Exeter: The formation of massive stars

Exeter: The formation of massive stars

Figure 1: Density cut through the simulation. The colour scale shows density (scaled between 10−19 and 10−14 g/cc). The arrows show the direction of the velocity field, and their colour the speed (between 0 and 100 km/s).

Stars form as molecular clouds gravitationally collapse and fragment. The formation of the most massive stars (generally understood to be those with masses greater than twenty times the mass of the Sun) is complicated by the presence of the intense protostellar radiation field, which produces outward forces which may hinder accretion and even lead to outflows. Incorporating the effects of the radiation field on the gas is a complex problem, involving a combination of microphysical effects (dust heating, scattering, ionisation) that require a polychromatic treatment of a radiation field.

 Figure 2: The line profile (upper left), integrated intensity map (upper right in Jy km/s/pixel), moment 1 map (lower left) and position velocity diagram (lower right in mJy/pixel), for the CH3CN J = 13 → 12, K = 3 line.

At Exeter we have developed a novel radiation hydrodynamic (RHD) code that treats the problem using a Monte Carlo method, splitting the radiation field into many millions of photon packets that propagate through the computational domain, undergoing absorptions, scatterings and re-emissions. We have run this code on DiRAC’s Complexity system to simulate the growth of a massive star. We find that the star forms via stochastic disc accretion and produces fast, radiation-driven bipolar cavities. The evolution of the envelope infall rate and the accretion rate on to the protostar are broadly consistent with observational constraints. After 35 kyr the star has a mass of 25 solar masses and is surrounded by a 7 solar-mass Keplerian disc of 1500 au radius (see Figure 1). Once again these results are consistent with those from recent high-resolution studies of discs around forming massive stars.

We are able to construct so called-synthetic observations from our RHD models. For example, molecular line simulations of a methyl cyanide (CH3CN) transition compare well with observations in terms of surface brightness and line width. These results indicate that it should be possible to reliably extract the protostellar mass from such observations (see Figure 2).