The Thermal Evolution of the Postshock Layer in Pregalactic Clouds
The Thermal Evolution of the Postshock Layer in Pregalactic Clouds
复制标题
银河系前云中震后层的热演化
DOI：
10.1143/ptp.100.63
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发表时间：
1998
影响因子：
--
通讯作者：
M. Yamada
中科院分区：
文献类型：
--
作者：
H. Susa;R. Nishi;H. Uehara;M. Yamada
我们重新研究了原始气体云中激波后层的热演化。通过时间尺度的比较，我们发现，原始气云中激波后区的演化路径可以从离子电离度-温度平面图上得到基本的理解。在不包括光解离和光电离的情况下，由图中得到的结果与密度无关。我们还认为，该图不仅是有关的情况下，稳定的激波后流，但也等容冷却气体。许多作者对原始气云的热演化进行了研究。15），24），3），17），9），20），29）几乎所有这些研究都与各种星系或原始恒星的形成有关。在这些论文中，星系被假设是从早期宇宙中存在的小密度扰动中生长出来的。由于不断增长的密度扰动的寒冷，形成的云，这是星系的祖先，在弹跳时期经历强烈的冲击加热。冲击加热也预计在结构形成的层次聚类方案。在这种情况下，预计在两个云之间的碰撞中会发生冲击，这两个云被困在与较大结构相关的引力势阱中。无论如何，在星系形成的时代，冲击加热是可以预期的。许多作者对原始气云中激波后层的空间结构和热演化进行了研究。8），26），27），14），23），10），1），30）在这些研究中，共同的和最重要的一点是氢分子的过量产生。例如，Shapiro和Kang（23）（以下简称SK）研究了定常激波后流的热演化。他们发现，激波后气流冷却得如此之快，以至于复合过程无法赶上冷却。其结果是，电离度仍然很高（Ye rv 10- 3），即使当温度已经下降到10 - 4 K以下。通过这些“遗留”电子，氢分子形成的过程
We re-examine the thermal evolution of the postshock layer in primordial gas clouds. Comparing the time scales, we find that the evolutionary paths of postshock regions in primordial gas clouds can be basically understood in terms of the diagram drawn in the ion ization degree vs temperature plane. The results obtained from the diagram are independent of the density in the case that we do not include photodissociation and photoionization. We also argue that the diagram is not only relevant to the case of the steady postshock flow, but also to the isochorically cooling gas. The thermal evolution of primordial gas clouds has been investigated by many authors. 15),24),3),17),9),20),29) Almost all of those studies have been concerned with the formation of various kinds of galaxies, or primordial stars. In those papers, galaxies are assumed to grow out of small density perturbations present in the early universe. Because of the coldness of the growing density perturbations, the formed clouds, which are the progenitors of galaxies, experience strong shock heating at the bouncing epoch. Shock heating is also expected in the hierarchical clustering scenario of structure formation. In this case, shocks are expected in the collision between two clouds which are trapped in the gravitational potential well associated with the larger structure. In any case, shock heating is expected in the era of galaxy formation. The spatial structure and the thermal evolution of the postshock layer in primordial gas clouds were investigated by many authors. 8), 26), 27),14),23),10),1),30) In those studies the common and most important point is the over production of hydrogen molecules. For example, in Shapiro and Kang 23) (hereafter SK), the thermal evolution of steady postshock flow is investigated. They found that the postshock flow cools down so fast that the recombination process cannot catch up with the cooling. As a result, the ionization degree remains high (Ye rv 10- 3 ), even when the temperature has dropped below 10 4 K. Feeded these "relic" electrons, hydrogen molecules form through the processes