Theoretical calculations of the H I , He I and He II free - bound continuous emission spectra

We present coefficients for the calculation of the continuous emission spectra of H I, He I and He II due to electron-ion recombination. Coefficients are given for photon energies from the first ionization threshold for each ion to the n = 20 threshold of hydrogen (36.5μ), and for temperatures 100 K ≤ T ≤ 10 K. The emission coefficients for He I are derived from accurate ab initio photoionization data. The coefficients are scaled in such a way that they may be interpolated by a simple scheme with uncertainties less than 1% in the whole temperature and wavelength domain. The data are suitable for incorporation into photoionisation/plasma codes and should aid with the interpretation of spectra from the very cold ionised gas phase inferred to exist in a number of gaseous clouds.