1. UV radiation from massive stars in Orion may promote the growth of planetesimals by removing dust-depleted gas from protoplanetary disks via photoevaporation. 2. As grains grow and sediment to the disk midplane, the gas-to-dust ratio at the surface increases. UV ablation then leaves behind a metal-enriched midplane. 3. The higher metallicity allows gravitational instability to rapidly form kilometer-scale planetesimals on shorter timescales than radial drift of solids. Some evidence supports this process occurring in Orion's proplyds.

1. 1University of Colorado, Boulder
2SouthWest Research Institute, Boulder
3Keck Observatory
4UCLA
5NASA, Ames
Prompt UV-Induced
Planetesimal Formation In Disks:
Proplyds to Planetesimals
John Bally1
Henry Throop2
Mark Kassis3
Mark Morris4
Ralph Shuping5

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

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)
Position
(mas)
Br  HeI

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)
Jet
Counter Jet
HST 2
Nebular Ha

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

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

13. Keck AO IR HST H-alpha
(Kassis et al. 2007)
2.12 m H2 0.63 m [OI]
=> Soft UV photo-heating of disk surface

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. 8 ; 10
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 Chandra COUP
Jet Star
1280 AU

18. 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)

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
GI unstable region
Photoevaporation On

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