Inhomogeneity within Local Interstellar Clouds

Abstract

Analysis of interstellar absorption lines observed in high-resolution Hubble Space Telescope spectra of nearby stars provides temperatures, turbulent velocities, and kinetic properties of warm interstellar clouds. A new analysis of 97 interstellar-velocity components reveals a wide range of temperatures and turbulent velocities within the Local Interstellar Cloud (LIC) and the nearby Cluster of Interstellar Clouds (CLIC). These variations appear to be random with Gaussian distributions. We find no trends of these properties with stellar distance or angles from the Galactic Center, magnetic field, the main source of extreme-UV radiation (the star ϵ CMa), the center of the LIC, or the direction of inflowing interstellar matter into the heliosphere. The spatial scale for temperature variations in the LIC is likely smaller than 5100 au, a distance that the Sun will traverse in 1000 yr. Essentially all velocity components align with known warm clouds. We find that within 4 pc of the Sun, space is completely filled with partially ionized clouds, but at larger distances space is only partially filled with partially ionized clouds. We find that the neutral hydrogen number density in the LIC and likely other warm clouds in the CLIC is about 0.10 cm^-3 rather than the 0.20 cm^-3 density that may be representative of only the immediate environment of the LIC. The ≤3000-12,000 K temperature range for the gas is wider than the predictions of thermal equilibrium theoretical models of the warm neutral medium and warm ionized medium, and the high degree of inhomogeneity within clouds argues against simple theoretical models. *All of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via https://doi.org/10.17909/gabn-m136. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support to MAST for these data is provided by the NASA office of Space Science via grant No. NAG5-7584 and by other grants and contracts.