Star & Planet Formation 2017 - SRONvdtak/psf2017_lecture8.pdf1. Radiation fields Cosmic Rays • cosmic rays (CR) penetrate down to column densities of 100 g cm-2 • CR ionize and

Star & Planet Formation 2017 Lecture 8: Disk chemistry

Review papers: Henning & Semenov 2013, Chem. Rev. van Dishoeck et al 2014, PP VI

Recap of Lecture 7 1.  Shakura-Sunyaev model

vertical structure: (almost) isothermal, hydrostatic radial structure: Keplerian, viscosity parameter origin of viscosity: MRI

2.  Disk evolution

disk spreading dead zones Toomre instability FU Orionis outbursts

3.  Effect of irradiation

viscous accretion & spreading flat disk does not match SED solution: flaring

4. Chiang-Goldreich model

assume surface ~few pressure scale height solve heating / cooling balance obtain h ~ r9/7 (or flatter if h/Hs .ne. C) CG97: warm surface, cool interior near-IR: puffed-up hot inner rim

5. Disk dispersal

viscous phase gap opening by EUV photoevaporation by FUV external irradiation

Course Outline Date Topic Reading 06-02-2017 I. Overview and context SP 1-4

10-02-2017 II. Molecular clouds SP 5-8 13-02-2017 III. Cloud stability and collapse SP 9+10 17-02-2017 IV. Pre-main sequence stars SP 11+12 20-02-2017 V. Extreme star formation SP 13-15 21-02-2017 VI. Observations of disks SP 16-19 03-03-2017 VII. Accretion disks PA 1 06-03-2017 VIII. Chemistry of disks PA 2 10-03-2017 IX. Particle motions in disks PA 3 13-03-2017 X. From dust to planetesimals (MM) PA 4 17-03-2017 XI. From planetesimals to planets (MM) PA 5 20-03-2017 XII. Planet migration (MM) PA 6 24-03-2017 XIII. Exoplanets PA 7 07-04-2017 Exam

Disk chemistry 1.  Radiation fields

I.  Stellar radiation II.  Interstellar radiation III.  Cosmic rays

2.  Reaction networks

3.  Gas phase chemistry

I.  Radiative association II.  Three-body reactions III.  Neutral-neutral reactions IV.  Ion-molecule reactions

4.  Photochemistry

I.  Photoionization II.  Photodissociation

5.  Gas-grain chemistry

I.  Adsorption II.  Thermal desorption III.  Photodesorption IV.  Cosmic-ray induced

desorption

6.  Grain surface chemistry

7.  Chemical disk structure

I.  Vertical layering II.  Disk ionization

8.  The midplane

I.  Planets II.  Meteorites III.  Comets

0. Introduction

Main drivers:

Chemical inventory in space

~190 known species

neutral & cat/anionic

an/organic

N = 2 – 13 & 60-70

Astrochemistry

0. Introduction

Main drivers:


Use chemistry to learn about physical conditions

Why Astrochemistry?

[Barnard 5 dark cloud core (observed ratios solid lines) and edge (dashed lines) compared to PDR models: Bensch 2006]

0. Introduction

Main drivers:


Use chemistry to learn about physical conditions Building blocks of Solar system

Why Astrochemistry?

0. Introduction

Main drivers:


Use chemistry to learn about physical conditions Building blocks of Solar system Study ‘exotic’ chemistry at conditions which laboratory cannot provide

Why Astrochemistry?

1. Radiation fields

[Henning & Semenov 2013]

Schematic view



2.  Chemical reaction networks







desorption




8.  The midplane


1. Radiation fields

Stellar radiation

FUV: large contribution from the stellar chromosphere (stellar activity) and hot accretion spots on the stellar surface

Optical: dust heating

FUV: driving chemistry

grain photoelectric heating

1. Radiation fields

Stellar radiation

two-temperature thin-thermal plasma model reproduces observed X-ray spectra of T Tauri stars (LX ~ 1030 erg/s) derived best-T parameters kT1 = 0.8 keV, kT2 = 0.2 keV, NH = 2.7×1020 cm-2 (foreground NH) Need 2 temperatures, but not far apart: gas not quite isothermal

1. Radiation fields

Interstellar radiation field ISRF estimates [Habing 1968, Draine 1978] G0 = 1.6 x 10-3 erg s-1 cm-2

[van Dishoeck et al. 2007]

Originates predominantly from hot O and B stars in our neighbourhood: flat intensity in the UV

Higher color temperature than stellar radiation field: important for photoionization and -dissociation

1. Radiation fields

Cosmic Rays •  cosmic rays (CR) penetrate down to column densities of 100 g cm-2 •  CR ionize and excite molecules •  excitation of molecular hydrogen through secondary electrons produces

local secondary UV radiation field – Absorption by dust grains – Surface reactions on grains – Dissociation and ionization of molecules All these processes occur deep inside the disk at high optical depth, but at a low level compared with the strongly UV illuminated disk surface










desorption




8.  The midplane


2. Chemical reaction networks

Chemistry Chemical equilibria behave very similar to level populations in atoms or molecules •  Both follow a rate equation approach (radiative and collisional rates

versus photorates, 2- and 3-body reactions) •  Like level populations can be in LTE or SE, there are different

chemical equilibria as well

Thermodynamic equilibrium: Time-dependent chemistry: rate equation Stationary chemistry: 0


