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1. Radiative feedback for supermassive star formation in a massive cloud with H2 molecules in an atomic-cooling halo

   https://doi.org/10.1093/mnras/staa3227  

   Sakurai, Yuya; Haiman, Zoltán; Inayoshi, Kohei (November 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Recent three-dimensional cosmological simulations of protogalaxy formation have suggested that supermassive stars (SMSs) can form in gas clouds in which H2 cooling is suppressed by dynamical heating prior to the activation of atomic cooling, but they stopped short of the following growth of a central protostar. Here, we examine whether accretion on the protostellar core in this cloud is sufficiently rapid, in the face of the radiation feedback, to produce an SMS. We perform one-dimensional radiation-hydrodynamical simulations of the hot collapsing cloud with non-equilibrium chemical reactions directly adopting the cloud properties from Wise et al. as an initial condition. We find that the stellar Lyman–Werner (LW) radiation from the SMS dissociates H2 in the inner regions of the gas flow, increasing gas temperature and thermal pressure, and temporarily stopping the accretion. However, this negative feedback ceases when the self-gravity and inward ram pressure force on larger scales push the gas inwards. The central protostar is unable to expand an H ii region due to the high density, and grows to a mass of $${\gtrsim}10^5\, {\rm M}_{\odot }$$. Our results suggests the successful formation of SMSs, and resulting massive ($${\sim}10^5\, {\rm M}_{\odot }$$) remnant black holes in the clouds, but need to be confirmed in two- or three-dimensional simulations. 

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2. Cradles of the first stars: self-shielding, halo masses, and multiplicity

   https://doi.org/10.1093/mnras/staa139  

   Skinner, Danielle; Wise, John H (March 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT The formation of Population III (Pop III) stars is a critical step in the evolution of the early Universe. To understand how these stars affected their metal-enriched descendants, the details of how, why and where Pop III formation takes place needs to be determined. One of the processes that is assumed to greatly affect the formation of Pop III stars is the presence of a Lyman–Werner (LW) radiation background, that destroys H2, a necessary coolant in the creation of Pop III stars. Self-shielding can alleviate the effect the LW background has on the H2 within haloes. In this work, we perform a cosmological simulation to study the birthplaces of Pop III stars, using the adaptive mesh refinement code enzo. We investigate the distribution of host halo masses and its relationship to the LW background intensity. Compared to previous work, haloes form Pop III stars at much lower masses, up to a factor of a few, due to the inclusion of H2 self-shielding. We see no relationship between the LW intensity and host halo mass. Most haloes form multiple Pop III stars, with a median number of four, up to a maximum of 16, at the instance of Pop III formation. Our results suggest that Pop III star formation may be less affected by LW radiation feedback than previously thought and that Pop III multiple systems are common. 

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3. Impact of gas spin and Lyman–Werner flux on black hole seed formation in cosmological simulations: implications for direct collapse

   https://doi.org/10.1093/mnras/stab3439  

   Bhowmick, Aklant K.; Blecha, Laura; Torrey, Paul; Kelley, Luke Zoltan; Vogelsberger, Mark; Nelson, Dylan; Weinberger, Rainer; Hernquist, Lars (November 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Direct collapse black holes (BHs) are promising candidates for producing massive z ≳ 6 quasars, but their formation requires fine-tuned conditions. In this work, we use cosmological zoom simulations to study systematically the impact of requiring: (1) low gas angular momentum (spin), and (2) a minimum incident Lyman–Werner (LW) flux in order to form BH seeds. We probe the formation of seeds (with initial masses of $$M_{\rm seed} \sim 10^4\!-\!10^6\, \mathrm{M}_{\odot }\, h^{-1})$$ in haloes with a total mass >3000 × Mseed and a dense, metal-poor gas mass >5 × Mseed. Within this framework, we find that the seed-forming haloes have a prior history of star formation and metal enrichment, but they also contain pockets of dense, metal-poor gas. When seeding is further restricted to haloes with low gas spins, the number of seeds formed is suppressed by factors of ∼6 compared to the baseline model, regardless of the seed mass. Seed formation is much more strongly impacted if the dense, metal-poor gas is required to have a critical LW flux (Jcrit). Even for Jcrit values as low as 50J21, no $$8\times 10^{5}~\mathrm{M}_{\odot }\, h^{-1}$$ seeds are formed. While lower mass ($$1.25\times 10^{4},1\times 10^{5}~\mathrm{M}_{\odot }\, h^{-1}$$) seeds do form, they are strongly suppressed (by factors of ∼10–100) compared to the baseline model at gas mass resolutions of $$\sim 10^4~\mathrm{M}_{\odot }\, h^{-1}$$ (with even stronger suppression at higher resolutions). As a result, BH merger rates are also similarly suppressed. Since early BH growth is dominated by mergers in our models, none of the seeds are able to grow to the supermassive regime ($$\gtrsim 10^6~\mathrm{M}_{\odot }\, h^{-1}$$) by z = 7. Our results hint that producing the bulk of the z ≳ 6 supermassive BH population may require alternate seeding scenarios that do not depend on the LW flux, early BH growth dominated by rapid or super-Eddington accretion, or a combination of these possibilities. 

