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Scattering And Absorption of Light in Planetary Regoliths
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Photoprocesses in protoplanetary disks.

Ewine F van Dishoeck1, Bastiaan Jonkheid, Marc C van Hemert

  • 1Leiden Observatory, P.O. Box 9513, 2300 RA, The Netherlands. ewine@strw.leidenuniv.nl

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Summary

Young stars emit intense ultraviolet radiation, shaping inner circumstellar disks into photon-dominated regions (PDRs). This study analyzes UV radiation effects on disk chemistry and structure, presenting new photodissociation data.

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Area of Science:

  • * Astrophysics
  • * Astrochemistry

Background:

  • * Circumstellar disks are exposed to intense ultraviolet (UV) radiation from young stars.
  • * Inner disks experience UV radiation enhanced by over seven orders of magnitude compared to the interstellar medium.
  • * This intense UV field creates conditions similar to dense photon-dominated regions (PDRs).

Purpose of the Study:

  • * To investigate the physical and chemical structure of circumstellar disks under intense UV radiation.
  • * To analyze parameters differentiating disk PDRs from traditional PDRs.
  • * To compute photodissociation and ionization rates for various UV radiation fields and grain sizes.

Main Methods:

  • * Analysis of UV radiation effects on disk chemistry, vertical structure, and gas temperature.
  • * Comparison of disk PDRs with traditional PDRs, considering factors like UV field shape, grain growth, and PAH absence.
  • * Computation of photodissociation and ionization rates using new cross-section data for various species and radiation fields.
  • * Presentation of infrared spectra from the Spitzer Space Telescope.

Main Results:

  • * Intense UV radiation significantly impacts disk chemistry, vertical structure, and gas temperature, particularly in surface layers.
  • * Key differences between disk PDRs and traditional PDRs identified, including UV field shape, grain growth, and absence of PAHs.
  • * New photodissociation cross sections for ions and other species were computed.
  • * Photodissociation and ionization rates were calculated for a range of UV radiation fields and grain sizes.
  • * Illustrative infrared spectra from Spitzer demonstrate disk properties.

Conclusions:

  • * The intense UV radiation from young stars creates unique PDR environments in circumstellar disks.
  • * Understanding these PDRs is crucial for comprehending disk evolution and planet formation.
  • * Accurate photoprocess modeling, including grain growth effects, is essential for interpreting observations of transitional disks like HD 141569A.