Near UV HST images of the remnant of SN 1885 ( S And ) in M31 show a 0 \farcs 70 \pm 0 \farcs 05 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 \pm 0.4 { pc } at a distance of 725 \pm 70 kpc , implying an average expansion velocity of 11000 \pm 2000 { km } { s } ^ { -1 } over 110 years .
Low-dispersion FOS spectra over 3200 – 4800 { \AA } reveal that the absorption arises principally from Ca ii H & K ( equivalent width \simeq 215 { \AA } ) , with weaker absorption features of Ca i 4227 { \AA } and Fe i 3720 { \AA } .
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 13100 \pm 1500 { km } { s } ^ { -1 } .
We estimate Ca i , Ca ii , and Fe i masses of 2.9 ^ { +2.4 } _ { -0.6 } \times 10 ^ { -4 } \mbox { $ { M } _ { \sun } $ } , 0.005 ^ { +0.016 } _ { -0.002 } \mbox { $ { M } _ { \sun } $ } , and 0.013 ^ { +0.010 } _ { -0.005 } \mbox { $ { M } _ { \sun } $ } respectively .
If the ionization state of iron is similar to the observed ionization state of calcium , M _ { CaII } / M _ { CaI } = 16 ^ { +42 } _ { -5 } , then the mass of Fe ii is 0.21 ^ { +0.74 } _ { -0.08 } \mbox { $ { M } _ { \sun } $ } , consistent with that expected for either normal or subluminous SN Ia .