Theoretical calculations of the HI, HeI and HeII free-bound continuou emission spectra

Abstract

We present coefficients for the calculation of the continuous emission spectra of HI, HeI and HeII 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 (36.5um), and for temperatures 100 K <= Te <=10^5 K. The emission coefficients for HeI 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.