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A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun
Traditionally, runaway stars are O- and B-type stars with large peculiarvelocities. We would like to extend this definition to young stars (upto ?50 Myr) of any spectral type and to identify those present in theHipparcos catalogue by applying different selection criteria, such aspeculiar space velocities or peculiar one-dimensional velocities.Runaway stars are important for studying the evolution of multiple starsystems or star clusters, as well as for identifying the origins ofneutron stars. We compile the distances, proper motions, spectral types,luminosity classes, V magnitudes and B-V colours, and we utilizeevolutionary models from different authors to obtain star ages. We studya sample of 7663 young Hipparcos stars within 3 kpc from the Sun. Theradial velocities are obtained from the literature. We investigate thedistributions of the peculiar spatial velocity and the peculiar radialvelocity as well as the peculiar tangential velocity and itsone-dimensional components and we obtain runaway star probabilities foreach star in the sample. In addition, we look for stars that aresituated outside any OB association or OB cluster and the Galactic planeas well as stars for which the velocity vector points away from themedian velocity vector of neighbouring stars or the surrounding local OBassociation/cluster (although the absolute velocity might be small). Wefind a total of 2547 runaway star candidates (with a contamination ofnormal Population I stars of 20 per cent at most). Thus, aftersubtracting these 20 per cent, the runaway frequency among young starsis about 27 per cent. We compile a catalogue of runaway stars, which isavailable via VizieR.
| Meeting the Cool Neighbors. V. A 2MASS-Selected Sample of Ultracool Dwarfs
We present the initial results of our effort to create a statisticallyrobust, volume-limited sample of ultracool dwarfs from the Two MicronAll Sky Survey Second Incremental Data Release. We are engaged in amultifaceted search for nearby late-type objects, and this is the firstinstallment of our search using purely photometric selection. The goalof this work is a determination of the low-mass star and brown dwarfluminosity function in the infrared. Here we outline the construction ofthe sample, dubbed 2MU2, and present our first results, including thediscovery of 186 M7-L6 dwarfs-47 of which are likely to be within 20 pcof the Sun. These results represent 66% of the ultracool candidates inour sample yet constitute a 127% increase in the number of ultracooldwarfs known within the volume searched (covering 40% of the sky out to20 pc). In addition, we have identified 10 M4-M6.5 objects that arelikely to be within 20 pc (or within 1 σ). Finally, based on theseinitial data, we present a preliminary luminosity function and discussseveral interesting features of the partial sample presented here. Onceour sample is complete, we will use our measured luminosity function toconstrain the mass function of low-mass stars and brown dwarfs.
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