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

C. Sánchez Contreras, J. Alcolea, V. Bujarrabal, A. Castro-Carrizo, L. Velilla Prieto, M. Santander-García, G. Quintana-Lacaci and J. Cernicharo. 2018. Through the magnifying glass: ALMA acute viewing of the intricate nebular architecture of OH 231.8+4.2. Astronomy and Astrophysics 618, DOI: 10.1051/0004-6361/201833632

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 (similar to 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 (CO)-C-12, (CO)-C-13, CS, SO, SO2, OCS, SiO, SiS, H3O+, (NaCl)-Cl-37, and CH3OH. The molecules (NaCl)-Cl-37 and CH3OH are first detections in OH231.8 + 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. QXPup is enshrouded within a compact (less than or similar to 60AU) parcel of dust and gas (clump S) in expansion (V-exp similar to 5-7 km s(-1)) that is displaced by similar to 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 QXPup’s vicinity. This outflow is oriented similarly to the large-scale nebula but the expansion velocities are about ten times lower (V-exp less than or similar to 35 km s(-1)). We deduce short kinematical ages for the SiO outflow, ranging from similar to 50-80 yr, in regions within similar to 150 AU, to similar to 400-500 yr at the lobe tips (similar to 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 alpha r). The mini-waist is characterized by extremely low velocities, down to similar to 1 km s(-1) at similar to 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 similar to 800-900 yr ago. We report the discovery of two large (similar to 8 » x 6 »), faint bubble-like structures (fish bowls) surrounding the central parts of the nebula. These are relatively old structures, although probably slightly (similar to 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 less than or similar to 80 yr bipolar ejections indicate that the collimated fast wind engine is still active at the core of this outstanding object.

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