Interstellar Medium Physics

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Interstellar Medium Physics Chemistry

• Gianfranco Vidali

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ISM

• atoms, molecules, dust
• How do we know?
• Where do they come from?
• What is their role in the ISM?

Over all the sky the sky! far, far out of
reach, studded, breaking out, the eternal stars
W.Whitman
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Tools

• From ISO (Infrared Space Observatory)

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The ISM
A tumultuous cloud Instinct with fire and nitre
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Atoms

• H, He
• SourceBig Bang
• Metals (O, C, N, Si, Fe, )
• Source interior of stars, supernovae

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Molecules

• AB ?C
• dN(C)/dtk N(A) N(B)
• Ion-molecule (10-9 cm3 s-1)
• Neutral-neutral (lt10-11 cm3 s-1)
• H2, CO, OH, CS
• CO2, H20, HCN
• H2CO, NH3, C2H2
• CH4, CH3OH, HCOOH, OCS
• PAH

21 cm
rotations
vibrations
electronic
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Dust

• Origin
• Novae
• Supernovae
• Stellar outflows
• Characteristics
• a0.1 mm Na-3.5
• Silicates, carbonaceous material (dep. on C/O)

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Dust

• Extinction of starlight by dust (Mie scattering)

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Carbon in Space
Source Charnley and Ehrenfreund, Ann. Rev.
Astron. Astroph.
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• Olivine (a silicate) (Mg, Fe)2 SiO4

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Role of ISM

• Star formation
• Early universe
• HH ?H2 e
• H2 H ?H2 H
• Current
• Cooling by H2, CO

..the Almighty Maker them ordain His dark
materials to create more Worlds
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Stellar UV sources

• Chemical balance regulates abundance of atoms,
  molecules, dust
• Diffuse clouds, dense clouds, circumstellar
  envelopes

Cosmic rays
shocks
Thermal IR sources
Dynamical shocks
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Molecular Hydrogen

• Coolant promotes star formation
• Tracer of warm gas
• Promotes interstellar chemistry
• H2 cr ? H2 e
• H2 H2 ? H3 H
• H3 O ? OH H2
• Weak quadrupolar transitions CO is a tracer

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The Molecular Hydrogen Problem

• HH?H2 not in the gas phase
• Other routes
• He?H-hn
• H-H?H2e requires ionized medium

b3Su
X1Sg

• HH?H2 on dust grains
• Salpeter, Hollenbach 1970
• dn(H2)/dtR n n(H)-b n(H2)

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Models

• Hollenbach and Salpeter (1970)
• Semiclassical sticking, quantum mechanical
  tunneling
• RH2 1/2 ( nH vH s x) g ng

density of grains
prob. of recomb.
sticking
grain cross-section
flux
Recombination efficiency ( molecules sec-1 cm-3)
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Application of surface science techniques to
astrophysical problems

• Measurement of hydrogen recombination and
  hydrogenation/oxidation reactions on surfaces of
  dust grain analogues
• Experimental Conditions
• Low kinetic energy of H atoms (gas phase atoms)
  200-300 K
• Low flux of H atoms lt1012 atoms/cm2/sec
• Low sample temperature (5-40 K)
• Low background pressure (10-10 torr)
• Experiment Schutte et al. (1976), King and Wise
  (1963)
• Not in astrophysically relevant conditions

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Our Research Program

• Experiments of molecular synthesis on
  interstellar dust grain analogues
• HH ? H2
• Measure H2 formation on
• Silicates (olivine) Ap.J. 475, L69 (1997)
  Ap.J.483, L131 (1997) first experiments to
  study H2 formation on dust grains analogues in
  astrophysically relevant conditions
• Carbonaceous Materials (amorphous carbon) AA
  344, 681 (1999)
• Amorphous Water Ice Ap.J. 548, L253 (2001)
  Ap.J. accepted (2002)
• COO?CO2
• Measure CO2 formation due to oxidation of CO-ice
  by atomic oxygen

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The Team

• Joe Roser
• Bob DAgostino
• Chris Nagele, Emily Watkins, Sam Palermo
• Sol Swords
• Ofer Biham
• Valerio Pirronello, Giulio Manico

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Experimental Apparatus
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Apparatus to study molecule formation on dust
grain analogues
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(No Transcript)
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Measurement Methods

