NAME
xstar - calculate photoionized model
USAGE
xstar [cfrac] [temperature] [lcpres] [pressure] [density]
[spectrum] [spectrum_file] [spectun] [trad] [luminosity]
[column_density] [rlogxi] [habund] [heabund] [cabund] [nabund]
[oabund] [neabund] [mgabund] [siabund] [sabund] [arabund] [caabund]
[feabund] [niabund] [modelname] [nsteps] [niter] [lwrite]
[lprint] [lstep] [emult] [taumax] [xeemin] [critf] [vturbi]
[npass]
DESCRIPTION
xstar is a tool for calculating the physical conditions and
emission and absorption spectra of photoionized gases.
It may be applied in a wide variety of astrophysical contexts.
It is assumed that a spherical gas shell surrounding a central
source of ionizing radiation absorbs some of this radiation and
reradiates it in other portions of the spectrum. xstar computes
the effects on the gas of absorbing this energy, and the spectrum
of reradiated light. Other sources of heat may exist, for example,
mechanical compression or expansion, or cosmic ray scattering and
xstar permits consideration of these effects as well. The user
supplies the shape and strength of the incident continuum, the
elemental abundances in the gas, its density or pressure, and
its thickness. The code returns the ionization balance and temperature,
opacity, and emitted line and continuum fluxes. These are stored as
fits files, with te exception of an ascii log file which contains
a slightly expanded version of the information sent to the screen.
The user is urged to refer to the xstar manual, which is
available as part of the xstar source tree, or online at
http://heasarc.gsfc.nasa.gov/docs/software/xstar/xstar.html
PARAMETERS
cfrac
This parameter determines whether the geometry is a
complete sphere or covers only part of the continuum source.
In the former case, photons escaping the cloud in the 'inward'
direction are assumed to reenter the cloud at the inner edge
owing to the assumption of spherical symmetry. Default is 1.0.
temperature
Define temperature, in units of $10^4 K$. If the parameter
niter is set to 0 then the temperature is fixed at this
value. Otherwise the value is used as a first guess in
calculating the thermal equilibrium value. If the pressure
is specified it is also used to calculate an initial guess at
the gas density, n=P/(kT), which is then used to calculate
Delta R_{max}=N/n. Default value is 1.
lcpres
This parameter chooses between constant density (value 0) and
constant pressure (value 1).
pressure
Define model pressure in dynes/cm/cm. Note that this quantity
represents the full isotropic pressure
(neutral atoms + ions + electrons + trapped line radiation)
instead of just the pressure due to hydrogen atoms and protons.
density
Define model gas density, $n$. This is actually the hydrogen nucleus
density, so that, e.g., the total particle density in a fully-ionized
plasma with solar abundances is $.3 n. Units are cm$^{-3}$.
The default value is 1 cm$^{-3}$.
spectrum
Define Spectrum. Choices and formats are similar to those used
by XSPEC and include pow, bbody, brems and file
spectrum_file
If the `file' option is chosen for the spectrum type, you must
provide a text file of the spectrum in your current working
directory. The first line of the text file must be the number of
energies listed in the table. The remaining lines are the energy
channel (in eV) and the flux in units of
photons cm$^{-2}$ s$^{-1}$ erg$^{-1}$ or
erg cm$^{-2}$ s$^{-1}$ erg$^{-1}$.
spectun
The appropriate units for the spectrum file specified above
(1=photons cm$^{-2}$ s$^{-1}$ erg$^{-1}$,
0=erg cm$^{-2}$ s$^{-1}$ erg$^{-1}$). Default is 0
trad
This parameter pulls double duty, used to enter the radiation
temperature (in keV) in the case of a black body input model, or for
the power-law index in energy in the case of a power-law model.
luminosity
Define model luminosity integrated between 1 and 1000 Ry.
Units are $10^{38}$ erg s$^{-1}$. Default value is 1.
column_density
Define model column density, N. Units are cm$^{-2}$.
rlogxi
Define initial value of the log (base 10) of the
model ionization parameter. If the density is
held constant, the Tarter, Tucker, Salpeter (1969) form is used:
xi = L/(nR^2). If the pressure is held constant, a version of
the Krolik, McKee, and Tarter (1981) form is used:
Xi = L/(4\pi c R^2 P).
\subsection{Abundances}
habund
Hydrogen atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
heabund
Helium atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
cabund
Carbon atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
nabund
Nitrogen atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
oabund
Oxygen atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
neabund
Neon atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
mgabund
Magnesium atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
siabund
Silicon atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
sabund
Sulfur atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
arabund
Argon atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
caabund
Calcium atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
feabund
Iron atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
niabund
Nickel atomic abundances relative to solar abundances as
defined in Grevesse (1996), with 1.0 being defined as the solar
value and the default.
modelname
Model name, an 80 character string.
nsteps
Used in calculating step size. See manual for definition.
niter
Number of iterations for thermal equilibrium. 0=constant
temperature.
lwrite
Write switch. 0=default.
lprint
Print switch. 0=default
emult
Courant multiplier, used in calculating step size. Value between
0 and 1 is recommended. Default is 0.5
taumax
Maximum optical depth used in step size calculation. See manual
for definition. Default is 5.
xeemin
Minumum allowed electron fraction.
critf
Ions abundance criterion for inclusion in multi-level calculation.
See manual for details.
vturbi
Turbulent velocity in km/s. Default is 1.
npass
Number of passes through global structure. Should be an odd number,
1 or 3. Default is 1.
EXAMPLES
Note that when commands are issued on the Unix command line,
strings containing special characters such as '[' or ']' must be
enclosed in single or double quotes.
1. A spherical, constant density cloud with a source
at its center. The source luminosity is 10$^{38}$ erg s$^{-1}$.
The ionization parameter at the inner edge of the cloud
is log($\xi$)=5. The ionizing spectrum is an optically
thin bremsstrahlung.
xstar cfrac=1. temperature=1000. pressure=0.03 density=1.e+9
spectrum='brems' trad=10. rlrad38=1. column=1.e+23
rlogxi=5. lcpres=0 habund=1. heabund=1. cabund=1. nabund=1.
oabund=1. neabund=1. mgabund=1. siabund=1. sabund=1.
arabund=1. caabund=1. feabund=1. niabund=0.
modelname="spherical cloud" npass=1 niter=99 critf=1.e-12
SEE ALSO
LAST MODIFIED
April 2002