Astronomy and Astrophysics, volume 310, 315-327 (1996/6-1)
The ionization fraction in dense clouds.
DE BOISANGER C., HELMICH F.P. and VAN DISHOECK E.F.
Abstract (from CDS):
We present submillimeter observations of various molecular ions toward two dense clouds, NGC 2264 IRS1 and W 3 IRS5, in order to investigate their ionization fraction. Analysis of the line intensity ratios by the way of statistical equilibrium calculations allows determination of the physical parameters: n(H2)~(1-2)x106cm–3 and Tkin∼50-100K. Column densities and abundances are also derived. Together, the abundances of the observed ions provide a lower limit to the ionization fraction, which is (2-3)x10–9 in both clouds. In order to better constrain the electron abundance, a simple chemical model is built which calculates the steady state abundances of the major positive ions, using the observed abundances wherever available. With reasonable assumptions, good agreement within a factor of two with the observations can be achieved. The calculated electron fraction is xe=(1.0-3.3)x10–8 in the case of NGC 2264 and xe=(0.5-1.1)x10–8 for W 3 IRS5. In the first case, the high abundance of N2H+ requires a rather high cosmic ray ionization rate >10–16s–1, even if all nitrogen is assumed to be in gas phase N2. For W 3 IRS5, ionized metals such as Fe+ and Mg+ could provide 60% of the electrons.
ISM: molecules - ISM: clouds - ISM: individual: W 3 IRS5 (W 3m), NGC 2264 IRS1 (RAFGL 989) - radio lines: ISM - ISM: abundances