Synthetic photometry of OB star clusters with stochastically sampled IMFs: analysis of models and HST observations

TL;DR

This study develops a library of synthetic photometry for star clusters with stochastically sampled IMFs, compares it with deterministic models and observations, and highlights the importance of stochastic effects and nebular emission in interpreting cluster properties.

Contribution

The paper introduces a new library of stochastic cluster models including nebular gas effects and compares them with deterministic models and real observations, emphasizing stochasticity's impact on age and extinction estimates.

Findings

Stochastic effects cause magnitude residuals up to 0.5 mag even at high masses.

Nebular emission can significantly brighten clusters and contribute over 50% in certain bands.

Age-dating and extinction estimates are highly uncertain for low-mass clusters due to stochastic sampling.

Abstract

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.