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unpublished - photometric data from the Spitzer Space Telescope in conjunction with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.
