Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
What a local sample of spectroscopic binaries can tell us about the field binary population
We study a sample of spectroscopic binaries (SBs) in the local solarneighbourhood (d<= 100 pc and MV<= 4) in an attempt tofind the distributions of the period, P, the primary mass,m1, and the mass ratio q(=m2/m1), aswell as the initial mass function (IMF) of the local population of fieldbinaries. The sample was collated using available SB data and theHipparcos catalogue, the latter being used for distances and to refernumbers of objects to fractions of the local stellar population as awhole. We use the better determined double-lined SBs (SB2s) to calibratea Monte Carlo approach to modelling the q distribution of thesingle-lined SBs (SB1s) from their mass functions, f(m), and primarymasses, m1. The totalq distribution is then found by addingthe observed SB2 distribution to the Monte Carlo SB1 distribution. Whilea complete sample is not possible, given the data available, we are ableto address important questions of incompleteness and parameter-specificbiases by comparing subsamples of SBs with different ranges in parameterspace. Our results show a clear peak in the q distribution of fieldbinaries near unity. This is dominated by the SB2s, but the flatdistribution of the SB1s is inconsistent with their components beingchosen independently at random from a steep IMF.
| VLA Observations of ζ Aurigae: Confirmation of the Slow Acceleration Wind Density Structure
Studies of the winds from single K and early M evolved stars indicatethat these flows typically reach a significant fraction of theirterminal velocity within the first couple of stellar radii. The mostdetailed spatially resolved information of the extended atmospheres ofthese spectral types comes from the ζ Aur eclipsing binaries.However, the wind acceleration inferred for the evolved primaries inthese systems appears significantly slower than for stars of similarspectral type. Since there are no successful theories for mass loss fromK and early M evolved stars, it is important to place strong empiricalconstraints on potential models and determine whether this difference inacceleration is real or an artifact of the analyses. We have undertakena radio continuum monitoring study of ζ Aurigae (K4 Ib + B5 V)using the Very Large Array to test the wind density model of Baade etal. that is based on Hubble Space Telescope (HST) Goddard HighResolution Spectrograph ultraviolet spectra. ζ Aur was monitored atcentimeter wavelengths over a complete orbital cycle, and fluxvariations during the orbit are found to be of similar magnitude tovariations at similar orbital phases in the adjacent orbit. Duringeclipse, the flux does not decrease, showing that the radio emissionoriginates from a volume substantially larger thanR3K~(150Rsolar)3 surroundingthe B star. Using the one-dimensional density model of the K4 Ibprimary's wind derived from HST spectral line profile modeling andelectron temperature estimates from previous optical and new HSTstudies, we find that the predicted radio fluxes are consistent withthose observed. Three-dimensional hydrodynamic simulations indicate thatthe accretion flow perturbations near the B star do not contributesignificantly to the total radio flux from the system, consistent withthe radio eclipse observations. Our radio observations confirm the slowwind acceleration for the evolved K4 Ib component. ζ Aur's velocitystructure does not appear to be typical of single stars with similarspectral types. This highlights the need for more comprehensivemultiwavelength studies for both single stars, which have been sadlyneglected, and other ζ Aur systems to determine if its windproperties are typical.
