CO enhancement by magnetohydrodynamic waves

Context. The formation of molecular gas in interstellar clouds is a slow process, but can be enhanced by gas compression. Magnohydrodynamic (MHD) waves can create compressed quasi-periodic linear structures, referred to as striations. Striations are observed at column densities where the atomic to molecular gas transition takes place. Aims. We explore the role of MHD waves in the CO chemistry in regions with striations within molecular clouds. Methods. We target a region with striations in the Polaris Flare cloud.We conduct a CO J=2-1 survey in order to probe the molecular gas properties. We use archival starlight polarization data and dust emission maps in order to probe the magnetic field properties and compare against the CO morphological and kinematic properties. We assess the interaction of compressible MHD wave modes with CO chemistry by comparing their characteristic timescales. Results. The estimated magnetic field is 38 - 76 μG. In the CO integrated intensity map, we observe a dominant quasi-periodic intensity structure, which tends to be parallel to the magnetic field orientation and has a wavelength of one parsec approximately. The periodicity axis is ∼ 17◦ off from the mean magnetic field orientation and is also observed in the dust intensity map. The contrast in the CO integrated intensity map is ∼ 2.4 times larger than the contrast of the column density map, indicating that CO formation is enhanced locally. We suggest that a dominant slow magnetosonic mode with estimated period 2.1 − 3.4 Myr, and propagation speed 0.30 − 0.45 km s−1, is likely to have enhanced the formation of CO, hence created the observed periodic pattern. We also suggest that, within uncertainties, a fast magnetosonic mode with period 0.48 Myr and velocity 2.0 km s−1 could have played some role in increasing the CO abundance. Conclusions. Quasiperiodic CO structures observed in striation regions may be the imprint of MHD wave modes. The Alfvénic speed sets the dynamical timescales of the compressible MHD modes, and determines which wave modes are involved in the CO chemistry.

研究背景：星际云内分子气体的形成过程较为缓慢，但可通过气体压缩得到增强。磁流体动力学（Magnetohydrodynamic, MHD）波可产生压缩的准周期性线性结构，这类结构被称为条纹结构（striations）。条纹结构在原子气体向分子气体跃迁的柱密度条件下被观测到。
研究目标：我们旨在探究磁流体动力学波对分子云内条纹结构区域中一氧化碳（CO）化学过程的作用。
研究方法：我们以北极星耀斑云（Polaris Flare cloud）中的条纹结构区域为观测目标。为探究分子气体性质，我们开展了CO J=2-1巡天观测。我们使用存档的星光偏振数据与尘埃辐射图，以探测磁场性质，并将其与CO的形态及运动学特征进行对比。通过对比特征时标，我们评估了可压缩磁流体动力学波模式与CO化学过程的相互作用。
研究结果：估算得到的磁场强度为38~76 μG。在CO积分强度图中，我们观测到占主导地位的准周期性强度结构，该结构大致平行于磁场取向，波长约为1秒差距。该周期性结构的轴线与平均磁场取向夹角约为17°，且在尘埃强度图中也能观测到这一结构。CO积分强度图的对比度约为柱密度图对比度的2.4倍，这表明CO的形成过程在局部得到了增强。我们推测，周期为2.1~3.4 Myr、传播速度为0.30~0.45 km·s⁻¹的主导慢磁声模，可能增强了CO的形成，从而形成了观测到的周期性图案。此外，在误差范围内，周期为0.48 Myr、速度为2.0 km·s⁻¹的快磁声模，也可能对CO丰度的提升起到了一定作用。
研究结论：条纹结构区域中观测到的准周期性CO结构，可能是磁流体动力学波模式的印记。阿尔文速度决定了可压缩磁流体动力学波模式的动力学时标，并决定了哪些波模式参与了CO的化学过程。