UV-Induced Planetesimal Formation in Disks: From Proplyds to Planetesimals

1. 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

2. OMC 1 Outflow(H2 t = 500 yr) BNKL Trapezium (L = 105 Lo t << 105 yr) (L = 105 Lo t < 105 yr) Hundreds of Proplyds OMC1-S (L = 104 Lo , t < 105 yr)

3. 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

4. Anatomy of a proplyd

6. HH 508 HST4

7. Microjet from a proplyd: HH 508 1Ori B: 4 low-mass companions! (Shuping et al. 2006)

8. Proplyd photo-ablation flows: dM/dt ~ 10-7 Mo yr -1 HST4 (LV 6), LV 1 (Shuping et al. 2006) Br  HeI Position (mas)

9. (Williams et al. 2005) Mdisk ~ 0.003 to 0.02 Mo HH 514 HST 2

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

11. UV photo-ablation of disks & planet formation: Smith, Bally, Licht, Walawender 05 d253-535 in M43

12. HST 10, 16, 17 1” = 500 AU HST 16 200 AU diameter HST 10 0.1 pc to O7 star 0.15 pc to O9.5 star HST 17 Bally et al. 98

13. 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)

14. Growing grains: Orion 114-426 (Throop et al. 2001)

15. Growing grains:Si 10 m feature(Shuping et al. 2006)

16. The Beehive proplyd; HH 240 irradiated jet Bally et al. 2005

17. d181-825 “Beehive” proplydChandra COUP Jet Star 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) 8; 10 20 cm 1280 AU

18. 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) d181-825 “Beehive” proplyd

19. 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 Throop & Bally 2007

21. 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.

22. 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!

23. 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

24. Sedimentation + Photo-Evaporation Self-irradiation Gap opened at r = GM/c2 Viscous evolution + Radial migration moves dust into gap Large dust:gas => planetesimals

25. Photoevaporation Off

26. Photoevaporation On Photoevaporation On

27. Photoevaporation On Photoevaporation On GI unstable region

28. 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

29. 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

30. The End

32. 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.

33. OMC 1 Outflow (H2 t = 500 yr) Orion Nebula BNKL Trapezium (L = 105 Lo t << 105 yr) (L = 105 Lo t < 105 yr) OMC1-S (L = 104 Lo , t < 105 yr)