BISTRO (B-Fields in Star-Forming Region Observations) JCMT Large Programme: First observing run

我们将使用POL-2绘制我们先前用Scuba-2和HARP-B绘制的古尔德带所有恒星形成区最密集部分的偏振图，以便(通过Chandrasekhar-Fermi方法)追踪所有这些区域中心的磁场方向和强度。这将解答目前恒星形成中悬而未决的问题，例如磁场和湍流对恒星形成的相对重要性，并将使我们能够测试目前的恒星形成模型，例如磁场向丝状结构输送的赫歇尔模型。在JCMT上搭载Scuba-2的POL-2是一台独特的设备，因为它是世界上唯一可以绘制附近恒星形成区域(如金牛座和蛇夫座)约1000AU尺度的冷致密核心和细丝内磁场的设备。这个大小尺度对于测试丝状恒星形成模型是至关重要的，因为我们从其他研究中知道，大尺度的场大致垂直于整个典型的丝状方向，但我们不知道在细丝本身的致密气体中的场发生了什么，也不知道在细丝中形成的核心内发生了什么。这对于理解正在发生的物理过程，以及区分恰当地包含磁场的恒星形成过程的模型是至关重要的。目前的假设是，磁场可能以螺旋的方式缠绕在灯丝周围，或者在密度最高的气体中转向平行于灯丝运行。我们将能够区分这两种情况。关于B场在形成原恒星后在塑造原恒星演化中所起的作用以及它对两极外流的影响，还存在争议。例如，最近的研究发现，在1000AU以下的尺度上，外流与磁场方向之间没有相关性，而在10000AU及以上的尺度上，外流与磁场方向之间存在相关性。对这种场形态上的明显冲突的一种解释是，对云层中心的环形包裹的B场进行了详细的模拟。这已经被用来解释在最近的一个模型中0级原恒星的早期盘形成，在该模型中，假设早期盘优先在与流出方向不对齐的场中形成。这种大规模同质数据集的遗留价值将是巨大的。未来所有关于磁场在恒星形成中的作用的模型都将被迫解释所有最近的恒星形成区域的观测结果。

We will use POL-2 to map the polarization in the densest parts of all of the Gould Belt star-forming regions that we have previously mapped with SCUBA-2 and HARP-B, in order to trace (via the Chandrasekhar-Fermi method) the magnetic field direction and strength in the centres of all of these regions. This will answer currently open questions in star formation, such as the relative importance of magnetic fields and turbulence to star formation, and will allow us to test current models of star formation, such as the Herschel model of magnetic funnelling onto filaments.POL-2 with SCUBA-2 on JCMT is a unique facility, as it is the only facility world-wide that can map the magnetic field within cold dense cores and filaments on scales of ~1000 AU in nearby star-forming regions, such as Taurus and Ophiuchus. This size scale is crucial for testing models such as the filamentary star formation model, because we know from other studies that the large-scale field lies roughly perpendicular to the overall typical filament directions, but we do not know what happens to the field within the dense gas of the filaments themselves, nor what happens within the cores that form in the filaments. This is crucial to understanding the physical processes taking place, and to discriminating between the models of the star formation process that properly incorporate magnetic fields. The current hypotheses are either that the field may wrap around the filament in a helical manner, or else turn to run parallel to the filament in the densest gas. We will be able to distinguish between these two scenarios.There is debate about the role that the B-field plays in shaping proto-stellar evolution once proto-stars have formed and its effect on bipolar outflows. For example, recent studies find no correlation between outflow and magnetic field directions on scales less than 1000 AU, while there is correlation on scales of ~10,000 AU and above. One explanation of this apparent conflict in the field morphology uses detailed modelling of toroidally wrapped B-fields at the centres of clouds. This has been used to explain early disk formation in Class 0 proto-stars in a recent model in which early disks are hypothesized to be formed preferentially in fields misaligned with the outflow directions. The legacy value of this large-scale homogeneous data-set will be huge. All future models of the role of magnetic fields in star formation will be forced to explain these observations in all of the nearest star-forming regions.