Thanks for the tip. Gazebo starts up. However, it still crashes after being up for 10-15 minutes and spits up the error message:

gazebo-2] process has died [pid 12540, exit code -11].

log files: /home/sosentos/.ros/log/18b1012e-52e7-11df-853d-00241dc18d36/gazebo-2*.log

Unfortunately there is nothing in the .log file other than a line about successful initialization,

Sarah