arXiv 1406.64762014-06-25astro-ph.GA

暴露于AGN辐射的气体云的演变。I.三维辐射水动力模拟

On the evolution of gas clouds exposed to AGN radiation. I. Three-dimensional radiation hydrodynamic simulations

Daisuke Namekata, Masayuki Umemura, Kenji Hasegawa

备注：: 33 pages, 36 figures, 6 tables; accepted for publication in Monthly Notices of the Royal Astronomical Society
DOI：: 10.1093/mnras/stu1271

摘要：

我们对活动星系核（AGN）照射的均匀尘埃气体云进行了三维辐射流体动力学模拟，以研究云的演化对电离参数$\mathcal{U}$和Str{ö}mgren数$\mathcal{N}_{S}$的依赖性。我们发现，演变可以分为两种情况下，根据$\mathcal{U}$。在低$\mathcal{U}$病例（$\mathcal{U}\approx 10^{-2}$）中，演变主要由光蒸发驱动。从被照射面发射速度为$100\operatorname{-}150\;\mathrm{km\;s^{-1}}$的近似球对称的蒸发流。云被D型激波压缩，由于光蒸发而失去质量，最后被$t\lesssim 1.5t_{\mathrm{sc}}$转变为致密的暗条。在高$\mathcal{U}$病例（$\mathcal{U}\approx 5\times 10^{-2}$）中，辐射压力抑制了从被照射面的中心部分的光蒸发，降低了光蒸发速率。由于尘埃上的辐射压力，来自被照射面外围的蒸发流转变成高速（$\lesssim 500\;\mathrm{km\;s^{-1}}$）气体风。云被辐射压力驱动的激波扫过，并被$t\approx t_{\mathrm{sweep}}$变成致密的气体盘。对于$\mathcal{U}$的两种情况，在这些致密区域中预期都有恒星形成。讨论了活动星系核辐射对日冕的影响。一个简单的估计表明，这些团块被摧毁的时间尺度比它们的轨道周期短。为了使团簇结构长期保持，需要对大多数团簇充分减弱入射辐射场，或者需要某种连续产生团簇的机制。

英文摘要：

We perform three-dimensional radiation hydrodynamic simulations of uniform dusty gas clouds irradiated by an active galactic nucleus (AGN) to investigate the dependence of evolution of clouds on the ionization parameter $\mathcal{U}$ and the Str{ö}mgren number $\mathcal{N}_{S}$. We find that the evolution can be classified into two cases depending on $\mathcal{U}$. In low $\mathcal{U}$ cases ($\mathcal{U}\approx 10^{-2}$), the evolution is mainly driven by photo-evaporation. A approximately spherically-symmetric evaporation flow with velocity of $100\operatorname{-}150\;\mathrm{km\;s^{-1}}$ is launched from the irradiated face. The cloud is compressed by a D-type shock with losing its mass due to photo-evaporation and is finally turned into a dense filament by $t\lesssim 1.5t_{\mathrm{sc}}$. In high $\mathcal{U}$ cases ($\mathcal{U}\approx 5\times 10^{-2}$), radiation pressure suppresses photo-evaporation from the central part of the irradiated face, reducing photo-evaporation rate. A evaporation flow from the outskirts of the irradiated face is turned into a high velocity ($\lesssim 500\;\mathrm{km\;s^{-1}}$) gas wind because of radiation pressure on dust. The cloud is swept by a radiation pressure-driven shock and becomes a dense gas disk by $t\approx t_{\mathrm{sweep}}$. Star formation is expected in these dense regions for both cases of $\mathcal{U}$. We discuss the influences of the AGN radiation on the clumpy torus. A simple estimate suggests that the clumps are destroyed in timescales shorter than their orbital periods. For the clumpy structure to be maintained over long period, the incident radiation field needs to be sufficiently weaken for most of the clumps, or, some mechanism that creates the clumps continuously is needed.

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