| The Impact of Space Experiments on our Knowledge of the Physics of the Universe
With the advent of space experiments it was demonstrated that cosmicsources emit energy practically across all the electromagnetic spectrumvia different physical processes. Several physical quantities givewitness to these processes which usually are not stationary; thosephysical observable quantities are then generally variable. Thereforesimultaneous multifrequency observations are strictly necessary in orderto understand the actual behaviour of cosmic sources. Space experimentshave opened practically all the electromagnetic windows on the Universe.A discussion of the most important results coming from multifrequencyphotonic astrophysics experiments will provide new inputs for theadvance of the knowledge of the physics, very often in its more extremeconditions. A multitude of high quality data across practically thewhole electromagnetic spectrum came at the scientific community'sdisposal a few years after the beginning of the Space Era. With thesedata we are attempting to explain the physics governing the Universeand, moreover, its origin, which has been and still is a matter of thegreatest curiosity for humanity. In this paper we will try to describethe last steps of the investigation born with the advent of spaceexperiments, to note upon the most important results and open problemsstill existing, and to comment upon the perspectives we can reasonablyexpect. Once the idea of this paper was well accepted by ourselves, wehad the problem of how to plan the exposition. Indeed, the exposition ofthe results can be made in different ways, following several points ofview, according to: - a division in diffuse and discrete sources; -different classes of cosmic sources; - different spectral ranges, whichimplies in turn a sub-classification in accordance with differenttechniques of observations; - different physical emission mechanisms ofelectromagnetic radiation; - different vehicles used for launching theexperiments (aircraft, balloons, rockets, satellites, observatories). Inorder to exhaustively present The Impact of Space Experiments on ourKnowledge of the Physics of the Universe it would then have beennecessary to write a kind of Encyclopaedia of the Astronomical SpaceResearch, which is not our desire. On the contrary, since our goal is toprovide an useful tool for the reader who has not specialized in spaceastrophysics and for the students, we decided to write this paper in theform of a review, the length of which can be still consideredreasonable, taking into account the complexity of the argumentsdiscussed. Because of the impossibility of realizing a complete pictureof the physics governing the Universe, we were obliged to select how toproceed, the subjects to be discussed the more or the less, or those tobe rejected. Because this work was born in the Ph.D. thesis of one of us(LSG) (Sabau-Graziati, 1990) we decided to follow the `astronomicaltradition' used there, namely: the spectral energy ranges. Although suchenergy ranges do not determine physical objects (even if in many casessuch ranges are used to define the sources as: radio, infrared, optical,ultraviolet, X-ray, γ-ray emitters), they do determine themethods of study, and from the technical point of view they define thetechnology employed in the relative experiments. However, since then wehave decided to avoid a deep description of the experiments, satellites,and observatories, simply to grant a preference to the physical results,rather than to technologies, however fundamental for obtaining thoseresults. The exposition, after an introduction (Section 1) and somecrucial results from space astronomy (Section 2), has been focussed intothree parts: the physics of the diffuse cosmic sources deduced fromspace experiments (Section 3), the physics of cosmic rays from ground-and space-based experiments (Section 4), and the physics of discretecosmic sources deduced from space experiments (Section 5). In this firstpart of the paper we have used the logic of describing the main resultsobtained in different energy ranges, which in turn characterize theexperiments on board space vehicles. Within each energy range we havediscussed the contributions to the knowledge of various kind of cosmicsources coming from different experiments. And this part is mainlyderived by the bulk of the introductory part of LSG's Ph.D. thesis. Inthe second part of the paper, starting from Section 6, we have preferredto discuss several classes of cosmic sources independently of the energyranges, mainly focussing the results from a multifrequency point ofview, making a preference for the knowledge of the physics governing thewhole class. This was decided also because of the multitude of new spaceexperiments launched in the last fifteen years, which would haverendered almost impossible a discussion of the results divided intoenergy ranges without weakening the construction of the entire puzzle.We do not pretend to cover every aspect of every subject consideredunder the heading of the physics of the universe. Instead a crosssection of essays on historical, modern, and philosophical topics areoffered and combined with personal views into tricks of the spaceastrophysics trade. The reader is, then, invited to accept this papereven though it obviously lacks completeness and the arguments discussedare certainly biased by a selection effect owed essentially to ourknowledge, and to it being of a reasonable length. Some parts of itcould seem, in certain sense, to belong to an older paper, in which the`news' is not reported. But this is owed to our own choice, just in fullaccord with the goals of the text: we want to present those resultswhich have, in our opinion, been really important, in the development ofthe science. These impacting results do not necessarily constitute thelast news. This text was formally closed just on the day of the launchof the INTEGRAL satellite: October 17, 2002. After that date onlyfinishing touches have been added.
| Spectral Classification of the Hot Components of a Large Sample of Stars with Composite Spectra, and Implication for the Absolute Magnitudes of the Cool Supergiant Components.
