Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| A fourth component in the young multiple system V 773 Tauri
I report on a new component in the pre-main sequence multiple systemV 773 Tauri. This second visual companion, V 773 TauC, with a projected separation of ~0farcs2 has been detected usingspeckle interferometry in the near-infrared. Repeated observations from1996 to 2002 show significant orbital motion and thus confirm thecharacter of the new companion as a gravitationally bound star. Togetherwith the two components of the spectroscopic binary V 773 Tau A and thepreviously known visual companion V 773 Tau B, the V 773 Tau systemappears as a young ``mini-cluster'' of four T Tauri stars within asphere of a radius less than 100 AU. V 773 Tau A, B and C form a triplesystem that is not hierarchic, but is apparently stable despite of this.The brightness of V 773 Tau C has probably increased over the lastyears, which may explain its non-detection in previous binary surveys.Based on observations collected at the German-Spanish AstronomicalCenter on Calar Alto, Spain.
| A multi-wavelength study of pre-main sequence stars in the Taurus-Auriga star-forming region
Although many lowmass pre-main sequence stars are strong X-ray sources,the origin of the X-ray emission is not well known. Since these objectsare variable at all frequencies, simultaneous observations in X-rays andin other wavelengths are able to constrain the properties of the X-rayemitting regions. In this paper, we report quasi-simultaneousobservations in X-rays, the optical, and the radio regime for classicaland weak-line T Tauri stars from the Taurus-Auriga star-forming region.We find that all detected T Tauri stars show significant night-to-nightvariations of the X-ray emission. For three of the stars, FMTau and CW Tau, both classical T Tauristars, and V773 Tau, a weak-line T Tauri star, thevariations are especially large. From observations taken simultaneously,we also find that there is some correspondence between the strength ofHα and the X-ray brightness in V773 Tau. Thelack of a strong correlation leads us to conclude that the X-rayemission of V773 Tau is not a superposition offlares. However, we suggest that a weak correlation occurs becausechromospherically active regions and regions of strong X-ray emissionare generally related. V773 Tau was detected at 8.46GHz as a weakly circularly polarised but highly variable source. We alsofind that the X-ray emission and the equivalent width of Hαremained unchanged, while large variations of the flux density in theradio regime were observed. This clearly indicates that the emittingregions are different. Using optical spectroscopy we detected a flare inHα and event which showed a flare-like light-curve of thecontinuum brightness in FM Tau. However, ROSAT didnot observe the field at the times of these flares. Nevertheless, aninteresting X-ray event was observed in V773 Tau,during which the flux increased for about 8 hours and then decreasedback to the same level in 5 hours. We interpret this as a long-durationevent similar to those seen on the sun and other active stars. In thecourse of the observations, we discovered a new weak-line T Tauri star,GSC-1839-5674. Results are also presented for several other stars in theROSAT field.
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| A new optical extinction law and distance estimate for the Taurus-Auriga molecular cloud
This paper presents optical spectrophotometry of field stars projectedon the Taurus-Auriga molecular cloud. We derive extinction laws forheavily reddened stars in this sample and show that the reddening lawthrough the dark cloud is nearly identical to the standard law forlambda-lambda(3600-6100) and AV approximately less than 3mag. Our spectroscopic parallaxes suggest a distance of 140 +/- 10 pcfor the northern portion of the cloud and show no compelling evidencefor a substantial variation in distance across the leading edge of thecloud.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| Fifth fundamental catalogue. Part 2: The FK5 extension - new fundamental stars
The mean positions and proper motions for 3117 new fundamental starsessentially in the magnitude range about 4.5 to 9.5 are given in thisFK5 extension. Mean apparent visual magnitude is 7.2 and is on average2.5 magnitudes fainter then the basic FK5 which has a mean magnitude of4.7. (The basic FK5 gives the mean positions and proper motions for theclassical 1535 fundamental stars). The following are discussed: theobservational material, reduction of observations, star selection, andthe system for the FK5 extension. An explanation and description of thecatalog are given. The catalog of 3117 fundamental stars for the equinoxand epoch J2000.0 and B1950.0 is presented. The parallaxes and radialvelocities for 22 extension stars with large forecasting effects aregiven. Catalogs used in the compilation of the FK5 fundamental catalogare listed.
| UBV photometry of stars whose positions are accurately known. IV
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987A&AS...68..211O&db_key=AST
| U, B, V, R, I stellar photometry in the field of the Taurus dark clouds
A program of photoelectric stellar photometry has been carried out inthe field of the Taurus dark clouds with a view to establishing thedistance of the clouds and the absorption law in that region of the sky.It is found that the distance of the dark clouds accompanying the Tau T1and Tau T3 associations is 132 plus or minus 10 pc. Within theabsorption region 0-4 m, the Taurus dark clouds are characterized by anearly normal absorption law (to within 5-10%). In the field of the darkclouds a group of F0-G0 stars close to the main sequence is found whosespace density is three to eight times that in the solar neighborhood.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Tauro |
| Ascensión Recta: | 04h14m31.66s |
| Declinación: | +27°57'35.1" |
| Magnitud Aparente: | 8.429 |
| Distancia: | 137.552 parsecs |
| Movimiento Propio en Ascensión Recta: | 36.2 |
| Movimiento Propio en Declinación: | -19.1 |
| B-T magnitude: | 8.9 |
| V-T magnitude: | 8.468 |
Catálogos y designaciones:
|
