<defunct>: too many from pipe

On FC7, I have a daemon that executes system commands via a
pipe constructed of, essentially:
   pipe()
   fork()
   execvp()
all wrapped in a C++ class.

I find, on ps -ef, that each system command leaves behind a
processes marked <defunct> that remains until the daemon is
stopped.  These are unaffected by kill -9 issued either from
the destructor of the pipe class, or by me on the command
line.  I have two questions:

1. Is there any way to get rid of these defunct processes
   other than stopping the daemon?

2. How many of these can pile up before my system crashes?

Thanks for your help.
Mike.