Prompt UV-Induced Planetesimal Formation in disks

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					Prompt UV-Induced Planetesimal Formation In Disks: Proplyds to Planetesimals
John Bally1 Henry Throop2
Mark Kassis3 Mark Morris4 Ralph Shuping5
1University

of Colorado, Boulder 2SouthWest Research Institute, Boulder 3Keck Observatory 4UCLA 5NASA, Ames

OMC 1 Outflow(H2 t = 500 yr)

BNKL Trapezium (L = 105 Lo t < 105 yr ) Hundreds of Proplyds OMC1-S (L = 104 Lo , t < 105 yr)

(L = 105 Lo t << 105 yr)

Main Point:
• Problem: How do grains grow from d < 100 cm (gravity un-important) to d ~ 1 - 100 km (gravity dominated)
c.f. Weidenschilling, S. J., & Cuzzi, J. N. 1993, PP3

- Grains not “sticky” - Collisions tend to fragment & bounce - Head-wind => radial drift of solids => fast growth • Grain growth + sedimentation + UV-photoablation  Mass-loss from disk is metal depleted  Retained disk becomes metal-enriched Gravitational instability => planetesimals
Youdin, A. N., & Shu, F. H. 2002, ApJ, 580, 494 Throop, H. B. & Bally, J, 2005, ApJ, 623, L149

Anatomy of a proplyd

HH 508

HST4

Microjet from a proplyd: HH 508

1Ori B: 4 low-mass companions!
(Shuping et al. 2006)

Proplyd photo-ablation flows:
HST4 (LV 6), LV 1

dM/dt ~ 10-7 Mo yr -1

(Shuping et al. 2006)
Br  HeI

Position (mas)

(Williams et al. 2005)

Mdisk ~ 0.003 to 0.02 Mo

HH 514

HST 2

HH 514 micro-jet in Orion: Ha, [HII] Nebular Ha Jet

(HST/STIS)

Counter Jet

HST 2

UV photo-ablation of disks & planet formation:

Smith, Bally, Licht, Walawender 05

d253-535 in M43

HST 10, 16, 17
HST 16 200 AU diameter

1” = 500 AU

HST 10

0.1 pc to O7 star 0.15 pc to O9.5 star HST 17 Bally et al. 98

Keck AO IR

HST H-alpha

2.12 m H2 0.63 m [OI] => Soft UV photo-heating of disk surface (Kassis et al. 2007)

Growing grains: Orion 114-426

(Throop et al. 2001)

Growing grains:

Si 10 m feature (Shuping et al. 2006)

The Beehive proplyd;

HH 240 irradiated jet

Bally et al. 2005

d181-825 “Beehive” proplyd

Chandra COUP

Jet

Star

8 ; 10

1280 AU

20 cm

kT ~ 0.57 keV & 3.55 keV NH ~ 8 x 1020 cm-2 (soft) NH ~ 6 x 1022 cm-2 (hard)

(Kastner et al. 2005, ApJS, 160, 511)

d181-825 “Beehive” proplyd

X-ray absorption: NH ~ 8 x1020 cm-2 But, foreground AV ~ 1 mag ! H-alpha:

ne(rI) = 2.6 x 104 cm-3 dM/dt = 2.8 x 10-7 Mo yr-1
Neutral Column: (from 50 AU, V = 3 km/s) NH(RI) = 2.2 x 1021 V3-1 r50-1

Photo-ablation flow metal depleted!
(Kastner et al. 2005, ApJS, 160, 511)

N-Body Dense-Cluster Simulations
NBODY6 code (Aarseth 2003) Stars: • N=1000 • Mstar = 500 Mo • Salpeter IMF • R0 = 0.5 pc • O6 star fixed at center • Gas: • Mgas = 500 o • R0 = 0.5 pc • Dispersal timescale ~2 Myr

QuickTime™ and a Sorenson Video 3 decompressor are needed to see this picture.

Throop & Bally 2007

QuickTime™ and a Sorenson Video 3 decompressor are needed to see this picture.

Flux History, Typical 1 Mo Star

• • • • •

Flux varies by 1000x Peak flux approaches 107 G0. Intense close encounters with core. There is no `typical UV flux.’ Impulsive processing.

Grain growth + Sedimentation + UV => km-sized planetesimals
Most stars form in clusters: A, B, O stars have strong (soft) UV Orbits => Stochastic external UV Self-irradiation (by accretion flows) Massive star death: blue supergiants, SN increase soft UV dose.

UV may promote planetesimal growth!

Photo-Evaporation Triggered Instability
• Gravitational collapse of dust in disk can occur if sufficiently low gas:dust ratio (Sekiya 1997; Youdin & Shu 2002)  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

Sedimentation + Photo-Evaporation

Self-irradiation

Gap opened at r = GM/c2 Viscous evolution + Radial migration moves dust into gap

Large dust:gas => planetesimals

Photoevaporation Off

Photoevaporation On

Photoevaporation On

Photoevaporation On

Photoevaporation On

GI unstable region

UV => Fast Growth of Planetesimals:
Grain growth => Solids settle to mid-plane UV => Remove dust depleted gas => High metallicity in mid-plane Gravity => Instability => 1 - 100 km planetesimals

- Fast Formation of 1 to 100 km planetesimals
Throop & Bally et al. 05

Conclusions
• UV + grain growth + sedimentation => Gravitational instability => planetesimals
• UV irradiation is stochastic: Orbital motion of low-mass stars Evolution of massive stars (3 - 40 Myr) MS => (blue/red) supergiant => SN Planets born as massive stars die

The End

UV Radiation may Trigger Planetissimal Formation!
UV radiation may not be hazardous for planet formation!
Throop & Bally (2005, ApJ, 623, L149) show that in evolved disks in which grains have grown and sedimented to the disk-midplane BEFORE being irradiated by an external UV source, photo-ablation can actually promote the growth of planetesimals! In a sedimented disk, the gas:dust ratio at the disk surface can be larger than in the ISM. Thus, when UV radiation heats and ablates the disk, it removes dust depleted material. This process leaves the surviving portion of the disk metal enriched. Increased metallicity and grais growth can lead to the prompt formation of kilometer-scale planetesimals by gravitational instability on a time much shorter than the radial drift time-scale for centimeter to meter-sized particles. Some indirect evidence for this process has been found in Chandra X-ray studies of Orion’s proplyds (see Kastner et al. 2005, ApJS, 160, 511). The X-ray extinction (determined from X-ray spectra) to the central stars of several of Orion’s large proplyds was fond to be considerably lower than what is inferred from the hydrogen column density to the star (derived from the measured radii of the proplyd ionization fronts). In retrospect, the fiducial UV penetration depth derived from the analysis of HST images of proplyds that was derived by Johnstone, Hollenbach, & Bally (1998, ApJ, 499, 758) also is consistent with a factor of 3 to 5 times lower dust:gas ratio than found in the generatl ISM. Thus, contrary to being hazardous, UV radiation fields may actually promote the first stages of planet formation.

Orion Nebula

OMC 1 Outflow (H2 t = 500 yr) BNKL (L = 105 Lo t << 105 yr)

Trapezium (L = 105 Lo t < 105 yr )

OMC1-S (L = 104 Lo , t < 105 yr)


				
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posted:8/4/2008
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