标题： 分子云的演化增长通过其红外明亮部分进行追踪 显示英文标题

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

摘要： 理解恒星如何形成、演化及其对分子云的影响，是理解全球范围内恒星形成为何如此低效的关键。 在本文中，我们使用红外明亮比例，$f_\text{IRB}$（给定分子云中相对于银河系8$\mu$m背景显得明亮的部分的比例）作为时间演化的代理，以测试随着恒星形成的进展，云属性如何变化。 我们将这一指标应用于我们通过赫歇尔-Hi-GAL巡天在银河系平面的$|l|<70^\circ$之间识别出的12,000个高质星光形成分子云。 我们发现，在恒星形成过程中，云并不是静态的。 相反，随着恒星形成的进行，分子云持续获得质量。 通过对云属性进行主成分分析，我们发现它们沿着两条路径演化，这两条路径由它们的质量增长区分。 大多数云（80%）的质量增长是$f_\text{IRB}$的函数，其质量增长四倍。 其余的20%经历了一个极端的增长时期，平均质量增长了150倍，在此期间，它们最初获得质量的速度足以超过其恒星形成。 对于所有云来说，只有当它们的一半面积变为恒星形成区域之后，才会发生质量损失。 我们预计这可能是由于恒星反馈和可能的银河剪切力造成的。 通过分析云的位置，我们认为质量增长速率可能与更大的银河环境有关。 总体而言，这些结果对我们假设分子云质量随时间固定时，在云尺度上评估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.