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Ýlgili Makaleler
Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| Can stellar wobble in triple systems mimic a planet?
The first extrasolar planets were detected by the measurement of thewobble of the parent star. This wobble leads to the periodic modulationof three observables: the radial velocity, the position on the sky andthe time of arrival of periodic signals. We show that the same wobble,and therefore the same modulation of the three observables, can be dueto the presence of a more distant binary stellar companion. Thus, the observation of the wobble does not, by itself, constitute a proof of aplanet detection. In particular, astrometric confirmation of a wobbledoes not necessarily provide a sufficient proof of the existence of aplanet candidate detected by radial velocity. Additional conditions,which we discuss here, must be fulfilled. We investigate the observedwobble for the planet candidates already detected and we find that, foreach case, a wobble due to a binary stellar companion can beexcluded.
However, for apparent Saturn-like planets in wideorbits, there may be an ambiguity in future detections, especially inspaceborne astrometric missions. We conclude that, in some cases, adefinitive proof of the presence of a planet requires furtherobservations such as direct imaging.
| Observed Orbital Eccentricities
For 391 spectroscopic and visual binaries with known orbital elementsand having B0-F0 IV or V primaries, we collected the derivedeccentricities. As has been found by others, those binaries with periodsof a few days have been circularized. However, those with periods up toabout 1000 or more days show reduced eccentricities that asymptoticallyapproach a mean value of 0.5 for the longest periods. For those binarieswith periods greater than 1000 days their distribution of eccentricitiesis flat from 0 to nearly 1, indicating that in the formation of binariesthere is no preferential eccentricity. The binaries with intermediateperiods (10-100 days) lack highly eccentric orbits.
| B Star Rotational Velocities in h and χ Persei: A Probe of Initial Conditions during the Star Formation Epoch?
Projected rotational velocities (vsini) have been measured for 216 B0-B9stars in the rich, dense h and χ Persei double cluster and comparedwith the distribution of rotational velocities for a sample of fieldstars having comparable ages (t~12-15 Myr) and masses (M~4-15Msolar). For stars that are relatively little evolved fromtheir initial locations on the zero-age main sequence (ZAMS) (those withmasses M~4-5 Msolar), the mean vsini measured for the h andχ Per sample is slightly more than 2 times larger than the meandetermined for field stars of comparable mass, and the cluster and fieldvsini distributions differ with a high degree of significance. Forsomewhat more evolved stars with masses in the range 5-9Msolar, the mean vsini in h and χ Per is 1.5 times thatof the field; the vsini distributions differ as well, but with a lowerdegree of statistical significance. For stars that have evolvedsignificantly from the ZAMS and are approaching the hydrogen exhaustionphase (those with masses in the range 9-15 Msolar), thecluster and field star means and distributions are only slightlydifferent. We argue that both the higher rotation rates and the patternof rotation speeds as a function of mass that differentiatemain-sequence B stars in h and χ Per from their field analogs werelikely imprinted during the star formation process rather than a resultof angular momentum evolution over the 12-15 Myr cluster lifetime. Wespeculate that these differences may reflect the effects of the higheraccretion rates that theory suggests are characteristic of regions thatgive birth to dense clusters, namely, (1) higher initial rotationspeeds; (2) higher initial radii along the stellar birth line, resultingin greater spin-up between the birth line and the ZAMS; and (3) a morepronounced maximum in the birth line radius-mass relationship thatresults in differentially greater spin-up for stars that become mid- tolate-B stars on the ZAMS.
