Hubble Space Telescope Images and Spectra of the Remnant of SN 1885 in M31

Near-UV Hubble Space Telescope images of the remnant of SN 1885 (S And) in M31 show a 070±005 diameter absorption disk silhouetted against M31's central bulge, at SN 1885's historically reported position. The disk's size corresponds to a linear diameter of 2.5±0.4 pc at a distance of 725±70 kpc, implying an average expansion velocity of 11,000±2000 km s^-1 over 110 yr. Low-dispersion Faint Object Spectrograph spectra over 3200-4800 Å reveal that the absorption arises principally from Ca II H and K (equivalent width ≃215 Å), with weaker absorption features of Ca I 4227 Å and Fe I 3720 Å. The flux at Ca II line center indicates a foreground starlight fraction of 0.21, which places SNR 1885 some 64 pc to the near side of the midpoint of the M31 bulge, comparable to its projected 55 pc distance from the nucleus. The absorption line profiles suggest an approximately spherically symmetric, bell-shaped density distribution of supernova ejecta freely expanding at up to 13,100±1500 km s^-1. We estimate Ca I, Ca II, and Fe I masses of 2.9×10⁻⁴ M_☉, 0.005 M_☉, and 0.013 M_☉, respectively. If the ionization state of iron is similar to the observed ionization state of calcium, M_{Ca II}/M_{Ca I}=16, then the mass of Fe II is 0.21 M_☉, consistent with that expected for either normal or subluminous SN Ia.