Dissecting the central regions of OH 231.8+4.2 with ALMA: A salty rotating disk at the base of a young bipolar outflow

Abstract

We present a set of Atacama Large Millimeter/submillimeter Array (ALMA) continuum and molecular line emission maps at ~1 mm wavelengths of OH 231.8+4.2. This is a well studied bipolar nebula around an asymptotic giant branch (AGB) star that is key in investigations of the origin of the remarkable changes in nebular morphology and kinematics during the short transition from the AGB to the planetary nebula (PN) phase. The excellent angular resolution of our maps (~20 mas ≈ 30 au) allows us to scrutinize the central nebular regions of OH 231.8+4.2, which hold the clues to unravel how this iconic object assembled its complex nebular architecture. We report, for the first time for this object and others of its kind (i.e.,pre-PN with massive bipolar outflows), the discovery of a rotating circumbinary disk selectively traced by NaCl, KCl, and H2O emission lines. This represents the first detection of KCl in an oxygen-rich (O-rich) AGB circumstellar envelope (CSE). The rotating disk, of a radius of ~30 au, lies at the base of a young bipolar wind traced by SiO and SiS emission (referred to as the SS-outflow), which also presents signs of rotation at its base. The NaCl equatorial structure is characterised by a mean rotation velocity of Vrot ~ 4 km s−1 and extremely low expansion speeds, Vexp ~ 3 km s−1. The SS-outflow has predominantly expansive kinematics, characterized by a constant radial velocity gradient of ~65km s−1 arcsec−1 at its base. Beyond r ~ 350 au, the gas in the SS-outflow continues its radial flow at a constant terminal speed of Vexp ~ 16 km s−1. Our continuum maps reveal a spatially resolved dusty disk-like structure perpendicular to the SS-outflow, with the NaCl, KCl, and H2O emission arising from the surface layers of the disk. Within the disk, we also identify an unresolved point continuum source, which likely represents the central Mira-type star QX Pup enshrouded by a ~3 R* component of hot, (~1400 K) freshly formed dust. The point source is slightly off-center (by ~6.6mas) from the disk centroid, enabling us to place the first constraints on the orbital separation and period of the central binary system, namely: a ~ 20 au and Porb ~ 55 yr, respectively. The formation of the dense rotating equatorial structure at the core of OH 231.8+4.2 is most likely the result of wind Roche lobe overflow (WRLOF) mass transfer from QX Pup to the main-sequence companion; this scenario is greatly favored by the extremely low AGB wind velocity, the relatively high mass of the companion, and the comparable sizes of the dust condensation radius and the Roche lobe radius deduced from our data. The Vexp∝ r kinematic pattern observed within the r ≲ 350 au inner regions of the SS-outflow suggest that we are witnessing the active acceleration of the companion-perturbed wind from QX Pup as it flows through low-density polar regions.