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Trigonometric parallaxes from the Southern Hemisphere
We present an outline of the main motivations, goals, procedures, andfirst scientifiic results of a dedicated astrometric program aimed atdetermining trigonometric parallaxes for new nearby (D < 25) stars inthe Southern sky. This program is being carried out at the Cerro TololoInter-American Observatory (CTIO), Chile.
| Astrometric light-travel time signature of sources in nonlinear motion. I. Derivation of the effect and radial motion
Context: .Very precise planned space astrometric missions and recentimprovements in imaging capabilities require a detailed review of theassumptions of classical astrometric modeling.Aims.We show thatLight-Travel Time must be taken into account in modeling the kinematicsof astronomical objects in nonlinear motion, even at stellardistances.Methods.A closed expression to include Light-Travel Time inthe current astrometric models with nonlinear motion is provided. Usinga perturbative approach the expression of the Light-Travel Timesignature is derived. We propose a practical form of the astrometricmodelling to be applied in astrometric data reduction of sources atstellar distances(d>1 pc).Results.We show that the Light-Travel Timesignature is relevant at μ as accuracy (or even at mas) depending onthe time span of the astrometric measurements. We explain howinformation on the radial motion of a source can be obtained. Someestimates are provided for known nearby binary systemsConclusions.Giventhe obtained results, it is clear that this effect must be taken intoaccount in interpreting precise astrometric measurements. The effect isparticularly relevant in measurements performed by the plannedastrometric space missions (GAIA, SIM, JASMINE, TPF/DARWIN). Anobjective criterion is provided to quickly evaluate whether theLight-Travel Time modeling is required for a given source or system.
| Visual Star Colours from Instrumental Photometry
In order to display graphically the visual colours of stars and otherastronomical objects, photometric broadband R, V, B colours are used toproxy for the r, g, b colours of the three visual sensors of the eye.From photometric Johnson B-V and V-R colour indices, R, V, and Bmagnitudes (V = 0) are calculated, and from these the respectivebrightnesses (r, v = 1 = g, and b) are calculated. After suitablenormalization these are then placed in a ternary diagram having r, g,and b as the vertices. All B-V and V-R are adjusted so that the Sunfalls in the same place as a blackbody at 5800 K. The resulting ternaryplot shows all of its objects (stars, planets) in their visual coloursat their relative positions in the ternary diagram. The star coloursdisplayed on a computer monitor screen or as a print with a colourprinter are more vivid than the usual visual impressions of isolatedstars, undoubtedly because of properties of the dark-adapted eye, butdouble-star pairs with contrasting colours correspond nicely totelescopic visual impressions.
| Magnetic field measurements on four yellow supergiants. I
Multiyear high precision measurements of the longitudinal component ofthe magnetic field (Be) of four supergiants are reported: Aqr (G0 Ib),Aqr (G2 Ib), Gem (G8 Ib), and Peg (K2 Ib). The best measurementaccuracy, =0.8 G, was achieved for Peg. A Monte Carlo method was used totest the reliability of the derived measurement errors. The differencesbetween the observational errors and the calculated Monte Carlo errorswere 3.2%. For Aqr and Aqr no statistically significant value of themagnetic field was recorded when averaged over a night. For eGem thefollowing overnight average values of the magnetic field were recordedon five nights: 11.1±2.7 G, 9.8±2.5 G, -10.5±3.0 G,38.1±7.4 G, and 5.3±1.5 G. For Peg the magnetic fieldrecorded over two nights was -5.3±0.9 G and - 2.7±0.8 G.
| Stellar activity cycles: observing the dynamo?
The enormous complexity of the atmospheric structure observed on the Sunmakes it very difficult to compare the Sun with ``solar-type stars''.Clearly, we need to identify parameters that can be observed on the Sunas well as on other stars which can be interpreted unambiguously. Themost widely accepted dynamo signature is the presence of an activitycycle, well documented for the Sun and for main-sequence stars due tothe Mount Wilson Ca II H&K project. Only recently have we detectedspatial information, differential rotation and possibly meridional flowson other stars and thereby adding another constraint for itsinterpretation within a dynamo theory. Again, the picture is notcomplete yet, despite that there is just a single main ingredient thatacts as the driving mechanism for activity in all atmospheric layers andthe convective envelope of a solar-type star: the dynamo-relatedmagnetic field. I stress the importance of mapping stellar surfaces asfingerprints of the underlying dynamo action over long periods of time.