A sample of 135 stars with composite spectra has been observed in thenear-UV spectral region with the Aurélie spectrograph at theObservatoire de Haute-Provence. Using the spectral classifications ofthe cool components previously determined with near infrared spectra, weobtained reliable spectral types of the hot components of the samplesystems. The hot components were isolated by the subtraction methodusing MK standards as surrogates of the cool components. We also derivedthe visual magnitude differences between the components usingWillstrop's normalized stellar flux ratios. We propose a photometricmodel for each of these systems on the basis of our spectroscopic dataand the Hipparcos data. We bring to light a discrepancy for the Gsupergiant primaries between the visual absolute magnitudes deduced fromHipparcos parallaxes and those tabulated by Schmidt-Kaler for the GIbstars: we propose a scale of Mv-values for these stars incomposite systems. By way of statistics, about 75% of the hot componentsare dwarf or subgiant stars, and 25% should be giants. The distributionin spectral types is as follows: 41% of B-type components, 57% of typeA, and 2% of type F; 68% of the hot components have a spectral type inthe range B7 to A2. The distribution of the ΔMv-valuesshows a maximum near 0.75 mag.
| A catalog of rotational and radial velocities for evolved stars. II. Ib supergiant stars
Rotational velocity vsin i and mean radial velocity are presented for asample of 231 Ib supergiant stars covering the spectral region F, G andK. This work is the second part of the large survey carried out with theCORAVEL spectrometer to establish the behavior of the rotation for starsevolving off the main sequence (De Medeiros & Mayor 1999). Thesedata will add constraints to the study of the rotational behavior inevolved stars, as well as solid information concerning tidalinteractions in binary systems and on the link between rotation,chemical abundance and activity in stars of intermediate masses. Basedon observations collected at the Haute-Provence Observatory,Saint-Michel, France and at the European Southern Observatory, La Silla,Chile Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/395/97
| A Large Spectral Class Dependence of the Wilson-Bappu Effect among Luminous Stars
The striking correlation between Ca II K-line emission width andabsolute visual magnitude has not previously been well calibrated forstars more luminous than giants. From a sample of binary systems fit toisochrones, we find deviations of more than 2 mag, correlated withspectral class, between these binarity Mv values and theWilson-Bappu relation. Additional Mv values derived fromHipparcos parallaxes are used to explore the systematics. The spectralclass dependence vanishes for K-line parameterlogW0<=1.80. Linear spectral class corrections are derivedfor the more luminous stars, with logW0>=2.00, while atable is provided for the transition region. The dispersion from theserelations is about +/-0.6 mag. This recalibration extends thedemonstrated applicability of the Wilson-Bappu technique toMv~=-5 or distance ~20 kpc.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Spectral Classification of Unresolved Binary Stars with Artificial Neural Networks
An artificial neural network technique has been developed to performtwo-dimensional spectral classification of the components of binarystars. The spectra are based on the 15 Å resolution near-infrared(NIR) spectral classification system described by Torres-Dodgen &Weaver. Using the spectrum with no manual intervention except wavelengthregistration, a single artificial neural network (ANN) can classifythese spectra with Morgan-Keenan types with an average accuracy of about2.5 types (subclasses) in temperature and about 0.45 classes inluminosity for up to 3 mag of difference in luminosity. The error intemperature classification does not increase substantially until thesecondary contributes less than 10% of the light of the system. Byfollowing the coarse-classification ANN with a specialist ANN, the meanabsolute errors are reduced to about 0.5 types in temperature and 0.33classes in luminosity. The resulting ANN network was applied to sevenbinary stars.
| High excitation emission lines in binary systems with roundchroms
An unexpected empirical fact, a dependence of the observed luminositiesin high excitation emission lines - 1240 NV, 1400 SiIV, 1550 CIV, 1640HeII - on the intercomponent distance a of RS CVn type close binarysystems, is revealed. It is assumed that those high excitation emissionlines are generated most probably in a cone-like region between theLagrangian point L_1 and the surface of the primary component of thesystem. The behavior of high excitation emission lines at various phasesof the eclipse in the case of two binary systems, SX Cas and 22 Vul,indicates the possibility of existence of such a `Lagrangian cone' inthe structure of common chromospheres - roundchroms - of close binarysystems as a main source of generation of high excitation emissionlines.
