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, NeII\_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 $179_{-6}^{+8}$ K at the exterior W2 filament to $355_{-25}^{+37}$ K in the central 3". The total dust mass summed over the features is $2.6\pm0.4\times10^{-5}$ $\mathrm{M}_\odot$. A multi-epoch analysis of mid-IR photometry of WR~112 over the past $\sim20$ yr 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 ($\sim1200$ km s$^{-1}$) "pinwheel"-shaped outflow driven by the central WC-OB colliding-wind binary. An upper limit of $\lesssim120$ 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 $\lesssim8\times10^{-6}$ $\mathrm{M}_\odot$ yr$^{-1}$. 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.