Probing the properties of nebular plasmas with optical and IR spectroscopy

M. J. Barlow (Dept of Physics & Astronomy, University College London)

Ionized nebulae provide spectacular probes of conditions that cannot easily be recreated in the laboratory. Their spectra are dominated by dipole-forbidden transitions from ions of carbon, nitrogen, oxygen, neon and sulphur, which are observed only because of the extremely low electron and ion densities (106 < Ne < 1011 m-3) that are encountered in typical nebulae. These heavy element forbidden lines are routinely used to determine electron temperatures and densities, and thereby ionic and elemental abundances, both in our own Galaxy and in other galaxies near and far. I will review recent progress in this field and how it both depends on, and hopefully will continue to stimulate, new
laboratory measurements and theoretical calculations.

Massive stars photoionize and eventually disperse their natal clouds of gas and dust but during their earliest phases these stars and their surrounding ionized nebulae can be difficult to observe at optical wavelengths, due to the obscuration by dust particles in the surrounding clouds out of which they formed. However, the amount of dust obscuration decreases rapidly with increasing wavelength, becoming negligible at sufficiently large infrared wavelengths. I will describe how recent space observatories have utilised advances in infrared technologies to observe a wide range of ionic infrared fine-structure emission lines, providing us with a less biased picture of the relative contributions of
star formation and massive central black holes to the overall energetics of galaxies.
Many ionized nebulae, particularly H II regions and planetary nebulae in the nearby Universe, are not heavily obscured by dust. Analyses of optical forbidden lines emitted by such nebulae typically yield electron temperatures of about 104 K. The advent of 8-m class telescopes equipped with highly sensitive CCD array detectors has enabled high signal to noise detections of large numbers of faint optical recombination lines emitted by the same C, N, O and Ne ions that emit bright forbidden lines. I will describe how comparisons made between the intensities of heavy element recombination and forbidden lines have provided tantalising evidence for the existence within some nebulae of ‘cold plasma’, with temperatures ranging from just a few thousand K down to as low as only a few hundred
K in some cases.

Talk presented at the 17th International Conference on Atomic Processes in Plasmas, Queen’s University Belfast, 19-22 July 2011.

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