On the Nature of the Protoplanetary Nebula OH\,231.8+4.2

Abstract: Aims. OH 231.8+4.2 , also known as the Rotten Egg or Calabash nebula is a protoplanetary nebula which is seen in the direction of the open cluster M 46. While an association of the nebula with the cluster has been suggested in the past, this has been never confirmed. Here, we present accurate trigonometric parallax and proper motion measurements using 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 using the Very Long Baseline Array at 4 epochs over 1 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 1 year, we measured an annual parallax of OH 231.8+4.2 to be 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 of M 46 from the 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 x 10^4 L_sol. However, there is a 15 km/s velocity difference between OH 231.8+4.2 and M 46 which could be caused by a past merger event.