The interface between the outer heliosphere and the inner lism: Morphology of the local interstellar cloud, its hydrogen hole, stromgren shells, and $^{60}$Fe accretion

TL;DR

This paper models the shape and composition of the local interstellar medium around the Sun, revealing a hydrogen-deficient region and proposing the influence of nearby stellar radiation and clouds on the heliosphere's environment.

Contribution

It provides a three-dimensional model of the local interstellar cloud and introduces the concept of a hydrogen hole, linking interstellar structures to heliospheric conditions and $^{60}$Fe influx.

Findings

Identification of a hydrogen hole near $eta$~CMa and Sirius B.

The Blue cloud forms a Str"omgren shell influencing the heliosphere.

Interstellar grain influx suggests nearby supernova activity.

Abstract

We describe the interface between the outer heliosphere and the local interstellar medium (LISM) surrounding the Sun. The components of the inner LISM are the four partially ionized clouds [the Local Interstellar Cloud (LIC), G cloud, Blue cloud, and Aql cloud] that are in contact with the outer heliosphere, and ionized gas produced by EUV radiation primarily from $\epsilon$~CMa. We construct the three-dimensional shape of the LIC based on interstellar line absorption along 62 sightlines and show that in the direction of $\epsilon$~CMa, $\beta$~CMa, and Sirius~B the neutral hydrogen column density from the center of the LIC is a minimum. We call this region the ``hydrogen hole''. In this direction, the presence of Blue cloud absorption and the absence of LIC absorption can be simply explained by the Blue cloud lying just outside of the heliosphere. We propose that the outer edge of the Blue cloud is a Str\"omgren shell driven toward the heliosphere by high pressures in the H~II region. We find that the vectors of neutral and ionized helium flowing through the heliosphere are inconsistent with the LIC flow vector, and that the nearby intercloud gas is consistent with ionization by $\epsilon$~CMa and other stellar sources without requiring additional sources of ionization or million degree plasma. In the upwind direction, the heliosphere is passing through an environment of several LISM clouds, which may explain the recent influx of interstellar grains containing $^{60}$Fe from supernova ejecta measured in Antarctica snow.