Understanding the Origin of the Magnetic Field Morphology in the Wide-binary Protostellar System BHR 71
Understanding the Origin of the Magnetic Field Morphology in the Wide-binary Protostellar System BHR 71
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DOI：
10.3847/1538-4357/ab5809
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发表时间：
2019-10
期刊：
影响因子：
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通讯作者：
C. Hull;V. J. M. L. Gouellec;J. Girart;J. Tobin;T. Bourke
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文献类型：
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作者：
C. Hull;V. J. M. L. Gouellec;J. Girart;J. Tobin;T. Bourke
我们展示了 1.3 毫米阿塔卡马大型毫米/亚毫米阵列对宽双星原恒星系统 BHR 71 IRS1 和 IRS2 的偏振尘埃发射的观测。 IRS1 具有看似与生俱来的沙漏形磁场。相比之下，IRS2 表现出的磁场受到其双极流出的影响。对于 IRS2，偏振主要局限于流出腔壁。沿着 IRS2 红移流出腔的北缘，偏振发射夹在流出物和由 N2D+ 追踪的冷致密气体细丝之间，没有检测到尘埃偏振。这表明 IRS2 增强极化的起源是流出空腔壁的照射，这使得尘埃颗粒能够相对于磁场对齐，但只能达到约 300 au 的深度，超过该深度，尘埃是冷的且非极化的。然而，为了使空腔壁中足够深的晶粒对齐，并产生 IRS2 中看到的高偏振分数，对齐的光子可能位于中红外到远红外范围内，这表明晶粒生长的程度超出了非常年轻的 0 类光源通常预期的程度。最后，对于 IRS1，我们看到一个狭窄的线性特征，具有高 (10%–20%) 极化分数和与双极流出腔无关的有序磁场。我们推测这个特征可能是磁化吸积流带；然而，这一点尚未得到稠密气体示踪剂的运动学观察的证实。
We present 1.3 mm Atacama Large Millimeter/submillimeter Array observations of polarized dust emission toward the wide-binary protostellar system BHR 71 IRS1 and IRS2. IRS1 features what appears to be a natal, hourglass-shaped magnetic field. In contrast, IRS2 exhibits a magnetic field that has been affected by its bipolar outflow. Toward IRS2, the polarization is confined mainly to the outflow cavity walls. Along the northern edge of the redshifted outflow cavity of IRS2, the polarized emission is sandwiched between the outflow and a filament of cold, dense gas traced by N2D+, toward which no dust polarization is detected. This suggests that the origin of the enhanced polarization in IRS2 is the irradiation of the outflow cavity walls, which enables the alignment of dust grains with respect to the magnetic field—but only to a depth of ∼300 au, beyond which the dust is cold and unpolarized. However, in order to align grains deep enough in the cavity walls, and to produce the high polarization fraction seen in IRS2, the aligning photons are likely to be in the mid- to far-infrared range, which suggests a degree of grain growth beyond what is typically expected in very young, Class 0 sources. Finally, toward IRS1 we see a narrow, linear feature with a high (10%–20%) polarization fraction and a well-ordered magnetic field that is not associated with the bipolar outflow cavity. We speculate that this feature may be a magnetized accretion streamer; however, this has yet to be confirmed by kinematic observations of dense-gas tracers.