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)