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Henry ThroopDepartment of Space Studies

Southwest Research Institute (SwRI)Boulder, Colorado

John BallyUniversity of Colorado

DPS Pasadena, October 2006

Gap formation in Young Circumstellar Disks due to Photoevaporation from

the Central star

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Photoevaporation from Central Star• Historically thought unimportant due to low UV flux from central

star and poor line-of-sight geometry.

• Hollenbach et al (1994) revived idea with indirect flux reflected off disk’s corona.

• Matsuyama et al (2003) found that PE would create gap, but only after disk is depleted by 10 Myr+ of viscous evolution.

• Alexander et al (2005, 2006) STIS measurements found photospheric UV flux from young solar-mass stars. Showed PE could make gaps, using direct illumination in flared disks.

• We add dust transport to latest PE model.

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Dust settles to midplane rapidly -- cm sizes in 105 yr

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Photoevaporation: UV flux heats gas to 1500K, heating and dissociating H2 and allowing it to leave disk via Jeans escape

ddt ~ R-5/2, for R > RG

ddt = 0, for R < RG

Gravitational radius RG ~ 2 AU

RG ~ 2 AU

RG RG

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• Photoevaporation creates gap outward of RG

• Gas is removed but dust is retained– Dust is at midplane and inaccessible to PE– Dust has grown large enough to be retained

RG RG

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Viscous evolution fills in gap with new dust, gas

RG RG

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Process continues: Gas is removed and dust is transported inward

RG RG

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Final state:Gas depleted in gapDust enhanced in gap

Dust:Gas ratio D/G increased to point that gravitational instability can rapidly form planetesimals (Skkiya 1997; Youdin & Shu 2002)

RG RG

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Disk Model• Disk

– MMSN disk surrounding solar-mass star– Flared disk– Viscous evolution, = 0.01 (e.g., Pringle 1981)

• Photoevaporation– Central star flux 1041 ionizing photons/sec– Alexander et al 2005; Hollenbach et al 1994.

• Dust Transport– Dust settles to midplane rapidly (< Myr)– Dust grains are radially transported viscously– Radial transport stops when gas is depleted– Dust:gas ratio 1:100

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QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

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Photoevaporation Off

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Photoevaporation OnPhotoevaporation On

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Photoevaporation OnPhotoevaporation On

GI unstable region

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Conclusions and Implications• Gas disk in region ~2-10 AU is depleted on Myr

timescales– Faster than viscous timescales, but compatible with

observations of disk lifetimes– Gap formation will be slower for disks > 1 MMSN

• Dust, planetesimals concentrated at gap• Planetesimals can be formed via GI, if not formed

already• May speed formation of gas giant cores, but limits

envelope accretion timescales• In dense clusters (OB associations), effects of PE are

additive!– External star: Removes disk from outside in (Johnstone et al

1998; Throop & Bally 2005)– Central star: Removes disks from inside out

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Planetesimal Formation• Two methods:

– Sticking– Gravitational Instability

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Photo-Evaporation Triggered Instability• Gravitational collapse of dust

in disk can occur if sufficiently low gas:dust ratio (Sekiya 1997; Youdin & Shu 2004)

g / d < 10 (I.e., reduction by 10x of

original gas mass)

• PE removes gas and leaves most dust– Grain growth and settling

promote this further

• Dust disk collapse provides a rapid path to planetesimal formation, without requiring particle sticking.

Throop & Bally 2005