The stellar remnant of SN 1181

We report observations and modelling of the stellar remnant and presumed double-degenerate merger of Type~Iax supernova SN 1181 AD. It is the only known bound stellar SN remnant and the only star with Wolf-Rayet features that is neither a planetary nebula central star nor a massive Pop I progenitor. We model the unique emission-line spectrum with broad, strong O VI and O VIII lines as a fast stellar wind and shocked, hot gas. Non-LTE wind modeling indicates a mass-loss rate of $\sim 10^{-6}\,\rm M_\odot yr^{-1}$ and a terminal velocity of $\sim$15,000 km s$^{-1}$, consistent with earlier results. O VIII lines indicate shocked gas temperatures of $T \simeq 4$ MK. We derive a magnetic field upper limit of $B<2.5$ MG, below earlier suggestions. The luminosity indicates a remnant mass of $1.2\pm0.2$ $\rm M_\odot$ with ejecta mass $0.15\pm0.05$ $\rm M_\odot$. Archival photometry suggests the stellar remnant has dimmed by $\sim$0.5 magnitudes over 100 years. A low Ne/O $<0.15$ argues against a O-Ne white dwarf in the merger. A cold dust shell is only the second detection of dust in a SN Iax and the first of cold dust. Our ejecta mass and kinetic energy estimates of the remnant are consistent with Type Iax extragalactic sources.