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A spectroscopic survey of the youngest field stars in the solar neighbourhood. I. The optically bright sample
Aims: We present the first results of an ambitious ground-basedobservation programme conducted on 1-4 m class telescopes. Our sampleconsists of 1097 active and presumably young stars, all of them beingoptical (Tycho Catalogue) counterparts of ROSAT All-Sky Survey X-raysources in the northern hemisphere. In this paper, we concentrate on theoptically brightest (V_T?slant 9.5 m) candidates (704 objects). Weacquired high-resolution optical spectroscopy in the H? and/orlithium spectral regions for 426 of such stars without relevant data inthe literature. We describe the star sample and the observations and westart to discuss the physical properties of the investigated stars. Methods: We used a cross-correlation technique and other toolsdeveloped by us to derive accurate radial and rotational velocities andto perform an automatic spectral classification for both single starsand double-lined systems. The spectral subtraction technique was used toderive chromospheric activity levels and lithium abundances. Weestimated the fraction of young single stars and multiple systems instellar soft X-ray surveys and the contamination by more evolvedsystems, like RS CVn binaries. We classified stars on the basis of theirlithium abundance and give a glimpse of their sky distribution. Results: The sample appears to be a mixture of quite youngPleiades-like and Hyades-like stars plus an older lithium-poorpopulation probably born within the last 1-2 Gyr. Seven stars with alithium abundance compatible with the age of IC 2602 (about 30 Myr) oreven younger were detected as well, although two appear to belithium-rich giants.The discovery of a large number of highly ormoderately lithium-rich giants is another outcome of the present survey. Conclusions: The contamination of soft X-ray surveys by oldsystems in which the activity level is enhanced by tidal synchronisationis not negligible, especially for K-type stars. Five stars with lithiumcontent close to the primordial abundance are probably associated withalready known moving groups in the solar neighbourhood. Some of them aregood post-T Tauri candidates according to their positions in the HRdiagram.Based on observations collected at the Observatoire de Haute Provence(France) and the Catania Astrophysical Observatory (Italy). Data for the426 stars is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/504/829
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
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| קבוצת-כוכבים: | נבל |
| התרוממות ימנית: | 18h29m34.90s |
| סירוב: | +29°58'04.6" |
| גודל גלוי: | 9.104 |
| תנועה נכונה: | -7.5 |
| תנועה נכונה: | -9.2 |
| B-T magnitude: | 9.839 |
| V-T magnitude: | 9.165 |
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