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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
| Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud
We present the ROSAT All-Sky Survey data in a 126 deg2 areain and around the CrA star forming region. With low-resolutionspectroscopy of unidentified ROSAT sources we could find 19 new pre-mainsequence stars, two of which are classical T Tauri stars, the othersbeing weak-lined. The spectral types of these new T Tauri stars rangefrom F7 to M6. The two new classical T Tauri stars are located towardstwo small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram withisochrones for an assumed distance of 130 pc, the distance of the mainCrA dark cloud. The new off-cloud weak-line T Tauri stars may haveformed in similar cloudlets, which have dispersed recently.High-resolution spectra of our new T Tauri stars show that they havesignificantly more lithium absorption than zero-age main-sequence starsof the same spectral type, so that they are indeed young. From thosespectra we also obtained rotational and radial velocities. For somestars we found the proper motion in published catalogs. The directionand velocity of the 3D space motion - south relative to the galaticplane - of the CrA T Tauri stars is consistent with the dark cloud beingformed originally by a high-velocity cloud impact onto the galacticplane, which triggered the star formation in CrA. We also present VRIJHKphotometry for most of the new T Tauri stars to derive theirluminosities, ages, and masses. Partly based on observations collectedat the 1.52 m and 3.5 m telescopes of the European Southern Observatory,Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and onobservations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope.
| Searching for old neutron stars with ROSAT. I. Soft X-ray sources in molecular clouds at high galactic latitude
This is the first in a series of three papers aiming to put strongobservational constraints on the number of old accreting neutron starsin the Galaxy. Old neutron stars have been predicted to be visible assoft X-ray sources. The brightest sources are expected where the densityof the interstellar medium is high. I present an X-ray survey of acomplete sample of molecular clouds at high galactic latitudes based onthe ROSAT All-Sky Survey. I detect 89 sources inside the projectedcontours of the molecular clouds and report on a comprehensiveidentification program. For all sources I searched astronomical catalogsand created finding charts. I also observed all sources at radiofrequencies, most of them at 1.4GHz and at 8GHz. For sources with brightcounterparts, optical spectra are presented. For fainter sources,optical CCD images are shown. Of all sources, 54 are securely identifiedand for the majority of the remaining sources likely identifications aregiven. I find no candidate for an old accreting neutron star in thissample. From this survey I derive an upper limit to the neutron stardensity of 50sr(-1) at a ROSAT count rate of 0.012s(-1) . Fig.\,9 andFig.\,10 are only and Table\,2 is additionally available in electronicform at the CDS via http://cdsweb.u-strasbg.fr/Abstract.html
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| קבוצת-כוכבים: | קשת |
| התרוממות ימנית: | 19h19m31.01s |
| סירוב: | -36°39'30.7" |
| גודל גלוי: | 7.273 |
| מרחק: | 49.358 פארסק |
| תנועה נכונה: | 41.3 |
| תנועה נכונה: | -96.8 |
| B-T magnitude: | 7.82 |
| V-T magnitude: | 7.319 |
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