Charging of Dust Grains in Photoionized Gas Marcos Hilsenrat

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Charging of Dust Grains in Photoionized
GasMarcos Hilsenrat
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Outline

• AGNs (Brief introduction)
• Gas interaction with ionizing radiation
• Scientific background
• Evidence for dust in AGNs
• Research goals
• Calculations and results
• Basic assumptions
• Calculation steps
• Results and discussion

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Active Galactic Nuclei

• Energetic phenomena in central regions of
  galaxies which cannot be attributed to stars.
• Strongest persistent sources of ionizing
  radiation in the Universe.
• The ionizing radiation is produced by gas
  accreting onto the central massive black hole.
• The two largest subclasses of AGNs are
• - Seyfert galaxies - LSeyfert Lgalaxy
• - Quasars - Lquasar 100 Lgalaxy

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Gas interaction with ionizing radiation

• The temperature and ionization state of
  photoionized gas are set by interaction with the
  radiation field.
• Low radiation intensity and energy ? gas tend to
  be cold and neutral.
• Photon flux and energy rises ? gas ionization
  state increases, but its temperature remains
  nearly constant at T 104 K because the line
  cooling effect (Osterbrock 1989).
• Above a critical ionization parameter, U nph/
  ne
• ? high ionization state ? inefficient line
  cooling
• ? runaway to Compton temperature, TC.
• For AGNs, TC 107 K (e.g. Krolik 1999).

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Gas interaction with ionizing radiation (cont)

• About 1 of the gas mass in the Galactic ISM is
  in the form of small solid grains (e.g. Whittet
  1992).
• Inelastic scattering of electrons on the grains
  is an ionization independent cooling mechanism
  which increases with Te ? dominant at high enough
  U.
• This cooling may stop the gas from reaching TC.
• Grain surface provides a recombination site for
  incident ions ? may lower the ionization state of
  the gas.
• From X-ray absorption spectra ? presence of
  highly, but not fully ionized plasma, not
  reaching TC (e.g. George et al. 1998).
• Dust is rapidly spattered at Te gt 106 K (Draine
  Salpeter 1979).
• Only if Te lt 106 K, dust may have a major effect
  on the temperature and ionization state of
  photoionized dusty gas in AGNs.

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Gas equilibrium temperature

• Simple analytic estimates suggest that Te may be
  kept below 106 K.
• (Gas Osterbrock 1989, Dust Draine 1978)
• For typical values in a pure H region, the level
  of ionization is
• x nHII/nHI ? 3 x 103 U
• As U increases, x increases, therefore
• nH nHII ? ?gas? 2 x 10-18x-1cm2
• ?dust? 1.4 x 10-21 cm2/ (H atom) is
  independent of U
• For U gt 3 x 10-3 ? ?dust gt ?gas

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Gas equilibrium temperature (cont.)
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Gas equilibrium temperature (cont.)
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Evidence for dust in AGNs
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Research goals

• Interactions of X-ray photons with grains
• Grain photoelectric yield
• Energy spectrum of ejected photoelectrons
• Heating and cooling rates of gas by dust
• Equilibrium grain charge as a function of grain
  size, composition, gas density and ionizing flux
• Gas equilibrium temperature

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Basic assumptions

• Ionizing source (relevant to AGNs)
• Luminosity 1044 erg s-1
• Distance 1 pc
• Shape of the radiation field
• Gas (normalizing parameters)
• Composition Hydrogen
• Temperature 104 K
• Density 1 cm-3
• Dust grains (observational data)
• Composition C (graphite) and (MgFe)SiO4
  (olivine) (e.g. Laor Draine 1993)
• Mass fraction ? mdust/ mgas ? 0.01
• Geometrical shape
• Size distribution

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Basic assumptions (cont.)

• Shape of the radiation field
• Power law spectrum (e.g. Peterson 1997)
• dnph/dE photons s-1 eV ? Eph-?
• 2 ? 3 ? ? 2.5, Eph 104 eV
• Grains geometrical shape
• Low polarization of extincted light (e.g. Draine
  Lee 1984) ? nearly spherical shape
• Grains size distribution
• Stellar light extinction ? MRN grain size
  distribution (Mathis, Rumpl Nordsieck 1977)
• dn(a)/da ? a-3.5, 0.005 ?m a 0.25 ?m
• Optical geometry approach
• Photons travel in straight lines, ignoring
  refraction and reflection inside the grain

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Basic assumptions (cont.)

• Photoelectron current from grain (e.g. Evans
  1991)
• Incoming current from ions and electrons
  impinging on grain from Draine Sutin (1987)
• In electrostatic equilibrium

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Calculation steps

• Grain composition and photon energy range
  bound-free absorption cross section
• Grain optical depth parameter and absorption
  efficiency
• Pathlength distribution function
• Energy of photoelectrons at their formation
  points, including Auger electrons
• Electrons range
• Energy distribution of photoelectrons at grain
  surface
• Integration over the power law ionizing continuum
  and next, over the MRN size distribution
• Current of impinging electrons and ions on grain
• Grain potential to obtain a zero total current

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Bound-free absorption cross section

• Calculated with the fit parameters given by
  Verner Yakovlev (1995)

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Grain absorption efficiency

• grain optical depth parameter ?? 2an??

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Absorption efficiency for extreme grain radius in
the MRN grain size distribution
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Absorption efficiencyaccurate vs. simplified
calculations

• Accurate to 20 above 10 eV, and to better
  than 1 above 100 eV.

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Pathlength distribution function Distribution of
distances traveled by photoelectrons from their
emission point to the grain surface (Voit
1991)Angular distribution of velocities (Agarwal
1991)

• 1. Distribution of dimensionless escape
  pathlengths
• 2. Probability of photoabsorption between ? and
  ? d ? for a given ??

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Pathlength distribution function (cont)

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Pathlength distribution function (cont)
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Electron energy deposition in matter

• Ee Eph Ebind
• R(Ee) Column density of the material necessary
  to stop an electron with initial energy Ee.
• Fittings from Ashley Anderson (1981) and Ashley
  (1990)

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Electron ranges(Comparison with Dwek Smith
(1996))
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Electron energy distribution on grain surface for
specific grain radius
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Energy carried away by escaping
electrons(Comparison with Dwek Smith (1996))
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Integration over the power law ionizing continuum
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Integration over the MRNgrain size distribution
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Incoming current (Draine Sutin (1987))Maximal
grain potential (Draine Salpeter (1979))
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Calculation of grain equilibrium charge
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Equilibrium charge and equilibrium current as
function of grain radius
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Grain equilibrium potentialHeating and cooling
rates

• Vg(graphite) (993.84 ? 11.13) V
  Vg(silicate) (2274.27 ? 41.03) V

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Minimal surviving grain size as function of the
distance to the continuum source
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Summary

• Flexible code
• Build in parameter and assumption
• Grain spherical shape
• Optical geometry approach
• Needed Experimental data in the relevant energy
  regime for electron ranges in matter.
• Inclusion of other physical effects
• Sticking coefficient
• Recombination rates on grain surface
• Grain size time dependence sputtering by ion
  impacts