The Role of the Critical Ionization Velocity Effect in Interstellar Space and the Derived Abundance of Helium

Gaussian analysis of new, high-angular-resolution interstellar 21-cm neutral hydrogen emission profile structure more clearly reveals the presence of the previously reported signature of the critical ionization velocity (CIV) of helium (34 km s
^-1
). The present analysis includes 1496 component line widths for 178 neutral hydrogen profiles in two areas of sky at galactic latitudes around -50°, well away from the galactic plane. The new data considered here allow the interstellar abundance of helium to be calculated, and the derived value of 0.095 ± 0.020 agrees extremely well with the value of 0.085 for the cosmic abundance based on solar data. Although the precise mechanisms that give rise to the CIV effect in interstellar space are not yet understood, our results may provide additional motivation for further theoretical study of how the mechanism operates.