.. SPDX-FileCopyrightText: 1992-2026 NWO-I/SRON Space Research Organisation Netherlands .. .. SPDX-License-Identifier: CC-BY-4.0 .. _sect:cx: CX: model for charge exchange plasmas ===================================== This model calculates the spectrum emitted from a hot plasma when it recombines with cold neutral materials. This model is based on three key assumptions: (1) it considers only single electron capture in a ion-neutral collision; (2) all cross section data are obtained only with a atomic hydrogen target, (3) electronic collisional excitation and recombination are ignored in the spectral calculation. More information can be found in `Gu et al. (2016) `_. Charge exchange cross sections ------------------------------ The CX cross section data used in the model are partly taken from literature, including quantum molecular-orbital close-coupling calculations for :math:`\mathrm C^{5+}` and :math:`\mathrm O^{6+}` by `Wu et al. (2012) `_ and `Nolte et al. (2012) `_, multi-channel Laudau-Zener results for :math:`\mathrm Fe^{25+}` and :math:`\mathrm Fe^{26+}` by `Mullen et al. (2016) `_, other data compilations for :math:`\mathrm C^{6+}` and :math:`\mathrm O^{8+}` by `Janev et al. (1993) `_, and the NIFS Charge Transfer Database (CHART) [1]_ for :math:`\mathrm Be^{4+}`, :math:`\mathrm B^{5+}`, :math:`\mathrm N^{7+}`, and :math:`\mathrm Ne^{10+}`. For CHART database, we extracted all the data, from both theoretical calculations and experiments (see a full list in Table 1 of `Gu et al. (2016) `_), and fitted them with Eq.2 of `Gu et al. (2016) `_ in the energy range of interests. In typical astrophysical velocity range (:math:`\sim 100-5000` km :math:`\mathrm{s}^{-1}`), the useful CHART data are usually from molecular-orbital and atomic-orbital close-coupling methods, and a few classical trajectory Monte Carlo calculations. All the above data are dependent on collision energy, and resolved to levels described by quantum number :math:`n` and :math:`l`. For ions not available in public sources, we developed a new method to interpolate by analyzing the known ions. First we used a scaling law to determine total cross section for each ion, and applied another scaling law to represent the :math:`n-` selectivity. The :math:`l-` dependence is approximated by one of the five empirical weighting functions presented in Eqs.\ :math:`4-8` of `Gu et al. (2016) `_. .. Warning:: The CX model only works with the updated atomic database set through the command ``var calc new``. .. Warning:: All Beryllium-like sequence ions are not included in the current version; will be available later. .. Warning:: We will keep updating the CX model when new data (especially for molecular targets) from theoretical calculations and experiments become available. Parameter description --------------------- The parameters of the CX model are: | ``norm`` : the normalisation, which is the emission measure :math:`Y \equiv n_{\mathrm H} n_{\mathrm nh} V` in units of :math:`10^{64}` :math:`\mathrm{m}^{-3}`, where :math:`n_{\mathrm H}` and :math:`n_{\mathrm nh}` are the Hydrogen densities of the ionized and neutral materials, respectively, and :math:`V` is the effective interaction volume. Default value: 1. | ``hden`` : Hydrogen density of the neutral materials in units of :math:`10^{20}` :math:`\mathrm{m}^{-3}` (or :math:`10^{14}` :math:`\mathrm{m}^{-3}`). Default value: :math:`10^{-14}`. | ``mode`` : Switch between a hot-cold interaction driven by thermal motion of hot plasma, and the one dominated by flow velocity. Default value: 2 (kinematic). | ``t`` : the ionization temperature of hot matter in keV. It is also used to approximate the thermal motion when mode is set to 1. Default value: 1. | ``sig`` : the width :math:`\sigma_T` of the gaussian emission measure profile. Default value: 0. (no temperature distribution i.e. isothermal) | ``sup`` : the width :math:`\sigma_T` of the high-temperature part of the gaussian emission measure profile. If larger than :math:`10^{-5}` keV, the sig parameter becomes the sigma value for the low-temperature end. Default value: 0 | ``logt`` : Switch between linear and logarithmic temperature scale for the gaussian emission measure profile. Default value: 1 (logarithmic) | ``zv`` : Collision velocity in unit of km :math:`\mathrm{s}^{-1}`, used when mode is set to 2. Default value: 100 | ``op`` : Switch between single and multiple collisions for each ion. In multiple collision case, one ion would continuously undergo CX and produce various emission lines, until it becomes neutral. Default: 1 (single) | ``wt`` : Weighting functions for subshell :math:`l-` population. When wt is set to 1, the :math:`l-` population is approximated by a series of empirical functions that switchs from one to another as a function of collision velocity. See `Gu et al. (2016) `_ for details. These empirical functions are defined in Eqs. :math:`4-8` of `Gu et al. (2016) `_, and will be selected when wt is set to :math:`2-6`, respectively. Default: 1 | ``vrms`` : RMS Velocity broadening in km/s (see :ref:`sect:turbulence`) | ``ref`` : reference element. Default value 1 (hydrogen). See above for more details. The value corresponds to the atomic number of the reference element. | ``01`` : Abundance of hydrogen (H, Z=1) in Solar units. Default 1. | ``02`` : Abundance of helium (He, Z=2) in Solar units. Default 1. | :math:`\ldots` ``30`` : Abundance of zinc (Zn, Z=30) in Solar units. Default 1. | ``file`` : Filename for the nonthermal distribution. If not present, nonthermal effects are not taken into account (default). *Recommended citation:* `Gu et al. (2016) `_. .. [1] http://dbshino.nifs.ac.jp/