The role of turbulence and magnetic fields is studied for star formation in molecular clouds. We derive and compare six theoretical models for the star formation rate (SFR)—the Krumholz & McKee (KM), Padoan & Nordlund (PN), and Hennebelle & Chabrier (HC) models, and three multi-freefall versions of these, suggested by HC—all based on integrals over the log-normal distribution of turbulent gas. We extend all theories to include magnetic fields and show that the SFR depends on four basic parameters: (1) virial parameter αvir; (2) sonic Mach number ; (3) turbulent forcing parameter b, which is a measure for the fraction of energy driven in compressive modes; and (4) plasma with the Alfvén Mach number . We compare all six theories with MHD simulations, covering cloud masses of 300 to 4 × 106 M ☉ and Mach numbers -50 and -∞, with solenoidal (b = 1/3), mixed (b = 0.4), and compressive turbulent (b = 1) forcings. We find that the SFR increases by a factor of four between and 50 for compressive turbulent forcing and αvir ~ 1. Comparing forcing parameters, we see that the SFR is more than 10 times higher with compressive than solenoidal forcing for simulations. The SFR and fragmentation are both reduced by a factor of two in strongly magnetized, trans-Alfvénic turbulence compared to hydrodynamic turbulence. All simulations are fit simultaneously by the multi-freefall KM and multi-freefall PN theories within a factor of two over two orders of magnitude in SFR. The simulated SFRs cover the range and correlation of SFR column density with gas column density observed in Galactic clouds, and agree well for star formation efficiencies SFE = 1%-10% and local efficiencies = 0.3-0.7 due to feedback. We conclude that the SFR is primarily controlled by interstellar turbulence, with a secondary effect coming from magnetic fields.
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机译:研究了湍流和磁场对分子云中恒星形成的作用。我们推导并比较了六个恒星形成率(SFR)理论模型-克鲁姆霍兹(Krumholz)和麦基(McKee)(KM),帕多安(Padoan)和诺德伦德(Nordlund)(PN)以及亨内贝尔和夏布里(HC)模型,以及这些模型的三个多自由度版本HC-所有这些均基于湍流气体对数正态分布的积分。我们将所有理论扩展到包括磁场,并证明了SFR取决于四个基本参数:(1)病毒参数αvir; (2)声马赫数; (3)湍流强迫参数b,它是压缩模式下驱动的能量分数的量度; (4)具有Alfvén马赫数的等离子体。我们将所有六种理论与MHD仿真进行了比较,它们覆盖了300至4×106 M cloud的云量,马赫数为-50和-∞,具有螺线管(b = 1/3),混合(b = 0.4)和压缩湍流( b = 1)强迫。我们发现,对于压缩湍流强迫和αvir〜1,SFR增加了4到50倍。在比较强迫参数时,我们发现,对于压缩,SFR比电磁强迫的SFR高10倍以上。与流体动力湍流相比,在强磁化,跨阿尔夫尼克湍流中,SFR和碎片均减少了两倍。在SFR中,所有多点KM理论和多点自由PN理论将所有模拟同时拟合到两个以上两个数量级的因子中。模拟的SFR涵盖了在银河云中观测到的SFR柱密度与气柱密度的范围和相关性,并且由于反馈,恒星形成效率SFE = 1%-10%和局部效率= 0.3-0.7非常吻合。我们得出的结论是,SFR主要受星际湍流控制,而次级影响则来自磁场。
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