Evolution of the light echo of SN 1991T

Schmidt et al. presented strong evidence (photometry and spectroscopy) that the late time optical emission of SN 1991T in the Virgo spiral NGC 4527 is caused by a light echo. Here, we present photometry with the Wide Field and Planetary Camera 2 and high-resolution imaging polarimetry and photometry with the Faint Object Camera on board the Hubble Space Telescope, which demonstrates that the feature is indeed a light echo of the original supernova. We show that the emission is spatially resolved, complex, and both growing in size and changing in morphology. The echo is slowly fading. Our primary interest is to use the echo for estimating the distance to the host galaxy geometrically (see 1994 work by Sparks). Given that the elapsed time since the supernova exploded is small and that the galaxy is relatively distant, the expected region of maximally polarized emission cannot be fully resolved as yet. However, we do find polarized emission at the center of the echo, and simple models may be used to yield a distance estimate. The models favor smaller distances, with approximate to 15 Mpc being the upper allowable distance, subject to caveats described in the text. The echo is consistent with being caused by a dust cloud of uniform density n similar to 0.9 cm(-3) and extending to approximate to 50 pc in front of the supernova. It is encouraging that even in a case very far from ideal, we can use this type of observation to derive a distance.