Chemical reaction networks

Rate equation (statistical) approach: gas phase chemistry Master equation (stochastic) approach: surface chemistry

species i particle density ni rate coefficient kijl photorates Γij, ζij

P is the probability that i particles of species 1 to N are on the grain surface


Chemical reaction databases

UMIST edition 5 (2012):

•  467 species connected through 6173 binary gas-phase reactions •  focus is on low (<300 K) temperature chemistry

Combustion database, Konnov (2000):

•  Experimental data from combustion experiments -> high T chemistry

Kinetic database for astrochemistry (KIDA):

•  interstellar medium and planetary atmospheres

OSU database (Ohio State University, E. Herbst)

+ many private ones …


Chemical reaction networks

[Thi & Bik 2005] High temperature chemistry in the inner disk

3. Gas phase chemistry

Reaction rates

Chemical reaction between species A and B: Reaction rate: Rate equation:

a, b, c, d are stoichiometric ratios

A mole (mol) = 6.022 x 1023 units

6.022 x 1023= Avogadro's constant [unit: particles mol–1 or mol–1]

Chemistry basics:

k is the rate constant [A], [B] are concentrations a+b gives the order of the reaction

destruction of A formation of A


Rate constants

Neutral-neutral reactions often follow Arrhenius law: To understand the Arrhenius law, let’s go back to collisional theory:

Ea is the activation energy

σAB is the collision cross section nA, nB are the species densities mr is the reduced mass

Steric factor P: reaction only occurs if the reactive ends of the molecules point towards each other

α, β, γ, are constants as tabulated in e.g. the UMIST database


Rate constants (2) Ion-molecule reactions often follow Langevin law: k ~ 10-9 cm3 s-1, independent of T To understand the Langevin law, use capture theory Idea: ion induces dipole in neutral upon approach long-range attraction scaling as R-4

reaction occurs if impact parameter small enough Interaction potential: where α is the polarizability of the neutral (of order a0

3) Overcome barrier if 0.5 µv2 > Veff, i.e., The reaction cross-section is σ = π b2

crit and the rate is independent of T !


Reaction types


Reaction types

k ~ 10-17 cm3/s


Reaction types

k ~ 10-10 … -11 cm3/s


Reaction types

k ~ 10-9 cm3/s


Reaction types

k ~ 10-6 cm3/s










desorption




8.  The midplane


4. Photochemistry

Photochemistry

k ~ 10-11 s-1

4. Photochemistry

Photochemistry

Photoionization is a continuous process: Photodissociation can be either continuous (over larger wavelength range - e.g. OH, O3) or discrete (via line absorption - e.g. H2, CO) Rate parametrization in UMIST:

λij wavelength of line fij transition probability ηij dissociation probability (0…1)

σ(λ) ionization cross section

5. Gas-grain chemistry

Gas-grain chemistry

Rate equation:

# ice species Ri

ads adsorption onto grain surface Ri

des,th thermal desorption from grain surface Ri

des,ph photodesorption from grain surface Ri

des,crph CR induced photodesorption from grain surface Ri

des,cr CR direct desorption from grain surface


Adsorption & desorption

active surface layers contain ni#

desorb particles


Adsorption & desorption grain surface area thermal velocity sticking coefficient (0-1)

vibrational frequency in potential well (1011 – 1013 Hz) binding energy (960 K for CO, 4800 K for water ice)

# active surface spots yield ~10-3


Cosmic-ray desorption

ζH2 is the cosmic ray ionization rate of H2 f(70 K) is the duty-cycle of a grain ratio between CR-hit and cooling rate

typical timescale for consecutive CR-hits is ~106 yr

cooling time ~10-5 s

6. Grain surface chemistry

Grain surface chemistry

This is the least understood chemistry ! Open problems:

•  composition of grain surfaces only poorly understood (physisorption versus chemisorption, roughness and porosity)

•  chemical composition unknown (mixed ices instead of pure ices)

•  surface mobility poorly constrained










desorption




8.  The midplane


7. Chemical disk structure

Henning & Semenov 2013

Schematic view


[Dullemond et al. 2007, PPV]

Schematic view


Qi et al 2013

Snowline observations

Depletion of CO in cold outer disk => enhancement of N2H+


van Dishoeck 2014, PP VI

The water cycle in disks

T Tauri disk


[ProDiMo: Woitke et al. 2009, Kamp et al. 2010] other model: Du & Bergin 2014

A model for the water distribution

high densities (n<H> > 1013 cm-3): all oxygen locked in gas phase water => carbon rich chemistry (CH4) long timescales !

warm temperatures (T > 150 K): oxygen locked in gas phase water AND CO (also CO2 ring ~ 0.3 AU)

moderate temperatures (150 K < T < 20 K): all oxygen locked in water ice => carbon in CH4 / CH4 ice (40 K)

low temperatures (T < 20 K): all oxygen locked in CO ice (R > 100 AU)

T Tauri disk


Disk ionization Dead zones have ne/ntot < 10-12

Hydrodynamics, gas-phase chemistry and dust modeling (grain surface processes and grain growth) all coupled!

recombination and charge transfer

turbulent and ionization timescales of similar order in the dead zone










desorption




8.  The midplane


8. The midplane

Planets, Meteorites & Comets

The planets, meteorites and comets of our Solar System present the fossil record of early stages of chemistry and dynamic evolution in the protosolar nebula (our own protoplanetary disk).

