Magnetic fields and the earliest stages of star cluster formation

恒星照亮了宇宙中的每一个星系，使我们能够研究它们在宇宙中的结构。尽管它们很重要，但恒星形成过程的许多方面仍然没有得到很好的约束。我们知道恒星的形成是由冷分子气体组成的大型星际云破碎和坍缩的结果。然而，我们不知道是什么触发了坍缩，是由引力不稳定性、湍流压缩、磁场扩散，还是三者的某种组合驱动的。星际分子云的湍流（通过测量光谱线发射宽度）和重力（通过测量气体质量）是常规的特征。然而，磁场的观测要复杂得多。但在过去的几年里，由于技术的进步，在寒冷的星际云中推导磁场的方向变得更容易了。这是通过观察弥漫在星际介质中的尘埃颗粒的亚毫米偏振发射来完成的。偏振光的方向直接告诉你磁场在天空平面上的方向。像JCMT上的POL2这样的仪器是这种最先进的观测的前沿。最近，Peretto等人（即将提交）表明，团块，即星团的祖先，与它们的母分子云动态地解耦，可能是它们的全局坍缩的结果。然而，目前尚不清楚这种坍缩是否源于磁场性质的变化。在这个博士项目的背景下，学生将分析全新的JCMT POL2观测到的一些团块（bbb10），表征其中的磁场形态。更大尺度分子云的磁场形态将通过使用普朗克和GPIPS数据来确定。连接不同的磁场数据集将使我们能够确定观察到的动态跃迁是否与磁场性质的跃迁相匹配，这可能表明它处于团块坍缩的起源。在第二步中，学生将在观测和云演化的数值模拟中描述团块磁场方向和气体速度梯度之间的相关性，目的是确定磁场在密度最大的区域是否受到重力的牵引。攻读博士学位的学生将发现自己处于恒星形成研究的前沿，并将处于一个极好的位置，以充分利用未来几年将开发的未来偏振仪器。

Stars light up every single galaxy across the Universe, allowing us to study their structure across the cosmos. Despite their importance, there are still many aspects of the star formation process that remain poorly constrained. We know that stars form as the result of the fragmentation and collapse of large interstellar clouds of cold molecular gas. However, we do not know what triggers that collapse, whether it is driven by gravitational instabilities, turbulent compression, magnetic field diffusion, or some combination of all three. The turbulence (via the measurement of the width of spectral line emission) and gravity (via the measurement of gas masses) of interstellar molecular clouds are routinely characterised. However, magnetic fields are a lot more complicated to observe. But in the past few years, as the result of technological advancements, it has become easier to derive the direction of magnetic fields in cold interstellar clouds. This is done by observing the sub-millimetre polarisation emission of dust grains that pervade the interstellar medium. The direction of the polarised light directly tells you in which direction the magnetic field is in the plane-of-the-sky. Instruments like POL2 on the JCMT is at the forefront of such state-of-the-art observations. Recently, Peretto et al. (to be submitted) showed that that clumps, i.e. the progenitors of stellar clusters, are dynamically decoupled from their parent molecular clouds, probably as the result of their global collapse. However, it is unclear if this collapse finds its origin in the change of magnetic field properties.In the context of this PhD project, the student will analyse brand new JCMT POL2 observations of a number of clumps (>10), characterise the magnetic field morphology in those. Morphology of the magnetic field in the larger scale molecular clouds will be determined by using Planck and GPIPS data. Connecting the different magnetic field datasets will allow us to determine whether or not the observed dynamical transition is matched by a transition in magnetic field properties which could be an indication that it is at the origin of the clump collapse.In a second step, the student will characterise the correlations between the clump magnetic field directions and gas velocity gradients, both in the observations and numerical simulations of cloud evolution, with the goal of determining whether magnetic fields are entrained by gravity in the densest regions or not. The student taking on this PhD will find themselves at the forefront of star formation research and will be in an excellent position to take full advantage of future polarisation instrumentations that will be developed in the next few years.