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 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 ne( C iiS ii) 0: 11_0: 10 0: 05 cm 3. 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 3. Given some simple assumptions, the range of observed electron densities translates into a range of thermal pressures, P/ k 3300_5500 1900 K cm 3. This work greatly expands the number of electron density measurements and provides important constraints on the ionization, abundance, and evolutionary models of the LISM.