Registration of H$_2$O and SiO masers in the Calabash Nebula, to confirm the Planetary Nebula paradigm

TL;DR

This study uses VLBI imaging to precisely align SiO and water masers in the Calabash Nebula, confirming the connection between the stellar core and outflow, and revealing recent, ongoing mass ejection events shaping the nebula.

Contribution

First robust astrometric registration of SiO and water masers in the Calabash Nebula, confirming their spatial relationship and recent outflow activity.

Findings

Water masers expand at 19 km/s

Expansion age is 38±2 years

Mass loss is ongoing and multiple ejection epochs are confirmed

Abstract

We report on the astrometric registration of VLBI images of the SiO and water masers in OH231.8+4.2, the iconic Proto-Planetary Nebula also known as the Calabash nebula, using the KVN and Source/Frequency Phase Referencing. This, for the first time, robustly confirms the alignment of the SiO masers, close to the AGB star, driving the bi-lobe structure with the water masers in the out-flow. We are able to trace the bulk motions for the water masers over the last few decades to be 19 km/s and deduce that the age of this expansion stage is 38$\pm$2 years. The combination of this result with the distance allows a full 3D reconstruction, and confirms that the water masers lie on and expand along the known large-scale symmetry axis and that the outflow is only a few decades old, so mass loss is almost certainly on-going. Therefore we conclude that the SiO emission marks the stellar core of the nebular, the water emission traces the expansion, and that there must be multiple epochs of ejection to drive the macro-scale structure.