Effects of the X-ray emission from young stars on the ionization level   of a fractal star forming cloud
  We present the initial results of a study aimed at computing the global effects of the X-ray ionization due to young stars within a molecular cloud that forms stars with a standard IMF at a rate accelerating with time. We model the gas distribution of the cloud as a fractal of dimension 2.3. We introduce the concept of a "Roentgen sphere" as the region around each PMS star where the ionization rate due to stellar X-rays exceeds that due to interstellar cosmic rays. Using values of the X-ray luminosity typical of T Tauri stars, we find that within the Roentgen radius the average ionization rate is increased by a factor 4 to 20 with respect to the value obtained from cosmic-rays. 
Andrea Lorenzani (1), Francesco Palla (1) ((1) Osservatorio   Astrofisico di Arcetri, Firenze, Italy)