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1. Physical Processes in Star Formation

   https://doi.org/10.1007/s11214-020-00693-8  

   Girichidis, Philipp; Offner, Stella S.; Kritsuk, Alexei G.; Klessen, Ralf S.; Hennebelle, Patrick; Kruijssen, J. M.; Krause, Martin G.; Glover, Simon C.; Padovani, Marco (June 2020, Space Science Reviews)
2. Massive star cluster formation: I. High star formation efficiency while resolving feedback of individual stars

   https://doi.org/10.1051/0004-6361/202348840  

   Polak, Brooke; Mac_Low, Mordecai-Mark; Klessen, Ralf S; Wei_Teh, Jia; Cournoyer-Cloutier, Claude; Andersson, Eric P; Appel, Sabrina M; Tran, Aaron; Lewis, Sean C; Wilhelm, Maite J_C; et al (October 2024, Astronomy & Astrophysics)

   The mode of star formation that results in the formation of globular clusters and young massive clusters is difficult to constrain through observations. We present models of massive star cluster formation using the TORCHframework, which uses the Astrophysical MUltipurpose Software Environment (AMUSE) to couple distinct multi-physics codes that handle star formation, stellar evolution and dynamics, radiative transfer, and magnetohydrodynamics. We upgraded TORCHby implementing the N-body code PETAR, thereby enabling TORCHto handle massive clusters forming from 10⁶M_⊙clouds with ≥10⁵individual stars. We present results from TORCHsimulations of star clusters forming from 10⁴,  10⁵, and 10⁶M_⊙turbulent spherical gas clouds (named M4, M5, M6) of radiusR= 11.7 pc. We find that star formation is highly efficient and becomes more so at a higher cloud mass and surface density. For M4, M5, and M6 with initial surface densities 2.325 × 10^1,2,3M_⊙pc⁻², after a free-fall time oft_ff= 6.7,2.1,0.67 Myr, we find that ∼30%, 40%, and 60% of the cloud mass has formed into stars, respectively. The end of simulation-integrated star formation efficiencies for M4, M5, and M6 areϵ_⋆ = M_⋆/M_cloud = 36%, 65%, and 85%. Observations of nearby clusters similar in mass and size to M4 have instantaneous star formation efficiencies ofϵ_inst ≤ 30%, which is slightly lower than the integrated star formation efficiency of M4. The M5 and M6 models represent a different regime of cluster formation that is more appropriate for the conditions in starburst galaxies and gas-rich galaxies at high redshift, and that leads to a significantly higher efficiency of star formation. We argue that young massive clusters build up through short efficient bursts of star formation in regions that are sufficiently dense (Σ ≥ 10²M_⊙pc⁻²) and massive (M_cloud≥ 10⁵M_⊙). In such environments, stellar feedback from winds and radiation is not strong enough to counteract the gravity from gas and stars until a majority of the gas has formed into stars. 

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3. Star cluster formation in Orion A

   https://doi.org/10.1093/pasj/psaa035  

   Lim, Wanggi; Nakamura, Fumitaka; Wu, Benjamin; Bisbas, Thomas G; Tan, Jonathan C; Chambers, Edward; Bally, John; Kong, Shuo; McGehee, Peregrine; Lis, Dariusz C; et al (May 2020, Publications of the Astronomical Society of Japan)

   null (Ed.)

   Abstract We introduce new analysis methods for studying the star cluster formation processes in Orion A, especially examining the scenario of a cloud–cloud collision. We utilize the CARMA–NRO Orion survey 13CO (1–0) data to compare molecular gas to the properties of young stellar objects from the SDSS III IN-SYNC survey. We show that the increase of $$v_{\rm {}^{13}CO} - v_{\rm YSO}$$ and Σ scatter of older YSOs can be signals of cloud–cloud collision. SOFIA-upGREAT 158 μm [C ii] archival data toward the northern part of Orion A are also compared to the 13CO data to test whether the position and velocity offsets between the emission from these two transitions resemble those predicted by a cloud–cloud collision model. We find that the northern part of Orion A, including regions ONC-OMC-1, OMC-2, OMC-3, and OMC-4, shows qualitative agreements with the cloud–cloud collision scenario, while in one of the southern regions, NGC 1999, there is no indication of such a process in causing the birth of new stars. On the other hand, another southern cluster, L 1641 N, shows slight tendencies of cloud–cloud collision. Overall, our results support the cloud–cloud collision process as being an important mechanism for star cluster formation in Orion A. 

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4. The Single-cloud Star Formation Relation

   https://doi.org/10.3847/2041-8213/abf564  

   Pokhrel, Riwaj; Gutermuth, Robert A.; Krumholz, Mark R.; Federrath, Christoph; Heyer, Mark; Khullar, Shivan; Thomas Megeath, S.; Myers, Philip C.; Offner, Stella S.; Pipher, Judith L.; et al (May 2021, The Astrophysical Journal Letters)

   null (Ed.)
5. The Star Formation History of the Milky Way’s Nuclear Star Cluster

   https://doi.org/10.3847/1538-4357/aca8ad  

   Chen, Zhuo; Do, Tuan; Ghez, Andrea M.; Hosek, Jr., Matthew W.; Feldmeier-Krause, Anja; Chu, Devin S.; Bentley, Rory O.; Lu, Jessica R.; Morris, Mark R. (February 2023, The Astrophysical Journal)

   Abstract We report the first star formation history study of the Milky Ways nuclear star cluster (NSC), which includes observational constraints from a large sample of stellar metallicity measurements. These metallicity measurements were obtained from recent surveys from Gemini and the Very Large Telescope of 770 late-type stars within the central 1.5 pc. These metallicity measurements, along with photometry and spectroscopically derived temperatures, are forward modeled with a Bayesian inference approach. Including metallicity measurements improves the overall fit quality, as the low-temperature red giants that were previously difficult to constrain are now accounted for, and the best fit favors a two-component model. The dominant component contains 93% ± 3% of the mass, is metal-rich ( $\overline{\left[\mathrm{M}/\mathrm{H}\right]}\sim 0.45$), and has an age of $5_{-2}^{+3}$Gyr, which is ∼3 Gyr younger than earlier studies with fixed (solar) metallicity; this younger age challenges coevolutionary models in which the NSC and supermassive black holes formed simultaneously at early times. The minor population component has low metallicity ( $\overline{\left[\mathrm{M}/\mathrm{H}\right]}\sim -1.1$) and contains ∼7% of the stellar mass. The age of the minor component is uncertain (0.1–5 Gyr old). Using the estimated parameters, we infer the following NSC stellar remnant population (with ∼18% uncertainty): 1.5 × 10⁵neutron stars, 2.5 × 10⁵stellar-mass black holes (BHs), and 2.2 × 10⁴BH–BH binaries. These predictions result in 2–4 times fewer neutron stars compared to earlier predictions that assume solar metallicity, introducing a possible new path to understand the so-called “missing-pulsar problem”. Finally, we present updated predictions for the BH–BH merger rates (0.01–3 Gpc⁻³yr⁻¹). 

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