| Empirical Luminosities and Radii of Early-Type Stars after Hipparcos
Using Hipparcos parallaxes we derive empirical luminosities and radii ofthe early-type stars for which the effective temperatures are known fromdirectly measured angular diameters and total absolute fluxes. Theempirical luminosities allow a direct comparison of the position ofthese stars in the fundamental HR diagram with evolutionary tracks. Thecomparison shows an overall agreement with the Y=0.30 and Z=0.02 trackscomputed with OPAL opacities and moderate amount of overshooting fromthe convective core. In addition, we present evidence that systematicerrors of the masses read off the evolutionary tracks are below 10%.Consequently, the surface gravities obtained from these "evolutionary"masses and the empirical radii are very nearly model-independent.Spectrographic and photometric observations of these stars can thereforebe used for verifying model atmospheres and calibrating photometric logg indices.
| A wind accretion wake in RW Hydrae?
RW Hydrae is an eclipsing detached binary star system, consisting of amass losing M-giant and a hot white dwarf on circular orbits. We analyzeUV observations of RW Hydrae. Approximately at phi = 0.78, clearlyunrelated to the primary eclipse, we detect in the UV light curve anevent with significantly reduced UV flux. The spectral characteristicsof this event indicate Rayleigh scattering due to a high column densityof neutral hydrogen in the line of sight to the hot white dwarf. Wemodel this observation in the framework of an accretion wake trailingthe white dwarf. This interpretation is analogous to comparable modelsfor zeta Aur systems and X-ray binaries. We find qualitative agreementbetween our 3D hydrodynamical accretion simulation and the observed UVlight curve of RW Hya. Based on observations with the InternationalUltraviolet Explorer and observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc. under NASA contract No. NAS5-26555.
| From OAO2 to HST. A quarter of a century of ultraviolet astronomy.
Not Available
| Spectral classifications in the near infrared of stars with composite spectra. III. Study of a sample of 137 objects with the Aurelie spectrograph
We provide spectral classifications for a sample of 137 stars mentionedas having composite spectra. The classifications were carried out on 33Angstroms /mm spectra in the region 8370 - 8870 Angstroms. Of these 137objects, 115 correspond in the infrared to cool stars (G, K or M) ofluminosity classes III, II and I; for 22 stars, we find only hot spectraof types B, A, F or Am, so that they do not fulfil our definition ofcomposite spectra. We detect four new Am stars, and one Am star (HD70826) turns out to be a composite spectrum object. As in Paper II, thecool components of composite spectra show a strong concentration in thevicinity of G8III. Based upon observations carried out at Observatoirede Haute-Provence (OHP).
| Ultraviolet and Optical Studies of Binaries with Luminous Cool Primaries and Hot Companions. V. The Entire IUE Sample
We have obtained or retrieved IUE spectra for over 100 middle- andlate-type giant and supergiant stars whose spectra indicate the presenceof a hot component earlier than type F2. The hot companions areclassified accurately by temperature class from their far-UV spectra.The interstellar extinction of each system and the relative luminositiesof the components are derived from analysis of the UV and opticalfluxes, using a grid of UV intrinsic colors for hot dwarfs. We find thatthere is fair agreement in general between current UV spectralclassification and ground-based hot component types, in spite of thedifficulties of assigning the latter. There are a few cases in which thecool component optical classifications disagree considerably with thetemperature classes inferred from our analysis of UV and opticalphotometry. The extinction parameter agrees moderately well with otherdeterminations of B-V color excess. Many systems are worthy of furtherstudy especially to establish their spectroscopic orbits. Further workis planned to estimate luminosities of the cool components from the dataherein; in many cases, these luminosities' accuracies should becomparable to or exceed those of the Hipparcos parallaxes.
| Giantic roundchroms of binary systems SX Cas and 22 Vul.
