the atmosphere is assumed to be twice that of the stellarradius. Expansion velocities are assumed to be as large as 50 meanthermal units. We have computed the lines using total optical depths10(3) , 10(4) and 10(5) at the line centre. The irradiation from thesecondary is assumed to be one, five and ten times the self radiation.The line fluxes in the line of sight are calculated by using the totalsource function which is the sum of the source functions due to selfradiation and that due to irradiation. We notice that the expansion ofthe medium produces P Cygni type profiles and the irradiation enhancesthe emission in the lines although the equivalent widths reduceconsiderably. Figures 1(1-6), 2(1-30), 3(1-8), 4(1-8), 5(1-6) and 6(1-8)are available in electronic form at the http://www.edpsciences.com
| Catalogue of stars in the northern Milky Way having H-alpha in emission
Not Available
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| Observations of Variable Stars with the R-Coronae Phenomenon and Other Unique Objects
Not Available
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Abundances, planetary nebulae, and stellar evolution
Among Henry Norris Russell's many achievements were his contributions tosolar and stellar spectroscopy, in particular, to an analysis of thechemical composition of the solar atmosphere. The question ofcomposition differences between stars was hotly debated; somedistinguished astronomers argued that all stars had the solarcomposition. Some early challenges to this doctrine are described.Determinations of chemical compositions of gaseous nebulae were muchmore difficult. If we observe the lines of a given chemical element inone ionization stage in a stellar spectrum, we can deduce readily theabundance of that element. No such luxury is available for a planetaryor diffuse gaseous nebula. We must measure lines of as many ionizationstages as we can. Furthermore, a nebula is an extended object. Oftendetailed spectroscopy is at hand only for narrow pencil columns takenthrough the image. Different observers use a variety of apertures.Fortunately it is possible to calculate theoretical spectra for anyarbitrary cross section taken through a symmetrical model, so UV,optical, and IR observations all can be compared properly with aprediction. The value of high-resolution spectra obtained withinstruments such as the Hamilton Echelle Spectrograph at LickObservatory is emphasized. Improved fluxes for weak but importanttransitions are found. Close blends of lines of different ions can beresolved, and checks can be made on predictions of atomic parameterssuch as Einstein A-values and collision strengths. High spectralresolution data have been obtained and reduced for 22 planetary nebulaeof varying size, structure, stellar population membership, dustiness,level of excitation, evolutionary status, and chemical compositions. Thepromise seems justified that with such extensive, high quality data,additional insights on nebular genesis and late states of stellarevolution can be found. The present survey is confined to nebulae ofhigh surface brightness, but among these are found some engaging objectssuch as NGC 7027, IC 4997, and NGC 6572.
| A study of dust shells around high latitude supergiants
A sample of 22 stars with infrared excess emission and many of whichhave supergiant-like spectra was surveyed in the submillimeter continuumat 438, 761 and 1100μm using the James Clerk Maxwell Telescope (JCMT,Mauna Kea, Hawaii). Most of the sources are at high galactic latitudesbut stars with similar properties at lower galactic latitudes are alsoincluded. For 16 sources it is believed that they are post AsymptoticGiant Branch stars (post-AGB stars) and 6 of them are detected. The restof the sample consists of 2 binaries, 1 Herbig Ae star, 1 Luminous BlueVariable and 2 sources for which the post-AGB nature is uncertain. Theobservations are combined with optical, near infrared and IRAS data andare fitted with a spherically symmetric and optically thin dust shellmodel. It is found that a large fraction of the excess emissionoriginates from relatively hot dust near the star. Only 2 sources lacksuch a hot dust component. For all stars with a hot dust component itwas found that the emission longward of 60μm cannot be explained withonly one dust shell. Several possible explanations are discussed and itis concluded that the extra excess at far IR and submm wavelengths iscaused by a second colder and thus more distant dust component. For thepost-AGB stars this component is believed to be the remnant AGB shell.The relatively low stellar temperatures (<10,000 K) in combinationwith the relatively large ages of the remnant AGB shell (>10^3^ yr)confirm earlier suggestions that many of these stars are very slowlyevolving towards higher temperatures, as expected for low mass post-AGBstars. Evidence was found that stars may stop losing mass and evolve offthe AGB at temperatures below 5,000 K (as assumed for the Schoenbernertracks) causing a slower evolution towards higher temperatures.
