eric herbst departments of physics chemistry and astronomy the ohio state university n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY PowerPoint Presentation
Download Presentation
ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

Loading in 2 Seconds...

play fullscreen
1 / 38

ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY - PowerPoint PPT Presentation


  • 130 Views
  • Uploaded on

Low-Temperature Gas-Phase & Surface Reactions in Interstellar Clouds. ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY. Dense Interstellar Cloud Cores. 10 K. 10(4) cm-3. Molecules seen in IR absorption and radio emission.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY' - nerice


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
eric herbst departments of physics chemistry and astronomy the ohio state university

Low-Temperature Gas-Phase &

Surface Reactions in Interstellar

Clouds

ERIC HERBST

DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY

THE OHIO STATE UNIVERSITY

slide2

Dense Interstellar Cloud Cores

10 K

10(4) cm-3

Molecules seen in IR absorption and radio emission

H2 dominant

sites of star formation

Cosmic rays create weak plasma

Fractional ionization < 10(-7)

cosmic elemental abundances
H = 1

He = 6.3(-2)

O = 7.4(-4) 1.8(-4)

C = 4.0(-4) 7.3(-5)

N = 9.3(-5) 2.1(-5)

S = 2.6(-5) 8.0(-8)

Si = 3.5(-5) 8.0(-9)

Fe = 3.2(-5) 3.0(-9)

Dust/gas = 1% by mass

Gas-phase abundances of heavy elements in clouds reduced.

Cosmic Elemental Abundances
some fractional abundances in tmc 1
CO 1(-4)

HCN 2(-8)

C4H 9(-8)

HCO+ 8(-9)

c-C3H2 1(-8)

HC9N 5(-10)

OH 2(-7)

NH3 2(-8)

HC3N 2(-8)

N2H+ 4(-10)

HNC 2(-8)

O2 < 8(-8)

Some Fractional Abundances in TMC-1
slide6

Water, CO, CO2

+ small grains and PAH’s

Water ice = 10(-4) of

Gas density

slide7

H2+ + e

Cosmic ray

O

efficient low t gas phasereactions
Efficient Low T Gas-PhaseReactions
  • Ion-molecule reactions
  • Radiative association reactions
  • Dissociative recombination reactions
  • Radical-radical reactions
  • Radical-stable reactions

Ea = 0

Exothermic

In areas of star formation, reactions with barriers occur.

ion molecule reactions
Experimental evidence down to a few K

Rate coefficients explained by classical “capture” models in most but not all instances.

ion-non polar (Langevin case)

Ion-Molecule Reactions

cm3 s-1

ion mol rx cont
Ion-polarIon-mol. Rx. (cont)

+ more complex state-specific models

remaining questions
Remaining Questions

1) Why are some reactions slow?

2) Is there a quantum limit?

radiative association
Radiative Association

, size, bond engy

Few ion trap measurements by Gerlich, Dunn down to 10 K

What is the 0 K limit?

What about competitive channels?

dissociative recombination reactions
Dissociative Recombination Reactions

Studied in storage rings down to “zero” relative energy; products measuredfor approx.10 systems

n=0.5, 1.5

Some systems studied: H3+, HN2+, HCNH+, H3O+, NH4+, CH5+ ,CnHm+

question
QUESTION
  • How large must ions be before the dominant process becomes radiative recombination? “statistical trapping”
  • Answer via statistical theories (RRKM): 20-30 atoms?????
slide15

Radical-radical Reactions

Detailed capture models by Clary, Troe

radical neutral rx cont
RADICAL-NEUTRAL RX (CONT)

CN + C2H2 HCCCN + H

YES

C + C2H2 C3H + H

YES

CCH + HCN  HCCCN + H

NO

Barrier cannot be guessed!!

attachment
Attachment

If enough large molecules with large electron affinities present, electrons may not exist! Note no competitive fragmentation channels.

formation of gaseous water
FORMATION OF GASEOUS WATER

H2 + COSMIC RAYS  H2+ + e

Elemental abundances: C,O,N = 10(-4); C<O

Elemental abundances: C,O,N = 10(-4); C<O

H2+ + H2 H3+ + H

H3+ + O  OH+ + H2

OHn+ + H2  OHn+1+ + H

H3O+ + e  H2O + H; OH + 2H, etc

formation of hydrocarbons
FORMATION OF HYDROCARBONS

H3+ + C  CH+ + H2

CHn+ + H2  CHn+1+ + H; n=1,2

CH3+ + H2  CH5+ + hn

CH5+ + e  CH4 + H (5%)

