Rescued from oblivion: detailed analysis of archival Spitzer data of SN 1993J

We present an extensive analysis of the late-time mid-infrared (mid-IR) evolution of the Type IIb SN 1993J from 10–26 yr post-explosion based on archival – mostly previously unpublished – photometric data from the Spitzer Space Telescope in conjunction with an archival InfraRed Spectrograph spectrum. SN 1993J is one of the best-studied supernovae (SNe) with an extensive decade-long multiwavelength data set published in various papers; however, its detailed late-time mid-IR analysis is still missing from the literature. Mid-IR data follow not just the continuously cooling SN ejecta but also late-time dust-formation and circumstellar-interaction processes. We provide evidence that the observed late-time mid-IR excess of SN 1993J can be described by the presence of two-component local dust with a dust mass of ∼(3.5–6.0) × 10 ⁻³  M ⊙ in the case of a partly silicate-based dust composition. The source of these components could be either newly formed dust grains or heating of pre-existing dust via ongoing circumstellar matter interaction also detected at other wavelengths. If it is newly formed, the dust is assumed to be located both in the unshocked inner ejecta and in the outer cold dense shell, just as in the Cassiopeia A remnant and also assumed in other dust-forming SNe a few years after explosion.