| Formation and Evolution of Binary Planetary Nebula Nuclei and Related Objects
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...418..343I&db_key=AST
| Improved effective temperatures for hydrogen-deficient binary stars
Effective temperatures are obtained for all four knownhydrogen-deficient binary stars, using published and archival photometryand new line-blanketed model atmospheres. These model atmosphereslargely resolve the well-known mismatch between the theoretical andobserved flux distribution for Upsilon Sgr, although a discrepancypersists at short wavelengths (less than 1500 A). The effectivetemperature obtained is in excellent agreement with that derived fromthe ionization equilibrium in the photosphere. The effectivetemperatures of all four stars are substantially revised from previousmeasurements, and are all within 500 K of 12,000 K. With a reportedradius of 60 solar radii for v Sgr, its luminosity becomes 63,000 solarluminosities. Assuming this radius for all objects, their distances areestimated, and lie between 1.4 and 9.6 kpc. All four objects show IRexcesses. These are modeled assuming a single-temperature blackbodycontinuum; the results are broadly consistent with previous analyses.Three objects exhibit a 1000-K dust shell of some 10 to 100 times thestellar radius, while a 3500-K excess in LSS 1922 is probably due to anearby K or M star.
| SAO stars with infrared excess in the IRAS Point Source Catalog
We have undertaken a search for SAO stars with infrared excess in theIRAS Point Source Catalog. In contrast to previous searches, the entireIRAS (12)-(25)-(60) color-color diagram was used. This selection yieldeda sample of 462 stars, of which a significant number are stars withcircumstellar material. The stars selected can be identified aspre-main-sequence stars, Be stars, protoplanetary systems, post-AGBstars, etc. A number of objects are (visual) binary stars.Characteristic temperatures and IR excesses are calculated and theirrelations to spectral type are investigated.
| Mid-ultraviolet and optical photometry of helium stars
Mid-ultraviolet and optical photometric analysis of helium stars arepresented. A linear relation exists between the effective temperaturederived from model atmospheres and (1965-V)0 index. The effectivetemperatures derived from (1965-V)0 index are somewhat higher than thatof MK spectral type estimates especially for late B-type helium objects.
| Model for R Coronae Borealis stars
An evolutionary model is proposed in which R Coronae Borealis stars arein a second common-envelope phase, with a low-mass companion orbitingwell inside the envelope. The high mass-loss rate in the firstcommon-envelope phase turns the massive star into a hydrogen-deficientstar. In the second common-envelope phase, as a result of shear inducedby the inner envelope rotation and possibly by excitedgravity-wave-induced turbulence, carbon and oxygen are being dredged upfrom the core into the envelope. This evolutionary scenario smoothlyconnects R CrB stars to the double-mass-loss-episodes evolutionaryscenario of hydrogen-deficient binaries suggested by Plavec (1973). Thismodel may have consequences for the nature of mass loss in R CoronaeBorealis stars: the orbiting companion breaks spherical symmetryallowing for nonradial pulsations and asymmetric mass loss. The opticalpolarization and IR observations may point to axisymmetric mass losswith the same axis of symmetry for the different mass-loss episodes.
| Single and binary star evolution
After presenting a general account of the observed global properties ofsingle stars of low, intermediate, and high mass, together with theirtheoretical Hertzsprung-Russell diagram evolution, attention is given tothe observed properties of various evolved close binaries and to anassessment of the value of comparisons between observation and crudetheory in characterizing the physics of mass transfer within interactingbinary systems. Detailed consideration is then undertaken of such topicsas stellar evolution in globular clusters, interior star changes due tonucleosynthesis and mixing, asymptotic giant branch stars ofintermediate mass, the response of white dwarfs in binary systems tomass accretion, and scenarios for binary star evolution tending towardclose white dwarf pairs.
| POST AGB candidates - Selection and IR properties
The properties of a group of stars is examined, that are selected on thebasis of the properties of two known post-AGB stars, HR 4049 and HD213985. It is shown that the stars from this sample that probably arepost-AGB stars all show IR excess. This excess can be due to hot (T =1000 K), cool (T = 300 K) or hot and cool dust. The stars showing thehot dust all have mass loss rates that are substantially larger than themass loss rates of the stars showing only cool dust. This indicates thatmass loss continues after the AGB at a substantial rate (about 10 to the-7th solar mass/yr). Selection criteria are given on the basis of whichpost-AGB stars can be selected from the literature.
