SN 1987A After 18 Years: Mid-Infrared GEMINI and SPITZER Observations of the Remnant

Abstract

We present high resolution 11.7 and 18.3um mid-IR images of SN 1987A obtained on day 6526 with T-ReCS attached to the Gemini telescope. The 11.7um flux has increased significantly since our last observations on day 6067. The images clearly show that all the emission arises from the equatorial ring (ER). Spectra obtained with Spitzer, on day 6184 with MIPS at 24um, on day 6130 with IRAC in the 3.6-8um region, and on day 6190 with IRS in the 12-37um region show that the emission consists of thermal emission from silicate dust that condensed out in the red giant wind of the progenitor star. The dust temperature is ~166K, and the emitting dust mass is ~2.6 x 10-6 Msun. Lines of [Ne II]12.82um and [Ne III]15.56um are clearly present, as well as a weak [Si II]34.8um line. We also detect two lines near 26um which we tentatively ascribe to [Fe II]25.99um and [O IV]25.91um. Comparison of the Gemini 11.7um image with X-ray images from Chandra, UV-optical images from HST, and radio synchrotron images obtained by the ATCA show generally good correlation of the images across all wavelengths. Because of the limited resolution of the mid-IR images we cannot uniquely determine the location or heating mechanism of the dust giving rise to the emission. The dust could be collisionally heated by the X-ray emitting plasma, providing a unique diagnostic of plasma conditions. Alternatively, the dust could be radiatively heated in the dense UV-optical knots that are overrun by the advancing supernova blast wave. In either case the dust-to-gas mass ratio in the circumstellar medium around the supernova is significantly lower than that in the general ISM of the LMC, suggesting either a low condensation efficiency in the wind of the progenitor star, or the efficient destruction of the dust by the SN blast wave.