Mixing Interstellar Clouds Surrounding the Sun

TL;DR

This paper reveals that the Sun moves through a mixed interstellar cloud composed of multiple nearby clouds, challenging previous assumptions that it resides within a single cloud, based on combined observational data.

Contribution

It introduces a new model showing the local interstellar medium as a mixture of clouds, supported by combined space-based measurements and analysis of interstellar velocities.

Findings

The Sun's velocity is consistent with a mixture of cloud velocities.

The local interstellar medium is a combined environment from multiple clouds.

Interactions between clouds increase local hydrogen density.

Abstract

On its journey through the Galaxy, the Sun passes through diverse regions of the interstellar medium. High-resolution spectroscopic measurements of interstellar absorption lines in spectra of nearby stars show absorption components from more than a dozen warm partially ionized clouds within 15 pc of the Sun. The two nearest clouds - the Local Interstellar Cloud (LIC) and Galactic (G) cloud - move toward each other. Their bulk heliocentric velocities can be compared with the interstellar neutral helium flow velocity obtained from space-based experiments. We combine recent results from Ulysses, IBEX, and STEREO observations to find a more accurate estimate of the velocity and temperature of the very local interstellar medium. We find that, contrary to the widespread viewpoint that the Sun resides inside the LIC, the locally observed velocity of the interstellar neutral helium is consistent with a linear combination of the velocities of the LIC and G cloud, but not with either of these two velocities. This finding shows that the Sun travels through a mixed-cloud interstellar medium composed of material from both these clouds. Interactions between these clouds explain the substantially higher density of the interstellar hydrogen near the Sun and toward stars located within the interaction region of these two clouds. The observed asymmetry of the interstellar helium distribution function also supports this interaction. The structure and equilibrium in this region require further studies using in situ and telescopic observations.