Classical T Tauri stars (CTTS) accrete material from their discs through their magnetospheres. The geometry of the accretion flow strongly depends on the magnetic obliquity, i.e., the angle between the rotational and magnetic axes. We aim at deriving the distribution of magnetic obliquities in a sample of 10 CTTSs. For this, we monitored the radial velocity variations of the HeI$\lambda$5876 line in these stars' spectra along their rotational cycle. HeI is produced in the accretion shock, close to the magnetic pole. When the magnetic and rotational axes are not aligned, the radial velocity of this line is modulated by stellar rotation. The amplitude of modulation is related to the star's projected rotational velocity, $v\sin i$, and the latitude of the hotspot. By deriving $v\sin i$ and HeI$\lambda$5876 radial velocity curves from our spectra we thus obtain an estimate of the magnetic obliquities. We find an average obliquity in our sample of 11.4$^{\circ}$ with an rms dispersion of 5.4$^{\circ}$. The magnetic axis thus seems nearly, but not exactly aligned with the rotational axis in these accreting T Tauri stars, somewhat in disagreement with studies of spectropolarimetry, which have found a significant misalignment ($\gtrsim 20^{\circ}$) for several CTTSs. This could simply be an effect of low number statistics, or it may be due to a selection bias of our sample. We discuss possible biases that our sample may be subject to. We also find tentative evidence that the magnetic obliquity may vary according to the stellar interior and that there may be a significant difference between fully convective and partly radiative stars.

中文翻译：

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吸积金牛 T 星的磁倾角

经典的金牛 T 星（CTTS）通过它们的磁层从它们的圆盘吸积物质。吸积流的几何形状很大程度上取决于磁倾角，即旋转轴和磁轴之间的角度。我们的目标是推导出 10 个 CTTS 样本中的磁倾角分布。为此，我们监测了这些恒星光谱中 HeI$\lambda$5876 线沿其旋转周期的径向速度变化。HeI 是在吸积激波中产生的，靠近磁极。当磁轴和旋转轴未对齐时，这条线的径向速度受恒星旋转调制。调制幅度与恒星的投影旋转速度 $v\sin i$ 和热点的纬度有关。通过从我们的光谱中推导出 $v\sin i$ 和 HeI$\lambda$5876 径向速度曲线，我们获得了磁倾角的估计值。我们在样本中发现平均倾斜度为 11.4$^{\circ}$，均方根色散为 5.4$^{\circ}$。因此，在这些吸积的金牛 T 星中，磁轴看起来几乎与旋转轴对齐，但与旋转轴并不完全对齐，这与光谱极化法的研究有些不同，后者发现了几个明显的错位（$\gtrsim 20^{\circ}$） CTTS。这可能仅仅是低数量统计的影响，或者可能是由于我们样本的选择偏差。我们讨论了我们的样本可能受到的偏见。