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 < 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> 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

11. Reddening curves • Mean extinction varies within, between galaxies • Reddening ~1/λ 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,Mg]SiO3) • olivine ([Fe,Mg]2SiO4) • 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 Poisson’s equation

19. 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

20. 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 < 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 > some ε

24. 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