Molecules and Dust

1
Molecules and Dust

• 1 April 2003
• Astronomy G9001 - Spring 2003
• Prof. Mordecai-Mark Mac Low

2
Molecule Formation

• Gas phase reactions must occur during collisions
  lasting lt 10-12 s
• Radiative association reactions
• have rate coefficients of only 108 s-1
• are faster if they involve at least one ion
• Adsorption onto dust allows far longer contact
  times, so slower reactions can proceed. Dust is
  a catalyst.

3
H2 Formation

• Hollenbach Salpeter (1971) computed H2
  formation rate on dust to be
• Molecule formation only proceeds quickly at high
  densities
• Experimental results by Piranello et al. group
  show slower rates on graphite, olivine, but not
  on amorphous ice.

4
UMIST rate database

• Best compilation of gas phase astrochemical rates
  currently at U Manchester (Le Teuff, Millar
  Markwick 1999) available at http//www.rate99.co.
  uk
• 12 elements, 396 species, and 4000 reactions,
  including T dependence. Also some
  photoionization and dissociation rates, and
  interactions with CRs.
• Gives rates in the form

5
Collisional Dissociation

• Electron collisions with molecules most important
  collisional dissociation mechanism
• Collisional dissociation
• Dissociative ionization
• Dissociative recombination most likely

AB e- ? A B e- AB e- ? A B
2e- AB e- ? A B
6
Photodissociation
Lyman, Werner bands in range 912 to 1105 Å

• UV excitation followed by fluorescent
  dissociation
• Self-shielding occurs in H2 when Lyman and Werner
  bands become optically thick
• Similar physics controls CO dissociation, but
  lower abundance makes CO more fragile

Spitzer, PPISM
7
Photodissociation Regions

• Shielded from H ionizing radiation, but exposed
  to lower energy UV and X-rays
• Dust is dominant absorber
• Contain nearly all atomic and molecular gas
• Origin of much of IR from ISM
• dust continuum
• PAH features
• fine structure lines

8
shock
Hollenbach Tielens 1999
9
Dust formation

• Stellar ejecta (time-dependent process)
• giants and AGB stars
• massive post-main-sequence stars
• novae and supernovae
• Composition of ejecta determine grains
• Oxygen-rich ejecta make silicates
• Carbon-rich ejecta make graphite and soot
• Silicates must also form in cooler ISM
• Ices freeze on in molecular cloud cores

10
Grain Destruction in Shocks

• Thermal sputtering by ions
• Most important if vs gt 400 km s-1
• Occurs over 105 yr for typical grains
• Stopping time tstop (106 yr) a-5(nv500)-1
• Only largest grains survive fast shocks
• Grain-grain collisions lead to a-3.3 power law
• Vaporization at high velocities
• Spallation and fragmentation
• Amorphous carbon at v gt 75 km s-1
• Silicates at v gt 175 km s-1
• Cratering at v gt 2 km s-1
• Coagulation

11
Reddening curves

• Mean extinction varies within, between galaxies
• Reddening 1/? in optical
• Bump due to small carbon grains

2175 Å bump
Dopita Sutherland
12
Grain distribution

• Properties of reddening curve can be fit by a
  size distribution of grains n(a) a-3.5 (Mathis,
  Rumple, Nordsieck 1977) with composition
• graphite
• silicon carbide (SiC)
• enstatite (Fe,MgSiO3)
• olivine (Fe,Mg2SiO4)
• iron, magnetite (Fe3O4)

13
Optical Properties
14
Dust Polarization
15
Mineralogy

• Wind density, velocity, imply grain mineralogy
• If the wind is oxygen rich
• fast, low density winds produce corundum (Al2O3),
  and perovskite (CaTiO3).
• higher density allows forsterite (Mg2SiO4) and
  enstatite (MgSiO3) mantles
• Iron reacts to form olivine (Fe2SiO4) and
  pyroxene (FeSiO3)
• Narrow mid-IR features observed
• Dust grains traced by isotopic anomalies to
  different stars.

16
PAHs

• Polycyclic aromatic hydrocarbons dominant species
  in carbon-rich winds.
• Gradual transition from flat PAHs to spherical
  soot
• 3-10 µm features prob. from mixture of PAHs

PAH formation in C-rich wind via H abstraction
and acetylene addition (Frenklach Feigelson
1989)
17
Assignments

• Finish Exercises 4 and 5
• Read Ballesteros-Paredes, Hartmann,
  Vázquez-Semadeni, 1999, ApJ, 527, 285

18
Gravity

• Fixed (or at least pre-defined) potential from a
  background mass distribution not part of the
  computation
• stars
• dark matter
• Self-consistent potential from the matter on the
  grid
• requires solution of Poissons equation

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Poisson Equation Solutions

• Poisson equation is solved subject to boundary
  conditions rather than initial conditions
• Several typical methods used in astrophysics
• uniform grid Fourier transform (FFT)
• particles
• direct summation (practical with hardware
  acceleration)
• tree methods
• particle-particle/particle-mesh (P3M)
• non-uniform/refined grids multigrid relaxation

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Finite Differencing
Numerical Recipes
21
Fourier transform solution
Numerical Recipes
22
Direct Summation

• Simplest and most accurate method of deriving
  potential from a particle distribution.
• Too bad its computational time grows as N2!
• Normally only practical for small N lt 100 or so
• GRAPE project attacks with brute force by putting
  expensive part
  in silicon on a special purpose, massively
  parallel chip

23
Tree Methods
Volker, Yoshida White 2001

• Tree is constructed with one pcle in each leaf
• Every higher node has equivalent monopole,
  quadrupole moments
• Potential computed by sum over nodes
• Nodes opened if close enough that error gt some e

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PPPM

• A grid covering all the particles is set up, with
  density in each zone interpolated from the
  particles in the zone.
• The potential on the grid is solved by any method
  (eg FFT)
• A local correction to the potential for each
  particle is then derived from direct summation of
  particles within its own grid cell
• An adaptive mesh can be used for very clumpy
  density distributions

25
Multigrid Relaxation
Saraniti et al. 1996

• Gauss-Seidel relaxation
• on multiple grids

• Relaxation methods solve
• Each timestep relaxes most strongly close to
  grid scale.
• By averaging onto coarser grids, larger-scale
  parts of solution can be found