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

¹ Centro de Astrobiología (CSIC-INTA), ESAC, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28691 Villanueva de la Cañada, Madrid, Spain
e-mail: csanchez@cab.inta-csic.es
² Observatorio Astronómico Nacional (IGN), Alfonso XII No 3, 28014 Madrid, Spain
³ Observatorio Astronómico Nacional (IGN), Ap 112, 28803 Alcalá de Henares, Madrid, Spain
⁴ Institut de Radioastronomie Millimetrique, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁵ Instituto de Física Fundamental (CSIC), C/ Serrano 123, 28006, Madrid, Spain

Received: 13 June 2018
Accepted: 27 July 2018

Abstract

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 (V_exp ~ 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 (V_exp ≲ 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 (V_exp ∝ 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 nebular components found in OH 231.8+4.2 as well as the properties and locus of the central binary system. The presence of ≲80 yr bipolar ejections indicate that the collimated fast wind engine is still active at the core of this outstanding object.