Hydroxyl Emission from Shock Waves in Interstellar Clouds

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Hydroxyl Emission from Shock Waves in
Interstellar Clouds

• Catherine Braiding

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Hydroxyl Emission in Interstellar Clouds

• Supernova Remnants Molecular Clouds
• OH Masers
• Shock Waves
• OH emission from Shock Waves
• Modelling OH
• Testing the Model
• Modelling Shock Waves
• Future Directions

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Molecular Clouds

• About half the gas in the Galaxy is found in
  clouds of dense gas.
• These are cold enough (10-30 K) to form
  molecules.
• Gravitational collapse causes star formation.
• The clouds are dispersed by ultraviolet
  radiation, stellar wings and supernovae.

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Supernova Remnants
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Supernovae

• Mark the death of massive stars (gt8Msun).
• Distribute energy and heavy elements into the
  interstellar medium.
• Frequently occur near molecular clouds, due to
  the short lifespan of massive stars.
• Cause shock waves to be driven into the molecular
  cloud.

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Supernovae Molecular Clouds
Wardle and Yusef-Zadeh (Science, volume 206, 2002)
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Supernovae Molecular Clouds

• Shock waves create compression and heating in the
  cloud.
• This can lead to star formation.
• The chemical composition of the gas is changed,
  as reactions between molecules are allowed to
  occur.
• It is difficult to positively identify this
  behaviour.

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Supernovae Molecular Clouds

• A signpost of the interaction is the OH 1720
  MHz maser.
• About 10 of supernova remnants possess maser
  spots.
• By studying the emission and absorption of other
  OH lines in shocked gas as well as the maser
  spots, can gain a better understanding of the
  interaction.

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OH Masers

• Microwave Amplification of Stimulated Emission
  Radiation
• Microwave analogue of a laser.
• Occur naturally in stellar atmospheres and
  interstellar space.
• Bright, compact spectral line sources.
• These occur at 1612, 1665, 1667 and 1720 MHz

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OH 1720 MHz Masers

• Not found in stellar atmospheres.
• Require specific physical conditions
• Density n 105 cm-3
• Temperature T 50100 K
• OH column density 1016 1017 cm-2
• The absence of a strong far-infrared continuum.
• Collisionally-pumped by H2

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OH Level Diagram
(Pavlakis Kylafis 1996, ApJ, 467, 300)
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Shock Waves

• These conditions are satisfied if the shock is a
  slow, continuous shock wave.
• The low ionisation level in the molecular cloud
  causes the magnetic pressure to exceed the
  thermal pressure by several orders of magnitude.
• When a slow shock passes through, the ions stream
  ahead of the shock wave in what is known as a
  magnetic precursor.

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Shock Waves

• image of J vs C type shocks

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Shock Waves

• In C-type shocks, ion-neutral collisions smooth
  out the viscous transition, so that an extended
  region of gas is heated.
• Critical velocity for C-type shocks is 40-50 km
  s-1.
• Supernova-driven shock waves travel at 25 km
  s-1.

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Shock Waves

• All of the OH produced within the shock at
  temperatures above 400 K is converted rapidly to
  water.
• O H2 ? OH H
• OH H2 ? H2O H

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Shock Waves

• The dissociation of water by ultraviolet
  radiation creates OH.
• H2O ? OH H
• X-rays from the supernova and cosmic rays induce
  a far-ultraviolet radiation field that is capable
  of dissociating water.

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Shock Waves

• How does one identify these shocks?
• OH 1720 MHz maser signpost
• OH also detected in absorption
• Known to be strong sources of H2 2.12 µm emission
• Contrast between CO emission in the both shocked
  and unshocked regions of the cloud

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Candy (G349.702) H2
J. S. Lazendic et. al. in preparation
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Candy (G349.702) OH
J. S. Lazendic et. al. in preparation
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Modelling the OH Emission

• Wardle (1999) showed that by including
  photodissociation in the oxygen chemistry, the OH
  column density produced was sufficient to form OH
  1720 MHz masers.
• This effect has not been examined in previous
  models.

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Oxygen Chemistry in a C-type Shock
(Wardle, ApJ, 525L101, 1999)
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Modelling the OH Emission

• Want to calculate the populations of the excited
  levels of OH for a given gas density, temperature
  and column density.
• Using this information, can then determine
  emission from one point in the gas.
• This can then be incorporated into shock
  calculations.

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Calculating the Level Populations

• The level populations change over time as
• equation
• These equations are integrated over a long period
  of time, so that many collisions and radiative
  transitions may occur, bringing the system to
  equilibrium.

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Calculating the Level Populations

• Data was provided for the Einstein A coefficients
  for the first 32 hyperfine-split levels of OH.
• Given the high temperatures found in shocked gas,
  more levels were required for the model.

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OH Level Diagram
(Pavlakis Kylafis 1996, ApJ, 467, 300)
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Calculating the Level Populations

• The HITRAN 96 database contained level energies
  for the first 100 split levels of OH.
• Unfortunately, it only contained rotational
  transitions from the first 72 levels.
• However, the code can easily be updated when more
  data comes to hand.

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Calculating the Level Populations

• The collisional rates used were obtained from
  Offer, Hemert and van Dishoeck, for transitions
  between the lowest 24 states.
• For the higher states, hard sphere rates were
  used.

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Testing the Level Population Code

• For low temperatures and densities, the level
  populations should be concentrated in the lower
  levels.
• In the limits of high temperature or density, the
  population distribution tends towards a Boltzmann
  distribution.

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Testing the Level Population Code

• insert picture here

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Future Directions

• The shock code needs to be optimised for better
  runtimes.
• The calculated emission needs to be tested.
• The dependence of the emission on the input
  parameters will be explored.
• The effect of the X-ray flux on the emission
  should be examined.

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Future Directions

• Calculations of the emission should then be
  compared with observations.

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Future Directions

• Further observations of supernova remnant /
  molecular cloud interactions would provide
  greater opportunity to test this theory of OH
  emission.
• The GREAT spectrometer on SOFIA will be capable
  of detecting the warm OH column density within
  C-type shocks.

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Future Directions
SOFIA will fly in 2004 (we hope).
(http//sofia.arc.nasa.gov)