| The Solar Neighborhood. XIII. Parallax Results from the CTIOPI 0.9 Meter Program: Stars with μ >= 1.0" yr-1 (MOTION Sample)
We present the first set of definitive trigonometric parallaxes andproper motions from the Cerro Tololo Inter-American Observatory ParallaxInvestigation. Full astrometric reductions for the program arediscussed, including methods of reference star selection, differentialcolor refraction corrections, and conversion of relative to absoluteparallax. Using data acquired at the 0.9 m telescope at CTIO, fullastrometric solutions and VRIJHKs photometry are presentedfor 36 red and white dwarf stellar systems with proper motions fasterthan 1.0" yr-1. Of these, 33 systems have their first evertrigonometric parallaxes, which comprise 41% of MOTION systems (thosereported to have proper motions greater than 1.0" yr-1) southof δ=0deg that have no parallaxes. Four of the systemsare new members of the RECONS 10 pc sample for which the first accuratetrigonometric parallaxes are published here: DENIS J1048-3956(4.04+/-0.03 pc), GJ 1128 (LHS 271, 6.53+/-0.10 pc), GJ 1068 (LHS 22,6.97+/-0.09 pc), and GJ 1123 (LHS 263, 9.02+/-0.16 pc). In addition, twored subdwarf-white dwarf pairs, LHS 193AB and LHS 300AB, are identified.The white dwarf secondaries fall in a previously uncharted region of theH-R diagram.
| Disk populations from HIPPARCOS kinematic data. Discontinuities in the local velocity distribution
The full space motions including radial velocities of a stellarsample drawn from HIPPARCOS catalogue are used to discriminatedifferentiated statistical behaviours that are associated with stellarpopulations in the solar neighbourhood. A sampling parameter is used tobuild a hierarchical set of nested samples, where a discontinuouspattern, based in a partition introduced by two normal distributions,scans the subsamples. Two quantities inform whether any subsample fitsproperly into the discontinuous model. A χ2 test measuresthe Gaussianity of both components, and the entropy of the mixtureprobability gives account of how informative the resulting segregationis. The less informative partition is the one with maximum populationentropy, which provides most representative kinematic parameters. Eachnew population merged to the cumulative subsample produces adiscontinuity in the plot entropy versus sampling parameter, that allowsto determine the number of populations contained in the whole sample.The resulting method has been named MEMPHIS, Maximum Entropy of theMixture Probability from HIerarchical Segregation. In addition to bothmain kinematic components, thin and thick disk, with respective velocitydispersions (28± 1, 16± 2, 13± 1) and (65±2, 39± 9, 41± 2) km s-1, two discretenon-Gaussian subcomponents are detected within the thin disk. Thesepopulations are identified with early-type and young disk stars.Moreover, a continuous old disk population is mixed with the foregoingsubcomponents composing all together the thin disk. Older thin diskstars have a velocity dispersion overlapping a wing of the thick disk.Although they could appear like an intermediate continuous population,nested subsamples distributions allow us to conclude that theydefinitively belong to the thin disk, and that a clear discontinuitydetaches thick from thin disk. Almost the same qualitative results, butwith less accuracy, are obtained whether MEMPHIS is applied tosubsamples from the Third Catalogue of Nearby Stars (CNS3). A dynamicmodel according to Chandrasekhar's approximation, under particularsymmetry hypotheses, allows to interpret the results. The non-vanishingvertex deviation lower for older stars of all Galactic components issuggesting that, at least, point-axial symmetry is required in order toexplain the local kinematic behaviour. According to this model, theoldest thick disk population, with no net radial movement, can beextrapolated, having heliocentric velocities of -76 ± 2 kms-1 in rotation, and -18 ± 1 km s-1 in theradial direction. Early-type stars show a worthy local singularity,nearly with no net radial motion, similarly to the oldest thick diskstars. Older populations half of the thin disk and the whole thickdisk share a common differential galactic movement, suggesting acommon dynamical origin for the rupture of the axial symmetry. Therelationship between the maximum stellar velocity of a sample and itsaverage age τ is discussed, finding an approximate relation|V|max ∝ τ. Local stellar populations can bedescribed from a Titius-Bode-like law for the radial velocitydispersion, σ1 = 6.6 (4/3)^x, so that for naturalvalues x=2,3,5,8 it determines average energy levels of discretepopulations, while for continuous intervals x≤ 5 and x≥ 7 itdescribes the velocity-age evolution of thin and thick disk components,according to x 1.5 ln τ.
