Our cloud-coud collision simulations attempt to capture the more realistic scenario in which low-density, atomic / quasi-molecular clouds collide in spiral arms to produce high density molecular clouds. Although the inital clouds are seeded with some low-level turbulence, this does not decay as much as in the isolated clouds, as the collision pumps kinetic energy into the gas. These simulations are performed with Arepo, and thus cover both magnitised and non-magnitised clouds.

Technical details

Physial properties in initial conditions

The clouds have 10⁴ solar masses, initial density of 10 cm^-3, and a radius of 19 pc. They are stirred with a turbulent field with power law scaling k^-4, comprising purely solenoidal modes. The turbulence has a mean velocity dispersion of 1 km/s. This turbulence is roughly transonic at our initial temperature of ~ 200-300 K. Clouds collide head on (along the x axis), which each cloud having a velocity of 3.75 km/s towards the other. The clouds are initially 19pc apart. The magnetic field is set to be B_x = 3 μG, and zero in the other dimentions. The clouds are embedded in the WNM with mean density of 0.1 cm^-3.

For the CO chemistry, we use the Nelson and Langer (1999) reduced network and for the H₂ chemistry we use the prescriptions in Glover and Mac Low (2007). More details can be found in Glover and Clark (2012b). The clouds start with a small ionisation fraction of H⁺ = 0.01, to account for the ionisation by cosmic rays, but otherwise the chemistry starts off with the hydrogen in the form of atomic hydrogen, carbon in the form of C⁺, and oxygen in its neutral form.

In the simulations listed below, we vary 1) the strength of the interstellar radiation field, quoted in Habing units, and expressed via G₀, 2) the cosmic ray ionisation rate (CRIR), which are assumed to be free-streaming in the simulations 3) the inital turbulent velocity field, to create different 'clouds'. We will add to the list of simulations below as more become available.