1. How do habitable planets acquire carbon? PAHs in Protoplanetary Disks
CU: March 19, 2008.
How do habitable planets acquire carbon? PAHs in Protoplanetary Disks
Dr. Monika Kress
Dept. of Physics and Astronomy
San Jose State University
Same question as for water.

2. Xander Tielens, NASA Ames
Michael Frenklach, UC Berkeley
Sean Raymond, CU Boulder
Vikki Meadows, UW Seattle

3. Origins & Astrobiology: Interstellar medium --> planets --> life?

4. What are PAHs?
PAHs in astrophysical environments
PAHs in meteorites
Protoplanetary disks
Model results: the soot line
Conclusions

5. What are PAHs? Strongly bound pi-bonded cyclic hydrocarbons
Prominent nonthermal emission features
Extremely stable:
oxidizing/reducing conditions
high temperatures
UV radiation
Wide range of environments:
the interstellar medium
evolved stars
planetary atmospheres
Meteorites
interplanetary dust particles
other galaxies
What are PAHs?

6. PAH and related compounds
Cyclopropene
Naphthalene
Phenanthrene
Benzene
Acenaphthene
Pyrene

7. PAHs in astrophysical environments

9. Tielens 2007; Peeters et al 2002

10. Disks around H Ae Be and T Tauri stars
Geers et al, A&A 2008
Disks around H Ae Be and T Tauri stars

11. PAHs are modified in disk environments
Boersma et al 2007
TY CrA (Class A)
HD (Class A-B)
HD (Class B)
Wavelength (from 6-10 m)

14. Composition of primitive meteorites

15. Types of meteorites
Beatty et al “The New Solar System”

17. TL,CM,CI,CR
Wt% CK LL H L CV CO
R,EL,EH: no H2O
Wt%

18. H2O and C in primitive meteorites
CI, CM, TL carbonaceous chondrites
~ 4% wt. carbon (bulk)
~10% water of hydration
Ordinary chondrites:
Carbon ~ 0.25%
H2O ~ 0.8%
~Same C/H2O as CI, CM, TL
CV carbonaceous chondrites
~ 0.5% wt. carbon
~0.15% water of hydration
C/H2O ~10x higher than other CC’s
Enstatite chondrites:
Carbon ~ 0.4%
H2O ~ 0

19. Carbonaceous chondrites contain abundant aromatic C

20. Cody & Alexander 2005

21. The parent bodies of meteorites: Asteroids

22. Reflectance spectrum of asteroid 832 Karin compared to L6 chondrite
S(IV)-type asteroid 584 Semiramis
L6 ordinary chondrite

23. Protoplanetary Disks and the Soot line

26. asteroids < 2.5 AU show no OH features in spectra

27. Anatomy of a planet-forming disk
The “snow line”: ~2.5 AU during meteorite formation

28. Model results: the soot line
Is it due to phase transition, or chemical reactions?

29. PAH Chemistry
Well studied under combustion conditions (P ~ 1atm, T ~ 1000 K).
Combustion kinetics model developed by M. Frenklach (UC Berkeley) for sooting flames.
Considers only H, C and O
The reaction mechanism is tailored to the lower P & T of disk.
Input abundances = cometary
Nebular conditions (C/O <1) => PAHs do not form

30. Cyclopropene AC3H4 Benzene A1 Naphthalene A2 Acenaphthene A2R5
Some important PAHs in the model
Cyclopropene
AC3H4
Benzene A1
Naphthalene A2
Acenaphthene
A2R5
Phenanthrene A3
Pyrene A4

31. T = 800 K: PAHs survive over nebular timescales

32. T = 950 K: PAHs are reacting over nebular timescales

33. Input: Pyrene, water, CO and H2 only. T = 1100 K
Lahuis (2007)
C2H2 ~ 10-6

34. 1100 K
1000 K
900 K

35. Meteorites agglomerated between 1-10 Myr (Bell et al 1997).
time
The snow line is due to a phase transition.
~106 yr
The soot line is also due to a phase transition!
Temperature profile for disks from Bell et al 1997.

36. Conclusions
High temperatures in the presence of water (OH, H) destroy PAHs rather than form them
=> presolar heritage
PAHs are destroyed by OH in the solar nebula at K (~2 AU at years, or from 2-4 AU at 105 years)
The “soot line” occurs at ~ 950 K: over nebular timescales PAHs will be destroyed.
=> explains bulk meteorite compositions
Chondritic PAHs may be an important indicator of the conditions characterizing
nebula temperature history
mixing processes
meteorite agglomeration

37. Further studies
Use PAH model and new generation of disk models & observations
To constrain the original composition of primitive meteorites
To isolate which meteoritic constituents are presolar and which are likely due to parent-body processing
To determine the link between the ISM and the compounds arriving on early Earth

38. VPL science
Use PAH model, and new generation of disk models, planet formation models, & observations
Given variations in disk evolution (i.e. how fast does it cool and disperse) and the luminosity of the star, exoterrestrial planets may have >> earth abundance of C and water, or much less.
What is the primordial composition (before heat and aqueous alteration) of meteorites that fell to early Earth? What fell when, and what was it made of?