标题： 分子云的红外亮分数所追踪的演化增长 显示英文标题

标题： Evolutionary growth of molecular clouds as traced by their infrared bright fraction

摘要： 理解恒星的形成、演化及其对分子云的影响是理解为何恒星形成在全球范围内如此低效的关键。 本文中，我们使用红外亮度过高部分（$f_\text{IRB}$，即在银河系背景（8$\mu$米波长）下显得明亮的分子云比例）作为时间演化的代理，测试了随着恒星形成过程的演化，云的属性如何变化。 我们将这一指标应用于通过赫歇尔-Hi-GAL巡天识别出的银河系平面上$|l|<70^\circ$区域内的 12,000 个大质量恒星形成分子云。 我们发现，在恒星形成过程中，分子云并非静止不变。 相反，分子云在恒星形成进展的同时不断获得质量。 通过对云的属性进行主成分分析，我们发现它们沿着两条由质量增益区分的路径演化。 大多数云（80%）的质量随$f_\text{IRB}$增长四倍。 剩下的 20% 经历了一个极端的增长阶段，平均质量增长了 150 倍，在此期间，它们最初以足够快的速度获得质量，从而超过了恒星形成速度。 对于所有云来说，只有当它们的一半面积成为恒星形成区域后，才开始发生质量损失。 我们预计恒星反馈和可能的星系剪切力对此负责。 通过分析云的位置，我们建议质量增长速率可能与更大的星系环境有关。 总之，这些结果对我们评估假设分子云质量在时间上固定时的云尺度恒星形成能力具有重要意义。 显示英文摘要

摘要： Understanding how stars form, evolve and impact molecular clouds is key to understanding why star formation is such an inefficient process globally. In this paper, we use the infrared bright fraction, $f_\text{IRB}$ (the fraction of a given molecular cloud that appears bright against the 8 $\mu$m Milky Way background) as a proxy for time evolution to test how cloud properties change as star formation evolves. We apply this metric to 12,000 high-mass star-forming molecular clouds we identify using the Herschel-Hi-GAL survey between $|l|<70^\circ$ on the Milky Way plane. We find clouds are not static while forming stars. Instead, molecular clouds continuously gain mass while star formation progresses. By performing principal component analysis on the cloud properties, we find that they evolve down two paths distinguished by their mass gain. Most clouds (80%) gain four times more mass as a function of $f_\text{IRB}$. The remaining 20% experience an extreme period of growth, growing in mass by a factor of 150 on average and during this period, they initially gain mass fast enough to outpace their star formation. For all clouds, it is only after half their area becomes star forming that mass loss occurs. We expect stellar feedback and potentially galactic shear is responsible. By analysing cloud positions, we suggest that the rate of mass growth may be linked to the larger galactic environment. Altogether, these results have strong implications on how we assess star forming ability on cloud scales when assuming molecular cloud masses are fixed in time.