Stagnant Shells in the Vicinity of the Dusty Wolf–Rayet–OB Binary WR 112

Abstract

We present high spatial resolution mid-infrared images of the nebula around the late-type carbon-rich Wolf–Rayet (WC)–OB binary system WR 112 taken by the recently upgraded VLT spectrometer and imager for the mid-infrared (VISIR) with the PAH1, Ne ii _2, and Q3 filters. The observations reveal a morphology resembling a series of arc-like filaments and broken shells. Dust temperatures and masses are derived for each of the identified filamentary structures, which exhibit temperatures ranging from K at the exterior W2 filament to K in the central 3″. The total dust mass summed over the features is 2.6 ± 0.4 × 10 −5 M ⊙ . A multi-epoch analysis of mid-IR photometry of WR 112 over the past ∼20 years reveals no significant variability in the observed dust temperature and mass. The morphology of the mid-IR dust emission from WR 112 also exhibits no significant expansion from imaging data taken in 2001, 2007, and 2016, which disputes the current interpretation of the nebula as a high expansion velocity (∼1200 km s −1 ) “pinwheel”-shaped outflow driven by the central WC–OB colliding-wind binary. An upper limit of ≲120 km s −1 is derived for the expansion velocity assuming a distance of 4.15 kpc. The upper limit on the average total mass-loss rate from the central 3″ of WR 112 is estimated to be ≲8 × 10 −6 M ⊙  year −1 . We leave its true nature as an open question, but propose that the WR 112 nebula may have formed in the outflow during a previous red or yellow supergiant phase of the central Wolf–Rayet star.