8. The midplane

Planets

assume that dust grains in the protosolar nebula completely vaporized and re-condensed under equilibrium conditions (and in equilibrium with the gas at any time) => condensation sequence as a function of temperature (pressure)

8. The midplane

Planets

[Lewis 1997] assume that dust grains in the protosolar nebula completely vaporized and re-condensated under equilibrium conditions (and in equilibrium with the gas at any time) => condensation sequence as a function of temperature (pressure) translate chemical composition (r) into a mean density

8. The midplane

Meteorites

[Encrenaz 2004]

Meteorites retain a fossil record of the solar nebula composition inside the snow line if they are not differentiated (i.e. not molten once they formed) CI chondrites (subclass of carbonaceous chondrites) show an elemental composition that is very close to our Sun => Common origin of Sun and material in the Solar System

8. The midplane

Meteorites

[Bergin 2009]

Elements with higher condensation temperature more likely ended up in planetesimals not unique:

This can be either attributed to condensation in the cooling protoplanetary nebula or to evaporation during heating events (flares and shocks)

8. The midplane

Comets

Comets originate in cooler parts of the Solar Nebula (beyond snow line) Cometary compositions resemble that of the interstellar medium

•  comets consist of pristine interstellar ices •  conditions under which comets formed in the outer Solar System

were very similar to those in interstellar clouds

Wide degree of inhomogeneity amongst short period comets (P < 200 yr, origin in the Kuiper Belt) as well as Oort cloud comets (P > 200 yr) Variations seen for example in CO, C2H6, CH4, CH3OH relative to water: systematic changes in chemistry in the comet forming zone

8. The midplane

Comets

[Bockelée-Morvan 2000]

Cometary compositions resemble that of the interstellar medium

•  comets consist of pristine interstellar ices

•  conditions under which comets formed in the outer Solar System were very similar to those in interstellar clouds

8. The midplane

Comets

Comets originate in cooler parts of the Solar Nebula (beyond snow line) Cometary compositions resemble that of the interstellar medium

•  comets consist of pristine interstellar ices •  conditions under which comets formed in the outer Solar System

were very similar to those in interstellar clouds

Wide degree of inhomogeneity amongst short period comets (P < 200 yr, origin in the Kuiper Belt) as well as Oort cloud comets (P > 200 yr) Variations seen for example in CO, C2H6, CH4, CH3OH relative to water -> systematic changes in chemistry in the comet forming zone

8. The midplane

Comets

[Bockelée-Morvan 2010]

Black/red: Range of measured abundances Right: Number of comets where species is detected

Variations seen for example in CO, C2H6, CH4, CH3OH relative to water

-> systematic changes in chemistry in the comet forming zone

8. The midplane

Henning & Semenov 2013

Schematic view

Next lecture:

Particle motions in a gaseous disk


[Ida & Lin 2008]

Disk ionization

Gas surface density Σg with the coupling effect of MRI and the ice line:

(a)  without enhancement in dust surface density Σd due to grain trapping (b)  with this effect solid lines Σg distributions for M = 10-7, 3×10-8, 10-8, 3×10-9, 10-9 M¤/yr from top to bottom, dotted and dashed lines ΣA (surface density of active layer) and Σd

Crucial parameters:

• metal abundance (e- donors)

•  grain abundance (recombination)

ice line


[Ida & Lin 2008]

Disk ionization



Crucial parameters:

• metal abundance •  (e- donors)


•  fraction Σg /ΣA

accumulating grains near the pressure maximum

ice line


Dust trapping in P-gradients

vgas < vkep vdust = vkep

dust feels headwind

€

vφ2

r=GM*

r2+1ρ∂P∂r lo

g P

log r


Dust trapping in P-gradients

vgas < vkep vdust = vkep

dust feels headwind

log r

€

vφ2

r=GM*

r2+1ρ∂P∂r lo

g P

Armitage 4.3; see Lecture 11


[Ida & Lin 2008]

Disk ionization



Crucial parameters:

• metal abundance •  (e- donors)


•  fraction Σg /ΣA

accumulating grains near the pressure maximum

ice line

Next lecture:

Disk Observations

Documents

Star & Planet Formation 2017 - SRONvdtak/psf2017_lecture8.pdf1. Radiation fields Cosmic Rays • cosmic rays (CR) penetrate down to column densities of 100 g cm-2 • CR ionize and