Is the Dust Cloud Around Lambda Orionis a Ring or a Shell, or Both?

Abstract

The dust cloud around λ Orionis is observed to be circularly symmetric with a large angular extent (≈ 8°). 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 λ Ori is composed of both ring and shell structures.

Protoplanetary disc truncation mechanisms in stellar clusters: comparing external photoevaporation and tidal encounters

“…The λ Orionis (λ Ori) star forming region is an OB association at a distance of around 420 pc from the Sun (Schlafly et al 2014). It began forming stars ∼ 5 Myr ago, and is located inside a shell-like structure of dust and gas which is thought to be the result of a supernova explosion ∼ 1 Myr ago (Dolan & Mathieu 2001;Lee et al 2015) . Its proximity makes it a good candidate for studying disc populations, and previously Hernández et al (2010) have used data from the Spitzer Space Telescope to observe disc fractions of ∼ 20% around M-type stars.…”

Detection of spectral variations of Anomalous Microwave Emission with QUIJOTE and C-BASS

“…1. More recent CO observations (Lang & Masheder 1998;Lang et al 2000) and simulations of the ring's morphology (Lee, Seon & Jo 2015) support the basic shell model with a secondary toroidal ring component, favouring the flattened progenitor molecular cloud hypothesis. The star λ Orionis, which dominates UV emission in the region, has a proper motion of ≈ 3 mas yr −1 (van Leeuwen 2007) and is offset about a degree from the geometrical centre of the ring (Dolan & Mathieu 2002).…”

An ALMA survey of $λ$ Orionis disks: from supernovae to planet formation

“…Although λ Ori is the only O-type star in the cluster, it has a B0V companion at 1900 au projected separation, and this binary system is accompanied by nine other B-type stars (some close binaries themselves) in a dense ∼2 pc radius clump in the cluster center (Murdin & Penston 1977). The interstellar dust and molecular gas in the region is largely confined to a ∼30 pc radius ring (or possibly shell; Lee et al 2015), which is centered on the clump of OB stars, encompasses the lower-mass population, and is rapidly expanding at ∼14 km s −1 (e.g., (2009,2010), which we use as the basis of our ALMA survey sample selection (Section 3.1). The inset zooms in on this region: the large circles are our ALMA detections color-coded by dust mass (Section 5.1), while the small gray circles are the non-detections (Section 4.2); the dotted lines represent 1 pc and 3 pc radial distances from the λ Ori system at the center of the cluster.…”