| On the ages of exoplanet host stars
We obtained spectra, covering the CaII H and K region, for 49 exoplanethost (EH) stars, observable from the southern hemisphere. We measuredthe chromospheric activity index, R'{_HK}. We compiled previouslypublished values of this index for the observed objects as well as theremaining EH stars in an effort to better smooth temporal variations andderive a more representative value of the average chromospheric activityfor each object. We used the average index to obtain ages for the groupof EH stars. In addition we applied other methods, such as: Isochrone,lithium abundance, metallicity and transverse velocity dispersions, tocompare with the chromospheric results. The kinematic method is a lessreliable age estimator because EH stars lie red-ward of Parenago'sdiscontinuity in the transverse velocity dispersion vs dereddened B-Vdiagram. The chromospheric and isochrone techniques give median ages of5.2 and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The medianage of F and G EH stars derived by the isochrone technique is 1-2 Gyrolder than that of identical spectral type nearby stars not known to beassociated with planets. However, the dispersion in both cases is large,about 2-4 Gyr. We searched for correlations between the chromosphericand isochrone ages and L_IR/L* (the excess over the stellarluminosity) and the metallicity of the EH stars. No clear tendency isfound in the first case, whereas the metallicy dispersion seems toslightly increase with age.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| Tidal Effects in Binaries of Various Periods
We found in the published literature the rotational velocities for 162B0-B9.5, 152 A0-A5, and 86 A6-F0 stars, all of luminosity classes V orIV, that are in spectroscopic or visual binaries with known orbitalelements. The data show that stars in binaries with periods of less thanabout 4 days have synchronized rotational and orbital motions. Stars inbinaries with periods of more than about 500 days have the samerotational velocities as single stars. However, the primaries inbinaries with periods of between 4 and 500 days have substantiallysmaller rotational velocities than single stars, implying that they havelost one-third to two-thirds of their angular momentum, presumablybecause of tidal interactions. The angular momentum losses increase withdecreasing binary separations or periods and increase with increasingage or decreasing mass.
| Spectra and Diagnostics for the Direct Detection of Wide-Separation Extrasolar Giant Planets
We calculate as a function of orbital distance, mass, and age thetheoretical spectra and orbit-averaged planet/star flux ratios forrepresentative wide-separation extrasolar giant planets (EGPs) in theoptical, near-infrared, and mid-infrared. Stellar irradiation of theplanet's atmosphere and the effects of water and ammonia clouds areincorporated and handled in a consistent fashion. We include predictionsfor 12 specific known EGPs. In the process, we derive physicaldiagnostics that can inform the direct EGP detection and remote sensingprograms now being planned or proposed. Furthermore, we calculate theeffects of irradiation on the spectra of a representative companionbrown dwarf as a function of orbital distance.
| Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems
In preparation for the advent of the Allen Telescope Array, the SETIInstitute has the need to greatly expand its former list of ~2000targets compiled for Project Phoenix, a search for extraterrestrialtechnological signals. In this paper we present a catalog of stellarsystems that are potentially habitable to complex life forms (includingintelligent life), which comprises the largest portion of the new SETItarget list. The Catalog of Nearby Habitable Systems (HabCat) wascreated from the Hipparcos Catalogue by examining the information ondistances, stellar variability, multiplicity, kinematics, and spectralclassification for the 118,218 stars contained therein. We also make useof information from several other catalogs containing data for Hipparcosstars on X-ray luminosity, Ca II H and K activity, rotation, spectraltypes, kinematics, metallicity, and Strömgren photometry. Combinedwith theoretical studies on habitable zones, evolutionary tracks, andthird-body orbital stability, these data are used to remove unsuitablestars from HabCat, leaving a residue of stars that, to the best of ourcurrent knowledge, are potentially habitable hosts for complex life.While this catalog will no doubt need to be modified as we learn moreabout individual objects, the present analysis results in 17,129Hipparcos ``habstars'' near the Sun (75% within 140 pc), ~2200 of whichare known or suspected to be members of binary or triple star systems.
| A Planetary Companion to HD 40979 and Additional Planets Orbiting HD 12661 and HD 38529
We report the detection of three extrasolar planets from the Lick andKeck observatories. The F8 V star HD 40979 has a companion with orbitalperiod P=263.1+/-3 days, eccentricity e=0.25+/-0.05, and velocitysemiamplitude K=101.2+/-5.6 m s-1. The inferred semimajoraxis is 0.83 AU and Msini=3.28MJup. Observations of planetarycompanions orbiting the G6 V star HD 12661 and the G4 IV star HD 38529have already been published, and here we report additional, longerperiod companions for both of these stars. The outer companion to HD12661 has Pc=1444.5+/-12.5 days, ec=0.20+/-0.04,and Kc=27.6+/-2.5 m s-1. Adopting a stellar massof 1.07 Msolar, we find Mcsini=1.57MJupand a semimajor axis of 2.56 AU. The second companion to HD 38529 hasPc=2174+/-30 days, ec=0.36+/-0.05, andKc=170.5+/-9 m s-1. The assumed mass of 1.39Msolar for HD 38529 yieldsMcsini=12.7MJup and a semimajor axis of 3.68 AU.Photometric observations at Fairborn Observatory reveal low-amplitudebrightness variations in HD 40979 and HD 38529 due to rotationalmodulation in the visibility of photospheric starspots, and they yieldrotation periods of 7.0 and 35.7 days, respectively, very different fromthe planetary orbital periods. The orbital parameters of these twosystems are compared with updated parameters for all of the knownmultiple-planet systems. Updated velocities are provided for the ΥAndromedae system.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. Keck time has been granted by both NASA and theUniversity of California.
