Resolving Decades of Periodic Spirals from the Wolf–Rayet Dust Factory WR 112

Abstract

Abstract WR 112 is a dust-forming carbon-rich Wolf–Rayet (WC) binary with a dusty circumstellar nebula that exhibits a complex asymmetric morphology, which traces the orbital motion and dust formation in the colliding winds of the central binary. Unraveling the complicated circumstellar dust emission around WR 112 therefore provides an opportunity to understand the dust formation process in colliding-wind WC binaries. In this work, we present a multi-epoch analysis of the circumstellar dust around WR 112 using seven high spatial resolution (FWHM ∼ 0.″3–0.″4) N-band (λ ∼ 12 μm) imaging observations spanning almost 20 yr and that includes images obtained from Subaru/COMICS in 2019 October. In contrast to previous interpretations of a face-on spiral morphology, we observe clear evidence of proper motion of the circumstellar dust around WR 112 consistent with a nearly edge-on spiral with a θ s = 55° half-opening angle and a ∼20 yr period. The revised near edge-on geometry of WR 112 reconciles previous observations of highly variable nonthermal radio emission that was inconsistent with a face-on geometry. We estimate a revised distance to WR 112 of kpc based on the observed dust expansion rate and a spectroscopically derived WC terminal wind velocity of km s−1. With the newly derived WR 112 parameters, we fit optically thin dust spectral energy distribution models and determine a dust production rate of M ⊙ yr−1, which demonstrates that WR 112 is one of the most prolific dust-making WC systems known.