  Dust emission at 8 micron has been extensively calibrated as an indicator of
current star formation rate for galaxies and ~kpc-size regions within galaxies.
Yet, the exact link between the 8 micron emission and the young stellar
populations in galaxies is still under question, as dust grains can be
stochastically heated also by older field stars. In order to investigate this
link, we have combined mid-infrared images from the Spitzer Space Telescope
with a published star cluster candidates catalog for the Local Group galaxy
M33. M33 is sufficiently close that the Spitzer's 8 micron images resolve
individual regions of star formation. Star clusters represent almost-single-age
stellar populations, which are significantly easier to model than more complex
mixtures of stars. We find a decrease in the 8 micron luminosity per unit
stellar mass as a function of age of the star clusters, with a large scatter
that is consistent with varying fractions of stellar light absorbed by dust.
The decrease and scatter both confirm findings based on more distant galaxies
and are well described by simple models for the dust emission of a young
stellar population. We conclude that the dust emission at 8 micron depends
sensitively on the age of the stellar population, out to at least the oldest
age analyzed here, ~400 Myr. This dependence complicates the use of the 8
micron emission as a star formation rate indicator, at least for small galactic
regions and individual star forming regions. By leveraging the Spitzer legacy,
this investigation paves the way for future explorations with the James Webb
Space Telescope.
