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A revisit to agglomerates of early-type Hipparcos stars
% We study the spatial structure and sub-structure of regions rich in{Hipparcos} stars with blue B_T-V_T colours. These regions, whichcomprise large stellar complexes, OB associations, and young openclusters, are tracers of on-going star formation in the Galaxy. TheDBSCAN (Density-Based Spatial Clustering of Applications with Noise)data clustering algorithm is used to look for spatial overdensities ofearly-type stars. Once an overdensity, ``agglomerate'', is identified,we carry out a data and bibliographic compilation of their star membercandidates. The actual membership in agglomerate of each early-type staris studied based on its heliocentric distance, proper motion, andprevious spectro-photometric information. We identify 35 agglomerates ofearly-type {Hipparcos} stars. Most of them are associated to previouslyknown clusters and OB associations. The previously unknown P Puppisagglomerate is subject of a dedicated study with Virtual Observatorytools. It is actually a new, nearby, young open cluster (d ˜ 470pc, age ˜ 20 Ma) with a clear radial density gradient. We list PPuppis and other six agglomerates (including NGC 2451 A, vdBH 23, andTrumpler 10) as new sites for substellar searches because of theiryouth, closeness, and spatial density. We investigate in detail thesub-structure in the Orion, CMa-Pup and Pup-Vel OB complexes(``super-agglomerates''). We confirm or discover some stellaroverdensities in the Orion complex, like the 25 Ori group, the Horseheadregion (including the σ Orionis cluster), and the η Orionisagglomerate. Finally, we derive accurate parallactic distances to thePleiades, NGC 2451 A, and IC 2391, describe several field early-typestars at d < 200 pc, and discuss the incompleteness of our search.
| The VELA star cloud. I - NGC 2547, TR 10, the Gamma Velorum system, and bright stars
The first results of an intermediate-band, large-scale photometricsurvey of the Vela star cloud are discussed. Attention is given to theluminosity and reddening, as well as the apparent or proper motions ofall the CPD stars near NGC 2547, the brightest stars in Trumpler 10, arandom selection of stars in the region of Gamma Velorum, and the HRstars. The similarity of the Alpha Persei cluster with the clusters inthe Vela sheet is shown, and it is found that the Vela sheet, at leastover the region surveyed, is nearly perpendicular to the line of sight.The probability is demonstrated that a thin sheet of coeval (2.5 x 10 tothe 7th yr) stars, some 425 pc distant and with a similar metalabundance, lies in front of a dense dark cloud.
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