Through the thick and thin: Electron density measurements of the local interstellar medium

In a comprehensive survey of observations from the high resolution spectrographs on board the Hubble Space Telescope (HST), the Goddard High Resolution Spectrograph (GHRS) and the Space Telescope Imaging Spectrograph (STIS), we have detected interstellar CII absorption for the 1334.5 angstrom ground-state transition and the 1335.7 angstrom (doublet) collisionally populated excited-state transition for 13 sight lines toward stars within 100 pal-sees. We derive column densities of CII in each of the states and use methods implemented by Wood & Linsky (1997) to measure the electron densities along the lines of sight. Electron densities in the local interstellar medium (LISM) are particularly difficult to ascertain, but are essential for understanding the ionization structure of the LIS 14 and density variations which impact the morphology of the heliosphere. To combat systematic error due to the saturation of the CII resonance line, we also use SII measurements as a proxy to determine column densities. Our sample consists of 13 stars, a significant improvement in the number of measurements that allows us to search for variations in density for the first time. The sample of electron densities appears consistent with a log-normal distribution, with an unweighted mean density of n(e) = 0.13(-0.07)(+0.15) cm(-3) and, based on SII as a proxy, n(e) (CIISII) = 0.11(-0.05)(+0.10) cm(-3).