1 April 2003 Astronomy G9001 - Spring 2003 Prof. Mordecai-Mark Mac Low
Molecule Formation Gas phase reactions must occur during collisions lasting < 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.
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.
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
Collisional Dissociation Electron collisions with molecules most important collisional dissociation mechanism - Collisional dissociation
- Dissociative ionization
- Dissociative recombination most likely
Photodissociation 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
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
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
Grain Destruction in Shocks Thermal sputtering by ions - Most important if vs > 400 km s-1
- Occurs over 105 yr for typical grains
- Stopping time τstop~ (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 > 75 km s-1
- Silicates at v > 175 km s-1
- Cratering at v > 2 km s-1
- Coagulation
Reddening curves Mean extinction varies within, between galaxies Reddening ~1/λ in optical Bump due to small carbon grains
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,Mg]SiO3)
- olivine ([Fe,Mg]2SiO4)
- iron, magnetite (Fe3O4)
Optical Properties
Dust Polarization
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.
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
Assignments Finish Exercises 4 and 5 Read Ballesteros-Paredes, Hartmann, & Vázquez-Semadeni, 1999, ApJ, 527, 285
Gravity Fixed (or at least pre-defined) potential from a background mass distribution not part of the computation Self-consistent potential from the matter on the grid - requires solution of Poisson’s equation
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
Finite Differencing
Fourier transform solution
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 < 100 or so GRAPE project attacks with brute force by putting expensive part in silicon on a special purpose, massively parallel chip
Tree Methods Every higher node has equivalent monopole, quadrupole moments Potential computed by sum over nodes Nodes opened if close enough that error > some ε
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
Multigrid Relaxation Relaxation methods solve Each “timestep” relaxes most strongly close to grid scale.
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