A NEW INTERSTELLAR MODEL FOR A NEW INTERSTELLAR MODEL FOR HIGH-TEMPERATURE HIGH-TEMPERATURE

TIME-DEPENDENT KINETICSTIME-DEPENDENT KINETICS

Nanase HaradaNanase HaradaEric HerbstEric Herbst

The Ohio State UniversityThe Ohio State University

June 24, 2006International Symposium

on Molecular Spectroscopy

Motivation

• Accretion disk around AGNs

• High-temperature

• High-density

• Radiation, cosmic-rays, etc….

Active galactic nucleus (AGN) :

Central region of a galaxy with very high luminosity, which is believed to be caused by a supermassive blackhole

Chemical Abundance Calculation

• Thermodynamic equilibrium

– - High-temperature, high-density

– (ex. stellar atmosphere)

• Kinetics

– - Low-temperature, low-density (ex. cold dense cloud)

– Time-dependent

– Time-independent

∆

k=αexp(- Ea/kT)

High-Temperature Kinetics Model

Addition and modification to the model:

• Reactions with high activation energy

• Endothermic reactions

• Temperature-dependence of H2 formation on dust grains

• Addition of reactions with H2O, NH3, etc

• Additional protonation reactions of ions

Hydrogenation of C, N, O, and CNMore abundant H2O, NH3, CH4, and

HCN

O H2OOH CN HCN

Hydrogenation reactions with barriers: rate k = α exp(- Ea/kT)

Ea = 820 KEa = 3140 K Ea = 1040 K

H2

H2

H2

T = 100 K

T = 800 K

n = 104 cm-3

Making Hydrocarbons

• Production of C2H2 (diagram by Agundez et al. 2008)

• C2H2 has been seen in protoplanetary disks (Carr and Najita 2008), and ultra-luminous infrared galaxies (ULIRGs) (Lahuis et al. 2007)

• Efficient production of carbon-chains by effective carbon-rich condition. ([C]/[O]>1)

Making Hydrocarbons?

• Production of C2H2 (diagram by Agundez et al. 2008)

C3H

C4H

C5H

C2H

3

C3H

2

C4H

2

C5H

2

C3H

3

C4H

3

C5H

3

H2

C

C

H2

Making Hydrocarbons?

T = 800 Kn = 104 cm-3

Abundances of Positive Ions

• HCO+ + H2O → CO + H3O+

• H3O+ can take over the dominant form of ions

800 K

100 K

n = 104cm-3

Application - AGN Accretion Disk

• <n> ~ 109(r/pc)-3cm-3

• T ~ 1000 (r/pc)-0.5 K

radio jet

molecular torus / disk

~ 100s pc

Application - AGN Accretion Disk

• <n> ~ 109(r/pc)-3cm-3

• T ~ 1000 (r/pc)-0.5 K

radio jet

molecular torus / disk

~ 100s pc radiation (X-ray)

Application - AGN Accretion Disk

• <n> ~ 109(r/pc)-3cm-3

• T ~ 1000 (r/pc)-0.5 K

radio jet

molecular torus / disk

~ 100s pc radiation (X-ray)

Star formation

→Supernovae→ Cosmic-ray

→ UV-photons

Application - AGN Accretion Disk

• <n> ~ 109(r/pc)-3cm-3

• T ~ 1000 (r/pc)-0.5 K

radio jet

molecular torus / disk

~ 100s pc radiation (X-ray)

Star formation

→Supernovae→ Cosmic-ray

→ UV-photons

Shock

AGN- Observation and

Theoretical ModelsObservations

• I(HCN)/I(CO) ~0.1-0.5, I(HCN)/I(HCO+) ≥ 2 (Kohno 2005)

• I(CN)/I(HCN)~0.6-2, I(HNC)/I(HCN)~0.5-0.7 (Aalto et al. 2006)

Models

• XDR and PDR Chemistry (Meijerink and Spaans 2005, Stäuber et al. 2005)

AGN Chemistry – preliminary result

• Condition for r~3pc

• T = 600 K, n = 4x107 cm-3

• ζ = 1x10-17 s-1

•Condition for r~30pc

•T = 200 K, n = 4x104 cm-3

• ζ = 1x10-17 s-1

Summary

• High abundance of H2O in high-temperature environment will change the entire chemistry, making it effectively C-rich.

• The inner midplane of an AGN disk may be affected by this high-temperature chemistry, leading to C2H2 and other carbon-chain abundances.

• In the inner AGN disk, lower CN/HCN and higher HCN/HCO+ abundance ratios, compared with the outer disk, are also expected.

• Advent of high resolution telescopes (ALMA, Herschel) will reveal the finer structure of the chemistry of the AGN disks.

Acknowledgement

• Prof. Eric Herbst

• Prof. Todd Thompson

• Dr. Valentine Wakelam

• Dr. Guillaume Pineau des Forêts

Thank you!