STScI Preprint #1351

Evolution of the Light Echo of SN 1991T

Based on observations with the NASA/ESA Hubble Space Telescope which is operated by AURA, Inc. under NASA contract NAS 5-26555 and by ST ScI grant GO-06713.01-95A.

Schmidt et al. (1994) presented strong evidence (photometry and spectroscopy) that the late time optical emission of SN 1991T in the Virgo spiral NGC 4527 is due to 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, Sparks (1994). Since the elapsed time since the supernova exploded is small, and the galaxy relatively distant, the expected region of maximally polarized emission cannot yet be fully resolved. 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 favour smaller distances, with 15 Mpc 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 ~ 0.9 cm^-3 extending to 50 pc in front of the supernova. It is encouraging that even in a case which is very far from ideal, we can use this type of observation to derive a distance.

Status:

Appeared in: The Astrophysical Journal

Affiliations:

¹ Space Telescope Science Institute, 3700 San Martin Drive Baltimore, MD 21218, USA
² Visiting Scholar, The John's Hopkins University, on sabbatical leave from Space Telescope Science Institute.
³ Affiliated to the Astrophysics Division of the European Space Agency, ESTEC, Noordwijk, The Netherlands.
⁴ Dept. of Physics, Astronomy University of Oklahoma Norman, OK 73019
⁵ Harvard-Smithsonian Center for Astrophysics 60 Garden Street Cambridge, MA 02138
⁶ Dipartimento di Astronomia Universitá di Padova Vicolo Osservatorio 5 35122-Padova, Italy