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

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, QCS, 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≲35km 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 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.

Time variations of H2O and SiO masers in the proto-Planetary Nebula OH 231.8+4.2

“…On the other hand, geometrically, the H 2 O maser shows the underlying bipolar outflow that probably produce the macro-scale structure of the nebula, which represents the innermost parts of the SiO outflow traed by SiO J = 7→6 and 29 SiO J = 8→6 lines (Sánchez Contreras et al 2018). The velocity extent of the H 2 O maser line may give the outflow velocity which makes up a sizable fraction of the terminal velocity.…”

“…There is good agreement between the values obtained from this thermal emission and the H 2 O maser as a tracer of shocked gas. For reference, the SiO v = 0, J = 7→6 and 29 SiO J = 8→7 (thermal) emission lines have been also recently detected to trace a compact bipolar outflow, where the expansion velocities are lower than 35 km s −1 (Sánchez Contreras et al 2018). This SiO compact outflow seems to be the large scale counterpart of the H 2 O maser bipolar flow.…”

Non-adiabatic dynamics in collisions of sodium and chlorine atoms and their ions

“…Being the first molecular bearer of a sodium atom detected in space, 12,13 the gas-phase NaCl molecules have been observed in various space environments. These observations are usually associated with stars in their final evolution stage (carbon-rich or oxygen-rich stars), where NaCl molecules briefly exist in the inner layers of expanding circumstellar shells 12,[14][15][16][17][18] before being eventually condensed onto dust grains. Recently, however, the gas-phase NaCl was also detected above the protostellar oxygen-rich disk around a massive accreting young star, 19 which was its first observation not related to evolved stars.…”

(Sub)mm-Wavelength Observations of Pre-Planetary Nebulae and Young Planetary Nebulae

“…Another example of nebular complexity disclosed by ALMA is found in the work by Sánchez Contreras et al [32] on OH 231.8+4.2, a remarkably fast bipolar outflow around a binary system formed by an AGB star and a main-sequence companion. As seen with ∼1 -resolution, the molecular outflow, which is rather massive (∼0.3 M ) and fast (V exp up to ∼400 km s −1 ), has an overall cylindrical shape.…”

“…2-0. 3-resolution, the nebular structure is significantly more complex, and several distint components that were previously unknown were identified [32]. The large and dense equatorial waist of OH 231.8+4.2 is now spatially resolved, which enables a proper characterization of its shape, and most importantly, its kinematics.…”