Constraining properties of dust formed in Wolf-Rayet binary WR 112 using mid-infrared and millimeter observations

TL;DR

This study combines ALMA and JWST observations to analyze dust properties in the Wolf-Rayet binary WR 112, revealing a bimodal grain size distribution dominated by nanometer-sized grains, which helps resolve previous conflicting estimates.

Contribution

First millimeter observations of WR 112 with ALMA combined with JWST data to analyze dust grain size distribution in a WC binary system.

Findings

Dust emission consistent with hydrogen-poor amorphous carbon grains

Majority of grains have radii below one micrometer, dominated by nanometer-sized grains

A bimodal grain size distribution best fits the observed spectral energy distribution

Abstract

Binaries that host a carbon-rich Wolf-Rayet (WC) star and an OB-type companion can be copious dust producers. Yet the properties of dust, particularly the grain size distribution, in these systems remain uncertain. We present Band 6 observations of WR 112 by the Atacama Large Millimeter/submillimeter Array telescope (ALMA), which are the first millimeter observations of a WC binary system capable of resolving its dust emission. By combining ALMA observations with James Webb Space Telescope (JWST) images, we were able to analyze the spatially resolved spectral energy distribution (SED) of WR 112. We found that the SEDs are consistent with emissions from hydrogen-poor amorphous carbon grains. Notably, our results also suggest that the majority of grains in the system have radii below one micrometer, and the extended dust structures are dominated by nanometer-sized grains. Among four parameterizations of the grain radius distribution that we tested, a bimodal distribution, with abundant nanometer-sized grains and a secondary population of 0.1-micron grains, best reproduces the observed SED. This bimodal distribution helps to reconcile the previously conflicting grain size estimates reported for WR 112 and for other WC systems. We hypothesize that dust destruction mechanisms such as radiative torque disruption and radiative-driven sublimation are responsible for driving the system to the bimodal grain size distribution.