| Stability of the HD 12661 Planetary System
In this paper we study the stability of the HD 12661planetary system in the framework of the N-body problem. Using theinitial conditions found and announced by the California & CarnegiePlanet Search Team, (http://exoplanets.org/almanacframe.html), weestimate the dynamical limits on orbital parameters that provide stable(quasi-periodic) motions of the system. We investigate the orbitalstability by combining the MEGNO indicator analysis with short-termintegrations of the orbital dynamics. The MEGNO technique, invented byCincotta & Simó (\cite{Cincotta2000}), makes it possible todistinguish efficiently between chaotic and regular dynamics of aconservative dynamical system. The orbital evolution, derivedsimultaneously with MEGNO, helps to identify sources of instability. Thenominal initial condition leads to a chaotic solution, with a Lyapunovtime =~ 1300 yr. In spite of this, the system motion seems to bebounded. This was examined directly, by long-term, 1 Gyr integrations.During this time, the eccentricities vary in the range (0.1, 0.4). Thesystem is locked in apsidal resonance with the critical argumentlibrating about 180deg, with a full amplitude varyingtypically between 40deg and 180deg. Using MEGNO,we found that the \pstar system evolves on a border of the 11:2 meanmotion resonance. This resonance is stable and results in aquasi-periodic evolution of the system. From the viewpoint of globaldynamics, the crucial factor for system stability is the presence of theapsidal resonance. We detected this resonance in a wide neighborhood ofthe initial condition in the space of orbital parameters of the system,and in wide ranges of relative inclination and masses of the planets.The center of libration can be 180deg (as in the nominalsystem) or 0deg. The regime depends on the initial values ofthe apsidal longitudes. Statistically, the system prefers almostexclusively one of these two resonance regimes. The \pstar system givesa very evident example of the dynamical role of secular resonances, andtheir influence on the stability of exosystems containing Jupiter-likeplanets. Data derived by numerical experiments are compared with theresults of Laplace-Lagrange secular theory. The analytical theory givesa crude approximation of the secular dynamics, because theeccentricities and masses of the planets are large, and the nominalsystem is near the 11:2 mean motion resonance.
| Astrophysics in 2001
During the year, astronomers provided explanations for solar topicsranging from the multiple personality disorder of neutrinos tocannibalism of CMEs (coronal mass ejections) and extra-solar topicsincluding quivering stars, out-of-phase gaseous media, black holes ofall sizes (too large, too small, and too medium), and the existence ofthe universe. Some of these explanations are probably possibly true,though the authors are not betting large sums on any one. The data oughtto remain true forever, though this requires a careful definition of``data'' (think of the Martian canals).
| Rotational Velocities of B Stars
We measured the projected rotational velocities of 1092 northern B starslisted in the Bright Star Catalogue (BSC) and calibrated them againstthe 1975 Slettebak et al. system. We found that the published values ofB dwarfs in the BSC average 27% higher than those standards. Only 0.3%of the stars have rotational velocities in excess of two-thirds of thebreakup velocities, and the mean velocity is only 25% of breakup,implying that impending breakup is not a significant factor in reducingrotational velocities. For the B8-B9.5 III-V stars the bimodaldistribution in V can be explained by a set of slowly rotating Ap starsand a set of rapidly rotating normal stars. For the B0-B5 III-V starsthat include very few peculiar stars, the distributions in V are notbimodal. Are the low rotational velocities of B stars due to theoccurrence of frequent low-mass companions, planets, or disks? Therotational velocities of giants originating from late B dwarfs areconsistent with their conservation of angular momentum in shells.However, we are puzzled by why the giants that originate from the earlyB dwarfs, despite having 3 times greater radii, have nearly the samerotational velocities. We find that all B-type primaries in binarieswith periods less than 2.4 days have synchronized rotational and orbitalmotions; those with periods between 2.4 and 5.0 days are rotating withina factor 2 of synchronization or are ``nearly synchronized.'' Thecorresponding period ranges for A-type stars are 4.9 and 10.5 days, ortwice as large. We found that the rotational velocities of the primariesare synchronized earlier than their orbits are circularized. The maximumorbital period for circularized B binaries is 1.5 days and for Abinaries is 2.5 days. For stars of various ages from 107.5 to1010.2 yr the maximum circularized periods are a smoothexponential function of age.