Not Available
| Temperatures for A0-K0 Supergiants from 13-Color Photometry
Observations on the 13-color photometric system are reported for 71A0-K0 supergiant stars brighter than V=6.0. Three independent methods todetermine the effective temperatures for A0-K0 supergiants from their13C photometry are discussed: 1) Calibrations between T$eff$and reddening-free indices are developed; temperatures were collectedfrom the literature for the calibration stars, and most of them werespectroscopically determined. 2) An empirical correlation betweenintrinsic colors in the 13C system and T$eff$ has beenderived. 3) The unreddened colors are compared with synthetic colorscalculated by Kurucz (1989); this leads to simultaneous estimates forT$eff$ and log $g$. The estimated uncertainties inT$eff$ by the three methods are comparable and areapproximately plus or minus 200-300K. (SECTION: Stars)
| The structure of roundchroms in close binary systems
The determination of common chromospheres - roundchroms - for 32 closebinary systems, all of RS CVn type, is described; the main parameters ofthese roundchroms - volumes, electron concentrations and masses, areestimated. Three types of roundchroms are established according to theirstructure. The empirical relationship between their electronconcentration n_e and intercomponent distance a - n_e = K a^-0.80,discovered earlier, is confirmed by data for over fifty close binarysystems. This law holds promise for the determination of component radiiof close binary systems and some parameters of their roundchroms.
| Common chromospheres - roundchroms - as a means for the study of binary systems
An independent method based on the concept of common chromospheres -roundchroms - is proposed for the determination of the radii of the maincomponents of RS CVn-type close binary systems. The essence of themethod is in the coincidence of the radius of the main component of thebinary system with the equipotential zero-velocity surface for somevalues of the Jacobi constant. As an illustration, the method is appliedto a sample of 15 RS CVn-type systems, and as a result the revised radiiof the main components in the systems are determined. The mainparameters, particularly the volumes, electron concentrations and massesof the roundchroms, are obtained as well. Empirical dependences of theelectron concentration in the roundchrom, n_e, and of the 2800 Mg iidoublet luminosity, L(Mg ii), on intercomponent distance a werediscovered, the first in the form n_e~a^-0.85 and the second in the formL(Mg ii)~a^1.66. The formation of a roundchrom in close binary systemsis inevitable, and the roundchrom is as essential a physical formationfor binary systems as are the corona and chromosphere for single stars.
| A critical test of stellar evolution and convective core `overshooting' by means of zeta Aurigae systems
Quantitative tests of late stellar evolution are presented by computingmodels with our evolutionary code to match the exact properties ofcertain zeta Aurigae eclipsing binaries and related non-eclipsingsystems. Those binaries have a late- type giant or supergiant primaryand their orbital inclination is well determined from either eclipses orspeckle orbits. They provide the only direct measurements of masses forsuch evolved stars, together with other well-determined physicalparameters. In the computations all effects of enhanced mixing beyondthe convective cores during central hydrogen burning stages, e.g., coreovershooting and any rotationally induced meridional mixing, arerepresented by a simple overshooting prescription. Its single parametercan be constrained to within 25 per cent of its value and leads to anovershooting length l_ov of ~0.24 H_P (pressure scaleheights) for 2.5Msolar, slightly increasing to ~0.32 H_P for 6.5 Msolar. Those valuesare required by our code to reproduce the well-determined luminositiesof the giants in or at the end of their blue loop. This new methodprovides the currently most sensitive test of the overshooting issue.
| Standard Stars for CCD Photometry in the Vilnius System
The results of seven-color photometry of 73 stars of magnitudes 9--13are given in 11 areas. The areas are of 10times 10 arcmin size at lowgalactic latitudes in the transparent parts of the Milky Way in theconstellations of Serpens, Aquila, Vulpecula, Lyra and Cygnus. Thesestars can be used as zero-point standards for future CCD photometry.
| Mesures de vitesses radiales. VIII. Accompagnement AU sol DU programme d'observation DU satellite HIPPARCOS
We publish 1879 radial velocities of stars distributed in 105 fields of4^{\circ} \times 4^{\circ}. We continue the PPO series \cite[(Fehrenbachet al. 1987;]{Feh87} \cite[Duflot et al. 1990, 1992 and 1995),]{Du90}using the Fehrenbach objective prism method. Table 1 only available inelectronic form at CDS via to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Radiation driven wind models for A, F and G supergiants.
