Is Dust Cloud around $\lambda$ Orionis a Ring or a Shell, or Both?

TL;DR

This study investigates the 3D structure of the dust cloud around λ Orionis, revealing it is composed of both a spherical shell and a toroidal ring, using radiative transfer modeling and inverse Abel transform analysis.

Contribution

The paper introduces a combined approach of 3D Monte Carlo radiative transfer modeling and inverse Abel transform to determine the complex shell and ring structure of the dust cloud.

Findings

The shell model reproduces UV scattered light radial profiles.

Negative density values indicate additional non-spherical clouds.

The combined structure of shell and ring best fits observations.

Abstract

The dust cloud around $\lambda$ Orionis is observed to be circularly symmetric with a large angular extent ($\approx$ 8 degrees). However, whether the three-dimensional (3D) structure of the cloud is shell- or ring-like has not yet been fully resolved. We study the 3D structure using a new approach that combines a 3D Monte Carlo radiative transfer model for ultraviolet (UV) scattered light and an inverse Abel transform, which gives a detailed 3D radial density profile from a two-dimensional column density map of a spherically symmetric cloud. By comparing the radiative transfer models for a spherical shell cloud and that for a ring cloud, we find that only the shell model can reproduce the radial profile of the scattered UV light, observed using the S2/68 UV observation, suggesting a dust shell structure. However, the inverse Abel transform applied to the column density data from the Pan-STARRS1 dust reddening map results in negative values at a certain radius range of the density profile, indicating the existence of additional, non-spherical clouds near the nebular boundary. The additional cloud component is assumed to be of toroidal ring shape; we subtracted from the column density to obtain a positive, radial density profile using the inverse Abel transform. The resulting density structure, composed of a toroidal ring and a spherical shell, is also found to give a good fit to the UV scattered light profile. We therefore conclude that the cloud around $\lambda$ Ori is composed of both ring and shell structures.