| Orbital Stability of Terrestrial Planets inside the Habitable Zones of Extrasolar Planetary Systems
We investigate orbital stability of terrestrial planets inside thehabitable zones of three stellar systems, i.e., 51 Peg, 47 UMa, and HD210277, with recently discovered giant planets. These systems havesimilar habitable zones; however, their giant planets have differentmasses and significantly different orbital parameters. It is shown thatstable orbits of terrestrial planets exist in the entire habitable zoneof 51 Peg as well as in the inner part of the habitable zone of 47 UMa,but no stable orbits are found in the habitable zone of HD 210277. Theobtained results allow us to draw general conclusions on the existenceof stable orbits in the habitable zones of newly found extra-solarplanetary systems.
| Orbital migration and the frequency of giant planet formation
We present a statistical study of the post-formation migration of giantplanets in a range of initial disk conditions. For given initialconditions we model the evolution of giant planet orbits under theinfluence of disk, stellar, and mass loss torques. We determine the massand semi-major axis distribution of surviving planets after diskdissipation, for various disk masses, lifetimes, viscosities, andinitial planet masses. The majority of planets migrate too fast and aredestroyed via mass transfer onto the central star. Most survivingplanets have relatively large orbital semi-major axes of several AU orlarger. We conclude that the extrasolar planets observed to date,particularly those with small semi-major axes, represent only a smallfraction ( ~ 25% to 33%) of a larger cohort of giant planets aroundsolar-type stars, and many undetected giant planets must exist at large(>1-2 AU) distances from their parent stars. As sensitivity andcompletion of the observed sample increase with time, this distantmajority population of giant planets should be revealed. We find thatthe current distribution of extrasolar giant planet masses implies thathigh mass (more than 1-2 Jupiter masses) giant planet formation must berelatively rare. Finally, our simulations imply that the efficiency ofgiant planet formation must be high: at least 10% and perhaps as many as80% of solar-type stars possess giant planets during their pre-mainsequence phase. These predictions, including those for pre-main sequencestars, are testable with the next generation of ground- and space-basedplanet detection techniques.
| Spectral line ratios as Teff indicators in solar-like stars
The ratios of spectral line depths are often used as indicators of thestellar effective temperature T_eff. In particular, Gray &Livingston (\cite{grayliv97a}) calibrated the temperature sensitivity ofthe ratios between the central depths of the line C I 538.032 nm andeither the Fe I 537.958 or the Ti II 538.103, making use of observedspectra of several solar-like stars. The ultimate reason for choosingthese lines was the subsequent application of their calibration to along series of solar data, collected at Kitt Peak (in disk-integratedlight) from 1978 to 1992, in order to get the T_eff variation of the Sunduring its 11-yr magnetic cycle (Gray & Livingston\cite{grayliv97b}). We propose a theoretical calibration that includes acareful treatment of convective transport and fits the stellar data verywell , showing, at the same time, that the empirical calibration of Grayand Livingston incorporates in the T_eff sensitivity an undesireddependence of line ratios on the surface gravities of the individualstars they used. A possible dependence of the calibration upon stellarrotation is also explored.
