New Nearby Accreting Young Stars and a First Estimate of the IMF for the TW Hydrae Association

Abstract

Planetary formation is inexorably linked to its parent process of star formation, which begins with the gravitationally collapses of a cold molecular cloud to form an opaque, dense clump (Adams et al. 1987). As the center of the infall envelope becomes denser and the temperature increases, the object stabilizes and becomes an embedded protostar with a circumstellar disk ( 200,000 years; Enoch et al. 2008). Stellar winds disperse the gaseous envelope, and the protostar continues to accrete from its circumstellar disk, sometimes powering bipolar out ows (Mundt & Fried 1983). This accretion timescale lasts for a few to 10 Myr (Fedele et al. 2010) during which the star is known as a classical T Tauri Star (cTTS). During this time the disk undergoes radical changes { dust particles grow and settle into the disk mid-plane, protoplanets form and dynamically interact, and strong stellar winds clear the system of lingering gas and dust (Muzerolle et al. 2000). To understand planetary formation we must therefore study stellar formation. Stars themselves are distributed over a wide mass spectrum: from >100 M down to ~0.07 M.. Below this limit, objects are not able to fuse hydrogen in the core and are known as brown dwarfs or "failed stars." The highest mass stars are short-lived and rare while low-mass stars (0.1 M. < M < 0.8 M.) are long-lived (lifetimes greater than the age of the Universe; Laughlin et al. 1997) and make-up 70% of stars in the Milky Way (Bochanski et al. 2010). Stellar evolution and hence, disk sizes and lifetimes, are heavily dependent on the stellar mass. Accretion and out ow processes taking place in low-mass stars appear to operate down into the lower mass regime of brown dwarfs as well (Mohanty et al. 2005).