Examining astrophysical gas-cloud collapse using an optical depth-scaled, x-ray-irradiated, carbon-foam sphere

When stellar radiation interacts with a molecular cloud, the cloud fate depends on the strength of the incident radiation and the radiation’s mean-free-path within the cloud [1]. Under the right conditions, the radiation compresses the cloud and star formation may occur. Where and when the stellar formation occurs in the cloud’s collapse are open questions. Direct observation of a cloud implosion with initial conditions is nearly impossible, making laboratory astrophysics a way to investigate.

This work describes laboratory experiments to study the radiation-driven implosion of clouds, using x-rays from a laser-irradiated, thin-gold foil as a surrogate star and a carbon-foam sphere as a surrogate cloud. The system’s optical depth was controlled by the foam density. Gold foil and sphere motions were imaged by x-ray radiography. Radiographic images show the formation of an interface between rarefied gold and carbon plasmas, a shock moving into the sphere, and a blunting of the initial sphere’s shape. Measurements show that the shock moved linearly around 64 um/ns into the sphere, and the gold-carbon interface formed by 2 ns at the sphere edge remained stationary. The deformation of the sphere was driven by the incident radiation and not by mechanical pressures applied by gold plasma. The blunting of the sphere was likely due to the geometric reduction of flux near the sphere’s poles. Higher x-ray flux near the sphere’s equator caused high compression and a faster shock, which flattened the sphere. We will discuss the results and implications of our observations.

[1] F. Bertoldi. ’The photoevaporation of interstellar clouds I Radiation Driven Implosion’. In The Astrophysical Journal 346.2 (1989), pp. 735-755.