Robert

The remnant of Supernova 1885 (S And), a probable Type~Ia supernova, can be seen in absorption against the bulge of the Andromeda galaxy, M31. Here we present Hubble Space Telescope optical and ultraviolet STIS spectra of S And taken in order to investigate the remnant’s three dimensional structure. Optical spectra covering 2900-5700 A, taken using six 0.2″ wide slit positions in two orientations, show broad Ca II H&K absorption extending out to at least 11 500 km/s consistent with previous HST narrow passband Ca II images of S And. We find
enhancement of Ca II absorption between expansion velocities of 2000 and 5000 km/s suggestive of a lumpy Ca-rich shell. These spectra, together with previous HST images, indicate a remnant with less than a 10 percent departure from purely spherical expansion, a layered abundance structure indicative of a detonation phase, and a clumpy and plume-like Fe distribution suggestive of Rayleigh-Taylor instabilities although significantly less than expected from hydrodynamic simulations.