早期宇宙恆星形成雲氣之物理特性

Abstract

紊流在恆星形成的過程中扮演了重要的角色，進而影響第一恆星的各種特徵。過往的模擬研究預測此類的恆星質量將大於當前所觀測到缺乏金屬成份之恆星，而其中的差異則可能是紊流所導致的。近期的研究顯示紊流將提高原始氣體雲的溫度，降低其中的恆星形成速率，並因而影響所形成恆星之質量。並且，過去的研究已顯示由吸積所造成的紊流在宇宙中是廣泛存在的。因此，於模擬之中加入驅動紊流能為我們提供寶貴的資訊。然而，該驅動紊流以及氣體雲之間的物理關係仍然處於尚未明瞭的狀態。 在這篇論文中，我們利用 GIZMO 進行多體的宇宙學流體模擬。同時，我們在模擬之中加入許多重要的物理機制，例如原始氣體雲的輻射冷卻機制及紊流的擴散。我們從 IllustrisTNG 計畫中的 TNG50-1 模擬，截取其中質量為 10^7 個太陽質量的迷你暗物質暈作為模擬的初始條件。藉由粒子切割的技術，我們在模擬中達到每個氣體粒子具有 0.1 個太陽質量的解析度。如此高的解析度將能使我們解析高密度氣體雲中的微小結構。在模擬完成後，我們對所有的模型進行數個物理量的分析，例如密度分佈、柯爾莫哥洛夫能量頻譜，以及紊流的馬赫數。我們發現紊流將在 1 kpc 的尺度開始形成，並在密度為 ∼ 2.78 × 10^−22 g cm^−3 處形成 M = 2.20 ± 1.67 馬赫的強度。 我們的研究旨在為使用人工紊流的宇宙學模擬提供合理的參數。藉由限制紊流強度，我們能夠進一步的找出第三星族恆星的質量範圍。未來的研究將會專注在分析模擬中產生的高密度氣體團，並試圖以此來建立或限制第三星族恆星的初始質量函數。

Turbulence is a pivotal factor in the star formation process, influencing the characteristics of the first star. Existing simulations have predicted the formation of stars with significantly greater mass than observed in metal-poor stars, potentially attributed to the absence of turbulence. Recent studies indicate that turbulence can raise the temperature of primordial clouds, thereby reducing the star-forming rate (SFR) and impacting the mass of the earliest stars in the universe. Additionally, research has identified accretion-driven turbulence as a universal cosmic phenomenon. While implementing driven turbulence in simulations can provide valuable insights, the underlying relationship between primordial turbulence and cloud dynamics remains unclear. We present N-body hydrodynamics (HD) cosmological simulations by utilizing the GIZMO code, which incorporates critical physics such as radiative cooling for primordial gas and turbulence diffusion. The initial conditions are derived from mini-halo within the TNG50-1 of IllustrisTNG project at z ~ 20, where the total mass is around 10^7 Msun. By employing particle-splitting techniques, we achieve the finest resolution of 0.1 Msun per gas particle, enabling us to unveil the intricate structure within dense clouds. Our analysis encompasses various physical properties, including density distributions, Kolmogorov spectrum, and the Mach number of the turbulence spanning the entirety of our simulations. We found out the scale of turbulence formed at the scale of ~ 1 kpc for mini-halos, and the Mach number reaches M = 2.20 ± 1.67 at ∼ 2.78 × 10^−22 g cm^−3. This finding could be the reference for the cosmological simulations that introduced artificial-driven turbulence. By constraining the strength of turbulence, we can further constrain the mass range of Pop. III stars. Future work will focus on the analysis of gas clumps in our model, and try to build or constrain the initial mass function of Pop. III stars.