xstar HEADAS help file

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.

Additional information is contained in 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).

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

LAST MODIFIED

April 2002