The Structure of the Local Interstellar Medium. V. Electron Densities

Abstract

We present a comprehensive survey of C iiabsorption 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 obtainedbyGHRS andSTISonboardHST,wesearched foralldetectionsof LISM Ciiabsorption.We identify13sightlineswith23individualCiiabsorption components,whichprovide electrondensitymeasurements. 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 iiline, constraints from independent temperature measurementsof theLISMgasbasedonlinewidthsof otherions,andthird,usingmeasuredSiicolumndensitiesasa proxyforCiicolumndensities.Thedistributionof electrondensitiesbasedonusingSiiasaproxyforCiiissimilarto 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) ¼ 0:11 þ0:10 � 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) ¼ 0:12 � 0:04 cm � 3 . Given some simple assumptions, the range of ob- served electron densities translates into a range of thermal pressures, P/k ¼ 3300 þ5500 � 1900 Kc m � 3 . This work greatly expands the number of electron density measurements and provides important constraints on the ionization, abun- dance, and evolutionary models of the LISM. Subject headingg atomic processes — ISM: abundances — line: profiles — solar neighborhood — techniques: spectroscopic — ultraviolet: ISM