| X-rays from α Centauri - The darkening of the solar twin
We present first results from five XMM-Newton observations of the binarysystem α Centauri, which has been observed in snapshot likeexposures of roughly two hours each during the last two years. In allour observations the X-ray emission of the system is dominated byα Cen B, a K1 star. The derived light curves of the individualcomponents reveal variability on short timescales and a flare wasdiscovered on α Cen B during one observation. A PSF fittingalgorithm is applied to the event distribution to determine thebrightness of each component during the observations. We perform aspectral analysis with multi-temperature models to calculate the X-rayluminosities. We investigate long term variability and possible activitycycles of both stars and find the optically brighter component αCen A, a G2 star very similar to our Sun, to have fainted in X-rays byat least an order of magnitude during the observation program, abehaviour never observed before on α Cen A, but rather similar tothe X-ray behaviour observed with XMM-Newton on HD 81809. We alsocompare our data with earlier spatially resolved observations performedover the last 25 years.
| Stability of planetary orbits in binary systems
Stability of S-type and P-type planetary orbits in binary systems ofdifferent mass and separation ratios is investigated. Criteria forstable, marginally stable and unstable planetary orbits are specified.These criteria are used to determine regions of stability of planetaryorbits in different binary systems with Jupiter-type planets. Theobtained results show that the regions of stability for S-type orbitsdepend on the distance ratio between the star and planet, and thestellar companions, in the range of 0.22 and 0.46, depending on the massratio. For P-type orbits, the regions of stability also depend on thatdistance ratio, in the range of 1.75 and 2.45, again depending on thethe mass ratio. Applications of these results to three observed binarysystems with giant planets, namely, τ Boo, HD 195019 and GJ 86, showthat the orbits of the giant planets in those systems can be classifiedas stable, as expected.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| The debris disc around τ Ceti: a massive analogue to the Kuiper Belt
An excess of far-infrared emission is seen towards the nearby G8V starτ Ceti, and this has been attributed to orbiting dust particlesgenerated in planetesimal collisions. A new 850-μm image shows thatthere is indeed such a debris disc, extending out to ~55 au (15 arcsec)radius. This is the first disc around a Sun-like star of latemain-sequence age to be confirmed by imaging. The dust mass is at leastan order of magnitude greater than in the Kuiper Belt, although thedimensions of the systems are very similar and the age of τ Cetiexceeds that of the Sun. Modelling shows that the mass in collidingbodies up to 10 km in size is around 1.2 Earth masses, compared with 0.1M⊕ in the Kuiper Belt, and hence the evolution aroundthe two stars has been different. One possibility is that τ Ceti haslost fewer comets from the outskirts of the system, compared with theSun. Alternatively, a greater number of comets could have been forcedout by a migrating planet, compared with the case of Neptune in theSolar system. Notably, the disc of τ Ceti fits the expected declinewith time compared to that of the younger nearby star ɛ Eridani.Among these three stars, the Sun would then be the case with the leastdust and a `minimal Kuiper Belt' - a situation which may be beneficialin terms of less bombardment and better stability for life.
| The dynamical stability of extra-solar planets in binary systems
The question about dynamical stability of extra-solar planets isconsidered in the frame of the general three-body problem, i.e. a planetin the binary system revolves around one of the components. The distancebetween the star's components is much longer than between the orbitingstar and the planet. In the differential equations with regard to theeccentricity and the argument of the perigee we used the Hamiltonianwithout the short-periodic terms, excluded by von Zeipel's method.The possible conditions of the dynamical stability of extra-solarplanets are presented by their orbital parameters -- the mutualinclination of orbits and the argument of the perigee of the planet.The theory has been applied to the systems Gliese 86, γ Cephei,and 61 Cygni. The results were verified by the numerical integration.