| The Mass Ratio of Nu-Sagittarii from Ultraviolet Radial Velocities
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990MNRAS.247..400D&db_key=AST
| Close binaries observed polarimetrically
Not Available
| The ultraviolet spectrum of the hydrogen-poor binary HD 30353
An analysis is made of the high- and low-resolution UV spectra of thehydrogen-poor star HD 30353. The stellar wind profiles of the resonancelines of N V, C V, and Si IV are found to be similar to those observedin Upsilon Sgr. However, HD 30353 shows shortward-shifted profiles alsoin the ground-level lines of Fe II, in the subordinate multiplet 3 of MgII, and in the excited lines of Fe III. The results suggest that theshell around HD 30353 may be more extended than that of Upsilon Sgr. TheUV excess in the flux shortward of 1800 A suggests that the companion isan early B-type star.
| The ultraviolet spectrum of the binary system KS Persei (HD 30353)
Nine high- and low-resolution IEU spectra at different orbital phasesduring different cycles, of the hydrogen-poor single-lines spectroscopicbinary KS Per (HD 30353) have been studied. The near-UV spectrum ischaracterized by the presence of several lines of once ionized metalswhich show the same orbital RV shifts observed in the visual range. TheMg II resonance lines present a sharp interstellar + circumstellar coreand an emission peak at about 83 and 92 km/s. The far UV spectrum ischaracterized by the presence of strong absorptions of the resonancelines of N V, C IV, Si IV, Si III. Direct determination of the ISextinction from the 2200 A feature gives E(B - V) = 0.33. A comparisonof corrected logarithmic continuous energy distribution with thetheoretical models by Kurucz, indicates that the companion is a hotstar, Te = 15,000 K; logg = 4. A comparison with recent UV work onUpsilon Sgr, shows that, KS Per is very similar to Upsilon Sgr.
| The formation and detectability of Be + white dwarf systems
The formation of Be + He star and Be + white dwarf binaries is discussedusing evolutionary calculations. A total number of 300-1000 rapidlyrotating B star + He star binaries and 100-300 rapidly B star + whitedwarf binaries should exist within 1 kpc of the sun. If the B star is aBe star, the Be + white dwarf systems produce X-rays due to accretion ofwind material of the equatorial wind of the Be star onto the whitedwarf. The estimated X-ray luminosities range between 10 to the 29th and10 to the 33rd erg/s. The late type Be stars Mu2 Cru and HR 4804, thathave X-ray luminosities of about 10 to the 32nd erg/s, are likely tohave accreting white dwarf companions. The Be stars with unseencompanions are good candidates for Be + He star binaries.
| Beta Cephei pulsation and the helium-strong stars
Six He-strong stars are identified which may be good candidates insearches for Beta Cephei pulsation based on their location in a plot ofbeta vs (u-b). The significance of detection of pulsational variabilityamong the He-peculiar stars in the study of excitation mechanisms forBeta Cephei stars is discussed.
| Investigations on binaries with more massive invisible components. I - The H-deficient system KS Persei
More accurate spectroscopic elements of the hydrogen-deficient variablestar KS Persei have been determined from new and old radial velocities.A lower mass function of 3.6 + or - 0.4 solar masses is obtained alongwith improved orbital elements, including a period = 362.8 + or - 0.1 d,periastron passage = J.D. 2435152 + or - 5, e = 0.30 + or - 0.03, omega= 277 deg + or - 6 deg, a sin i = 228 + or - 8 x 10 to the 6th km. Itsmore massive, invisible companion, formerly proposed as a black holecandidate, is now recognized as a hot star.
| Photometric observations and interpretation of the extremely hydrogen-deficient binary Upsilon Sagittarii
The results of a detailed photometric investigation of the extremelyhydrogen-deficient binary Upsilon Sgr are presented. The nature of thequasi-periodic 20-day variation previously reported can be understood interms of radial pulsation of the supergiant primary component. Fromobservational and theoretical considerations, the secondary componentcan best be understood as an early B-type main-sequence star with a massof about 11 solar masses, much higher than previously thought. UpsilonSgr does not appear to be capable of providing an evolutionary link tothe R CrB and extreme helium stars. Drastic orbital shrinkage during afuture common-envelope episode could draw the binary componentssufficiently close together to result in a Type I supernova.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Perseo |
| Ascensione retta: | 04h48m53.35s |
| Declinazione: | +43°16'32.1" |
| Magnitudine apparente: | 7.757 |
| Distanza: | 10000000 parsec |
| Moto proprio RA: | -2 |
| Moto proprio Dec: | -1.2 |
| B-T magnitude: | 8.252 |
| V-T magnitude: | 7.798 |
Cataloghi e designazioni:
|
