Investigating the nature of the protoplanetary nebula OH 231.8+4.2

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge MA 02138, USA

^⋆ Corresponding authors: brunthal@mpifr-bonn.mpg.de, ykchoi@mpifr-bonn.mpg.de, reid@cfa.harvard.edu

Received: 30 April 2025
Accepted: 13 August 2025

Abstract

Aims. The protoplanetary nebula OH 231.8+4.2, also known as the Rotten Egg or Calabash nebula, is seen in the direction of the open cluster M 46. While an association between the nebula and the cluster has been suggested in the past, it was never confirmed. Here, we present accurate trigonometric parallax and proper motion measurements using very-long-baseline interferometry (VLBI) observations of masers in the nebula and Gaia DR3 data for the cluster.

Methods. We observed 22 GHz H₂O and 43 GHz SiO masers around OH 231.8+4.2, with the Very Long Baseline Array (VLBA) in four epochs over the course of one year. We also calculated the parallax and proper motion of the open star cluster M 46 using Gaia DR3 data.

Results. Based on astrometric monitoring for one year, we measured an annual parallax for OH 231.8+4.2 of 0.65 ± 0.01 mas (stat.) ± 0.02 mas (syst.), corresponding to a distance of 1.54 ± 0.05 kpc. This agrees well with the parallax for M 46 drawn from Gaia DR3 data, which is 0.639 ± 0.001 mas (stat.) ±0.010 mas (syst.). We re-estimated the luminosity of OH 231.8+4.2 to be 1.4 × 10⁴ L_⊙. However, there is a 15km s⁻¹ velocity difference between OH 231.8+4.2 and M 46, which could potentially have been caused by a past merger event.