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

Rogelio Orozco-Duarte; Aida Wofford; Alba Vidal-García; Gustavo Bruzual; Stephane Charlot; Mark R. Krumholz; Stephen Hannon; Janice Lee; Timothy Wofford; Michele Fumagalli; Daniel Dale; Matteo Messa; Eva K. Grebel; Linda Smith; Kathryn Grasha; David Cook

doi:10.1093/mnras/stab2988

paper

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

arXiv:2110.05595 · doi:10.1093/mnras/stab2988

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, $M_{i} = 1 0^{3}$ , $1 0^{4}$ , and $1 0^{5}$ M $_{⊙}$ ; $t = 1$ , 3, 4, and 8 Myr; $Z = 0.014$ and $Z = 0.002$ ; and log(U $_{S}$ ) =-2 and -3. We compare the library with predictions from deterministic models and observations of isolated low-mass ( $< 1 0^{4}$ M $_{⊙}$ ) 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 $α$ -LEGUS surveys. 1) For model magnitudes that only account for the stars: a) the residual |deterministic mag - median stochastic mag| can be $\geq 0.5$ mag, even for $M_{i} = 1 0^{5}$ M $_{⊙}$ ; and b) the largest spread in stochastic magnitudes occurs when Wolf-Rayet stars are present. 2) For $M_{i} = 1 0^{5}$ M $_{⊙}$ : 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 $M_{i} \sim 1 0^{3}$ M $_{⊙}$ and $Z = 0.002$ for $M_{i} \sim 1 0^{3} - 1 0^{5}$ M $_{⊙}$ . 4) For low-mass clusters, the V-band extinction derived with stochastic models significantly depends on the value of log(U $_{S}$ ). 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.

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

Abstract

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