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

TL;DR

This study uses Hubble Space Telescope spectra to analyze the layered structure and composition of supernova remnant SN 1885, supporting a delayed detonation model and revealing insights into its explosion mechanism.

Contribution

First detailed optical and UV spectral analysis of SN 1885's remnant, providing evidence for a layered structure and supporting a delayed detonation explosion model.

Findings

Layered Ca-rich structure with an outer shell

Presence of Fe-rich plumes indicating deflagration phase

Remnant morphology inconsistent with merger scenarios

Abstract

We present optical and ultraviolet spectra of SN 1885 (S And), visible in absorption against the bulge of the Andromeda galaxy (M31), using the Hubble Space Telescope's STIS spectrograph to probe the three dimensional arrangement of the supernova debris. Spectra covering 2900 to 5700 A taken using six 0.2" slit positions in two orientations show broad Ca II absorption with blue and red radial velocities out to at least 11,500 km/s, consistent with HST Ca II images of S And. Enhanced Ca II absorption is seen between 2000 to 6000 km/s suggestive of a Ca II-rich shell. The spectra also show strong, asymmetric Ca I 4227 A absorption extending out to +12,400 km/s, along with weak Fe I 3720 A absorption in a shell with velocities between 2000 and 9000 km/s. Ultraviolet spectra obtained revealed weak broad absorption shortward of 3000 A consistent with model predictions. The STIS spectra, together with previous HST images, show a layered structure with a well defined Ca-rich outer edge indicative of a delayed detonation phase. The remnant's clumpy inner Ca-rich shell plus only a handful of Fe-rich plumes is unlike morphologies expected from dynamical or violent merger scenarios. The small number of Fe-rich plumes suggest their formation during a deflagration phase by Rayleigh-Taylor instabilities but less well developed without extended mixing as expected from hydrodynamic calculations. The suppression of strong Rayleigh-Taylor instabilities is possibly the result of strong magnetic fields. We propose SN 1885 was an off-center, delayed detonation and slightly subluminous SN Ia similar to SN 1986g.