Star Formation Group
8-Sep-2004
H. Throop

Photo-evaporation and circumstellar disks

Model:
We have a numerical model which simulates evolution of a disk undergoing
photo-evaporation.

Physics involved:
  o Photo-evaporation removes disk material in EUV, FUV outflows
    according to model of Johnstone et al.

  o Gas outflow also entrains dust grains, depending on size, distance

  o Dust grains have two possible vertical distributions:
    o Well-mixed: they are entrained uniformly with the gas
    o Settled: Following model of Sekiya (1998), dust grains sediment
      into a dense layer at midplane because of finite mass-loading
      capacity of dust.

  o Gravitational instability: If dust:gas surface density ratio is
    increased ~10x from initial, then dust becomes unstable against
    local collapse (Youdin & Shu 2002).

  o Youdin & Shu used radial drift to trigger this instability (dust
    is dragged inward via differential gas/dust rotation, while gas
    stays radially fixed).  We haven't included this yet, but our
    two processes will add, in general.

Results:
  o In the Sekiya vertical profiles, gas is preferentially removed
    over dust.  The dust:gas surface density increases beyond critical.
    Planetesimals can then form via instability.

Implications:
  o Orion disks were thought to have been rapidly destroyed, and thus
    poor sites to form planets.  However, photo-evaporation may actually
    *encourage* planetesimal growth, rather than prevent it.

  o Few 10^5 yr timescales at 1-20 AU [checking this out].

  o Other instabilities and processes may also occur (e.g., Boss gas
    instability; Youdin & Shu 2002; Ward radial drift instability 2004)

  o Grain growth makes particles harder to lose -> instability easier
    to achieve.

Additional processes to look at:
  o Grain growth: enhances process
  o Photo-sputtering of ices: removes Kuiper Belt
  o Production / loss of organics: do Orion disks retain organic
    content > solar?
  o Are some disks in Orion (e.g., 114-426) already depleted of
    gases, leaving only dust subdisk?
  o Grain charging and partially ionized outflow
  o Photo-evaporation from central star

Throop & Bally ApJL in progress.