2008ApJ...683..207R

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 obtained by GHRS and STIS on board 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. 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 lognormal distribution and an unweighted mean value of n_e(C II_SII_)=0.11+0.10^_–0.05/cm3. 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 n_e(LIC)=0.12±0.04/cm3. Given some simple assumptions, the range of observed electron densities translates into a range of thermal pressures, P/k=3300⁺⁵⁵⁰⁰_–1900 K/cm3. This work greatly expands the number of electron density measurements and provides important constraints on the ionization, abundance, and evolutionary models of the LISM.