H I -H 2 transition: Exploring the role of the magnetic field: A case study toward the Ursa Major cirrus
H I -H 2 transition: Exploring the role of the magnetic field: A case study toward the Ursa Major cirrus
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H I -H 2 转变：探索磁场的作用：大熊卷云的案例研究
DOI：
10.1051/0004-6361/202142512
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发表时间：
2022
影响因子：
6.5
通讯作者：
Kypriotakis, J. A.
中科院分区：
文献类型：
--
作者：
Skalidis, R.;Tassis, K.;Panopoulou, G. V.;Pineda, J. L.;Gong, Y.;Mandarakas, N.;Blinov, D.;Kiehlmann, S.;Kypriotakis, J. A.
星际扩散介质(ISM)中的原子气体是以丝状结构组织起来的。这些结构通常含有寒冷而密集的分子团块。银河系磁场被认为在这些团块的形成中起着重要的作用。目的我们的目标是探索磁场在HI-H跃迁过程中的作用。方法我们将一个弥漫的ISM丝状云对准大熊座卷云，在那里气体从原子到分子的跃迁。我们利用光学偏振观测探讨了云团的磁场特性。我们对不同物种进行了多波长光谱观测，以探索云的气相性质。为了探测云团的分子含量，我们观测了CO(J=1−0)和(J=2−1)谱线。我们还获得了对[C II]157.6微米发射线的观测，以追踪CO暗H_2气体，并估计云的平均体积密度。其中一个区域主要是原子，而另一个区域主要是分子气体，尽管其中大部分是CO暗。两个区域之间的估计天空平面磁场强度在不确定范围内保持不变，在13-30微克范围内。总磁场强度不随密度而变化。这意味着气体沿着场线被压缩。我们还发现湍流是跨AlfvéNic的，MA≈为1。在分子区域，我们发现了一个不对称的CO团簇，它的短轴比其长轴的角度更接近平均磁场方向，偏离了24°。除了靠近CO团块的区域外，HI速度梯度总体上垂直于平均磁场方向，在那里它们趋于平行。这种现象很可能与气体经历引力坠落有关。靶云的磁场形态在原子区平行于云的HI柱密度结构，而在分子区趋于垂直于HI结构。另一方面，磁场形态似乎与该过渡云的总柱密度形状(包括原子气体和分子气体)形成较小的偏移角。结论在发生H I-H H转变的目标云中，湍流是跨AlfvéNic的，因此磁场在云动力学中起着重要的作用。原子气体可能会优先沿着磁力线聚集，并在分子气体可能形成的地方产生过高的密度。用云团的总柱密度形状而不是它的HI柱密度形状可以更好地探测磁场形态。
ContextAtomic gas in the diffuse interstellar medium (ISM) is organized in filamentary structures. These structures usually host cold and dense molecular clumps. The Galactic magnetic field is considered to play an important role in the formation of these clumps.AimsOur goal is to explore the role of the magnetic field in the HI-H2transition process.MethodsWe targeted a diffuse ISM filamentary cloud toward the Ursa Major cirrus where gas transitions from atomic to molecular. We probed the magnetic field properties of the cloud with optical polarization observations. We performed multiwavelength spectroscopic observations of different species in order to probe the gas phase properties of the cloud. We observed the CO (J= 1−0) and (J= 2−1) lines in order to probe the molecular content of the cloud. We also obtained observations of the [C ii] 157.6µm emission line in order to trace the CO-dark H2gas and estimate the mean volume density of the cloud.ResultsWe identified two distinct subregions within the cloud. One of the regions is mostly atomic, while the other is dominated by molecular gas, although most of it is CO-dark. The estimated plane-of-the-sky magnetic field strength between the two regions remains constant within uncertainties and lies in the range 13–30 µG. The total magnetic field strength does not scale with density. This implies that gas is compressed along the field lines. We also found that turbulence is trans-Alfvénic, withMA≈ 1. In the molecular region, we detected an asymmetric CO clump whose minor axis is closer, with a 24° deviation, to the mean magnetic field orientation than the angle of its major axis. The H i velocity gradients are in general perpendicular to the mean magnetic field orientation except for the region close to the CO clump, where they tend to become parallel. This phenomenon is likely related to gas undergoing gravitational infall. The magnetic field morphology of the target cloud is parallel to the H i column density structure of the cloud in the atomic region, while it tends to become perpendicular to the H i structure in the molecular region. On the other hand, the magnetic field morphology seems to form a smaller offset angle with the total column density shape (including both atomic and molecular gas) of this transition cloud.ConclusionsIn the target cloud where the H i–H2transition takes place, turbulence is trans-Alfvénic, and hence the magnetic field plays an important role in the cloud dynamics. Atomic gas probably accumulates preferentially along the magnetic field lines and creates overdensities where molecular gas can form. The magnetic field morphology is probed better by the total column density shape of the cloud, and not its H i column density shape.