The Structure of the Local Interstellar Medium V: Electron Densities

We present a comprehensive survey of C II absorption detections toward stars within 100 pc in order to measure the distribution of electron densities present in the local interstellar medium (LISM). Using high spectral resolution observations of nearby stars obtained by the Goddard High-Resolution Spectrograph (GHRS) and the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST), we searched for all detections of LISM C II absorption. We identify 13 sight lines with 23 individual C II absorption components, which provide electron density measurements, the vast majority of which are new. We employ several strategies to determine more accurate C II column densities from the saturated C II resonance line, including, constraints of the line width from the optically thin C II line, constraints from independent temperature measurements of the LISM gas based on line widths of other ions, and third, using measured S II column densities as a proxy for C II column densities. The distribution of electron densities based on using S II as a proxy for C II is similar to the distribution based on carbon alone, while significantly tighter, and proves to be a promising technique to avoid grossly overestimating the C II column density based on the saturated line profile. The sample of electron densities appears consistent with a log-normal distribution and an unweighted mean value of ne(C IISII) = 0.11 +0.10 −0.05 cm. Seven individual sight lines probe the Local Interstellar Cloud (LIC), and all present a similar value for the electron density, with a weighted mean of ne(LIC) = 0.12±0.04 cm . Two clouds, the NGP and Gem clouds, show similar electron density properties as the LIC. The Hyades Cloud, a decelerated cloud at the leading edge of the platoon of LISM clouds, has a significantly higher electron density than the LIC. Observed toward G191-B2B, the high electron density may be caused by the lack of shielding from such a strong radiation source.