We present mid-infrared ( MIR ) spectroscopy of a Type II-plateau supernova , SN 2004dj , obtained with the Spitzer Space Telescope , spanning 106–1393 d after explosion .
MIR photometry plus optical/near-IR observations are also reported .
An early-time MIR excess is attributed to emission from non-silicate dust formed within a cool dense shell ( CDS ) .
Most of the CDS dust condensed between 50 d and 165 d , reaching a mass of 0.3 \times 10 ^ { -5 } M _ { \odot } .
Throughout the observations much of the longer wavelength ( > 10 \leavevmode \nobreak \mu m ) part of the continuum is explained as an IR echo from interstellar dust .
The MIR excess strengthened at later times .
We show that this was due to thermal emission from warm , non-silicate dust formed in the ejecta .
Using optical/near-IR line-profiles and the MIR continua , we show that the dust was distributed as a disk whose radius appeared to be shrinking slowly .
The disk radius may correspond to a grain destruction zone caused by a reverse shock which also heated the dust .
The dust-disk lay nearly face-on , had high opacities in the optical/near-IR regions , but remained optically thin in the MIR over much of the period studied .
Assuming a uniform dust density , the ejecta dust mass by 996 d was ( 0.5 \pm 0.1 ) \times 10 ^ { -4 } M _ { \odot } , and exceeded 10 ^ { -4 } M _ { \odot } by 1393 d. For a dust density rising toward the center the limit is higher .
Nevertheless , this study suggests that the amount of freshly-synthesized dust in the SN 2004dj ejecta is consistent with that found from previous studies , and adds further weight to the claim that such events could not have been major contributors to the cosmic dust budget .