| On the Flux of Extrasolar Dust in Earth's Atmosphere
Micron-size extrasolar dust particles have been convincingly detected bysatellites. Larger extrasolar meteoroids (5-35 μm) have most likelybeen detected by ground-based radar at Arecibo and New Zealand. Wepresent estimates of the minimum detectable particle sizes and thecollecting areas for both radar systems. We show that particles largerthan ~10 μm can propagate for tens of parsecs through theinterstellar medium, opening up the possibility that ground-based radarsystems can detect AGB stars, young stellar objects such as T Tauristars, and debris disks around Vega-like stars. We provide analyticaland numerical estimates of the ejection velocity in the case of a debrisdisk interacting with a Jupiter-mass planet. We give rough estimates ofthe flux of large micrometeoroids from all three classes of sources.Current radar systems are unlikely to detect significant numbers ofmeteors from debris disks such as β Pictoris. However, we suggestimprovements to radar systems that should allow for the detection ofmultiple examples of all three classes.
| Nearby stars of the Galactic disk and halo. III.
High-resolution spectroscopic observations of about 150 nearby stars orstar systems are presented and discussed. The study of these and another100 objects of the previous papers of this series implies that theGalaxy became reality 13 or 14 Gyr ago with the implementation of amassive, rotationally-supported population of thick-disk stars. The veryhigh star formation rate in that phase gave rise to a rapid metalenrichment and an expulsion of gas in supernovae-driven Galactic winds,but was followed by a star formation gap for no less than three billionyears at the Sun's galactocentric distance. In a second phase, then, thethin disk - our ``familiar Milky Way'' - came on stage. Nowadays ittraces the bright side of the Galaxy, but it is also embedded in a hugecoffin of dead thick-disk stars that account for a large amount ofbaryonic dark matter. As opposed to this, cold-dark-matter-dominatedcosmologies that suggest a more gradual hierarchical buildup throughmergers of minor structures, though popular, are a poor description forthe Milky Way Galaxy - and by inference many other spirals as well - if,as the sample implies, the fossil records of its long-lived stars do notstick to this paradigm. Apart from this general picture that emergeswith reference to the entire sample stars, a good deal of the presentwork is however also concerned with detailed discussions of manyindividual objects. Among the most interesting we mention the bluestraggler or merger candidates HD 165401 and HD 137763/HD 137778, thelikely accretion of a giant planet or brown dwarf on 59 Vir in itsrecent history, and HD 63433 that proves to be a young solar analog at\tau200 Myr. Likewise, the secondary to HR 4867, formerly suspectednon-single from the Hipparcos astrometry, is directly detectable in thehigh-resolution spectroscopic tracings, whereas the visual binary \chiCet is instead at least triple, and presumably even quadruple. Withrespect to the nearby young stars a complete account of the Ursa MajorAssociation is presented, and we provide as well plain evidence foranother, the ``Hercules-Lyra Association'', the likely existence ofwhich was only realized in recent years. On account of its rotation,chemistry, and age we do confirm that the Sun is very typical among itsG-type neighbors; as to its kinematics, it appears however not unlikelythat the Sun's known low peculiar space velocity could indeed be thecause for the weak paleontological record of mass extinctions and majorimpact events on our parent planet during the most recent Galactic planepassage of the solar system. Although the significance of thiscorrelation certainly remains a matter of debate for years to come, wepoint in this context to the principal importance of the thick disk fora complete census with respect to the local surface and volumedensities. Other important effects that can be ascribed to this darkstellar population comprise (i) the observed plateau in the shape of theluminosity function of the local FGK stars, (ii) a small thoughsystematic effect on the basic solar motion, (iii) a reassessment of theterm ``asymmetrical drift velocity'' for the remainder (i.e. the thindisk) of the stellar objects, (iv) its ability to account for the bulkof the recently discovered high-velocity blue white dwarfs, (v) itsmajor contribution to the Sun's 220 km s-1 rotationalvelocity around the Galactic center, and (vi) the significant flatteningthat it imposes on the Milky Way's rotation curve. Finally we note ahigh multiplicity fraction in the small but volume-complete local sampleof stars of this ancient population. This in turn is highly suggestivefor a star formation scenario wherein the few existing single stellarobjects might only arise from either late mergers or the dynamicalejection of former triple or higher level star systems.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| High-amplitude, long-term X-ray variability in the solar-type star HD 81809: The beginning of an X-ray activity cycle?