We investigate the effects of radiation pressure on the atmospheres ofA, F and G-supergiants by calculating hydrodynamical model atmospheresfor stars with 5500<=T_eff_<=9500K. In the subsonic part of thewind, the radiation pressure by continuum and lines from Kurucz (1992,ATLAS 6 program) is taken into account. In the supersonic part of thewind, the radiation pressure is expressed in terms of the forcemultiplier formalism (Castor et al. 1975ApJ...195..157C) with thecorrection for the finite disk taken into account. The temperaturestructure is from the T(τ) relation of blanketed model atmospheres.The predicted mass loss rates of the A-supergiants agrees excellentlywith the observed values. However the predicted terminal velocities areabout a factor 3 higher than observed. We discuss several possiblecauses for this discrepancy. The most likely one is a change in theforce multiplier parameter α of the line radiation force fromabout 0.5 in the lower parts of the wind to a much smaller value ofabout 0.1 throughout most of the wind. This might be the result of achange in the ionization of the wind with distance, or a decoupling ofthe line driven ions in the wind from the ambient gas. The predictedmass loss rate of the G-type supergiant 22Vul, which is the onlyG-supergiant with a reliable mass loss rate, is a factor 10^5^ smallerthan observed. This is probably due to the fact that G-supergiants havechromospheres, which were not taken into account in our model. Ourmodels for F-supergiants could not be compared with observations becausethere are no reliable empirical mass loss rates or terminal windvelocities for normal F-supergiants. The F-supergiants ρCas andHR8752 have highly variable mass loss rates which obviously cannot beexplained by our models. We conclude that mass loss from A-typesupergiants is most likely due to a line driven wind but that the massloss from G-supergiants is not. It is interesting to find the spectraltype between F0 and G3 where the radiation driven wind models break downand to compare that with the type where the chromospheres becomenoticeable. The high opacity in the hydrogen ionization zone produces anet outward force in those layers. This gives rise to a pressureinversion in the subsonic part of the atmosphere, but does not lead tohigh mass loss rates.
| Non-LTE model chromospheres of zeta Aurigae stars
Plane-parallel, one-component, non-local thermodynamic equilibrium(LTE), semi-empirical model chromospheres have been constructed for theprimary stars in the zeta Aurigae systems HR 6902 and 22 Vul. This hasbeen accomplished by means of radiative transfer calculations undertakenwith a non-LTE multilevel radiative transfer code, MULTI, and curves ofgrowth applied to chromospheric eclipse spectra. The former are thesubject of this paper, which is divided into two parts. The firstbriefly describes the observations and derivation of basic stellarparameters, while the second concerns the radiative transfercalculations and conclusions which follow from them. In particular, thecalculated model photospheres plus chromospheres are used to investigatethe relative contribution of primary and secondary stars to the overallradiation field. The radiation field of the hot secondary isapproximated by use of an appropriate Kurucz model atmosphere. Althoughthe hot companion is wholly responsible for the ionization of themetals, this is not the case as far as excitation is concerned. Use of a47-level Fe atomic model demonstrates the intimate convolution betweenatomic and atmospheric physics. The chromospheres calculated byradiative transfer methods are compared with those derived empiricallyby curves of growth and are found to be comparable in the line-formingregion of CaII H & K, MgII h & k and the many FeII lines.Finally, owing to the effects of radiative transitions from high-lyinglevels which are enhanced by the radiation field of the hot companion,it is found that lines resulting from transitions from the a^4F, a^4Dand a^4P levels of FeII may not be used to derive an inner windtemperature for 22 Vul. This is the first time that zeta Aurigaechromospheres have been the subject of such intensive radiative transfercalculations.
| A Catalogue of Correlations Between Eclipsing Binaries and Other Categories of Double Stars
Among the 9110 stars in The Bright Star Catalogue, there are 225eclipsing or ellipsoidal variables. A search has been made for these incatalogues of spectroscopic binaries, visual double or multiple stars,speckle interferometry, occulation binaries, and galatic clusters. Themajority of the photometric binaries are also members of groups ofhigher multiplicity. The variables are in systems ranging from one to 91stars, five on the average. 199 are either spectroscopic binaries (SB)or stars with variable radial velocity, with orbital periods known for160. Photometric periods are lacking for 48 while SB periods areavailable for 23 of these. Observers with photoelectric equipment areencouraged to plan observations to test if the SB periods are consistentwith photometric data. Observers are likewise encouraged to examinethose stars for which the photometric and SB periods appear to beinconsistent. Parallaxes are available for 86 of the stars, 41 of themindicating distances nearer than 50 parsecs.
| The measurement of mass loss rates of binary systems enveloped in winds or expanding shells.
