  Core-collapse supernovae (CC SNe), especially Type II-Plateau ones, are
thought to be important contributors to cosmic dust production. SN 2004dj, one
of the closest and brightest SN since 1987A, offered a good opportunity to
examine dust formation processes. To find signs of newly formed dust, we
analyze all available mid-infrared (MIR) archival data from the Spitzer Space
Telescope. We re-reduce and analyze data from IRAC, MIPS and IRS instruments
obtained between +98 and +1381 days after explosion, generating light curves
and spectra for each epoch. Observed spectral energy distributions are fitted
with both analytic and numerical models, using the radiative-transfer code
MOCASSIN for the latter ones. We also use imaging polarimetric data obtained at
+425 days by the Hubble Space Telescope. We present convincing evidence of dust
formation in the ejecta of SN 2004dj from MIR light curves and spectra.
Significant MIR excess flux is detected in all bands between 3.6 and 24 um. In
the optical, a ~0.8% polarization is also detected at a 2-sigma level, which
exceeds the interstellar polarization in that direction. Our analysis shows
that the freshly-formed dust around SN 2004dj can be modeled assuming a nearly
spherical shell containing amorphous carbon grains, cooling from ~700 K to ~400
K between +267 and +1246 days. Persistent excess flux has been found above 10
um, which is explained by a cold (~115 K) dust component. If this cold dust is
of circumstellar origin, it is likely to be condensed in a cool, dense shell
between the forward and reverse shocks. Pre-existing circumstellar dust is less
likely, but cannot be ruled out. An upper limit of ~8x10^{-4} M_sun was derived
for the dust mass, which is similar to previously published values for other
dust-producing SNe.
