Optical and UV Spectra of the Remnant of SN 1885 (S And) in M31*

Abstract

Abstract We present multi-slit, 1D and 2D optical and UV spectra of the remnant of supernova 1885 (SN 1885; S And) taken using the Hubble Space Telescope’s Imaging Spectrograph (HST/STIS). These spectra of this probable subluminous Type Ia remnant, seen in silhouette against the central bulge of the Andromeda galaxy (M31), show strong and broad absorptions from neutral and singly ionized species of calcium, magnesium, and iron but with strikingly different distributions. Calcium H and K absorption indicates spherically distributed Ca-rich ejecta, densest in a lumpy shell expanding at 2000–6000 km s−1. Equally broad but weaker Ca i 4227 Å absorption is seen to extend out to velocities of ∼13,000 km s−1. Magnesium-rich ejecta in the remnant are detected for the first time through Mg i 2852 Å and Mg ii 2796, 2803 Å absorptions concentrated in a shell with expansion velocities from to at least 10,000 km s−1. Fe i 3720 Å absorption is detected as two discrete blueshifted and redshifted absorptions suggestive of an Fe i shell with expansion velocities of ±2000–8000 km s−1. Weak Fe ii resonance absorptions in the wavelength region 2300–2700 Å are consistent with prior HST UV images showing Fe ii–rich ejecta confined to a small number of optically thick plumes. The presence of such iron plumes extending out from the remnant’s core plus layered shells of calcium and magnesium point to a delayed-detonation explosion. The spectra also suggest a roughly spherical explosion, contrary to that expected by a merger or collision of two white dwarfs. We conclude that SN 1885 likely was an off-center, delayed-detonation explosion leading to a subluminous SN Ia similar to SN 1986G.