Polarized dust continuum emission has been observed with Atacama Large Millimeter/submillimeter Array in an increasing number of deeply embedded protostellar systems. It generally shows a sharp transition going from the protostellar envelope to the disc scale, with the polarization fraction typically dropping from ${\sim } 5{{\ \rm per\ cent}}$ to ${\sim } 1{{\ \rm per\ cent}}$ and the inferred magnetic field orientations becoming more aligned with the major axis of the system. We quantitatively investigate these observational trends using a sample of protostars in the Perseus molecular cloud and compare these features with a non-ideal magnetohydrodynamic disc formation simulation. We find that the gas density increases faster than the magnetic field strength in the transition from the envelope to the disc scale, which makes it more difficult to magnetically align the grains on the disc scale. Specifically, to produce the observed ${\sim } 1{{\ \rm per\ cent}}$ polarization at ${\sim } 100\, \mathrm{au}$ scale via grains aligned with the B-field, even relatively small grains of $1\, \mathrm{\mu m}$ in size need to have their magnetic susceptibilities significantly enhanced (by a factor of ∼20) over the standard value, potentially through superparamagnetic inclusions. This requirement is more stringent for larger grains, with the enhancement factor increasing linearly with the grain size, reaching ∼2 × 104 for millimetre-sized grains. Even if the required enhancement can be achieved, the resulting inferred magnetic field orientation in the simulation does not show a preference for the major axis, which is inconsistent with the observed pattern. We thus conclude that the observed trends are best described by the model where the polarization on the envelope scale is dominated by magnetically aligned grains and that on the disc scale by scattering.

中文翻译：

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极化尘埃热发射从原恒星外壳到圆盘尺度的转变

在越来越多的深度嵌入的原恒星系统中，使用阿塔卡马大毫米/亚毫米阵列已观察到偏振尘埃连续谱发射。它通常显示从原恒星包络到圆盘尺度的急剧转变，极化分数通常从 ${\sim } 5{{\ \rm per\ cent}}$ 下降到 ${\sim } 1{{\ \rm per\ cent}}$ 和推断的磁场方向变得更加与系统的主轴对齐。我们使用英仙座分子云中的原恒星样本定量研究这些观测趋势，并将这些特征与非理想磁流体动力学圆盘形成模拟进行比较。我们发现在从封套到圆盘刻度的过渡过程中，气体密度的增加速度快于磁场强度，这使得在磁盘刻度上磁性对齐颗粒变得更加困难。具体来说，为了在 ${\sim } 100\ 处产生观察到的 ${\sim } 1{{\ \rm per\ cent}}$ 极化，\mathrm{au}$ 通过与 B 场对齐的晶粒进行缩放，甚至$1\, \mathrm{\mu m}$ 的相对较小的晶粒需要使其磁化率显着高于标准值（约 20 倍），这可能是通过超顺磁性夹杂物来实现的。这一要求对于较大的晶粒更为严格，增强因子随晶粒尺寸线性增加，对于毫米尺寸的晶粒达到 ∼2 × 104。即使可以实现所需的增强，在模拟中得到的推断磁场方向也不会显示出对主轴的偏好，这与观察到的模式不一致。