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arXiv 1206.6493astro-ph.GAastro-ph.CO
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电离参数作为H II区和星暴星系辐射和风压的诊断

Ionization Parameter as a Diagnostic of Radiation and Wind Pressures in H II Regions and Starburst Galaxies

Sherry C. C. Yeh, Christopher D. Matzner

摘要:

电离参数U对于测量来自大质量星团的辐射压力反馈是潜在有用的,因为它反映了辐射与气体压力的比率,并且很容易从中红外线比率导出。我们考虑几种影响,确定在HII地区和星系的U的表观值。上限由辐射压力对气体的压缩设定。对于给定的辐射强度,来自恒星风的压力和中性团块的存在都会降低U。辐射最强烈的区域被电离光子的内部尘埃吸收而选择性地变暗,导致星系尺度上的观测偏差。我们探索这些影响的分析和数值,并用它们来解释以前的观测结果。我们发现,辐射约束设置的上限log_10 U = -1在个别地区看到。未分辨的星爆显示的最大值约为-2.3。虽然较低,但这也与其HII区域的大部分以辐射为主相一致,考虑到用于解释未解决区域的不同技术,并考虑到尘埃吸收造成的偏差。我们推断,许多个别的,强烈的照明区域不能由恒星风占主导地位,即使在银河系尺度上平均,冲击风压不可能比辐射压力大。因此,绝大多数HII区不可能是绝热风泡。我们的模型意味着金属丰度的物理结构和尘埃衰减的辐射为主的地区,这两者都应该有很大的变化,在一个临界金属丰度约二十分之一的太阳。

英文摘要:

The ionization parameter U is potentially useful for measuring radiation pressure feedback from massive star clusters, as it reflects the radiation-to-gas-pressure ratio and is readily derived from mid-infrared line ratios. We consider several effects which determine the apparent value of U in HII regions and galaxies. An upper limit is set by the compression of gas by radiation pressure. The pressure from stellar winds and the presence of neutral clumps both reduce U for a given radiation intensity. The most intensely irradiated regions are selectively dimmed by internal dust absorption of ionizing photons, inducing observational bias on galactic scales. We explore these effects analytically and numerically, and use them to interpret previous observational results. We find that radiation confinement sets the upper limit log_10 U = -1 seen in individual regions. Unresolved starbursts display a maximum value of ~ -2.3. While lower, this is also consistent with a large portion of their HII regions being radiation dominated, given the different technique used to interpret unresolved regions, and given the bias caused by dust absorption. We infer that many individual, strongly illuminated regions cannot be dominated by stellar winds, and that even when averaged on galactic scales, shocked wind pressures cannot be large compared to radiation pressure. Therefore, most HII regions cannot be adiabatic wind bubbles. Our models imply a metallicity dependence in the physical structure and dust attenuation of radiation-dominated regions, both of which should vary strongly across a critical metallicity of about one-twentieth solar.

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