4.1.9. 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).
4.1.9.1. 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 and
by
Wu et al. (2012)
and Nolte et al. (2012),
multi-channel Laudau-Zener results for
and
by Mullen et al. (2016), other data
compilations for
and
by
Janev et al. (1993),
and the NIFS Charge Transfer Database
(CHART) [1] for
,
,
, and
. 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
(
km
), 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
and
.
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 selectivity. The
dependence
is approximated by one of the five empirical weighting functions
presented in Eqs.
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.
4.1.9.2. Ion concentration¶
There are several reasons why hot plasma at the interface with cold matter may not
be in exact collisional ionization equilibrium. The parameter cmod
controls whether
CIE or NEI is assumed for the calculation of charge exchange spectrum. By default,
cmod
is set to 1, corresponding to CIE. When cmod
is set to 2, the plasma
is treated in NEI, and the parameter pt
and u
, together with t
, defines the
ionization balance in the same way as in the nei
model. For a recombining plasma,
pt
should be chosen higher than t
.
4.1.9.3. Parameter description¶
The parameters of the CX model are:
- norm:
the normalisation, which is the emission measure
in units of
, where
and
are the Hydrogen densities of the ionized and neutral materials, respectively, and
is the effective interaction volume. Default value: 1.
- hden:
Hydrogen density of the neutral materials in units of
(or
). Default value:
.
- 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.
- cmod:
Switch between a CIE and NEI charge balance for the hot plasmas. Default value: 1 (CIE).
- pt:
Pre-shock temperature same as
t1
in theneij
model. Only applied whencmod
is 2.- u:
Ionization parameter
. Only applied when
cmod
is 2.- zv:
Collision velocity in unit of km
, 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
population. When wt is set to 1, the
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.
of Gu et al. (2016), and will be selected when wt is set to
, respectively. Default: 1
- vrms:
RMS Velocity broadening in km/s (see Definition of the micro-turbulent velocity in SPEX)
- 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.
- 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).