| Microstructure of the Local Interstellar Cloud and the Identification of the Hyades Cloud
We analyze high-resolution UV spectra of the Mg II h and k lines for 18members of the Hyades to study inhomogeneity along these proximate linesof sight. The observations were taken by the Space Telescope ImagingSpectrograph (STIS) instrument on board the Hubble Space Telescope(HST). Three distinct velocity components are observed. All 18 lines ofsight show absorption by the Local Interstellar Cloud (LIC); 10 starsshow absorption by an additional cloud, which we name the Hyades Cloud;and one star exhibits a third absorption component. The LIC absorptionis observed at a lower radial velocity than predicted by the LICvelocity vector derived by Lallement & Bertin in 1992 and Lallementet al. in 1995[vpredicted(LIC)-vobserved(LIC)=2.9+/-0.7 kms-1], which may indicate a compression or deceleration at theleading edge of the LIC. We propose an extension of the Hyades Cloudboundary based on previous HST observations of other stars in thegeneral vicinity of the Hyades, as well as ground-based Ca IIobservations. We present our fits of the interstellar parameters foreach absorption component. The availability of 18 similar lines of sightprovides an excellent opportunity to study the inhomogeneity of thewarm, partially ionized local interstellar medium (LISM). We find thatthese structures are roughly homogeneous. The measured Mg II columndensities do not vary by more than a factor of 2 for angular separationsof <~8°, which at the outer edge of the LIC correspond tophysical separations of <~0.6 pc.
| 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
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| On the origin of the O and B-type stars with high velocities. II. Runaway stars and pulsars ejected from the nearby young stellar groups
We use milli-arcsecond accuracy astrometry (proper motions andparallaxes) from Hipparcos and from radio observations to retrace theorbits of 56 runaway stars and nine compact objects with distances lessthan 700 pc, to identify the parent stellar group. It is possible todeduce the specific formation scenario with near certainty for twocases. (i) We find that the runaway star zeta Ophiuchi and the pulsarPSR J1932+1059 originated about 1 Myr ago in a supernova explosion in abinary in the Upper Scorpius subgroup of the Sco OB2 association. Thepulsar received a kick velocity of ~ 350 km s-1 in thisevent, which dissociated the binary, and gave zeta Oph its large spacevelocity. (ii) Blaauw & Morgan and Gies & Bolton alreadypostulated a common origin for the runaway-pair AE Aur and mu Col,possibly involving the massive highly-eccentric binary iota Ori, basedon their equal and opposite velocities. We demonstrate that these threeobjects indeed occupied a very small volume ~ 2.5 Myr ago, and show thatthey were ejected from the nascent Trapezium cluster. We identify theparent group for two more pulsars: both likely originate in the ~ 50 Myrold association Per OB3, which contains the open cluster alpha Persei.At least 21 of the 56 runaway stars in our sample can be linked to thenearby associations and young open clusters. These include the classicalrunaways 53 Arietis (Ori OB1), xi Persei (Per OB2), and lambda Cephei(Cep OB3), and fifteen new identifications, amongst which a pair ofstars running away in opposite directions from the region containing thelambda Ori cluster. Other currently nearby runaways and pulsarsoriginated beyond 700 pc, where our knowledge of the parent groups isvery incomplete.
| Albedo and Reflection Spectra of Extrasolar Giant Planets
We generate theoretical albedo and reflection spectra for a full rangeof extrasolar giant planet (EGP) models, from Jovian to 51 Pegasi classobjects. Our albedo modeling utilizes the latest atomic and molecularcross sections, Mie theory treatment of scattering and absorption bycondensates, a variety of particle size distributions, and an extensionof the Feautrier technique, which allows for a general treatment of thescattering phase function. We find that, because of qualitativesimilarities in the compositions and spectra of objects within each offive broad effective temperature ranges, it is natural to establish fiverepresentative EGP albedo classes. At low effective temperatures(Teff<~150 K) is a class of ``Jovian'' objects (class I)with tropospheric ammonia clouds. Somewhat warmer class II, or ``watercloud,'' EGPs are primarily affected by condensed H2O.Gaseous methane absorption features are prevalent in both classes. Inthe absence of nonequilibrium condensates in the upper atmosphere, andwith sufficient H2O condensation, class II objects areexpected to have the highest visible albedos of any class. When theupper atmosphere of an EGP is too hot for H2O to condense,radiation generally penetrates more deeply. In these objects, designatedclass III or ``clear'' because of a lack of condensation in the upperatmosphere, absorption lines of the alkali metals, sodium and potassium,lower the albedo significantly throughout the visible. Furthermore, thenear-infrared