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Understanding Ionized Gas

Here on earth we are aware of ultraviolet (UV) radiation in space because of the dangers it poses to our health; the atmosphere is our shield against its damaging effects. Most astronomical environments are also bathed in UV radiation. In many places -- near young hot stars, for example, or in the vicinity of black holes accreting material -- that UV light is much more intense than the corresponding radiation that we get from the sun, and often there is little protection. UV light is potent because each photon carries enough energy to knock electrons off an atom, turning neutral atoms into charged "ions," and thereby altering their chemical and physical behaviors. Ions in space are essential probes of extremely hot environments.

Ions themselves can emit light across the electromagnetic spectrum. In order to unravel the physical conditions in a region of ionized gas, scientists rely on models and computer codes that follow all of the energetic processes at work as ultraviolet light first ionizes atoms into ions, and then as the ions themselves encounter other ions or radiation, emit light themselves, and so on. For over a decade, astronomers have had considerable success with simple models that assume that the geometry of these regions changed only in one direction, as it might in passing across an infinitely broad but thin slab of material.
Needless to say, this approximation to real, three-dimensional objects has always been recognized as a serious limitation to the full picture.

A team of three SAO astronomers, Barbara Ercolano, Jeremy Drake, and John Raymond, along with a colleague of theirs, have just announced the availability of a new, three-dimensional computer program for tracking radiation and its effects in ionized gas. The new code, called MOCASSIN (for Monte Carlo Simulations of Ionized Nebulae), was built on earlier, simpler versions of the code, and takes advantage of powerful new computer technology. The team reports having tested and confirmed the reliability of MOCASSIN by comparing its predictions against those of several other current, standard codes. The new program, which is now publicly available, also incorporates the most recent and reliable physical parameters for the behaviors of highly ionized atoms. The program will finally allow scientists to model asymmetric, complex three-dimensional ionized regions, for example, those found in jets erupting from the vicinity of black holes, and to accurately interpret the wealth of information encoded in the spectral lines emitted by the highly charged, ionized atoms in these regions.