The Owl and other strigiform nebulae: multipolar cavities within a filled shell

TL;DR

This study reveals that the Owl Nebula has a complex multipolar structure rather than a simple bipolar shape, introduces a new class of similar nebulae called strigiform, and provides detailed 3D modeling and morphological analysis.

Contribution

It identifies a new class of strigiform nebulae with unique morpho-kinematic features and proposes their common characteristics based on detailed spectroscopic and imaging data.

Findings

Owl Nebula has a multipolar, tripolar structure with inner cavities.

Identified three similar nebulae forming a new strigiform class.

Cavities may be relics of earlier stellar wind activity.

Abstract

We present the results of long-slit echelle spectroscopy and deep narrow-band imaging of the Owl Nebula (NGC 3587), obtained at the \textit{Observatorio Astron\'omico Nacional, San Pedro M\'artir}. These data allow us to construct an iso-velocity data cube and develop a 3-D morpho-kinematic model. We find that, instead of the previously assumed bipolar dumbbell shape, the inner cavity consists of multi-polar fingers within an overall tripolar structure. We identify three additional planetary nebulae that show very similar morphologies and kinematics to the Owl, and propose that these constitute a new class of \textit{strigiform} (owl-like) nebulae. Common characteristics of the strigiform nebulae include a double-shell (thin outside thick) structure, low-luminosity and high-gravity central stars, the absence of a present-day stellar wind, and asymmetric inner cavities, visible in both optical and mid-infrared emission lines, that show no evidence for surrounding bright rims. The origin of the cavities is unclear, but they may constitute relics of an earlier stage of evolution when the stellar wind was active.