Astrochemical Models Eric Herbst Departments of Chemistry and Astronomy University of Virginia
Chemical Models Gas-phase reactions 1000 s of reactions Grain-surface reactions Abundances, columns, spectra uncertainties Observations Physical conditions, history Limited by chemical knowledge
Uncertainty &Sensitivity Methods Help to determine which reactions to study in the lab or theoretically Wakelam et al. 2010
Sources Modeled Diffuse clouds Cold cores (10 K) Pre-stellar cores Hot Cores (200 K) Outflows Shocks Protoplanetary disks PDR s; XDR s IRDC s Circumstellar envelopes Protoplanetary nebulae Planetary nebulae AGN disks Earliest clouds Exo-planetary atmospheres
Some Types of Networks/Models Low temperature (< 10-100 K) gas-phase + H 2 formation (UMIST; osu-uva; KIDA) Gas-grain (10 K 300 K) to handle cold and hot cores + warm-up High temperature (300-800 K) gas phase + H 2 formation (Harada) PDR (high external photon flux) (Meudon) Shock network (hot and brief)
Methods of Solution Coupled deterministic rate equations Stochastic methods mainly for dust chemistry with small numbers of reactive species per dust particle (discrete + fluctuations): master equation and Monte Carlo simulation Macroscopic vs Microscopic Coupling of gas and dust chemistries
The Taurus Dark Cloud. n = 104 cm-3 T = 10 K Exotic & unsaturated molecules in gas. Common species in ice mantles. Low temperature gas-phase and gas-grain models.
Chemical Processes in Cold Cores T=10 K, n = 10(4) cm-3 cold gas-phase chemistry consisting of exothermic reactions without activation energy ion-molecule chemistry plus some neutral reactions involving radicals. Source of ionization: cosmic rays (rate ζ) surface chemistry converts H into H 2 and builds up mantles of ices
o/p ratio? T = 10-20 K Experiments, calculations
Formation of Ices In Cold Dense Clouds H Non-thermal desorption CO CH 3 OH H H 2 O OH O Build-up to 100 monolayers of ice Sapporo, Leiden, Paris
Low temperature growth to COMs in ice:!" $!" # & "%!)"# # #!""# "# "!# ( "# $# # $!" # &! #!%" ' #'!" # & '# $!%"!# (!# $ "# $#!# (!#" $#!# $!" # & #!%!%" " & "%!)!#%" #!#"!#"!!!# ( *!# $ + $ "# $#!# (!# $!#" $#!# $!#!#" $# #! $!#" # &!!)!#%"! & #! #!%",-,*! $ # $ +!" Possible extension to larger molecules by atomic addition reactions at 10 K (Charnley, private comm. See also Hasegawa et al. 1992 for organo-nitrogen chemistry.) Little laboratory confirmation except for methanol and ethanol production. # $# $# '# $!#" #"!# $!#",-,*! $ #. +!" $# $# $# '# $!# $ "# #"!# $!# $ "#,-,*! $ #. +!#"#
Orion KL; Spectra of COM s
Hot Core Chemistry: gas-grain model Desorption nonthermal and thermal 10 20 K Cold phase +accretion + surface chemistry (Hrich) leads to saturated ices Surface chemistry (H-poor) involving radicals (formed by photolysis) 100-300 K Saturated organic molecules such as ethers, alcohols (COM s) Garrod et al. (2008); Laas et al. (2011)
High Temperature Network Should work up to 800 K (limited mainly by formation of H2 on dust) Classes of reactions added/improved: 1. ion-polar neutral reactions 2. Reactions with barriers, especially involving H2. 3. Reverse endothermic reactions 4. Proton and charge exchange
