The Chemical Distribution in a Subluminous Type Ia Supernova: Hubble Space Telescope Images of the SN 1885 Remnant*

摘要

SN 1885 was a probable subluminous SN Ia that occurred in the bulge of the Andromeda galaxy, M31, at a projected location 16'' from the nucleus. Here we present and analyze Hubble Space Telescope images of the SN 1885 remnant seen in absorption against the M31 bulge via the resonance lines of Ca I, Ca II, Fe I, and Fe II. Viewed in Ca II H and K line absorption, the remnant appears as a nearly black circular spot with an outermost angular radius of 0.40'' ± 0.025'', implying a maximum linear radius of 1.52 ± 0.15 pc at M31's estimated distance of 785 ± 30 kpc and hence a 120 yr average expansion velocity of 12,400 ± 1400 km s-1. The strongest Ca II absorption is organized in a broken ring structure with a radius of 0.2'' (=6000 km s-1) with several apparent absorption "clumps" of an angular size around that of the image pixel scale of 0.05'' (=1500 km s-1). Ca I and Fe I absorption structures appear similar except for a small Fe I absorption peak displaced 0.1'' off-center of the Ca II structure by a projected velocity of about 3000 km s-1. Analyses of these images using off-center, delayed-detonation models suggest a low 56Ni production similar to the subluminous SN Ia explosion of SN 1986G. The strongly lopsided images of Ca I and Fe I can be understood as resulting from an aspherical chemical distribution, with the best agreement found using an off-center model viewed from an inclination of ~60°. The images require a central region of no or little Ca but with iron group elements indicative for burning under sufficiently high densities for electron capture to take place, i.e., burning prior to a significant preexpansion of the WD.