The same except you check for your nick NOT being +a by prefixing the $nick() identifier with a "!" - !$nick(#,$me,!)
Code:
on *!:part:#channel:{
  If $nick == celtic^star && [color:red]![/color]$nick(#,$me,!) { [color:blue]chanserv protect # $me[/color] }
}
You might have to change the command in blue