A ram-pressure threshold for star formation
A ram-pressure threshold for star formation
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恒星形成的冲压压力阈值
DOI：
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发表时间：
2016
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通讯作者：
A. Whitworth
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作者：
A. Whitworth
在湍流碎片中，星星的形成发生在会聚流产生的凝聚中。凝聚体必须足够大，密度足够大，温度足够低，才能在引力作用下不稳定，因此它们开始收缩;然后它们必须足够快地辐射出热能，以使自引力保持主导地位，从而继续收缩。对于局部恒星形成云中的金属丰度和温度，只有当气体与尘埃热耦合时，第二个要求才能得到有力的满足，因为这提供了在连续光谱的整个带宽上辐射的能力，而不仅仅是在几个离散的光谱线上。这就转化为一个有力的星星形成的临界值，可以写成最小冲压压力PCRIT = 40 × 10−11达因。PCRIT与温度无关，对应于分子氢数密度nH2.FLOW和速度vFLOW满足nH2.FLOWv2FLOW <$800 cm −3（kms−1）2的流动。这又对应于活跃星星形成的最小分子氢柱密度NH2.CRIT <$4 × 1021 cm −2（<$CRIT <$100 M <$pc−2），以及最小视消光AV，CRIT <$9等。当超过这个临界值时，恒星形成暗条的特征直径和线密度是2 RFIL <$0.1 pc和μFIL <$13 M <$pc−2。星前核的特征直径和质量是2 RCORE <$0.1 pc和MCORE <$1 M <$。我们还表明，破碎的冲击压缩层可能会开始，而创建层的收敛流仍在进行中，我们强调，在这种情况下，现象和特征尺度的层的碎片是从根本上不同于传统的预先存在的层。
In turbulent fragmentation, star formation occurs in condensations created by converging flows. The condensations must be sufficiently massive, dense and cool to be gravitationally unstable, so that they start to contract; and they must then radiate away thermal energy fast enough for self-gravity to remain dominant, so that they continue to contract. For the metallicities and temperatures in local star-forming clouds, this second requirement is only met robustly when the gas couples thermally to the dust, because this delivers the capacity to radiate across the full bandwidth of the continuum, rather than just in a few discrete spectral lines. This translates into a threshold for vigorous star formation, which can be written as a minimum ram pressure PCRIT ∼ 4 × 10−11 dyne. PCRIT is independent of temperature, and corresponds to flows with molecular hydrogen number density nH2.FLOW and velocity vFLOW satisfying nH2.FLOWv2FLOW≳800cm−3(kms−1)2. This in turn corresponds to a minimum molecular hydrogen column density for vigorous star formation, NH2.CRIT∼4×1021cm−2 (ΣCRIT ∼ 100 M⊙ pc−2), and a minimum visual extinction AV, CRIT ∼ 9 mag. The characteristic diameter and line density for a star-forming filament when this threshold is just exceeded – a sweet spot for local star formation regions – are 2RFIL ∼ 0.1 pc and μFIL ∼ 13 M⊙ pc−2. The characteristic diameter and mass for a prestellar core condensing out of such a filament are 2RCORE ∼ 0.1 pc and MCORE ∼ 1 M⊙. We also show that fragmentation of a shock-compressed layer is likely to commence while the convergent flows creating the layer are still ongoing, and we stress that, under this circumstance, the phenomenology and characteristic scales for fragmentation of the layer are fundamentally different from those derived traditionally for pre-existing layers.