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 ALMA continuum and molecular line emission maps at $\sim$1 mm of OH 231.8, a well studied bipolar nebula around an AGB star. The excellent angular resolution of our maps ($\sim$20 mas) allows us to scrutinise the central nebular regions of OH 231.8, which hold the clues to unravel how this iconic object assembled its complex nebular architecture. We report, for the first time in this object and others of its kind, the discovery of a rotating circumbinary disk selectively traced by NaCl, KCl, and H${}_2$O emission lines. This represents the first detection of KCl in an oxygen-rich AGB circumstellar envelope. The rotating disk, of radius $\sim$30 au, lies at the base of a young bipolar wind traced by SiO and SiS emission, which also presents signs of rotation at its base. The NaCl equatorial structure is characterised by a mean rotation velocity of $\sim$4 km s${}^{-1}$ and extremely low expansion speeds, $\sim$3 km s${}^{-1}$. The outflow has a predominantly expansive kinematics characterised by a constant radial velocity gradient of $\sim$65 km s${}^{-1}$ arcsec${}^{-1}$ at its base. Beyond $r$$\sim$350 au, the gas in the outflow continues radially flowing at a constant terminal speed of $\sim$16 km s${}^{-1}$. Our continuum maps reveal a spatially resolved dust disk-like structure perpendicular to the outflow, with the NaCl, KCl and H${}_2$O emission arising from the disk's surface layers. Within the disk, we also identify an unresolved point continuum source, which likely represents the central Mira-type star QX Pup enshrouded by a $\sim$3 $R_{\star }$ component of hot ($\sim$1400 K) freshly formed dust. The point source is slightly off-centered from the disk centroid, enabling us for the first time to place constraints to the orbital separation and period of the central binary system, $a$$\sim$20 au and $P_{\mathrm{orb}}$$\sim$55 yr, respectively. (abridged).

arXiv comment30 pages, 18 figures. Accepted for publication in Astronomy & Astrophysics (on 19/06/2022)

References in corpus (45)

• Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium
• Post-Asymptotic Giant Branch Evolution of Low- and Intermediate-Mass Stars. Preliminary Results
• New models for the evolution of Post-Asymptotic Giant Branch stars and Central Stars of Planetary Nebulae
• The Origin and Shaping of Planetary Nebulae: Putting the Binary Hypothesis to the Test
• An HST Snapshot Survey of Proto-Planetary Nebulae Candidates: Two Types of Axisymmetric Reflection Nebulosities
• Young Planetary Nebulae: Hubble Space Telescope Imaging and a New Morphological Classification System
• Preplanetary Nebulae: An HST Imaging Survey and a New Morphological Classification System
• On the reliability of mass-loss-rate estimates for AGB stars
• A Mid-Infrared Imaging Survey of Proto-Planetary Nebula Candidates
• Wide Field CCD photometry around nine open clusters
• Extended rotating disks around post-AGB stars
• Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
• Mass Transfer and Disc Formation in AGB Binary Systems
• ALMA observations of the nearby AGB star L2 Puppis - I. Mass of the central star and detection of a candidate planet
• New insights into the dust formation of oxygen-rich AGB stars
• Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tau as seen by Herschel/HIFI
• The wind of W Hya as seen by Herschel - I. The CO envelope
• An Interferometric Spectral-Line and Imaging Survey of VY Canis Majoris in the 345 GHz Band
• Further ALMA observations and detailed modeling of the Red Rectangle
• Slowly, slowly in the wind: 3D hydrodynamical simulations of wind mass transfer and angular-momentum loss in AGB binary systems
• An extensive grid of DARWIN models for M-type AGB stars I. Mass-loss rates and other properties of dust-driven winds
• The evolution of DARWIN: current status of wind models for AGB stars
• Imaging the Radio Photospheres of Mira Variables
• Gas phase atomic metals in the circumstellar envelope of IRC+10216
• Detection of Keplerian dynamics in a disk around the post-AGB star AC Her
• High-resolution observations of IRAS 08544-4431. Detection of a disk orbiting a post-AGB star and of a slow disk wind
• Bipolar Planetary Nebulae from Outflow Collimation by Common Envelope Evolution
• Exploring the origin of clumpy dust clouds around cool giants. A global 3D RHD model of a dust-forming M-type AGB star
• A second post-AGB nebula that contains gas in rotation and in expansion: ALMA maps of IW Car
• New N-bearing species towards OH231.8+4.2: HNCO, HNCS, HC${}_3$N and NO
• Wind morphology around cool evolved stars in binaries: the case of slowly accelerating oxygen-rich outflows
• Dust and molecular shells in asymptotic giant branch stars - Mid-infrared interferometric observations of R Aql, R Aqr, R Hya, W Hya and V Hya
• Orion Source I's disk is salty
• Molecular ions in the O-rich evolved star OH231.8+4.2: HCO${}^{+}$,H${}^{13}$CO${}^{+}$ and first detection of SO${}^{+}$, N${}_2$H${}^{+}$, and H${}_3$O${}^{+}$
• An unusual face-on spiral in the wind of the M-type AGB star EP Aquarii
• SPH modelling of companion-perturbed AGB outflows including a new morphology classification scheme
• ALMA detection of a tentative nearly edge-on rotating disk around the nearby AGB star R~Doradus
• Through the magnifying glass: ALMA acute viewing of the intricate nebular architecture of OH231.8+4.2
• Keplerian disks and outflows in post-AGB stars: AC Herculis, 89 Herculis, IRAS 19125+0343, and R Scuti
• Shaping the envelope of the asymptotic giant branch star W43A with a collimated fast jet
• Models of the Hydrodynamic Histories of post-AGB Stars. I. Multiflow Shaping of OH231.8+04.2
• Structure and dynamics of the inner nebula around the symbiotic stellar system R Aqr
• An alternative approach to modelling a cosmic void and its effect on the cosmic microwave background
• A rotating fast bipolar wind and disk system around the B[e]-type star MWC 922
• The Open Cluster NGC 2437 (Messier 46)

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