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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
| Maximum separations among cataloged binaries
The paper classifies many of the widest common-motion binaries listed inthe Aitken catalog and list 72 physical pairs with known photoelectricphotometry, 31 physical pairs without good photometry, and 27 opticalpairs. As a function of primary types, the physical systems have upperlimits to their separations that are exceeded by some of the opticalpairs. The fact that optical pairs occur with larger separations impliesthat the limits are real ones and not just catalog limitations. Thoselimits (in AU) are expressed by 2500 M1 exp 1.54 for B5-KO main-sequenceprimaries. The same limits hold for the Trapezium and hierarchicalsystems studied previously.
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | ג'ירף |
| התרוממות ימנית: | 03h24m13.20s |
| סירוב: | +67°27'23.7" |
| גודל גלוי: | 8.927 |
| תנועה נכונה: | 54.2 |
| תנועה נכונה: | -9 |
| B-T magnitude: | 9.667 |
| V-T magnitude: | 8.989 |
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