A Slowly Expanding Disk and Fast Bipolar Outflow from the S Star π1Gruis

Abstract

We study the molecular outflow of the nearby evolved S star π1 Gru. We imaged the outflow in CO J = 2-1 and dust continuum with the Submillimeter Array. The CO emission was detected over a very broad velocity width of ~90 km s-1. Our high-resolution images show that the outflow at low velocities (≤15 km s-1) is elongated east-west and at high velocities (≥25 km s-1) is displaced north (at redshifted velocities) and south (blueshifted velocities) of center as defined by the dust continuum source. We model the spatial-kinematic structure of the low-velocity outflow as a flared disk with a central cavity of radius 200 AU and an expansion velocity of 11 km s-1, inclined by 55° to our line of sight. We attribute the high-velocity component to a bipolar outflow that emerges perpendicular to this disk with a velocity of up to ~45 km s-1. This high-velocity outflow may play an important role in shaping the gas envelope previously ejected by the AGB star and thus produce a bipolar morphology when the object evolves into a proto-planetary nebula.