JAC Style guide: template
 The Ionization Fraction in Dense Clouds
The ionization fraction of dense interstellar clouds is a crucial parameter of interstellar physics, since it
influences the efficiency with which magnetic fields couple with the gas and ultimately controls the rate of
collapse and star formation. Moreover, electrons and ions play a major role in interstellar chemistry.
Stimulated by the sensitive receivers at submillimeter wavelengths and new laboratory measurements of the
dissociative recombination rate coefficients, de Boisanger, Helmich and van Dishoeck (1996, A&A in
press) have taken a new look at this problem. Using the JCMT, they have observed several different ions
and their corresponding neutrals in two dense clouds, NGC 2264 and W3 IRS5. N2H+ is particularly strong
toward NGC 2264: the figure shows a clear detection of the high-excitation 5 -> 4 line at 465.824
GHz. Both clouds have n(H2) ÷ (1 - 2) 10(6) cm(-3) and T(kin) ~ 50 - 100 K. Together, the abundances of the
observed ions provide a lower limit to the ionization fraction of (2 - 3) 10(-9) in both clouds.
In order to better constrain the electron abundance, a simple chemical model has been constructed, which
uses observed abundances wherever available. The resulting electron fraction is x(e) = (1 - 3) 10(-8) in the case
of NGC 2264 and xe = (0.5 - 1.1) 10(-8) for W3 IRS5. In the first case, the high abundance of N2H+ requires a
high (cosmic ray) ionization rate of >10(-16) s(-1), even if all nitrogen is assumed to be in gas-phase N2. For
W3 IRS5, ionized metals such as Fe+ and Mg+ could provide ~60% of the electrons.
C. de Boisanger{1,2}, F.P. Helmich(1) and E.F. van Dishoeck(1)
1 Leiden Observatory, The Netherlands
2 CEA, Bruyères-le-Châtel, France
Figure 1: Observed N2H+ 5 - 4 spectrum toward NGC 2264 obtained with the JCMT and receiver C2.
Last Modification Date 1996/08/13 - Last Modification Author: gdw
Graeme Watt (gdw)
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