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Hydroxyl Emission from Hydroxyl Emission from Shock Waves in Shock Waves in
Interstellar CloudsInterstellar Clouds
Catherine Braiding
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
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.
Supernova Remnants
Supernovae
• Mark the death of massive stars (>8Msun).
• 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.
Supernovae + Molecular Clouds
Wardle and Yusef-Zadeh (Science, volume 206, 2002)
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.
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.
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
OH 1720 MHz Masers
• Not found in stellar atmospheres.
• Require specific physical conditions:– Density: n ~ 105 cm-3 – Temperature: T ~ 50–100 K
– OH column density 1016 – 1017 cm-2 – The absence of a strong far-infrared continuum.
• Collisionally-pumped by H2
(Pavlakis & Kylafis 1996, ApJ, 467, 300)
OH Level Diagram
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.
Shock Waves
• *image of J vs C type shocks*
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.
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
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.
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
Candy (G349.7+02) – H2
J. S. Lazendic et. al. in preparation
Candy (G349.7+02) – OH
J. S. Lazendic et. al. in preparation
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.
Oxygen Chemistry in a C-type Shock
(Wardle, ApJ, 525:L101, 1999)
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.
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.
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.
(Pavlakis & Kylafis 1996, ApJ, 467, 300)
OH Level Diagram
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.
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.
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.
Testing the Level Population Code
• *insert picture here*
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.
Future Directions
• Calculations of the emission should then be compared with observations.
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.
Future Directions
SOFIA will fly in 2004 (we hope).
(http://sofia.arc.nasa.gov)
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