A distance estimate based on angular expansion for the planetary nebula NGC 6881

TL;DR

This paper estimates the distance to the planetary nebula NGC 6881 using high-resolution radio observations and angular expansion measurements, providing a new method for distance determination in planetary nebulae.

Contribution

It introduces a distance estimation technique based on angular expansion measurements from radio observations, applied to NGC 6881, and compares it with optical spectroscopy data.

Findings

Distance to NGC 6881 is estimated at 1.6 ± 0.5 kpc.

Expansion rate of the nebula's core is 2.1 ± 0.7 mas/yr.

The method aligns with previous estimates and suggests possible association with nearby star-forming regions.

Abstract

In this paper, we report on high angular resolution radio observations of the planetary nebula NGC 6881 obtained with the Very Large Array at a wavelength of 6 cm. The emission appears to be the superposition of a roundish core and a point-symmetric bipolar structure elongated along a position angle of about 145$^\circ$. This is strongly reminiscent of the morphology seen in H$\alpha$ and [NII] images. A comparison between VLA observations obtained in 1984 and 1994 clearly reveals the expansion of the core of the nebula, at a rate of 2.1 $\pm$ 0.7 mas yr$^{-1}$. Assuming that the expansion velocity in the plane of the sky (determined from these measurements) and the expansion velocity along the line of sight (estimated from optical spectroscopy available in the literature) are equal, we find a distance to NGC 6881 of 1.6 $\pm$ 0.5 kpc $\pm$ 0.3 kpc, where the first error reflects the uncertainty on the expansion, and the second error comes from the potential difference between pattern and material speeds. This distance is compatible with (but does not necessarily imply) an association of NGC 6881 with the nearby HII region Sh 2-109 and, more generally, the Cygnus star-forming region.