Revealing Efficient Dust-Formation at Low Metallicity in Extragalactic
  Carbon-Rich Wolf-Rayet Binaries
  We present Spitzer/IRAC observations of dust formation from six extragalactic
carbon-rich Wolf-Rayet (WC) binary candidates in low-metallicity (Z
$\lesssim0.65$ Z$_\odot$) environments using multi-epoch mid-infrared (IR)
imaging data from the SPitzer InfraRed Intensive Transients Survey (SPIRITS).
Optical follow-up spectroscopy of SPIRITS~16ln, 19q, 16df, 18hb, and 14apu
reveals emission features from C IV $\lambda5801\text{-}12$~and/or the C III-IV
$\lambda4650$ He II $\lambda4686$~blend that are consistent with early-type WC
stars. We identify SPIRITS~16ln as the variable mid-IR counterpart of the
recently discovered colliding-wind WC4+O binary candidate, N604-WRXc, located
in the sub-solar metallicity NGC 604 H II~region in M33. We interpret the
mid-IR variability from SPIRITS~16ln as a dust-formation episode in an
eccentric colliding-wind WC binary. SPIRITS~19q, 16df, 14apu, and 18hb exhibit
absolute [3.6] magnitudes exceeding one of most IR-luminous dust-forming WC
systems known, WR~104 (M$_\mathrm{[3.6]}\lesssim-12.3$). An analysis of dust
formation in the mid-IR outburst from SPIRITS~19q reveals a high dust
production rate of $\dot{M}_d\gtrsim2\times10^{-6}$ M$_\odot$ yr$^{-1}$, which
may therefore exceed that of the most efficient dust-forming WC systems known.
We demonstrate that efficient dust-formation is feasible from early-type WC
binaries in the theoretical framework of colliding-wind binary dust formation
if the systems host an O-type companion with a high mass-loss rate
($\dot{M}\gtrsim1.6\times10^{-6}$ M$_\odot$ yr$^{-1}$). This efficient
dust-formation from early-type WC binaries highlights their potential role as
significant sources of dust in low-metallicity environments.