• Irradiation of sample with thermal energy H atoms
• Measurement of hydrogen recombination events
• Measurement of H2 formation due to fast
  processes, due to
• Eley-Rideal ("prompt") reaction
• Fast diffusion on surface of grain analogue
• Thermal Programmed Desorption, to
• Desorb molecules that have already formed on the
  surface
• Accelerate the diffusion of H atoms and favour
  the recombination process

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H adsorption and measurement of H2 due to the
fast reaction process
Tbeam150-200 K
Mass discriminating detector
T 5 - 20 K
samples olivine ((Fe,Mg)2 SiO4), amorphous
carbon, ice, etc.
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Temperature Programmed Desorption
To desorb molecules already formed on surface or
to set atoms in motion
detector
temperature
time
heat
Temperature ramp
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Hydrogen recombination reaction

• Thermal desorption trace HD from olivine (a
  silicate) as a function of exposure
  (sub-monolayer coverage)
• Ap.J. 1997
• Learn about reaction kinetics and rates

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Analysis of Temperature Programmed Desorption
results

• Desorption rate (Polanyi-Wigner)
• R (t) nb n(t)b exp (-Ed/kT)
• Order of desorption
• b0 desorption from multilayer
• b1 direct or molecular desorption
• b2 associative desorption

Order of desorption
Desorption energy barrier
Adatom density
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Rate equations

• d nH/dt F (1- nH - nH2)- pH nH - 2 a nH2
• d nH2/dt a m nH2- pH2 nH2
• pH n exp(-EH/kT) - desorption rate
• a n exp(-Ed/kT) - diffusionrecomb. rate
• RH2 (t) (1- m) a nH2 pH2 nH2

Attempt frequency
Recombination rate
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Analysis of rate equations

• Steady state conditions (dnH, H2/dt0)
• RH2 1/2 (nH vH s x) ng
• Indep. of H coverage - linear in flux
• Applicable when mobility is high (agtgtpH/F pH
  1/tH)
• RH2 1/2 (nH vH s x tH)2 ng a g
• Quadratic in H coverage - quadratic in flux
• Applicable when mobility is slow or coverage is
  low (altltpH/F)
• Kinetics
• Fit to experimental desorption curves
• Obtain physically relevant parameters
• Construct plot of recombination efficiency as a
  function of T and for a range of H fluxes

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Hydrogen recombination reaction

• Molecular hydrogen recombination efficiency on
  different dust grain analogues

amorphous carbon
water ice
olivine
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Influence of ice morphology on HsurfaceDsurface?H
D reaction

• Desorption of HD from amorphous ice (Roser et
  al., ApJ 02)
• high density, low density, gas phase deposited

• Recombination efficiency on amorphous ice
  surfaces (Roser et al., ApJ 02)
• high density
• low density
• gas-phase deposited

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Experiment-ISM connections

• Theoretical and computational methods connecting
  laboratory data to actual processes in the ISM
• Model hydrogen recombination reactions in the ISM
  using laboratory results
• Ap.J. 553, 595 (2001) Ap.J. 522, 305 (1999)
  MNRAS 296, 869 (1998)

Amorphous carbon
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Example II Oxidation reaction of CO

• The Problem
• Solid CO2 more abundant than explained by
  gas-phase reactions
• Solid CO2 can be made by UV in CO- and O2rich
  ices
• However, solid CO2 is seen in quiescent regions
  no UV

Whittet et al, AA, 1998 Spectrum towards Elias16

• Can solid CO2 be made by
• COice Ogas ? CO2 ice ?

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Oxidation of CO ice by atomic O
O
CO

• Roser et al., Ap.J. 2001

100 layers CO O CO/O 5.6-21
100 layers H2O
substrate
CO2
CO
O
heat
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Current Research Study of the energetics of H2
formation

• Goal
• Measurement of excitation state of molecular
  hydrogen formed on dust grain analogues
• Techniques
• Time-of-flight detection to measure the
  translational energy of molecules
• (21) REMPI (Resonance Enhance MultiPhoton
  Ionization) to measure the roto-vibrational state
  of molecules leaving the dust grain analogue

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Time-of-flight measurements

• In the time-of-flight experiment, the desorbing
  flux is chopped by a rotating mechanical wheel,
  see adjacent sketch. The time that a pulse of
  molecules takes to go from the chopper to the
  detector is measured and the kinetic energy
  calculated.
• Of the 4.5 eV energy released in the
  recombination reaction, it is not known
  quantitatively the partition of the in
  roto-vibration vs. translation of the molecule.
  Guess estimates of the amount of translational
  energy range from thermal energy (20 K) to 1 eV.
• The challenge is to measure the velocity
  distribution of the molecules exiting the surface
  during the brief time ( a few tens of sec.) of
  the TPD run. This imposes stringent requirements
  on abating the residual gas background pressure.
• Such experiment has not been done before under
  these conditions.

