We argue that massive stars are the dominant sources of energy for the turbulent motions within giant molecular clouds and that the primary agent of feedback is the expansion of H Ⅱ regions within the cloud volume. This conclusion is suggested by the low efficiency of star formation and corroborated by dynamical models of H Ⅱ regions. We evaluate the turbulent energy input rate in clouds more massive than 3.7 x 10~5 solar mass, for which gravity does not significantly affect the expansion of H Ⅱ regions. Such clouds achieve a balance between the decay of turbulent energy and its regeneration in H Ⅱ regions; summed over clouds, the implied ionizing luminosity and star formation rate are roughly consistent with the Galactic total. H Ⅱ regions also photoevaporate their clouds: we derive cloud destruction times somewhat shorter, than those estimated by Williams & McKee. The upper mass limit for molecular clouds in the Milky Way may derive from the fact that larger clouds would destroy themselves in less than 1 crossing time. The conditions within starburst galaxies do not permit giant molecular clouds to be supported or destroyed by H Ⅱ regions, and this may explain some aspects of the starburst phenomenon.
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机译：我们认为大质量恒星是巨大分子云内湍流运动的主要能量来源，而反馈的主要动力是云体积内HⅡ区的扩展。这个结论是由于恒星形成效率低而提出的，并得到了HⅡ区动力学模型的证实。我们评估了比3.7 x 10〜5太阳质量更大的云中的湍流能量输入速率，对于重力来说，重力对HⅡ区的扩展没有显着影响。这种云在湍流能量的衰减与其在HⅡ区的再生之间达到平衡。总的来说，暗含的电离光度和恒星形成率与银河系总数大致一致。 HⅡ区也使云层汽化：我们得出的云破坏时间比威廉姆斯和麦基估计的要短一些。银河系中分子云的质量上限可能来自以下事实：较大的云会在不到1个穿越时间内破坏自身。爆炸星系中的条件不允许HⅡ区支持或破坏巨大的分子云，这可以解释爆炸现象的某些方面。
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