Can photo evaporation trigger planetesimal formation
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Can Photo-Evaporation Trigger Planetesimal Formation?. Henry Throop John Bally SWRI Univ.Colorado / CASA DPS 12-Oct-2004. Orion Nebula. Photo-evaporation (PE) by external O/B stars removes disks on 10 5 -10 6 yr timescales.

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Can photo evaporation trigger planetesimal formation l.jpg
Can Photo-Evaporation Trigger Planetesimal Formation?

Henry Throop John Bally

SWRI Univ.Colorado / CASA

DPS 12-Oct-2004

Orion nebula l.jpg
Orion Nebula

Photo-evaporation (PE) by external O/B stars removes disks on 105-106 yr timescales.

OB associations like Orion are rare but large – majority of star formation in the galaxy probably occurs in regions like this.

Photo-evaporation by extrnal O and B stars

4 O/B stars, UV-bright, 105 solar luminosities

2000 solar-type stars with disks

Photo evaporation and gravitational instability l.jpg
Photo-Evaporation and Gravitational Instability

  • Problem: Planetary formation models explain grain growth on small sizes (microns) and large (km) but intermediate region is challenging.

  • Youdin & Shu (2002) find that enhancing dust:gas surface density ratio by 10x in settled disk allows gravitational instability of dust grains to form km-scale planetesimals.

  • Can photo-evaporation (PE) encourage this enhancement, and thus allow the rapid formation of planetesimals?

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Model of Photoevaporation

  • Our model is the first to examine dust and gas separately during photo-evaporation, and is the first to incorporate GI into photo-evaporation calculations.

  • 2D code which tracks gas, ice, dust around solar-mass star.

  • Processes:

    • Grain growth (microns-cm)

    • Vertical settling

    • Photo-evaporation

    • Dust gravitational instability

  • Photo-evaporation heats gas and removes from top down and outside in

    • Gas is preferentially removed

    • Dust in midplane is shielded and retained

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Effect of Sedimentation on PE

  • Case I: Dust and gas well-mixed (no settling); 0.02 Msol

  • Model result: Disk is evaporated inward to 2 AU after 105 yr

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Effect of Sedimentation on PE

  • Case I: Dust and gas well-mixed (no settling); 0.02 Msol

  • Model result: Disk is evaporated inward to 2 AU after 105 yr

Hashed: critical density for GI

  • Case II: Dust grows and settles to midplane

  • Model result: Disk is evaporated inward, but leaves significant amount of dust at midplane (40 Earth masses outside 2 AU)

  • Dust has sufficient surface density to collapse via GI

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Modeling Results


  • Photo-evaporation can sufficiently deplete gas in 2-100 AU region to allow remaining dust to collapse via GI.

  • Gas depletion depends on a sufficient quiescent period ~ 105 yr for grains to settle before photo-evaporation begins.

  • Disk settling depends on low global turbulence, and is not assured.

0 yr: Low-mass star with disk forms

105 yr: Grains grow and settle

105 yr: O stars turn on

106 yr: Gas disk is lost, allowing planetesimals to form from disk

Conclusions l.jpg

  • Photo-evaporation may not be so hazardous to planet formation after all! In this model, it actually encourages planetesimal formation.

  • Did Solar System form near an OB association?

    • Rapid gas dispersal may not allow for formation of giant planets.

    • Final distribution of rock, ice, gas may depend strongly on time of O stars to turn on, and speed of disk dispersal.

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Star Formation and Photo-Evaporation (PE)

  • The majority of low-mass stars in the galaxy form near OB associations, not in dark clouds (ie, Orion is the model, not Taurus)

  • PE by FUV and EUV photons removes disks from outside edge inward, on 106 yr timescales.

  • PE is caused by external O and B stars – not the central star.

  • In Orion, typical low-mass star age is 106 yr, but O star age is 104 yr – disks have had a quiescent period before PE begins.

Implications l.jpg

  • Coagulation models of grain growth have difficulty in the cm-km regime. This model allows for that stage.

  • Model explains how planets could be common, in spite of fact that majority of low-mass stars form near OB associations.