Anomalous Neutral Hydrogen Column Densities in Local Interstellar Medium Clouds* * Based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS AR-09525.01A. These observations are associated with programs #12475 and #12596.

Abstract

Analysis of high-resolution spectra of the hydrogen and deuterium Lyα lines provides measurements of interstellar neutral hydrogen column densities N(H i) to 113 stars within 50 pc of the Sun. Plots of N(H i) versus distance through the LIC, G, Mic, and other clouds in the local interstellar medium (LISM) show very interesting properties. For the LIC and G clouds, nearly all of the observed neutral hydrogen occurs within 3 or 4 pc of the Sun with no significant additional neutral hydrogen at larger distances out to 50 pc, except for several sight lines with anomalously high N(H i). Scatter about the mean hydrogen column density in the LIC is several times larger than the measurement errors. We evaluate several possible sources of the high N(H i) sight lines. Seven sight lines with anomalously high N(H i) are aligned perpendicular to the line connecting the centers of the LIC and G clouds where the two clouds merge creating a mixed cloud region. The high N(H i) in these seven sight lines can be explained by their paths through an irregularly shaped mixed cloud region where the neutral hydrogen number density is the sum for both clouds. However, the most recent nearby supernova explosion created a shell that is seen in nearly the same direction perpendicular to the LIC/G axis and may also explain these seven high N(H i) sight lines. Other possible explanations for the high N(H i) sight lines include interstellar shocks, wakes produced by stars moving rapidly through the LISM, and regions where other clouds may overlap.