Alicia Soderberg
Bates College
HST observations show that the nitrogen-rich circumstellar material surrounding SN1987A is a significant source of contamination in ground-based photometry for the SN debris. We analyze six epochs of WFPC2 PC images (1994.8--1999.0) in broad-band filters F336W, F439W, F555W, F675W, and F814W (HST UBVRI) and four epochs in narrow-band filters F656N, F658N, and F502N. Our corrected light curves demonstrate that flux from the circumstellar ring dominates flux from the debris in the ground-based data set after day 1195 (JD = 2448047). On day 3268, the magnitudes for the SN debris were U,B,V,R,I = 19.75, 19.85, 19.97, 18.91, 19.30. The magnitudes for the ring (including the faint star superimposed on the ring) were U,B,V,R,I = 18.29, 19.21, 18.36, 16.44, 18.41.
We have fit the slow change in the ring magnitudes over the last five years to provide an extrapolation back to the epochs of ground-based observations. These extrapolations are used to remove the ring flux from the ground-based photometric measurements, which yields a uniform set of UBVRIJHK light curves for the debris of SN1987A over its past 12 year evolution.