Magnetic fields are expected to play an important role in accretion processes for circumstellar disks. Measuring the magnetic field morphology is difficult, especially since polarimetric (sub)millimeter continuum observations may not trace fields in most disks. The Goldreich-Kylafis (GK) effect suggests that line polarization is perpendicular or parallel to the magnetic field direction. We attempt to observe CO(2-1), $^{13}$CO(2-1), and C$^{18}$O(2-1) line polarization toward HD 142527 and IM Lup, which are large, bright protoplanetary disks. We use spatial averaging and spectral integration to search for signals in both disks, and detect a potential CO(2-1) Stokes $Q$ signal toward both disks. The total CO(2-1) polarization fractions are 1.57 $\pm$ 0.18% and 1.01 $\pm$ 0.10% for HD 142527 and IM Lup, respectively. Our Monte Carlo simulations indicate that these signals are marginal. We also stack Stokes parameters based on the Keplerian rotation, but no signal was found. Across the disk traced by dust of HD 142527, the 3$\sigma$ upper limits for $P_{\text{frac}}$ at 0.5$^{\prime\prime}$ ($\sim$80 au) resolution are typically less than 3% for CO(2-1) and $^{13}$CO(2-1) and 4% for C$^{18}$O(2-1). For IM Lup, the 3$\sigma$ upper limits for these three lines are typically less than 3%, 4%, and 12%, respectively. Upper limits based on our stacking technique are up to a factor of $\sim$10 lower, though stacking areas can potentially average out small-scale polarization structure. We also compare our continuum polarization at 1.3 mm to observations at 870 $\mu$m from previous studies. The polarization in the northern dust trap of HD 142527 shows a significant change in morphology and an increase in $P_{\text{frac}}$ as compared to 870 $\mu$m. For IM Lup, the 1.3 mm polarization may be more azimuthal and has a higher $P_{\text{frac}}$ than at 870 $\mu$m.

中文翻译：

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两个原行星盘中的低水平一氧化碳线极化：HD 142527 和 IM Lup

磁场有望在星周盘的吸积过程中发挥重要作用。测量磁场形态很困难，尤其是因为极化（亚）毫米连续谱观测可能无法在大多数磁盘中追踪磁场。Goldreich-Kylafis (GK) 效应表明线极化垂直或平行于磁场方向。我们尝试观察 CO(2-1)、$^{13}$CO(2-1) 和 C$^{18}$O(2-1) 线极化朝向 HD 142527 和 IM Lup，它们很大，明亮的原行星盘。我们使用空间平均和光谱积分来搜索两个磁盘中的信号，并检测到两个磁盘的潜在 CO(2-1) Stokes $Q$ 信号。HD 142527 和 IM Lup 的总 CO(2-1) 极化分数分别为 1.57 $\pm$ 0.18% 和 1.01 $\pm$ 0.10%。我们的蒙特卡罗模拟表明这些信号是边缘的。我们还根据开普勒旋转来堆叠斯托克斯参数，但没有发现信号。在 HD 142527 尘埃追踪的磁盘上，$P_{\text{frac}}$ 在 0.5$^{\prime\prime}$ ($\sim$80 au) 分辨率下的 3$\sigma$ 上限通常是CO(2-1) 和 $^{13}$CO(2-1) 低于 3%，C$^{18}$O(2-1) 低于 4%。对于 IM Lup，这三行的 3$\sigma$ 上限通常分别小于 3%、4% 和 12%。基于我们的堆叠技术的上限最多低 10 美元，但堆叠区域可能会平均化小规模极化结构。我们还将我们在 1.3 mm 处的连续极化与之前研究中 870 $\mu$m 处的观察结果进行了比较。与 870 $\mu$m 相比，HD 142527 北部尘埃陷阱的极化显示出形态的显着变化和 $P_{\text{frac}}$ 的增加。对于 IM Lup，1.3 mm 极化可能更具方位角，并且比 870 $\mu$m 具有更高的 $P_{\text{frac}}$。