Two populations of transition discs?
  We examine the distribution of transition discs as a function of mm flux. We confirm that as expected in any model in which most primordial discs turn into transition discs and in which mm flux declines with time, transition discs have lower mm fluxes on average than primordial discs. However, we find that the incidence of transition discs does not, as expected, fall monotonically towards large mm fluxes and we investigate the hypothesis that these mm bright transition discs may have a distinct physical origin. We find that mm bright transition discs occupy a separate region of parameter space. Transition discs in the bright mm sub-sample have systematically higher accretion rates and inner hole radii than those in the faint mm sub-sample, along with being systematically weighted to earlier spectral types. 
James Owen and Cathie Clarke
astro-ph.EP astro-ph.SR