Molecules and Dust 1 April 2003

Molecules and Dust

• 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.

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

Collisional Dissociation

• Electron collisions with molecules most important collisional dissociation mechanism

• Collisional dissociation
• Dissociative ionization
• Dissociative recombination most likely

Photodissociation

• 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

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

• Ices freeze on in molecular cloud cores

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

• 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.

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

• stars
• dark matter

• Self-consistent potential from the matter on the grid

• requires solution of Poisson’s equation

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

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

• 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 > 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.

• By averaging onto coarser grids, larger-scale parts of solution can be found