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Measurement of the roto-vibrational energy of H2

• Of the 4.5 eV energy released in the
  recombination reaction, some is available to the
  molecule as roto-vibrational energy. Estimates of
  this energy vary greatly.
• The experiment consists in probing the quantum
  state of the desorbing hydrogen molecules.
  Because vibrational states of H2 lie in the UV,
  the measurement of the roto-vibrational state is
  challenging.
• We use the (21) REMPI (Resonance Enhanced
  MultiPhoton Ionization). The molecule is taken to
  an electronically excited state by the absorption
  of two photons. Here the molecule absorbs another
  photon that removes an electron. The molecular
  ion is then collected by a detector (a
  channel-plate), see adjacent diagrams.

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Specifics of the detection of roto-vibrational
energy levels

• The light from a NdYAG laser (1089 nm) is
  doubled and sent to a dye laser for tuning. The
  600 nm light is then sent to a non-linear
  crystal that convert visible light into a 200 nm
  and a 300 nm beams. The molecule absorbs a 200 nm
  photon that takes it to a virtual state. If the
  molecule absorbs another photon, then it can go
  in an electronically excited state, see diagram.
  From there, the absorption of a 300 nm photon
  ionizes the molecule. Thats the explanation for
  the (21) nomenclature.
• The challenge is to have a beam of photons
  intense enough so the molecule can absorb two
  photons virtually simultaneously. Furthermore,
  because the generation of tunable laser light at
  200 nm requires the use of the non-linearity of
  special crystals, the process is inherently
  inefficient and the experiment needs powerful
  lasers.

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Molecular hydrogen formation on dust grain
analogues in ISM conditions

• Study of molecular hydrogen formation on
  amorphous ices found in various interstellar
  environments.
• Study of the role of ice morphology and UV
  processing on H2 formation.
• Comparison of recombination efficiency due to
  surface or near-surface processes with competing
  mechanisms, such as cosmic rays and UV photons.
  SeeAp.J. 548, L243 (2001).
• Study of evolution of morphology of icy grains
  through astrophysical environments (Ap.J.,
  accepted - 2002)

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Summary of accomplishments and future directions

• We showed that
• Measurement of hydrogen recombination and CO
  oxidation reactions on dust grain analogues can
  explain processes occurring in the ISM
• Challenges
• Composition, morphology of dust poorly known
• Partition of reaction energy between new-born
  molecule and solid
• Excitation of molecule ejected into the gas
  phase theoretical estimates vary greatly
• Role of energy deposited in the ISM

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Analysis of experimental results

• Second order kinetics (b2)
• R (t) n2 n(t)2 exp (-Ed/kT)
• Ed effective activation energy barrier for
  formation of H2 and desorption
• Examples
• E 26 meV (olivine) 45 meV (amorphous carbon)
• n2 10-3 cm2/s
• Derivation of Ed
• dR(t)/dt0 (max of desorption rate), TT0at
• Ed/kTmaxln(Tmax2/a) ln (Ed/kn) b1
• Ed/kTmaxln(n Tmax2/a) ln (Ed/2kn) b2

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Details of calculations

• Numerical integration of rate equations
• Fit to ALL TPD curves for each surface with 4
  parameters
• activation energy for H desorption E1
• activation energy for H2desorption E2
• activation energy for H diffusion E0
• fraction of H2 desorbing m
• Results
• E0, E1, E2 tens of meV higher for a-carbon
• E0, E2 well determined
• Recombination efficiency
• R (recombination rate)/ F/2 (desorption rate)

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Results and Analysis

• Second order kinetics
• Rate equations
• Numerical integration of rate equations
• Fit to ALL TPD curves for each surface with 4
  parameters
• Fit to experimental desorption curves
• Obtain physically relevant parameters
• Construct plot of recombination efficiency as a
  function of T and for a range of H fluxes

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Basic processes of atom -surface interaction
applied to astrochemsitry 1. Prompt reaction /
Eley-Rideal reaction

• Significant only if most of the surface is
• covered with adsorbed atoms

Direct hit
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Basic processes of atom -surface interaction
applied to astrochemsitry 1. Indirect
mechanism(Langmuir-Hinshelwood)

• Sticking
• Diffusion
• Reaction
• Desorption