The Three‐dimensional Structure of the Warm Local Interstellar Medium. I. Methodology

Abstract

In this first in a series of papers, we develop a methodology for constructing three-dimensional models of the local interstellar cloud (LIC) and adjacent warm clouds in the local interstellar medium (LISM). Our models are based on the column density of neutral hydrogen gas (N) inferred primarily from measurements of the deuterium column density toward nearby stars obtained from the analysis of Hubble Space Telescope ultraviolet spectra. We also use values of N inferred from spectra of hot white dwarfs and B-type stars obtained by the Extreme Ultraviolet Explorer satellite. These very different methods give consistent results for the three white dwarf stars in common. We assume that along each line of sight all interstellar gas moving with a speed consistent with the LIC velocity vector has a constant density, N = 0.10 cm-3, and extends from the heliosphere to an edge determined by the value of N moving at this speed. A number of stars have velocities and/or depletions that indicate absorption by other warm clouds in their lines of sight. On this basis α Cen A and B and probably also Ind lie inside the Galactic center (G) cloud, HZ 43 and 31 Com lie inside what we call the north Galactic pole cloud, and β Cet is located inside what we call the south Galactic pole cloud. We show the locations of these clouds in Galactic coordinates. The Sun is located very close to the edge of the LIC toward the Galactic center and the north Galactic pole. The absence of Mg II absorption at the LIC velocity toward α Cen indicates that the distance to the edge of the LIC in this direction is ≤0.05 pc and the Sun should leave the LIC and perhaps enter the G cloud in less than 3000 yr. Comparison of LIC and total values of N toward pairs of stars with separations between 09 and 20° reveals a pattern of good agreement so long as both stars lie within 60 pc of the Sun. Thus the LIC and perhaps also other nearby warm clouds have shapes that are smooth on these angular scales. In our second paper we will therefore fit the shape of the LIC with a set of smooth basis functions (spherical harmonics).