We present Spitzer IRS and IRAC observations of the young supernova remnant E0102 ( SNR 1E0102-7219 ) in the Small Magellanic Cloud .
The infrared spectra show strong lines of Ne and O , with the [ Ne ii ] line at 12.8 \mu m having a large velocity dispersion of 2,000-4,500 km s ^ { -1 } indicative of fast-moving ejecta .
Unlike the young Galactic SNR Cas A , E0102 lacks emission from Ar and Fe .
Diagnostics of the observed [ Ne III ] line pairs imply that [ Ne III ] emitting ejecta have a low temperature of 650 K , while [ Ne V ] line pairs imply that the infrared [ Ne V ] emitting ejecta have a high density of \sim 10 ^ { 4 } cm ^ { -3 } .
We have calculated radiative shock models for various velocity ranges including the effects of photoionization .
The shock model indicates that the [ Ne v ] lines come mainly from the cooling zone , which is hot and dense , whereas [ Ne II ] and [ Ne III ] come mainly from the photoinization zone , which has a low temperature of 400 - 1000 K. We estimate an infrared emitting Ne ejecta mass of 0.04 M _ { \odot } from the infrared observations , and discuss implications for the progenitor mass .
The spectra also have a dust continuum feature peaking at 18 \mu m that coincides spatially with the ejecta , providing evidence that dust formed in the expanding ejecta .
The 18 \mu m-peak dust feature is fitted by a mixture of MgSiO _ { 3 } and Si dust grains , while the rest of the continuum requires either carbon or Al _ { 2 } O _ { 3 } grains .
We measure the total dust mass formed within the ejecta of E0102 to be \sim 0.014 M _ { \odot } .
The dust mass in E0102 is thus a factor of a few smaller than that in Cas A .
The composition of the dust is also different , showing relatively less silicate and likely no Fe-bearing dust , as is suggested by the absence of Fe-emitting ejecta .