In a continuing effort to investigate the role of magnetic fields in evolved low and intermediate mass stars (principally regarding the shaping of their envelopes), we present new ALMA high resolution polarization data obtained for the nebula OH 231.8+4.2. We found that the polarized emission likely arises from aligned grains in the presence of magnetic fields rather than radiative alignment and self scattering. The ALMA data show well organized electric field orientations in most of the nebula and the inferred magnetic field vectors (rotated by 90 degrees) trace an hourglass morphology centred on the central system of the nebula. One region in the southern part of OH 231.8+4.2 shows a less organized distribution probably due to the shocked environment. These findings, in conjunction with earlier investigations (maser studies and dust emission analysis at other scales and wavelengths) suggest an overall magnetic hourglass located inside a toroidal field. We propose the idea that the magnetic field structure is closely related to the architecture of a magnetic tower and that the outflows were therefore magnetically launched. While the current dynamical effect of the fields might be weak in the equatorial plane principally due to the evolution of the envelope, it would still be affecting the outflows. In that regard, the measurement of the magnetic field at the stellar surface, which is still missing, combined with a full MHD treatment are required to better understand and constrain the events occurring in OH 231.8+4.2.

中文翻译：

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ALMA 揭示了 OH 231.8+4.2 中磁场几何结构的相干性

为了继续研究磁场在演化的低质量和中等质量恒星中的作用（主要是关于它们的包层的形状），我们提出了新的 ALMA 高分辨率极化数据，该数据是为星云 OH 231.8+4.2 获得的。我们发现极化发射可能来自磁场存在下排列的晶粒，而不是辐射排列和自散射。ALMA 数据显示大部分星云的电场方向组织良好，推断的磁场矢量（旋转 90 度）追踪以星云中央系统为中心的沙漏形态。OH 231.8+4.2 南部的一个区域显示出较不有序的分布，这可能是由于受到震动的环境。这些发现，结合早期的研究（微波激射研究和其他尺度和波长的尘埃发射分析）表明，整体磁性沙漏位于环形场内。我们提出的想法是磁场结构与磁塔的结构密切相关，因此流出物是通过磁力发射的。虽然主要由于包络线的演变，赤道平面中磁场的当前动力学效应可能较弱，但它仍会影响流出。在这方面，需要对仍然缺失的恒星表面磁场进行测量，并结合完整的 MHD 处理，以更好地理解和约束 OH 231.8+4.2 中发生的事件。