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arXiv 1803.09348astro-ph.HE
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星周圆盘中的Cosmic-ray电离

Cosmic-ray ionisation in circumstellar discs

Marco Padovani, Alexei V. Ivlev, Daniele Galli, Paola Caselli

摘要:

银河宇宙射线是星际气体电离的普遍来源,与紫外线和X射线光子以及自然放射性竞争,确定分子云和周星盘中电子、离子和带电尘埃颗粒的丰度。我们模拟了银河系宇宙射线的不同成分与气体柱密度的传播。我们的研究集中在高密度的传播,在几克厘米$^{-2}$以上,特别是有关的内部地区的坍缩云和星周盘。初级和次级CR粒子(质子和较重的原子核,电子,正电子和光子)的传播计算在连续减速近似,扩散近似,或灾难性的近似,通过采用不同的传输制度的匹配程序。适当制度的选择取决于主要损失过程的性质,模拟为连续或灾难性的。CR电离速率由CR质子及其低于$\approx 130$ g cm $^{-2}$的次级电子和由高于$\approx600$ g cm $^{-2}$的光子衰变产生的电子/正电子对确定。我们表明,一个适当的描述的粒子运输是必不可少的计算在后一种情况下的电离率,因为电子/正电子差分通量敏感地依赖于质子和光子的通量。我们的研究结果表明,CR电离率在高密度环境中,如,例如,坍缩分子云的内部或星周盘的中平面,比以前假设的要大。它不会随着柱密度的增加而呈指数下降,而是由于控制次级粒子的产生和传播的不同过程的相互作用而遵循更复杂的行为。

英文摘要:

Galactic cosmic rays are a ubiquitous source of ionisation of the interstellar gas, competing with UV and X-ray photons as well as natural radioactivity in determining the fractional abundance of electrons, ions and charged dust grains in molecular clouds and circumstellar discs. We model the propagation of different components of Galactic cosmic rays versus the column density of the gas. Our study is focussed on the propagation at high densities, above a few g cm$^{-2}$, especially relevant for the inner regions of collapsing clouds and circumstellar discs. The propagation of primary and secondary CR particles (protons and heavier nuclei, electrons, positrons, and photons) is computed in the continuous slowing down approximation, diffusion approximation, or catastrophic approximation, by adopting a matching procedure for the different transport regimes. A choice of the proper regime depends on the nature of the dominant loss process, modelled as continuous or catastrophic. The CR ionisation rate is determined by CR protons and their secondary electrons below $\approx 130$ g cm$^{-2}$ and by electron/positron pairs created by photon decay above $\approx600$ g cm$^{-2}$. We show that a proper description of the particle transport is essential to compute the ionisation rate in the latter case, since the electron/positron differential fluxes depend sensitively on the fluxes of both protons and photons. Our results show that the CR ionisation rate in high-density environments, like, e.g., the inner parts of collapsing molecular clouds or the mid-plane of circumstellar discs, is larger than previously assumed. It does not decline exponentially with increasing column density, but follows a more complex behaviour due to the interplay of different processes governing the generation and propagation of secondary particles.

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