Laboratory Experiments Needed Gas Phase reactions: radiative association and attachment; ion-ion recombination; high temperature reactions Surface/Ice reactions: individual reactions involving atoms and molecules, and involving pairs of radicals; non-thermal desorption, photochemistry; bulk diffusion; high temperature reactions Theory: molecular dynamics on ices; non-thermal desorption mechanisms; improved Monte Carlo methods for surface chemistry
NGC 1068 (20 million pc distant) AGN disk around black hole; molecules detected at high T (up to 1000 K) X-ray emission in red First use of high T gas network
Harada et al. 2010; 2012 (under review) HCN in a dense, thin AGN Disk (vertical slice)
New ALMA Data
Orion Outflow Source: Strong PDR Rimmer and Herbst, in prep. Goal is to explain high abundances of OH + and H2O + reported by Gupta et al. 2010 with Herschel HIFI (far- infrared absorption) Ultra-high FUV field as well as large X-ray flux in gas-grain time-dependent PDR model. Conditions: 400 K (gas) and 95 K(dust) + little extinction lead to high abundances of electrons and [H] [H 2 ]. Gas-phase chemistry
Detection of COMs in Cold Cores TMC-1 etc methanol, acetaldehyde L1689B et al. Bacmann et al.: acetaldehyde, methyl formate, dimethyl ether; 7 new sources B1-b Cernicharo et al. Also methoxy radical. Small fractional abundances (10(-9) 10(-11)) How produced? Gas-phase? Ices? Radical-radical chemistry does not take place below about 20-25 K.
I. Production of Complex Saturated Molecules in Cold Cores: Some Suggestions 1. Cosmic rays can raise the temperature of a normal dust particle to 70 K (Aikawa et al.) 2. Efficient non-thermal desorption followed by gas-phase processes efficient at 10 K (Vasyunin). 3. Consider gas-grain chemistry occurring as a translucent cloud (T > 20 K) collapses into dense cold core rather than only during the cold core stage (Vasyunin).
Production of Complex Saturated Molecules in Cold Cores (cont.) 4. Low temperature surface processes of Charnley and Hasegawa et al. (atomic addition) followed by nonthermal desorption 5. Low-temperature photochemistry detected in laboratory (8K, Cernicharo et al. 2012) on photolyzed ices may be due to non-thermal diffusion, known as the hot atom mechanism. Models to be run (Oberg, Herbst).
Acknowledgments Sources of Funding: NSF: astronomy, chemistry, ALMA NASA: astrobiology, Herschel Recent group members: Paul Rimmer, Nanase Harada, Donghui Quan, Yezhe Pei, George Hassel, Anton Vasyunin, Tatiana Vasyunina, Qiang Chang, Dawn Graninger, Chris Shingledecker
Diffuse Clouds Detected with Herschel in THz Region (absorption) CH+,CH, CCH, HF, HCl, OH+, H2O+, H3O+, H2O, NH, NH2, NH3, H2Cl+, HCl+ OPR H 2 O + = 4.8 towards galactic center X Warm dense material around sources Cool and relatively diffuse clouds in spiral arms can absorb continuua from warmer distant objects in the THz region T = 50-100 K; n = 10-10 3 cm -3 H H 2
FORMATION OF GASEOUS WATER AND HYDROXYL H 2 + COSMIC RAYS H 2 + + e H 2 + + H 2 H 3 + + H H 3 + + O OH + + H 2 OH n + + H 2 OH n+1 + + H H 3 O + + e H 2 O + H; OH + 2H, etc + longer pathways to unsaturated organic species At higher temperatures, fewer constraints on reactions.
Extended Chemistry H2 + CRP H2+ + e + CRP H2+ + H2 H3+ + H O + H3+ OH+ + H2 OH+ + H2 H2O+ + H H2O+ + e OH + H; O + H2 H2O+ + H2 H3O+ + H H3O+ + e H2O + H; OH + H2, OH + 2H H2O + hν H2O+ + e OH + hν OH+ + e H2 + Xrays H..
Cold Core Low-mass Star Formation Isothermal collapse Pre-stellar Core Exotic chemistry T = 10 K n = 10 4 cm -3 Star + Disk Protostar adiabatic Cold envelope hot core exoplanets 100-300 K Terrestrial chemistry