Photoionization of Hydrogen in Atmospheres of Magnetic Neutron Stars
Photoionization of Hydrogen in Atmospheres of Magnetic Neutron Stars
复制标题
磁中子星大气中氢的光电离
DOI：
10.1086/304250
复制
发表时间：
1997
期刊：
影响因子：
--
通讯作者：
G. Pavlov
中科院分区：
文献类型：
--
作者：
A. Potekhin;G. Pavlov
中子星的强磁场（B ~ 1012-1013 G）特征使得原子的所有性质强烈依赖于其广义动量的横向分量 K⊥。特别是，光电离过程被大幅修改：（1）与静止原子相比，阈值能量降低，（2）横截面值显着变化，（3）运动违反了对静止原子有效的选择规则，从而允许新的光电离通道。为了计算光电离截面，我们首次对初始和最终原子态进行了精确的数值处理。这使我们能够考虑准束缚（自电离）原子态以及不同电离通道的耦合。我们将先前的考虑范围（仅限于与相对较小的 K⊥ 值相对应的所谓中心态）扩展到原子运动的任意状态。我们将横截面与原子在 K 上的热分布进行折叠。对于中子星大气的典型温度，平均横截面与静止原子的横截面有很大不同。特别是，原子的热运动强烈地拓宽了光电离边缘；这种“磁展宽”超出了通常的多普勒展宽几个数量级。原子的偏心状态产生光电离截面的低能成分。随着温度升高到 105.5 K 以上，密度降低到 1 g cm-3 以下，即中年中子星大气中预期的条件，这种新成分会显着增长。
The strong magnetic fields (B ~ 1012-1013 G) characteristic of neutron stars make all the properties of an atom strongly dependent on the transverse component K⊥ of its generalized momentum. In particular, the photoionization process is modified substantially: (1) threshold energies are decreased as compared with those for an atom at rest, (2) cross section values are changed significantly, and (3) selection rules valid for atoms at rest are violated by the motion so that new photoionization channels become allowed. To calculate the photoionization cross sections, we employ, for the first time, exact numerical treatment of both initial and final atomic states. This enables us to take into account the quasi-bound (autoionizing) atomic states as well as coupling of different ionization channels. We extend the previous consideration, restricted to the so-called centered states corresponding to relatively small values of K⊥, to arbitrary states of atomic motion. We fold the cross sections with the thermal distribution of atoms over K. For typical temperatures of neutron star atmospheres, the averaged cross sections differ substantially from those of atoms at rest. In particular, the photoionization edges are strongly broadened by the thermal motion of atoms; this “magnetic broadening” exceeds the usual Doppler broadening by orders of magnitude. The decentered states of the atoms give rise to the low-energy component of the photoionization cross section. This new component grows significantly with increasing temperature above 105.5 K and decreasing density below 1 g cm-3, i.e., for the conditions expected in atmospheres of middle-aged neutron stars.