Not Available
| Optical spectra of ζ Aurigae systems. VIII. The chromosphere of HR6902.
We analyze chromospheric spectra of the ζ Aur system HR6902observed during its 1987 and 1989 eclipses, deriving empirical columndensities, number densities and ionization characteristics for 5distinct heights within the giant primary's chromosphere. Columndensities for CaII are determined from K-line profiles and for otherions by a curve-of-growth technique, modelling iteratively the scaleheights of the line-forming regions according to the known geometry ofthe eclipses and dimensions of the system. We find evidence for theexistence of two plasma components in and beyond the chromosphere thatare physically different. The inner component has a low turbulence(v_tur_=15km/s); its ion column densities decrease steeply outwards andseem to outline the geometrically-thin chromosphere itself. There issome evidence that the geometrical extent of this component varies fromone eclipse to another. The outer component has a significantly higherturbulence (v_tur_=45km/s) and a scale height that is nearly an order ofmagnitude larger. Because its column density is much lower, we have onlybeen able to observe it by our technique in the strongest chromosphericlines (i.e. CaII); it extends much farther outwards than the innercomponent - probably into regions beyond the top of the chromosphere,for which IUE observations give evidence of plasma at transition-regiontemperatures (=~10^5^). We consider the evolutionary status of the twocomponents, and conclude that the primary star is an ordinary3.9Msun_ blue-loop giant that is 2.0x10^8^years old. Wecompare the chromosphere of HR6902 with those of other ζ Aur giantsand discuss the meaning of the `dividing lines' as tracers of the verydifferent conditions which are encountered in G and K (super)giantchromospheres.
| Hybrid stars and the reality of "dividing lines" among G to K bright giants and supergiants.
We present results of pointed ROSAT PSPC observations of 15 hybridstars/candidates, which have been analyzed in a homogenous way. 7 ofthese stars were observed in X-rays for the first time. 12 out of 15hybrid stars have been detected as X-ray sources, some of them close tothe detection limit. We conclude that essentially all hybrid stars asdefined by the simultaneous presence of transition region line emissionand cool stellar winds are X-ray sources if exposed sufficiently deep.The X-ray luminosities of hybrid stars cover a range between 2x10^27^and ~10^30^erg/s. Their X-ray surface fluxes can be as low as =~20erg/cm^2^/s and thus considerably lower than those of normal luminosityclass (LC) III giants. X-ray spectra of hybrid stars tend to be harderthan that of normal LC III giants, moreover, the X-ray brightest starshave the hardest spectra. We find that for K II giants the normalizedX-ray flux versus C IV flux obeys a power law with an exponent a=2.9,steeper than among normal giants (1.5). Hybrid K II stars are X-rayunderluminous by a factor of 5 to 20 compared to LC III giants at thesame level of normalized CIV flux f_CIV_/f_bol_; hybrid G supergiantsare even more X-ray deficient. We reanalyze the CaII wind dividing lineand find it vertical at B-V=1.45 for LC III giants. It is nearlyhorizontal between B-V=1.45 and 1.0 (at M_bol_=~-2...-3), and not welldefined for supergiants with B-V<1.0. We therefore suggest thatpossibly all LC II and Ib G and K giants are hybrid stars and that the"dividing line" concept in its simplest form is not valid for G/K giantsbrighter than M_bol_=~-2. Hybrid stars are supposed to be evolvedintermediate mass stars and their coronal activity may in principle bedetermined by the individual history of each star.
| Professional-amateur collaboration in variable star research: IV. One possibility for amateur/professional collaboration
Not Available
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Лисица |
| Ректацензија: | 20h15m30.20s |
| Deклинација: | +23°30'31.0" |
| Apparent магнитуда: | 5.15 |
| Даљина: | 1333.333 parsecs |
| Proper motion RA: | 2.2 |
| Proper motion Dec: | -6.5 |
| B-T magnitude: | 6.464 |
| V-T magnitude: | 5.289 |
Каталог и designations:
|
