5.3.3 Fundamental Model Parameters

• real teff:
  The effective temperature is one of the basic model parameters and is specified e.g. with
  real teff  f=F13.3 b=4  n='Effective Temperature' u=K
   3500.0
  
  (for a relatively cool star). Note that the actual effective temperature can only be determined a posteriori and that the entropy of the instreaming entropy (see below) is more important than teff itself. In fact, teff is only used to control material properties at the outer boundary. Its value should be close to the expected effective temperature of the model.

• character grav_mode:
  Gravity is another characteristic of a stellar atmosphere. The type (or geometry) of the external gravity field has to be specified e.g. with
  character grav_mode  f=A80 b=80  n='Type of gravity field' &
    c0='constant/central'
  central
  
  Three values are possible so far:
  • constant: In the standard ``solar'' case the constant gravity specified with real grav is directed downward in x3 direction.
  • localboxtide: This activates a simple model for the action of a tidal wave on convection in a local box model. It adds to the case of constant gravity a potential  $\hat{\Phi}_\mathrm{ex}$ which is harmonic in space and time according to
    

    $$\Phi_\mathrm{ex} = \hat{\Phi}_\mathrm{ex} \cos(k_\mathrm{h} x1 + \omega t).$$ (92)

    
    
    It is intended to mimic a travelling tidal wave. Since the setup is considered experimental the parameters are set by the C_test parameters according:
    

    $\hat{\Phi}_\mathrm{ex}$	=	C_test1 (amplitude of potential variations)
    $k_\mathrm{h}$	=	C_test2 (horizontal wavenumber)
    $\omega$	=	C_test3 (frequency)

    
    Boundary conditions: in order to be compatible with periodic lateral boundaries $k_\mathrm{h}$ should be an integer multiple of $2\pi$ times the inverse lateral box size.
  • central: For the ``supergiant'' case a central potential is assumed with an origin at x=0. The stellar mass as well as inner and outer smoothing radius have to be specified.

• real grav:
  In the case of a constant gravity the amount of the acceleration has to specified with
  real grav  f=E15.8 b=4  n='Gravity' u=cm/s^2
  27500.0
  
  Setting this value to zero switches off gravity (oh wonder).

• real mass_star:
  In the case of a central the mass (in cgs units) of the star has to be specified with
  real mass_star f=E15.8 b=4 n='Stellar Mass'              u=g
   9.94500e+33
  

• real r0_grav:
  To avoid the central singularity in a 1/r potential it is smoothed in the center to give a central potential of 1/r0_grav, specified with
  real r0_grav   f=E15.8 b=4 n='Inner Smoothing Radius'    u=cm
   9.45833e+12
  
  This parameter should always be non-zero for a central potential.

• real r1_grav:
  The density in an atmosphere in hydrostatic equilibrium can decline to very low values. To artificial enlarge the pressure (and density) scale height in the outer layers of the star (the corners of the box) the gravity can be reduced by defining the potential at infinity to be 1/r1_grav, specified with
  real r1_grav   f=E15.8 b=4 n='Outer Smoothing Radius'    u=cm &
    c0='0.0: Not used'
   11.35000e+13
  
  Setting this parameter to zero gives the usual 1/r behavior of the potential in the outer layers but also chooses another smoothing formula in the central part (where real r0_grav is relevant). But a value somewhat larger than the remotest corner of the box effectively cancels this artificial smoothing in the outer layers without changing the formula for the potential.

• real r1_rad:
  For a ``Star-in-a-Box'' and particularly when only ``simple'' ray directions are allowed in the radiation transport step the temperature in the outer corners of the box tends to become very small. To artificially increase the effect of radiative heating the parameter real r1_rad can specify a radius beyond which only positive contributions of the radiative energy transport to the energy budget are taken into account. This ruins the conservativity of the code in these layers and should be applied only in very remote corners which are then considered only as sort of extended boundary region but not as part of the ``real'' model. The parameter can be specified e.g. with
  real r1_rad   f=E15.8 b=4 n='Outer radiation transport radius'    u=cm &
    c0='0.0: Not used'
    8.00000e+13
  
  A value of 0.0 (default) or below deactivates this feature.

• real nu_rotation:
  To transform onto a coordinate system rotation around the x3 axis, a rotation rate can be specified with e.g.
  real nu_rotation   f=E15.8 b=4 n='Rotation frequency'    u=1/s
   0.0
  
  The potential is modified by adding terms due to a centrifugal force. In addition, a Coriolis force are applied during the hydrodynamics step.