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Deep imaging survey of young, nearby austral stars . VLT/NACO near-infrared Lyot-coronographic observations
Context. High contrast and high angular resolution imaging is theoptimal search technique for substellar companions to nearby stars atphysical separations larger than typically 10 AU. Two distinctpopulations of substellar companions, brown dwarfs and planets, can beprobed and characterized. As a result, fossile traces of processes offormation and evolution can be revealed by physical and orbitalproperties, both for individual systems and as an ensemble. Aims:Since November 2002, we have conducted a large, deep imaging, survey ofyoung, nearby associations of the southern hemisphere. Our goal isdetection and characterization of substellar companions with projectedseparations in the range 10-500 AU. We have observed a sample of 88stars, primarily G to M dwarfs, younger than 100 Myr, and within 100 pcof Earth. Methods: The VLT/NACO adaptive optics instrument of theESO Paranal Observatory was used to explore the faint circumstellarenvironment between typically 0.1 and 10''. Diffraction-limitedobservations in H and K_s-band combined with Lyot-coronagraphy enabledus to reach primary star-companion brightness ratios as small as10-6. The existence of planetary mass companions couldtherefore be probed. We used a standardized observing sequence toprecisely measure the position and flux of all detected sources relativeto their visual primary star. Repeated observations at several epochsenabled us to discriminate comoving companions from background objects. Results: We report the discovery of 17 new close (0.1-5.0'')multiple systems. HIP 108195 AB and C (F1 III-M6), HIP 84642 AB (a~14AU, K0-M5) and TWA22 AB (a~1.8 AU; M6-M6) are confirmed comovingsystems. TWA22 AB is likely to be a rare astrometric calibrator that canbe used to test evolutionary model predictions. Among our completesample, a total of 65 targets were observed with deep coronagraphicimaging. About 240 faint companion candidates were detected around 36stars. Follow-up observations with VLT or HST for 83% of these starsenabled us to identify a large fraction of background contaminants. Ourlatest results that pertain to the substellar companions to GSC08047-00232, AB Pic and 2M1207 (confirmed during this survey andpublished earlier), are reviewed. Finally, a statistical analysis of ourcomplete set of coronagraphic detection limits enables us to placeconstraints on the physical and orbital properties of giant planetsbetween typically 20 and 150 AU.Table 8 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/509/A52
| UBV(RI)C photometry of Hipparcos red stars
We present homogeneous and standardized UBV(RI)C photometryfor nearly 550 M stars selected from the Hipparcos satellite data baseusing the following selection criteria: lack of obvious variability (noHipparcos variability flag); δ<+10°(V-I)>1.7 and Vmagnitude fainter than about 7.6. Comparisons are made between thecurrent photometry, other ground-based data sets and Hipparcosphotometry. We use linear discriminant analysis to determine aluminosity segregation criterion for late-type stars, and principalcomponent analysis to study the statistical structure of the colourindices and to calibrate absolute magnitude in terms of (V-I) for thedwarf stars. Various methods are used to determine the mean absolutemagnitude of the giant stars. We find 10 dwarf stars, apparentlypreviously unrecognized (prior to Hipparcos) as being within 25pc,including five within 20pc.
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