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. Xander Tielens, NASA Ames
Michael Frenklach, UC Berkeley Sean Raymond, CU Boulder Vikki Meadows, UW Seattle Origins & Astrobiology: Interstellar medium --> planets --> life?
What are PAHs? PAHs in astrophysical environments PAHs in meteorites Protoplanetary disks Model results: the soot line Conclusions 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? PAH and related compounds
Cyclopropene Naphthalene Phenanthrene Benzene Acenaphthene Pyrene PAHs in astrophysical environments Tielens 2007; Peeters et al 2002 Disks around H Ae Be and T Tauri stars
Geers et al, A&A 2008 Disks around H Ae Be and T Tauri stars 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) Composition of primitive meteorites Types of meteorites Beatty et al The New Solar System TL,CM,CI,CR Wt% CK LL H L CV CO R,EL,EH: no H2O Wt% 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 CCs Enstatite chondrites: Carbon ~ 0.4% H2O ~ 0 Carbonaceous chondrites contain abundant aromatic C
Cody & Alexander 2005 The parent bodies of meteorites: Asteroids
Reflectance spectrum of asteroid 832 Karin compared to L6 chondrite
S(IV)-type asteroid 584 Semiramis L6 ordinary chondrite Protoplanetary Disks and the Soot line asteroids < 2.5 AU show no OH features in spectra Anatomy of a planet-forming disk
The snow line: ~2.5 AU during meteorite formation Model results: the soot line
Is it due to phase transition, or chemical reactions? PAH Chemistry Well studied under combustion conditions (P ~ 1atm, T ~ 1000 K). Combustion kinetics model developed byM. 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 PAHs do not form 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 T = 800 K: PAHs survive over nebular timescales T = 950 K: PAHs are reacting over nebular timescales Input: Pyrene, water, CO and H2 only. T = 1100 K
Lahuis (2007) C2H2 ~ 10-6 1100 K 1000 K 900 K 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. 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 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 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?