albedo is negligible, primarily because of strongCH4 and H2O molecular absorption andcollision-induced absorption (CIA) by H2 molecules. In thoseEGPs with exceedingly small orbital distance (``roasters'') and 900K<~Teff<~1500 K (class IV), a tropospheric silicatelayer is expected to exist. In all but the hottest(Teff>~1500 K) or lowest gravity roasters, the effect ofthis silicate layer is likely to be insignificant because of the verystrong absorption by sodium and potassium atoms above the layer. Theresonance lines of sodium and potassium are expected to be salientfeatures in the reflection spectra of these EGPs. In the absence ofnonequilibrium condensates, we find, in contrast to previous studies,that these class IV roasters likely have the lowest visible and Bondalbedos of any class, rivaling the lowest albedos of our solar system.For the small fraction of roasters with Teff>~1500 Kand/or low surface gravity (<~103 cm s-2 classV), the silicate layer is located very high in the atmosphere,reflecting much of the incident radiation before it can reach theabsorbing alkali metals and molecular species. Hence, the class Vroasters have much higher albedos than those of class IV. In addition,for class V objects, UV irradiation may result in significant alkalimetal ionization, thereby further weakening the alkali metal absorptionlines. We derive Bond albedos (AB) andTeff estimates for the full set of known EGPs. A broad rangein both values is found, with Teff ranging from ~150 tonearly 1600 K, and AB from ~0.02 to 0.8. We findthat variations in particle size distributions and condensation fractioncan have large quantitative, or even qualitative, effects on albedospectra. In general, less condensation, larger particle sizes, and widersize distributions result in lower albedos. We explore the effects ofnonequilibrium condensed products of photolysis above or withinprincipal cloud decks. As in Jupiter, such species can lower the UV/bluealbedo substantially, even if present in relatively small mixing ratios.
| Modeling Visual Photometry I: Preliminary Determination of Visual Bandpass
This paper addresses the issue of spectral sensitivity as part of theoverall visual model. A mathematical theory and observational method arepresented for the determination of color coefficient with error term foran individual observer.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| Spectroscopic observations of some Be/B stars at high galactic latitudes
Spectral types, rotational velocities, and radial velocities wereestimated for eight Be and two non-emission B stars at high galacticlatitudes from CCD spectra, and their distances from the galactic planecalculated. All of the objects appear to be spectroscopically normal;for the Be stars, at least, there is no reason to evoke an unusualorigin for this sample of stars. (SECTION: Stars)
| Possible Observational Criteria for Distinguishing Brown Dwarfs from Planets
The difference in formation process between binary stars and planetarysystems is reflected in their composition, as well as orbitalarchitecture, particularly in their orbital eccentricity as a functionof orbital period. It is suggested here that this difference can be usedas an observational criterion to distinguish between brown dwarfs andplanets. Application of the orbital criterion suggests that, with threepossible exceptions, all of the recently discovered substellarcompanions may be brown dwarfs and not planets. These criterion may beused as a guide for interpretation of the nature of substellar-masscompanions to stars in the future.
| A catalogue of [Fe/H] determinations: 1996 edition
A fifth Edition of the Catalogue of [Fe/H] determinations is presentedherewith. It contains 5946 determinations for 3247 stars, including 751stars in 84 associations, clusters or galaxies. The literature iscomplete up to December 1995. The 700 bibliographical referencescorrespond to [Fe/H] determinations obtained from high resolutionspectroscopic observations and detailed analyses, most of them carriedout with the help of model-atmospheres. The Catalogue is made up ofthree formatted files: File 1: field stars, File 2: stars in galacticassociations and clusters, and stars in SMC, LMC, M33, File 3: numberedlist of bibliographical references The three files are only available inelectronic form at the Centre de Donnees Stellaires in Strasbourg, viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5), or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| 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.
| The local distribution of NA I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures.
| Speckle observations of visual and spectroscopic binaries. V.
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| Takýmyýldýz: | Zincirli Prenses |
| Sað Açýklýk: | 00h49m48.80s |
| Yükselim: | +41°04'44.0" |
| Görünürdeki Parlaklýk: | 4.53 |
| Uzaklýk: | 208.333 parsek |
| özdevim Sað Açýklýk: | 22.3 |
| özdevim Yükselim: | -18.2 |
| B-T magnitude: | 4.343 |
| V-T magnitude: | 4.496 |
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