1 University of Colorado, Boulder 2 SouthWest Research Institute, Boulder 3 Keck Observatory 4 UCLA 5 NASA, Ames Prompt UV-Induced Prompt UV-Induced Planetesimal Formation In Disks: Proplyds to Planetesimals John Bally 1 Henry Throop 2 Mark Kassis 3 Mark Morris 4 Ralph Shuping 5

 Trapezium (L = 10 5 L o t < 10 5 yr ) OMC 1 Outflow    t = 500 yr) (L = 10 5 L o t << 10 5 yr)  NKL  OMC1-S (L = 10 4 L o, t < 10 5 yr) Hundreds of Proplyds

Main Point: Main Point: Problem: How do grains grow from d < 100 cm (gravity un-important) to d ~ 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  1 Ori B: 4 low-mass companions! (Shuping et al. 2006)

Proplyd photo-ablation flows: dM/dt ~ M o yr -1 HST4 (LV 6), LV 1 (Shuping et al. 2006) Position (mas) Br  HeI

(Williams et al. 2005) M disk ~ to 0.02 M o HH 514 HST 2

HH 514 micro-jet in Orion: H , [HII] (HST/STIS) Jet Counter Jet HST 2 Nebular H 

d in M43 UV photo-ablation of disks & planet formation: Smith, Bally, Licht, Walawender 05

HST 10 HST 17 HST 10, 16, 17 1” = 500 AU Bally et al. 98 HST AU diameter HST AU diameter 0.1 pc to O7 star 0.15 pc to O9.5 star 0.1 pc to O7 star 0.15 pc to O9.5 star

Keck AO IR HST H-alpha (Kassis et al. 2007) 2.12  m H  m [OI] => Soft UV photo-heating of disk surface

Growing grains : Orion (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

8 ; cm kT ~ 0.57 keV & 3.55 keV N H ~ 8 x cm -2 (soft) N H ~ 6 x cm -2 (hard) (Kastner et al. 2005, ApJS, 160, 511) d “Beehive” proplyd Chandra COUP Jet Star 1280 AU

d “Beehive” proplyd X-ray absorption: N H ~ 8 x10 20 cm -2 But, foreground A V ~ 1 mag ! H-alpha: n e (r I ) = 2.6 x 10 4 cm -3 dM/dt = 2.8 x M o yr -1 Neutral Column: (from 50 AU, V = 3 km/s) N H (R I ) = 2.2 x V 3 -1 r  Photo-ablation flow metal depleted! (Kastner et al. 2005, ApJS, 160, 511)

N-Body Dense-Cluster Simulations NBODY6 code (Aarseth 2003) Stars: N=1000 M star = 500 M o Salpeter IMF R 0 = 0.5 pc O6 star fixed at center Gas: M gas = 500 o R 0 = 0.5 pc Dispersal timescale ~2 Myr Throop & Bally 2007

Flux History, Typical 1 M o Star Flux varies by 1000x Peak flux approaches 10 7 G 0. Intense close encounters with core. There is no `typical UV flux.’ Impulsive processing.

Grain growth + Sedimentation + UV Grain growth + Sedimentation + UV => km-sized planetesimals => 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/c 2 Viscous evolution + Radial migration moves dust into gap Large dust:gas => planetesimals

Photoevaporation Off

Photoevaporation On

GI unstable region Photoevaporation On

UV => Fast Growth of Planetesimals: Grain growth => Solids settle to mid-plane UV => Remove dust depleted gas => High metallicity in mid-plane Gravity => Instability => km planetesimals - Fast Formation of 1 to 100 km planetesimals Throop & Bally et al. 05

Conclusions Conclusions UV + grain growth + sedimentation => Gravitational instability => planetesimals UV irradiation is stochastic: Orbital motion of low-mass stars Evolution of massive stars ( 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.

 NKL  Trapezium  OMC1-S (L = 10 5 L o t << 10 5 yr) (L = 10 4 L o, t < 10 5 yr) (L = 10 5 L o t < 10 5 yr ) OMC 1 Outflow   t = 500 yr) Orion Nebula