SN 1987A after 18 Years: Mid‐Infrared Gemini andSpitzerObservations of the Remnant

We present high resolution 11.7 and 18.3um mid-IR images of SN 1987A obtainedon day 6526 with T-ReCS attached to the Gemini telescope. The 11.7um flux hasincreased significantly since our last observations on day 6067. The imagesclearly 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 withIRAC in the 3.6-8um region, and on day 6190 with IRS in the 12-37um region showthat the emission consists of thermal emission from silicate dust thatcondensed out in the red giant wind of the progenitor star. The dusttemperature 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 [SiII]34.8um line. We also detect two lines near 26um which we tentatively ascribeto [Fe II]25.99um and [O IV]25.91um. Comparison of the Gemini 11.7um image withX-ray images from Chandra, UV-optical images from HST, and radio synchrotronimages obtained by the ATCA show generally good correlation of the imagesacross all wavelengths. Because of the limited resolution of the mid-IR imageswe cannot uniquely determine the location or heating mechanism of the dustgiving rise to the emission. The dust could be collisionally heated by theX-ray emitting plasma, providing a unique diagnostic of plasma conditions.Alternatively, the dust could be radiatively heated in the dense UV-opticalknots that are overrun by the advancing supernova blast wave. In either casethe dust-to-gas mass ratio in the circumstellar medium around the supernova issignificantly lower than that in the general ISM of the LMC, suggesting eithera low condensation efficiency in the wind of the progenitor star, or theefficient destruction of the dust by the SN blast wave.Comment: 50 pages; 3 tables; 18 figures. New version significantly modified. New figures (all of them appended). To appear in ApJ v648n1, October 10, 200