Astrochemical Models

Eric Herbst

Departments of Chemistry and Astronomy

University of Virginia

Chemical Models

Grain-surface reactions

Abundances, columns, spectra

uncertainties

Observations

Physical conditions, history

Limited by chemical knowledge

Gas-phase reactions

1000’s of reactions

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 + H2 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 + H2 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

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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 H2 and builds

up mantles of ices

Experiments, calculations

T = 10-20 K

o/p ratio?

Formation of Ices In Cold Dense Clouds

H O

OH

H H2O

CO

Build-up to 100 monolayers of ice

CH3OH

Non-thermal desorption

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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.

Low temperature growth to COMs in ice:

Orion KL; Spectra of COM’s

Saturated organic molecules such as ethers, alcohols (COM’s)

Hot Core Chemistry: gas-grain model

Cold phase +accretion + surface chemistry (H-rich) leads to saturated ices

Desorption – non-thermal and thermal

Surface chemistry (H-poor) involving radicals (formed by photolysis)

10 – 20 K

100-300 K

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)

X-ray emission in red First use of high T gas network

AGN disk around black hole; molecules detected at high T (up to 1000 K)

HCN in a dense, thin AGN Disk (vertical slice)

Harada et al. 2010; 2012 (under review)

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] ≈ [H2]. 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 non-thermal 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

X

Cool and relatively diffuse clouds in spiral arms can absorb continuua from warmer distant objects in the THz region

T = 50-100 K; n = 10-103 cm-3

H ≥ H2

CH+,CH, CCH, HF, HCl, OH+, H2O+, H3O+, H2O, NH, NH2, NH3, H2Cl+, HCl+

Diffuse Clouds Detected with Herschel in THz Region (absorption)

OPR H2O+ = 4.8 towards galactic center

Warm dense material around sources

FORMATION OF GASEOUS WATER AND HYDROXYL

H2 + COSMIC RAYS H2+ + e

H2+ + H2 H3

+ + H H3

+ + O OH+ + H2 OHn

+ + H2 OHn+1+ + H

H3O+ + e H2O + 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 Pre-stellar Core

Protostar Star + Disk

T = 10 K n = 104 cm-3

Isothermal collapse

adiabatic

hot core

100-300 K

Cold envelope

Low-mass Star Formation

Exotic chemistry

exoplanets Terrestrial chemistry