星際介質熱力學之數值實驗

Abstract

在本研究中，我們調查了冷星際介質(ISM)的冷卻和加熱過程對其熱力學和動力學的影響。狀態方程描述了形成恆星的分子雲中冷ISM的溫度和密度之間的關係，當數量密度小於每立方公分 10^4 時，其多變熱指數為 γ ∼ 0.7，當數量密度大於每立方公分 n< 10^4時，其多變指數為γ ∼ 1。然而，觀測結果與理論預測存在些許差異，因為這些理論假設加熱或冷卻時間尺度比動態過程時間尺度短因而不考慮壓縮或膨脹所做的功。為了解決這個問題，我們使用自適應網格(AMR) 數值程式 RAMSES 進行數值模擬，以找出有效的動態指數。我們的模擬採用分子雲中主要的發射線冷卻函數和參數化冷卻函數。當使用分子雲中主要的發射線冷卻函數時，分子雲迅速達到平衡。然而，當把冷卻功率降低以模擬低金屬豐度環境時，分子雲偏離了狀態方程的描述。我們還探討了參數化冷卻函數的使用，並找到了滿足分子雲坍縮的特定參數設置，也就是有效的多變指數需小於 4/3。我們的結果顯示，在研究原始冷 ISM 時，應將動力過程所做的功納入理論模型中，因為在低金屬豐度環境的情況下冷卻效率不高。總結來說，我們的研究強調了對冷 ISM 的動力學和熱力學的更全面的理解的需要，以及考慮冷卻和加熱過程對這些系統的影響的重要性。In this study, we investigated the impact of cooling and heating processes on the thermodynamics and dynamics of the cold interstellar medium (ISM). The widely accepted polytropic equation of state describes the relationship between temperature and density of the cold ISM in star-forming molecular clouds, with an effective polytropic index of γ ∼ 0.7 when n< 10^4 and γ ∼ 1 when n > 10^4. However, there are discrepancies between observational results and theoretical predictions. Most theoretical studies assume that the heating or cooling timescales are smaller than the dynamical timescale and do not account for the work done by compression or expansion. To address this issue, we use the adaptive mesh refinement (AMR) code RAMSES to conduct numerical simulations and determine the effective polytropic index. Our simulations adopt the physical coolingfunction and parametric cooling function. When physical cooling functions are used, the molecular cloud rapidly reaches equilibrium. However, when the cooling power is reduced to mimic low metallicity abundance, the molecular cloud deviates from the equilibrium temperature. We also explore the use of a parametric cooling function and found the corresponding equation of state given the cooling function of the medium. Our results suggest that when studying the primordial hot ISM, where the cooling is less effective, the work done by dynamical processes should be included in the theoretical model since cooling is not efficient in low metallicity scenarios. Overall, our study highlights the need for a more comprehensive. understanding of the dynamics and thermodynamics of the cold ISM and the importance of considering the effects of cooling and heating together with dynamical processes in these systems.