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

TL;DR

This work resolves the distance to OH 231.8+4.2 by combining VLBI maser astrometry ($p = 0.65 \,\pm\, 0.01$ stat $\pm\, 0.02$ sys mas) and Gaia DR3 data for M 46, yielding $d \approx 1.54$ kpc and $\varpi_{\rm cl} \approx 0.639$ mas for the cluster. The luminosity is revised to $1.4 \times 10^4 L_\odot$, and while the parallax distance confirms cluster membership, the nebula shows a notable proper motion and radial-velocity offset that cannot be easily explained by simple ejection from the cluster. A stellar-merger scenario offers a cohesive explanation for the kinematic anomaly, chemical peculiarities, and bipolar outflow geometry, paralleling features seen in other merger-formed systems. The results underscore the importance of precise astrometry in constraining evolutionary histories of complex late-type systems and their dynamic interactions within clusters.

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.

Figures (6)

• Figure 1: Spectra of the H$_2$O masers toward OH 231.8+4.2 produced by scalar averaging the data over all time and baseline. Four different lines represent different epochs.
• Figure 2: Spatial distribution of the H$_2$O masers in OH 231.8+4.2 at all epochs (top left: 1; top right: 2; bottom left: 3; bottom right: 4). The LSR velocities of the maser spots are indicated by the color bar to the right. The reference spot at $V_{\rm LSR}$ = 25.52 km s$^{-1}$ is located at (0,0). The blue and green stars in the bottom right map show the location of the SiO maser spots at 27.26 and 32.23 km s$^{-1}$, respectively.
• Figure 3: The median separation between the northern and southern water maser cluster in the observations from Desmurs (open square), Leal-Ferreira (filled square), Dodson2018 (blue x) and our own data (green x). The blue line is a linear fit to all data.
• Figure 4: Parallax and proper motion fit for OH 231.8+4.2. For each plot, points with error bars are position measurements of the H$_2$O maser spot at $V_{\rm LSR}$ of 25.52 km s$^{-1}$ relative to the background quasar J0746--1555. Left panel: motion of the maser on the sky. Middle panel: the change of position eastward (solid line) and northward (dashed line) as a function of time. The northward data have been shifted downward for clarity. Right panel: same as the middle panel after removing the best-fitting linear proper motion.
• Figure 5: Upper panel: Parallaxes of individual stars in M 46 plotted against the apparent G magnitude of the star. The horizontal line marks the average parallax. Lower panel: Same as the upper panel, but removing stars with astrometric_excess_noise_sig>2. The average parallax changed by 8 $\mu$as.