标题： 蟹状星云周围H$α$-晕的观测和理论限制 显示英文标题

标题： Observational and theoretical constraints for an H$α$-halo around the Crab Nebula

摘要： 我们搜寻了蟹状星云周围快速移动的H$\alpha$ 外壳。 这样一个外壳可以解释这个超新星遗迹缺失的质量，并且携带足够的动能使SN 1054成为正常的II型事件。 通过MPG/ESO 2.2米望远镜上的WFI和NOT上的MOSCA获得了深部H$\alpha$ 图像。 这些数据与源自具有抛射气体运动、恒定温度、恒定电离度以及密度剖面幂律分布的外壳模型的理论预期进行了比较。 我们达到了表面亮度极限为$5\times10^{-8} ergs s^{-1} cm^{-2} sr^{-1}$。 检测到了一个晕，但它的表面亮度比我们的再结合辐射和尘埃散射模型预测的要高得多。 只有Ly$\beta$的碰撞激发部分退激到H$\alpha$才能解释这样的振幅。 我们表明所看到的晕是由点扩散函数（PSF）散射引起的，因此与真实的外壳无关。 我们还研究了在蟹状星云中心方向上高能H$\alpha$气体的光谱探测可行性。 发射光谱的建模显示，在复杂的H$\alpha$线光谱环境中，这种气体很容易逃脱检测。 点扩散函数（PSF）散射显著污染了我们的数据，阻止了对预测的快速外壳的检测。 一个实际的光环，其观测峰值流量约为$2\times10^{-7} ergs s^{-1} cm^{-2} sr^{-1} $，仍然可以容纳在我们的误差范围内，但我们的模型预测其表面亮度要低四倍。 8米级望远镜可以在有足够精确的点扩散函数（PSF）模型的情况下明确检测到这样的流量。 最后，我们注意到，点扩散函数散射也会影响其他研究领域，在这些领域中，人们正在寻找明亮且延展的目标周围的微弱光环。 显示英文摘要

摘要： We searched for a fast moving H$\alpha$ shell around the Crab nebula. Such a shell could account for this supernova remnant's missing mass, and carry enough kinetic energy to make SN 1054 a normal Type II event. Deep H$\alpha$ images were obtained with WFI at the 2.2m MPG/ESO telescope and with MOSCA at the 2.56m NOT. The data are compared with theoretical expectations derived from shell models with ballistic gas motion, constant temperature, constant degree of ionisation and a power law for the density profile. We reach a surface brightness limit of $5\times10^{-8} ergs s^{-1} cm^{-2} sr^{-1}$. A halo is detected, but at a much higher surface brightness than our models of recombination emission and dust scattering predict. Only collisional excitation of Ly$\beta$ with partial de-excitation to H$\alpha$ could explain such amplitudes. We show that the halo seen is due to PSF scattering and thus not related to a real shell. We also investigated the feasibility of a spectroscopic detection of high-velocity H$\alpha$ gas towards the centre of the Crab nebula. Modelling of the emission spectra shows that such gas easily evades detection in the complex spectral environment of the H$\alpha$-line. PSF scattering significantly contaminates our data, preventing a detection of the predicted fast shell. A real halo with observed peak flux of about $2\times10^{-7} ergs s^{-1} cm^{-2} sr^{-1} $ could still be accomodated within our error bars, but our models predict a factor 4 lower surface brightness. 8m class telescopes could detect such fluxes unambiguously, provided that a sufficiently accurate PSF model is available. Finally, we note that PSF scattering also affects other research areas where faint haloes are searched for around bright and extended targets.