7.1.3 Fundamental model parameters

7.1.3.1 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 this parameter specifies the average output flux only for local models with closed radiative lower boundary and that in many cases the actual effective temperature can only be determined a posteriori. For other lower (or inner boundary conditions) the entropy of the instreaming material s_inflow (see below) is more important than teff itself. See Sect. 7.1.4.4 for different heating modes controled with heat_mode.

7.1.3.2 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:

• central: For the global ``star-in-a-box'' case, a central potential is assumed with origin at x=0. The stellar mass mass_star (Sect.7.1.3.4) as well as some characteristic radii (r0_grav, r1_grav, r2_grav, r1_grav, r0_core) have to be specified.
• constant: In the standard local ``box-in-a-star'' case, the constant gravity specified with grav (Sect.7.1.3.3) 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
  

  $$\Phi_\mathrm{ex} = \hat{\Phi}_\mathrm{ex} \cos(k_\mathrm{h} x1 + \omega t).$$ (1)

  
  
  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.

7.1.3.3 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.

7.1.3.4 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

7.1.3.5 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 of a global model. For local models, this parameter specifies the depth of the lower heating and damping region, see Sect. 7.1.4.4.

7.1.3.6 real r1_grav

The density in an atmosphere in hydrostatic equilibrium can decline to very low values. To artificially 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 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.

7.1.3.7 real r2_grav

Using r2_grav instead of r1_grav means that a potential function more appropriate for a polytropic stellar interior model is used. It can be set, e.g. with

real r2_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 means the potential parameterized with r1_grav is used. This parameter is similar to r1_grav, i.e., it enlarges the pressure scale height in the outer layers but does not change the smoothing formula in the center. This parameter is only effective if r1_grav is set to zero.

7.1.3.8 real r0_core

To insert energy in a sphere different with a radius other than r0_grav (the default), the heating radius r0_core can be specified separately, e.g. with

real r0_core   f=E15.8 b=4 n='Core Radius'    u=cm
 9.45833e+12

If this parameter is not specified or set to zero r0_grav is used as the radius of the core.

7.1.3.9 character centrifugal_force

Usually, a centrifugal force is applied if nu_rotation$\ne$0. To switch it off even for a non-zero rotation rate, the parameter centrifugal_force can be used, e.g. by setting

character centrifugal_force  f=A80 b=80  n='Switch on/off the centrifugal force' &
  c0='on: default, off: no centrifugal force, even for non-zero nu_rotation'
off

Two alternative values are possible so far:

• on: This is the default and can be set even for non-rotating objects.
• off: This skips the application of the centrifugal force in case of a non-zero rotation rate. In this case, only the Coriolis force is applied.

By default, the centrifugal force is activated if the rotation rate nu_rotation is non-zero, but only for global models as the centrifugal force for local models would in effect only modify the value of the gravity constant.

7.1.3.10 real nu_rotation

To transform onto a rotating coordinate-system a rotation rate can be specified with e.g.

real nu_rotation   f=E15.8 b=4 n='Rotation frequency'    u=1/s
 0.01

The potential is modified by adding terms due to a centrifugal force unless centrifugal_force is set to off. In addition, a Coriolis force is applied during the hydrodynamics step.

7.1.3.11 real ar_rotationaxis

The rotation axis can be specified with

real ar_rotationaxis f=E10.4 b=4 p=1 d=(1:3) n='Rotation axis' u=1
1.0
0.0
0.0

The default value is (0.0,0.0,1.0) and should be used for rotating global models. A different axis only makes sense for local models.