Through the magnifying glass: ALMA acute viewing of the intricate nebular architecture of OH 231.8+4.2

We present continuum and molecular line emission ALMA observations of OH 231.8+4.2, a well studied bipolar nebula around an asymptotic giant branch (AGB) star. The high-angular resolution (~0.′′2–0.′′3) and sensitivity of our ALMA maps provide the most detailed and accurate description of the overall nebular structure and kinematics of this object to date. We have identified a number of outflow components previously unknown. Species studied in this work include 12CO, 13CO, CS, SO, SO2, OCS, SiO, SiS, H3O+, Na37Cl, and CH3OH. The molecules Na37Cl and CH3OH are first detections in OH 231.8+4.2, with CH3OH being also a first detection in an AGB star. Our ALMA maps bring to light the totally unexpected position of the mass-losing AGB star (QX Pup) relative to the large-scale outflow. QX Pup is enshrouded within a compact (≲60 AU) parcel of dust and gas (clump S) in expansion (Vexp ~ 5–7 km s−1) that is displaced by ~ 0.′′6 to the south of the dense equatorial region (or waist) where the bipolar lobes join. Our SiO maps disclose a compact bipolar outflow that emerges from QX Pup’s vicinity. This outflow is oriented similarly to the large-scale nebula but the expansion velocities are about ten times lower (Vexp ≲ 35 km s−1). We deduce short kinematical ages for the SiO outflow, ranging from ~50–80 yr, in regions within ~150 AU, to ~400–500 yr at the lobe tips (~3500 AU). Adjacent to the SiO outflow, we identify a small-scale hourglass-shaped structure (mini-hourglass) that is probably made of compressed ambient material formed as the SiO outflow penetrates the dense, central regions of the nebula. The lobes and the equatorial waist of the mini-hourglass are both radially expanding with a constant velocity gradient (Vexp ∝ r). The mini-waist is characterized by extremely low velocities, down to ~1 km s−1 at ~150 AU, which tentatively suggest the presence of a stable structure. The spatio-kinematics of the large-scale, high-velocity lobes (HV lobes), and the dense equatorial waist (large waist) known from previous works are now precisely determined, indicating that both were shaped nearly simultaneously about ~800–900 yr ago. We report the discovery of two large (~8′′ × 6′′), faint bubble-like structures (fish bowls) surrounding the central parts of the nebula. These are relatively old structures, although probably slightly (~100–200 yr) younger than the large waist and the HV lobes. We discuss the series of events that may have resulted in the complex array of ne