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4. The role of radiation and halo mergers in Pop III star formation

   https://doi.org/10.1093/mnras/stad3167  

   Correa Magnus, Lilia; Smith, Britton D.; Khochfar, Sadegh; O’Shea, Brian W.; Wise, John H.; Norman, Michael L.; Turk, Matthew J. (October 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present a study of the co-evolution of a population of primordial star-forming minihaloes at Cosmic Dawn. In this study, we highlight the influence of individual Population III stars on the ability of nearby minihaloes to form sufficient molecular hydrogen to undergo star formation. In the absence of radiation, we find the minimum halo mass required to bring about collapse to be ∼105 M⊙, this increases to ∼106 M⊙ after two stars have formed. We find an inverse relationship between halo mass and the time required for it to recover its molecular gas after being disrupted by radiation from a nearby star. We also take advantage of the extremely high resolution to investigate the effects of major and minor mergers on the gas content of star-forming minihaloes. Contrary to previous claims of fallback of supernova ejecta, we find minihaloes evacuated after hosting Pop III stars primarily recover gas through mergers with undisturbed haloes. We identify an intriguing type of major merger between recently evacuated haloes and gas-rich ones, finding that these ‘mixed’ mergers accelerate star formation instead of suppressing it like their low-redshift counterparts. We attribute this to the gas-poor nature of one of the merging haloes resulting in no significant rise in temperature or turbulence and instead inducing a rapid increase in central density and hydrostatic pressure. This constitutes a novel formation pathway for Pop III stars and establishes major mergers as potentially the primary source of gas, thus redefining the role of major mergers at this epoch. 

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5. Radiative feedback on supermassive star formation: the massive end of the Population III initial mass function

   Toyouchi, Daisuke; Inayoshi, Kohei; Li, Wenxiu; Haiman, Zoltán; Kuiper, Rolf (June 2022, The astrophysical journal)

   Supermassive stars (SMSs) with masses of 𝑀∗ ≃ 104–105 M⊙ are invoked as possible seeds of high-redshift supermassive black holes, but it remains under debate whether their protostar indeed acquires sufficient mass via gas accretion overcoming radiative feedback. We investigate protostellar growth in dynamically heated atomic-cooling haloes (ACHs) found in recent cosmological simulations, performing three-dimensional radiation hydrodynamical (RHD) simulations that consider stellar evolution under variable mass accretion. We find that one of the ACHs feeds the central protostar at rates exceeding a critical value, above which the star evolves in a cool bloating phase and hardly produces ionizing photons. Consequently, the stellar mass reaches 𝑀∗ 􏰁 104 M⊙ unimpeded by radiative feedback. In the other ACH, where the mass supply rate is lower, the star spends most of its life as a hot main-sequence star, emitting intense ionizing radiation. Then, the stellar mass growth is terminated around 500 M⊙ by photoevaporation of the circumstellar disk. A series of our RHD simulations provide a formula of the final stellar mass determined either by stellar feedback or their lifetime as a function of the mass supply rate from the parent cloud in the absence of stellar radiation. Combining the results with the statistical properties of SMS-forming clouds in high-redshift quasar progenitor haloes, we construct a top-heavy mass distribution of primordial stars over 𝑀∗ ≃ 100–105 M⊙, approximately following a power-law spectrum of ∝ 𝑀−1.3 with a steeper decline at 𝑀 􏰁 2 × 104 M . Their massive BH remnants would be ∗∗⊙ further fed via the dense debris disk, powering “milli-quasars" with a bolometric luminosity of 𝐿bol 􏰁 1043 erg s−1. 

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