Constructing a ZAHB model
-------------------------

To construct a ZAHB model requires a few steps:

1. Start with a star that is massive enough to avoid the helium flash,
evolve it until it has a large enough core (it has to be doing steady
helium burning), then terminate the run.

2. Strip the hydrogen envelope from the last model of the previous run
until you are left with a low-mass helium burning star.

3. Bring this model into equilibrium, so that the code can reliably start
from it and accrete new material onto the model.

4. Test it!

Running "make" in this directory causes each of these steps to be run (ok,
so you need to run "make test" manually):

1. The input files m225.run and m225.dat evolve a star of approximately
2.25 solar masses from the ZAMS until it has a core mass of of 0.4 solar masses

2. The input files strip.run and strip.dat are setup to take the last
output model of the m225 run and strip the mass down to 0.39 solar masses,
eliminating almost all of the hydrogen envelope.

3. The input files relax.run and relax.dat take the last model from that
run and let it evolve until the end of time while doing absolutely nothing.
This is overkill, it could be run for less time.

These three steps will be completed in sequence and produce a file
"zahb.mod" that can be used for post-helium flash model construction.
It can be tested by running "make test". CHECK THE OUTPUT OF THIS RUN TO
MAKE SURE EVERYTHING LOOKS AS YOU EXPECT.
To install it in the stars/input/ directory, you can run "make install".
The file is then also renamed zahb$Z.mod.

Finally, you can run "make clean" to eliminate all temporary files.

Environment variables:
 Z - metallicity
 evpath - path to evolution code, assumed to be $PWD/../../ if not set
 EV - path to the ev program, assumed to be $evpath/code/ev if not set

Example:
Make a ZAHB starting model for Z=0.0001, install it and clean up
afterwards:
$ make Z=0001
$ make Z=0001 install
$ make clean
Or in one line:
$ make Z=0001 install && make clean

Evert Glebbeek, 25 September 2008
