The TW Hydrae Association: The nearest region of recent star formation.

摘要

A recently identified group of T Tauri stars, located far from any known molecular cloud, now constitutes the nearest known region of recent star formation. A comprehensive study of this group, dubbed the "TW Hydrae Association" (hereafter TWA), is presented. Spectroscopic observations of X-ray bright, late-type stars has identified 14 new TWA systems, bringing the total known population to 21 systems containing 36 stars. An additional 23 unconfirmed TWA candidates are also identified.; Detailed observations and analysis of each TWA member have enabled derivation of fundamental properties. The TWA's age is estimated at 10 Myr based on theoretical pre-main-sequence isochrones. Kinematics-including distance, proper motion and radial velocity-are measured and Galactic space motions are computed. The uniformity of space motion confirms the TWA stars formed together from a molecular cloud which has since dissipated. The space motions also imply the TWA is kinematically related to other young, nearby groups which make up the Local Association. The TWA may have originated in a giant cloud complex which also formed the eta Cha and Sco-Cen OB associations.; The TWA possesses a unique combination of age, (intermediate between T Tauri stars in molecular clouds and young nearby open clusters), and distance, (∼3 times closer than commonly studied star formation regions), characteristics which make it an ideal environment to study forming planetary systems. For example, thermal emission from dust particles was detected at 850mum near five TWA stars. Large fluxes imply a substantial reservoir of cold dust which is undetected at shorter wavelengths. Interferometric measurements imply the disk orbiting TW Hya has radius ≳ 200 AU. Also identified is a ∼20 MJupiter brown dwarf orbiting TWA 5.; Searches for additional nearby, unrecognized stellar groups have been initiated. A control survey of X-ray bright M-type stars revealed only one very young star which characterize the TWA. However, a survey of stars selected based on kinematics revealed a slightly older (∼40 Myr) analogue of the TWA, dubbed the "Tucanae Association". Even including these discoveries, the local census of young stars and our understanding of recent star formation in the solar neighborhood, are far from complete.