H$\alpha$ Morphologies of Star Clusters in 16 LEGUS Galaxies:
  Constraints on HII region evolution timescales
  The analysis of star cluster ages in tandem with the morphology of their HII
regions can provide insight into the processes that clear a cluster's natal
gas, as well as the accuracy of cluster ages and dust reddening derived from
Spectral Energy Distribution (SED) fitting. We classify 3757 star clusters in
16 nearby galaxies according to their H$\alpha$ morphology (concentrated,
partially exposed, no emission), using Hubble Space Telescope (HST) imaging
from the Legacy ExtraGalactic Ultraviolet Survey (LEGUS). We find: 1) The mean
SED ages of clusters with concentrated (1-2 Myr) and partially exposed HII
region morphologies (2-3 Myr) indicate a relatively early onset of gas clearing
and a short (1-2 Myr) clearing timescale. 2) The reddening of clusters can be
overestimated due to the presence of red supergiants, which is a result of
stochastic sampling of the IMF in low mass clusters. 3) The age-reddening
degeneracy impacts the results of the SED fitting - out of 1408 clusters with
M$_*$ $\geq$ 5000 M$_{\odot}$, we find that at least 46 (3%) have SED ages
which appear significantly underestimated or overestimated based on H$\alpha$
and their environment, while the total percentage of poor age estimates is
expected to be several times larger. 4) Lastly, we examine the dependence of
the morphological classifications on spatial resolution. At HST resolution, our
conclusions are robust to the distance range spanned by the sample (3-10 Mpc).
However, analysis of ground-based H$\alpha$ images shows that compact and
partially exposed morphologies frequently cannot be distinguished from each
other.