We present the initial results from our XMM-Newton program aimed atsearching for X-ray activity cycles in solar-type stars. HD 81809 is aG2-type star (somewhat more evolved than the Sun, and with a lessmassive companion) with a pronounced 8.2 yr chromospheric cycle, asevident from from the Mt. Wilson program data. We present here theresults from the initial 2.5 years of XMM-Newton observations, showingthat large amplitude (a factor of ≃10) modulation is present inthe X-ray luminosity, with a clearly defined maximum in mid 2002 and asteady decrease since then. The maximum of the chromospheric cycle tookplace in 2001; if the observed X-ray variability is the initial part ofan X-ray cycle, this could imply a phase shift between chromospheric andcoronal activity, although the current descent into chromospheric cycleminimum is well reflected into the star's X-ray luminosity. Theobservations presented here provide clear evidence for the presence oflarge amplitude X-ray variability coherent with the activity cycle inthe chromosphere in a star other than the Sun.
| Reduction of CCD observations of visual binaries using the ``Tepui'' function as PSF
429 CCD measurements of relative positions and magnitude differences inV and R photometric bands for 165 visual double and multiple stars aregiven. CCD frames were taken at the 1.52 m Spanish telescope of theSpanish-German Center of Astronomy at Calar Alto (Almería,Spain). During the reduction process a ``Tepui" function was used as thePSF function.Tables 1 and 2 are 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/416/811
| Optical interferometry in astronomy
Here I review the current state of the field of optical stellarinterferometry, concentrating on ground-based work although a briefreport of space interferometry missions is included. We pause both toreflect on decades of immense progress in the field as well as toprepare for a new generation of large interferometers just now beingcommissioned (most notably, the CHARA, Keck and VLT Interferometers).First, this review summarizes the basic principles behind stellarinterferometry needed by the lay-physicist and general astronomer tounderstand the scientific potential as well as technical challenges ofinterferometry. Next, the basic design principles of practicalinterferometers are discussed, using the experience of past and existingfacilities to illustrate important points. Here there is significantdiscussion of current trends in the field, including the new facilitiesunder construction and advanced technologies being debuted. This decadehas seen the influence of stellar interferometry extend beyond classicalregimes of stellar diameters and binary orbits to new areas such asmapping the accretion discs around young stars, novel calibration of thecepheid period-luminosity relation, and imaging of stellar surfaces. Thethird section is devoted to the major scientific results frominterferometry, grouped into natural categories reflecting these currentdevelopments. Lastly, I consider the future of interferometry,highlighting the kinds of new science promised by the interferometerscoming on-line in the next few years. I also discuss the longer-termfuture of optical interferometry, including the prospects for spaceinterferometry and the possibilities of large-scale ground-basedprojects. Critical technological developments are still needed to makethese projects attractive and affordable.
| Submillimeter Images of the Closest Debris Disks
Debris disks are the cold remnant material seen around nearby stars,where dust is thought to be continually regenerated by collisions ofasteroids and comets. We discuss some new observations of disks aroundclose sun-like stars. Some have detectable dust up to the late mainsequence, although the sun's Kuiper Belt contains much less material.Given the small distance to which we could detect sun-like systems, wediscuss whether dusty disks could be the norm --- this has implicationsfor searches for other habitable terrestrial planets.
| Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity
Collisions between the winds of solar-like stars and the localinterstellar medium result in a population of hot hydrogen gassurrounding these stars. Absorption from this hot H I can be detected inhigh-resolution Lyα spectra of these stars from the Hubble SpaceTelescope. The amount of absorption can be used as a diagnostic for thestellar mass-loss rate. We present new mass-loss rate measurementsderived in this fashion for four stars (ɛ Eri, 61 Cyg A, 36 Oph AB,and 40 Eri A). Combining these measurements with others, we study howmass loss varies with stellar activity. We find that for the solar-likeGK dwarfs, the mass loss per unit surface area is correlated with X-raysurface flux. Fitting a power law to this relation yieldsM~F1.15+/-0.20X. The active M dwarf Proxima Cenand the very active RS CVn system λ And appear to be inconsistentwith this relation. Since activity is known to decrease with age, theabove power-law relation for solar-like stars suggests that mass lossdecreases with time. We infer a power-law relation ofM~t-2.00+/-0.52. This suggests that the solar wind may havebeen as much as 1000 times more massive in the distant past, which mayhave had important ramifications for the history of planetaryatmospheres in our solar system, that of Mars in particular. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| Diffraction-limited CCD imaging with faint reference stars
By selecting short exposure images taken using a CCD with negligiblereadout noise we obtained essentially diffraction-limited 810 nm imagesof faint objects using nearby reference stars brighter than I=16 at a2.56 m telescope. The FWHM of the isoplanatic patch for the technique isfound to be 50 arcseconds, providing ~ 20% sky coverage around suitablereference stars.
