Relativistic AGN jets I. The delicate interplay between jet structure, cocoon morphology and jet-head propagation
Relativistic AGN jets I. The delicate interplay between jet structure, cocoon morphology and jet-head propagation
复制标题
DOI：
--
复制
发表时间：
2013
期刊：
影响因子：
--
通讯作者：
S. Walg;S. Walg;Abraham Achterberg;S. Markoff;R. Keppens;Z. Meliani
中科院分区：
文献类型：
--
作者：
S. Walg;S. Walg;Abraham Achterberg;S. Markoff;R. Keppens;Z. Meliani
天体物理喷流显示出强烈的径向结构迹象。他们认为，喷流的内部区域，即喷流脊椎，由低密度、快速移动的气体组成，而喷流的外部区域由密度更高、移动速度更慢的气体组成，称为喷流鞘。此外，如果喷流携带角动量，所产生的离心力会导致径向分层。目前的观测不能完全分辨出放射状结构，因此人们对它的实际轮廓知之甚少。我们给出了三个2.5D的活动星系核喷流模型，其中两个被赋予了放射状结构。第一个模型是均匀射流，是唯一不携带角动量的模型；第二个模型是具有等温状态方程的脊鞘射流；第三个射流模型是(分段)等时等容脊鞘射流，射流脊部和射流鞘的密度是恒定但不同的。在本文中，我们研究了径向层结对喷流完整性、不同喷流组分之间的混合以及喷流头部和周围蚕茧的整体形态的影响。我们考虑已经活跃了23个月的稳定喷流。在模拟的最后时刻，所有喷流沿喷流轴发展出相同数量的强烈内激波。这些激波是在喷流头喷出涡流时产生的。我们发现，这三种喷流在喷头的整个过程中都保持了稳定。等温射流在喷头保持了部分结构的完整性，在那里仍然可以区分喷流脊椎和喷流护套材料。在这种情况下，在射流内的射流脊椎和射流护套之间的混合是相当低效的。另一方面，等容射流会在喷射后相当快地失去其结构射流的完整性。在其喷头处，几乎没有保持结构，喷流的中心部分主要由喷流鞘材料组成。在这种情况下，喷射脊椎和喷射护套材料在喷射内有效地混合。我们发现，从简单的理论预测来看，这三种模型的传播速度都低于预期。我们认为这是由于喷流与周围介质碰撞的横截面增大所致。结果表明，在这些模型中，均质射流的有效表面积是均匀射流的16倍，等容射流的有效表面积是等容射流的30倍，而等温射流的有效表面积可达40倍。
Astrophysical jets reveal strong signs of radial structure. They suggest that the inner region of the jet, the jet spine, consists of a low-density, fast-moving gas, while the outer region of the jet consists of a more dense and slower moving gas, called the jet sheath. Moreover, if jets carry angular momentum, the resultant centrifugal forces lead to a radial stratification. Current observations are not able to fully resolve the radial structure, so little is known about its actual profile. We present three active galactic nuclei jet models in 2.5D of which two have been given a radial structure. The first model is a homogeneous jet, the only model that does not carry angular momentum; the second model is a spine–sheath jet with an isothermal equation of state; and the third jet model is a (piecewise) isochoric spine–sheath jet, with constant but different densities for jet spine and jet sheath. In this paper, we look at the effects of radial stratification on jet integrity, mixing between the different jet components and global morphology of the jet-head and surrounding cocoon. We consider steady jets that have been active for 23 Myr. All jets have developed the same number of strong internal shocks along their jet axis at the final time of simulation. These shocks arise when vortices are being shed by the jet-head. We find that all three jets maintain their stability all the way up to the jet-head. The isothermal jet maintains part of its structural integrity at the jet-head where the distinction between jet spine and jet sheath material can still be made. In this case, mixing between jet spine and jet sheath within the jet is fairly inefficient. The isochoric jet, on the other hand, loses its structural jet integrity fairly quickly after the jet is injected. At its jet-head, little structure is maintained and the central part of the jet predominantly consists of jet sheath material. In this case, jet spine and jet sheath material mix efficiently within the jet. We find that the propagation speed for all three models is less than expected from simple theoretical predictions. We propose this is due to an enlarged cross-section of the jet which impacts with the ambient medium. We show that in these models, the effective surface area is 16 times as large in the case of the homogeneous jet, 30 times as large in the case of the isochoric jet and can be up to 40 times as large in the case of the isothermal jet.