Synthetic photometry of OB star clusters with stochastically sampled
  IMFs: analysis of models and HST observations
  We present a pilot library of synthetic NUV, U, B, V, and I photometry of
star clusters with stochastically sampled IMFs and ionized gas for initial
masses, $M_i=10^3$, $10^4$, and $10^5$ M$_{\odot}$; $t=1$, 3, 4, and 8 Myr;
$Z=0.014$ and $Z=0.002$; and log(U$_{\rm S}$) =-2 and -3. We compare the
library with predictions from deterministic models and observations of isolated
low-mass ($<10^4$ M$_{\odot}$) star clusters with co-spatial compact H\2
regions. The clusters are located in NGC 7793, one of the nearest galaxies
observed as part of the \hst LEGUS and H$\alpha$-LEGUS surveys. 1) For model
magnitudes that only account for the stars: a) the residual |deterministic mag
- median stochastic mag| can be $\ge0.5$ mag, even for $M_i=10^5$ M$_{\odot}$;
and b) the largest spread in stochastic magnitudes occurs when Wolf-Rayet stars
are present. 2) For $M_i=10^5$ M$_{\odot}$: a) the median stochastic mag with
gas can be $>$1.0 mag more luminous than the median stochastic magnitude
without gas; and b) nebular emission lines can contribute with $>50\%$ and
$>30\%$ to the total emission in the V and I bands, respectively. 3) Age-dating
OB-star clusters via deterministic tracks in the U-B vs. V-I plane is highly
uncertain at $Z=0.014$ for $M_i\sim10^3$ M$_{\odot}$ and $Z=0.002$ for
$M_i\sim10^3-10^5$ M$_{\odot}$. 4) For low-mass clusters, the V-band extinction
derived with stochastic models significantly depends on the value of
log(U$_{\rm S}$). 5) The youngest clusters tend to have higher extinction. 6)
The majority of clusters have multi-peaked age PDFs. 7) Finally, we discuss the
importance of characterising the true variance in the number of stars per mass
bin in nature.