| The Race to Measure the Cosmos
Not Available
| Galileo's measurement of the diameter of a star, and of the eye's pupil
Galileo tried to measure the diameter of the star Vega (Alpha Lyrae)using a piece of rope, and realising that he needed to take into accountthe finite size of the pupil of the eye, he measured that too. Not onlyis it fun to try to repeat these experiments for yourself, they also actas a great introduction to practical historical astronomy.
| Measurements of the General Magnetic Fields of Active Late-Type Stars
Measurements of the general magnetic fields of active, late-type starsobtained with the 2.6-m Sha n telescope of the Crimean AstrophysicalObservatory are reported. Statistically significant values of themagnetic fields of ∈ Eri and 61 Cyg A have been detected. Themagnetic fields range from -10 to 21 G. The magnetic-field values for∈ Eri and 61 Cyg A exceed the solar field by an order of magnitude;∈ Eri is more active than the Sun, while 61 Cyg A has approximatelythe same activity level as the Sun. There is some evidence forvariations in the general magnetic fields of these stars associated withtheir axial rotation.
| Optical Interferometry
The field of optical and infrared (IR) interferometry has seen rapidtechnical and scientific progress over the past few years. A number ofinstruments capable of precise visibility measurements have been built,and closure-phase imaging with multitelescope arrays has beendemonstrated. Astronomical results from these instruments includemeasurements of stellar diameters and their wavelength dependence, limbdarkening, stellar surface structure, and distances of Cepheids and ofNova Cygni 1992. Precise stellar masses have been obtained frominterferometric observations of spectroscopic binaries, andcircumstellar disks and shells have been resolved. Searches forsubstellar companions and extrasolar planets with interferometricastrometry will begin soon. Nulling interferometry will enable studiesof exozodiacal disks from the ground and the detection andcharacterization of terrestrial extrasolar planets from space. Thesedevelopments are reviewed, as well as progress in some key technologicalareas.
| 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
| CCD measurements of visual double stars from the German-Spanish Astronomical Center at Calar Alto
342 CCD measurements of relative positions and magnitude differences for145 visual double stars are presented. Observations were carried out atthe 1.23m telescope of the German-Spanish Astronomical Center at CalarAlto (Spain), all of them in V and R photometric bands.
| Friedrich Wilhelm Bessel (1784-1846) and the Russian Astronomy
In the first half of the 19th century the foundations of stellarastronomy were established thanks to the German astronomer, geodeticistand mathematician Friedrich Wilhelm Bessel. Internationally estimed andin close relationship with scientists in many countries, especially inRussia, Bessel (although not yet 30 years old) in 1814 became a foreignmember of the Petersburg Academy of Sciences. Wilhelm Struve, directorof the Dorpat observatory, was in close contact with him. Together theydiscussed problems of observational procedures and the use ofinstruments. In 1817 Struve bought the same Reichenbach meridian circleas Bessel used in Königsberg. Both ordered their refractors fromthe famous Fraunhofer workshop in Munich. %(9-inch for Dorpat). Besselwas also involved in the high precision Russian geodetic survey whichstarted in 1816 and which succeeded in connecting the Russian andwestern European triangulation networks. Struve tried to measureparallaxes using the bright star Vega (α Lyrae); his results werepublished in 1837. Also in 1837 Bessel, using his Fraunhofer heliometerand the star 61 Cygni, found a result close to modern values, lateracknowledged with the Gold Medal of the Royal Astronomical Society. In1834 Struve was named director of the new Russian observatory inPulkovo, St. Petersburg. Struve discussed the plans with Bessel and bothacquired a Repsold meridian circle. The next aim, begun in the 1840s --after the first determination of stellar distances -- was thedistribution of stars in space (in the Milky Way). For this, catalogueswith not only accurate stellar positions but also brightnesses werenecessary. Thanks to Bessel's thorough reformation of measuring andreduction methods, making possible high accuracy telescopicobservations, Struve was able to produce important results in stellarastronomy.
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