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

Orozco-Duarte, Rogelio and Wofford, Aida and Vidal-García, Alba and Bruzual, Gustavo and Charlot, Stephane and Krumholz, Mark R. and Hannon, Stephen and Lee, Janice and Wofford, Timothy and Fumagalli, Michele and Dale, Daniel and Messa, Matteo and Grebel, Eva K. and Smith, Linda and Grasha, Kathryn and Cook, David (2022) Synthetic photometry of OB star clusters with stochastically sampled IMFs: analysis of models and HST observations. Monthly Notices of the Royal Astronomical Society, 509 (1). pp. 522-549. ISSN 0035-8711. doi:10.1093/mnras/stab2988.

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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 = 10³, 10⁴, and 10⁵ 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 (<10⁴ M⊙) star clusters with co-spatial compact H II regions. The clusters are located in NGC 7793, one of the nearest galaxies observed as part of the HST LEGUS and Hα-LEGUS surveys. (1) For model magnitudes that only account for the stars: (a) the residual |deterministic mag - median stochastic mag| can be ≥0.5 mag, even for M_i = 10⁵ M⊙; and (b) the largest spread in stochastic magnitudes occurs when Wolf–Rayet stars are present. (2) For M_i = 10⁵ 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 per cent and >30 per cent to the total emission in the V and I bands, respectively. (3) Age-dating OB-star clusters via deterministic tracks in the U-B versus V-I plane is highly uncertain at Z = 0.014 for M_i ∼ 10³ M⊙ and Z = 0.002 for M_i ∼ 10³–10⁵ M⊙. (4) For low-mass clusters, the V-band extinction derived with stochastic models significantly depends on the value of log(US). (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 characterizing the true variance in the number of stars per mass bin in nature.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper ItemMikulski Archive for Space Telescopes ItemLEGUS
Orozco-Duarte, Rogelio0000-0003-2193-3005
Wofford, Aida0000-0001-8289-3428
Krumholz, Mark R.0000-0003-3893-854X
Hannon, Stephen0000-0001-9628-8958
Lee, Janice0000-0002-2278-9407
Fumagalli, Michele0000-0001-6676-3842
Messa, Matteo0000-0003-1427-2456
Grebel, Eva K.0000-0002-1891-3794
Grasha, Kathryn0000-0002-3247-5321
Cook, David0000-0002-6877-7655
Additional Information:© 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2021 October 8. Received 2021 September 8; in original form 2021 April 1. Published: 19 October 2021. We thank the jury of ROD’s M.S. thesis (E. Terlevich, S. Sánchez, L. Aguilar, S. Srinivasan) as well as M. Cerviño and B. Elmegreen for comments and suggestions which have greatly improved the quality of this paper. ROD and AW acknowledge the support of UNAM via grant agreement PAPIIT no. IA-102120. AVG, SC and GB acknowledge support from the ERC via an Advanced Grant under grant agreement no. 321323-NEOGAL. AVG also aknowledges support from the ERC Advanced Grant MIST (FP7/2017-2022, No 742719). MRK acknowledges support from the Australian Research Council’s Future Fellowship funding scheme, award FT180100375, and from resources and services provided by the National Computational Infrastructure (NCI), which is supported by the Australian Government. Data Availability: The HST data underlying this article are available in the Mikulski Archive for Space Telescopes at, and can be accessed with the data set identifiers 13364 and 13773. LEGUS high level science products can be found at
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Universidad Nacional Autónoma de México (UNAM)IA-102120
European Research Council (ERC)321323
European Research Council (ERC)742719
Australian Research CouncilFT180100375
Subject Keywords:methods: data analysis – stars: luminosity function, mass function – (ISM:) HII regions – galaxies: ISM– galaxies: stellar content
Issue or Number:1
Record Number:CaltechAUTHORS:20220107-252979700
Persistent URL:
Official Citation: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, Synthetic photometry of OB star clusters with stochastically sampled IMFs: analysis of models and HST observations, Monthly Notices of the Royal Astronomical Society, Volume 509, Issue 1, January 2022, Pages 522–549,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:112797
Deposited By: Tony Diaz
Deposited On:09 Jan 2022 03:08
Last Modified:09 Jan 2022 22:14

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