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Wolfgang Brandner (Caltech - JPL/IPAC), Michael Kunkel, Christoph Leinert (MPIA Heidelberg), Rainer K\"ohler, Hans Zinnecker (AI Potsdam), Andrea Moneti (ISO-SOC/ESA)
Using data from a variety of satellite missions (HST, HIPPARCOS, ISO, ROSAT) and ground-based telescopes, we have carried out an extensive study of the low-mass pre-main-sequence population in Upper Scorpius, the youngest subgroup of the Scorpius-Centaurus OB association.
ROSAT helped to pinpoint young, active stars. Optical spectroscopy was then used to search for signs of Lithium absorption as an indication of youth. As a result about 100 weak-line and/or post T Tauri stars could be identified as likely members of Upper Scorpius. HIPPARCOS parallaxes and proper motions for a small subset of the stars confirm that they are indeed members of the association.
The spectroscopy in combination with photometry shows that the low-mass stars are coeval to the 5 to 7 Myr old population of high-mass stars in Upper Scorpius. Ground-based IR speckle observations and observations with HST/NICMOS reveal a high incident of close, previously unresolved binaries. This bias might explain why a prior study by Walter et al.\ (1994) deduced an age of only 1 Myr for the low-mass population of Upper Scorpius.
No close, substellar companions were detected by HST/NICMOS. This is a strong indication that the infrared excess at wavelengths between 6\,\mum and 15\,\mum, as detected by ISO, is due to evolved, remnant circumstellar disks, which might be in the process of forming planetesimals.
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