The Low-Mass Stellar Content of the Scorpius-Centaurus OB Association

Astronomy and Astrophysics – Astronomy

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Scientific paper

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 mu m and 15 mu m, as detected by ISO, is due to evolved, remnant circumstellar disks, which might be in the process of forming planetesimals.

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