Normal, Nascent and Stalled Pre-Planetary Nebulae

Abstract

The formation and shaping of planetary nebulae (PNe) is probably the most exciting, yet least understood issue in the late evolution of (1–8) $M_{\odot}$ stars. PNe evolve from the envelopes of AGB stars through the supposedly very short ($\lesssim10^3$ yr) pre-PN (or PPN) phase (Balick & Frank 2002). In 1998, in a radical departure from the then long-standing theoretical paradigm for PN formation, Sahai & Trauger (1998) proposed that as most stars evolve off the AGB, they drive collimated fast winds that sweep up and shock the AGB circumstellar envelope, producing the observed dramatic changes in circumstellar geometry and kinematics from the AGB to the PN phase. Pre-Planetary Nebulae (PPNs) have traditionally been understood as rare objects that represent a transitory phase in the evolution of AGB stars to PNs. In recent years, mainly due to high-resolution imaging surveys with HST, it has become possible to start studying the detailed physical properties for a statistically significant number of these objects. Here we provide a brief report of results from our several large surveys of PPNs with HST (and supporting ground-based observations).