Dust in RCW 58: clues to common envelope channel formation?

TL;DR

This study characterizes dust in the RCW 58 nebula around WR 40, revealing a ring-like structure with large dust grains and high dust-to-gas ratios, supporting a common envelope formation scenario.

Contribution

It provides the first detailed IR and radio modeling of RCW 58's dust and gas, proposing a ring morphology and common envelope origin for the nebula.

Findings

RCW 58 has a ring-like structure with a mass of about 2.5 solar masses.

Two dust grain populations are present: small grains throughout and large grains further from the star.

The nebula's properties support a common envelope formation channel.

Abstract

We present a characterization of the dust in the Wolf-Rayet (WR) nebula RCW 58 around the WN8h star WR 40 using archival infrared (IR) observations from WISE and Herschel and radio observations from ATCA. We selected two clumps, free from contamination from material along the line of sight and located towards southern regions in RCW 58, as representative of the general properties of this WR nebula. Their optical, IR and radio properties are then modelled using the photoionization code Cloudy, which calculates a self-consistent spatial distribution of dust and gas properties. Two populations of dust grains are required to model the IR SED: a population of small grains with sizes 0.002-0.01 $\mu$m, which is found throughout the clumps, and a population of large grains, with sizes up to 0.9 $\mu$m, located further from the star. Moreover, the clumps have very high dust-to-gas ratios, which present a challenge for their origin. Our model supports the hypothesis that RCW 58 is distributed in a ring-like structure rather than a shell, and we estimate a mass of $\sim$2.5 M$_\odot$. This suggests that the mass of the progenitor of WR 40 was about $\approx40^{+2}_{-3}$ M$_\odot$. The ring morphology, low nebular mass, large dust grain size and high dust-to-gas ratio lead us to propose that RCW 58 has formed through a common envelope channel, similar to what has been proposed for M 1-67.