 CH3 + 2H (70%)

CH5+ + CO  CH4 + HCO+

slide20

Solved kinetically; thermodynamics useless!

t=0; atoms except for H2

Latest network – osu.2003 – contains over 300 rapid neutral-neutral reactions. Rate coefficients estimated by Ian Smith and others for many of these. Verification needed!!

nature of solution for a homogeneous time independent cloud
Nature of Solution for a homogeneous, time-independent cloud

“early time if O- rich”

fi

Small species (CO)

Large species (HC9N)

0.1

10

Time (Myr)

nature of solution for a homogeneous time independent cloud1
Nature of Solution for a homogeneous, time-independent cloud

“early time if O- rich”

fi

Found in pre-stellar cores

accretion

Small species (CO)

Large species (HC9N)

0.1

10

Time (Myr)

low temperature surface chemistry on amorphous surfaces
Low Temperature Surface Chemistry on Amorphous Surfaces
  • 1) Mechanisms (diffusive [Langmuir-Hinshelwood], Eley-Rideal, hot atom, impurity site)
  • 2) Dependence on size, mantle, fluffy nature, energy parameters
  • 3) Rate equations vs. stochastic treatments
  • 4) non-thermal desorption (cosmic rays)
slide25

Edes

Ediff

“physisorption”

(diffusion)

desorption diffusion
Desorption & Diffusion

for heavies

Desorption via evaporation and cosmic-ray heating.

kdiff = khop/N; N is the number of binding sites

For H, tunneling can occur as well.

H diffuses the fastest and dominates the chemistry.

slide27

TYPES OF SURFACE REACTIONS

REACTANTS: MAINLY MOBILE

ATOMS AND RADICALS

A + B AB association

H + H H2

H + X XH (X = O, C, N, CO,

etc.)

WHICH CONVERTS

O  OH  H2O

C CH  CH2  CH3  CH4

N  NH  NH2  NH3

CO  HCO  H2CO  H3CO  CH3OH

X + Y XY (CO + O  CO2) ??????????

experiments on cold surfaces
Experiments on cold surfaces
  • Vidali et al. Formation of H2 on silicates, carbon, and amorphous ice; LH mechanism characterized and energies obtained; formation of CO2; energy partitioning of hydrogen product (also UCL group)
  • Ediff(H, olivine) = 287 K; Ediff(H, carbon) = 511 K
  • But whole analysis of data has been questioned by others, who feel that both tunneling and some chemisorption sites are involved!!!!!
  • Hiraoka et al. Formation of ices (CH4, H2O,NH3, H2CO)
  • Watanabe et al. Formation of methanol
  • Danish group formation of H2
modelling diffusive surface chemistry
MODELLING DIFFUSIVE SURFACE CHEMISTRY

Rate Equations

The rate coefficient is obtained by

Method accurate if N>1

Biham et al. 2001

stochastic methods
STOCHASTIC METHODS

Based on solution of master equation, which is a kinetic-type equation in which one calculates not abundances but probabilities that certain numbers of species are present. Can solve directly (Hartquist, Biham) or via Monte Carlo realization (Charnley).

stochastic states
Unfortunately, with more than one reactive surface species, one must compute joint probabilities Stochastic States

so that

the computations require significant computing power. It is necessary to impose cutoffs on the ni and the total number of surface species considered.

More simple fix: modified rate method

new gas grain stochastic deterministic model
New Gas-Grain Stochastic-Deterministic Model
  • Stantcheva & Herbst (2004)
  • Gas-phase chemistry solved by deterministic rate equations, while surface chemistry solved by solution of master equation. Some results quite different from total deterministic approach.
results surfaces
RESULTS: surfaces
  • From observations of grain mantles, the dominant species in the ice are water, CO, CO2, and occasionally methanol.
  • The models at 10 K and a gas density of 10(4) cm-3 are able to reproduce the high abundance of water, seem to convert CO into methanol too efficiently, and tend to underestimate the amount of CO2. Results sensitive to density.
  • The modified rate method reproduces the master equation approach at 10 K, but the normal rate method can be in error.
slide37

% Agreement in TMC-1

Gas-phase species

Roberts & Herbst 2002

some conclusions
Some Conclusions
  • 1) Low-temperature chemistry in interstellar clouds (both gas-phase and surface) partially understood only.
  • 2) Chemistry gives us many insights into the current state and history of sources
  • 3) More work on “cold chemistry” is clearly needed to make our mirror into the cosmos more transparent.