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 H 2 O 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 V rot ∼4 km s −1 and extremely low expansion speeds, V exp ∼3 km s −1 . The SS-outflow has predominantly expansive kinematics, characterized by a constant radial velocity gradient of ∼65 km 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 V exp ∼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 H 2 O 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 ∼6.6 mas) 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 P orb ∼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 V exp ∝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.

The Binary and the Disk: The Beauty is Found within NGC3132 with JWST

“…The currently adopted wind Roche-lobe Overflow (wRLOF) mechanism of hydrodynamical interaction between a companion and a stellar wind in the red giant phase predicts strong effects when the separation of both stars is small, typically under 50-100 au (Mohamed & Podsiadlowski 2012;Chen et al 2017;Kim et al 2019). The circumstellar gas at such distances is still being accelerated and shows a relatively low velocity, which is a basic ingredient for allowing strong interaction effects, including the formation of rotating circumbinary disks and symbiotic phenomena (e.g., Sánchez Contreras et al 2022). Rotation in inner suborbital regions can also be induced in those cases of strong interaction (e.g., Bermúdez-Bustamante et al 2020).…”

Salt-bearing Disk Candidates around High-mass Young Stellar Objects

“…Salts have been detected in CRL 2688 (Highberger et al 2003), IRAS+10216 (Cernicharo & Guelin 1987), IK Tauri, VY Canis Majoris (Milam et al 2007;Decin et al 2016), VX Sgr (mentioned in passing in Danilovich et al 2021), andOH231.8+44.2 (Sánchez Contreras et al 2022). Sánchez Contreras et al (2022) discovered a brinary disk surrounding the post-AGB mass transfer system OH231.8+4.2, highlighting the similarity between birth and death among moderately massive stars. However, there are also some nondetections in well-surveyed sources, including the S-type AGB stars W Aql and π 1 Gru (Homan et al 2020;Danilovich et al 2021).…”